Chemistry Notes for NEET Chapter 27: Aldehydes and Ketones PDF
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These notes cover aldehydes and ketones, including their structures, properties, and various methods of preparation. The document explains different reactions and mechanisms involved in the formation of these compounds.
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60 Chapter E3 27 Aldehydes and Ketones ID Introduction Carbonyl compounds are of two types, aldehydes and ketones. Both have a carbon-oxygen double bond often called as carbonyl group. C D YG Both aldehyde and Carbonyl ketonesgroup possess the same general formula Cn H 2n O. Structure : Carbonyl car...
60 Chapter E3 27 Aldehydes and Ketones ID Introduction Carbonyl compounds are of two types, aldehydes and ketones. Both have a carbon-oxygen double bond often called as carbonyl group. C D YG Both aldehyde and Carbonyl ketonesgroup possess the same general formula Cn H 2n O. Structure : Carbonyl carbon atom is joined to three atoms by sigma bonds. Since these bonds utilise sp 2 -orbitals, they lie in the same plane and are 120° apart. The carbon-oxygen double bond is different than carboncarbon double bond. Since, oxygen is more electronegative, the electrons of the bond are attracted towards oxygen. Consequently, oxygen attains a partial negative charge and carbon a partial positive charge making the C O bond polar. The high values of dipole moment, U (2.3 – 2.8D) cannot be explained only on the basis of inductive effect and thus, it is proposed that carbonyl group is a resonance hybrid of the following two structures. ST C O 120° 120° C O 120° || R CH 2 OH R C H agents Primary alcohol Aldehyde Mild oxidising agents are (a) X 2 (Halogen) (b) Fenton reagent ( FeSO 4 H 2 O2 ) U O O Mild oxidising (c) K 2 Cr2 O7 / H (d) Jones reagent (e) Sarret reagent (f) MnO 2 (g) Aluminium tertiary butoxide [ Al(O C(CH 3 )3 )3 ] When the secondary alcohols can be oxidised to ketones by aluminium tert-butoxide, [(CH 3 )3 CO ]3 Al the reaction is known as oppenauer oxidation. Unsaturated secondary alcohols can also be oxidised to unsaturated ketones (without affecting double bond) by this reagent. The yield of aldehydes is usually low by this methods. The allylic alcohols can be converted to aldehydes by treating with oxidising agent pyridinium chloro-chromate (C5 H 5 NH CrO3 Cl ). It is abbreviated as PCC and is called Collin's reagent. This reagent is used in non-aqueous solvents like CH 2 Cl 2 (dichloro methane). It is prepared by mixing pyridine, CrO3 and HCl in dichloromethane. This is a very good reagent because it checks the further oxidation of aldehydes to carboxylic acids and is suitable method for preparing ,-unsaturated aldehydes. (ii) Dehydrogenation of 1° and 2° alcohols by Cu/300° or Ag/300°C. O || C O Cu / 300 C R CH 2 OH R C H H 2 OH | O || -bond R CH R ' R C R ' H 2 -bond (2) From carboxylic acids Cu / 300 C (i) Distillation of Ca, Ba, Sr or Th salts of monobasic acids O Preparation of carbonyl compounds (1) From alcohols OH (i) By oxidation. O | || Mild oxidising R CH R ' R C R ' Secondary alcohol agents Ketone || (RCOO )2 Ca (R' COO )2 Ca 2 R C R ' 2CaCO 3 Thus in the product, one alkyl group comes from one carboxylic acid and other alkyl group from other carboxylic acid. Calcium salts of dibasic acid (1, 4 and higher) on distillation give cyclic ketones. (iii) Wacker process || C H2 C O Distillation Ca | CH 2 C O || PdCl2 / HOH (a) CH 2 CH 2 CH 3 CHO O+CaCO air / Cu 2Cl2 Ethene 3 O Cyclopropanone O O || Distillation O C (CH 2 )5 COO Ca || (b) R CH CH 2 R C CH 3 O PdCl 2 / HOH air/Cu 2 Cl 2 Alkyl ethene (5) From alkynes O H O/HgSO /H SO Cyclohexanone 2 (ii) Decarboxylation or Dehydration of acids by MnO/ 300°C. (a) This reaction takes place between two molecules of carboxylic acids. Both may be the same or different. 4 2 R – C – CH 4 R–CC–H (i) SiO BH 2 3 R – CH – CHO Case I : When both molecules are HCOOH O || || H C OH H COO H MnO CO 2 HOH H C H 300 C formic acid 2 formaldehyde O || || HCOOC H Aldehyde O O || || Carboxylic acid D YG Ketone 2 5 R – C – R' (Ketone) O (Excess) (i) HCN R–C–H (ii) H O/H MnO / 300 C R C OH R COO H R C R CO 2 HOH (Aldehyde) O R – MgX U Case III : When none of the molecule is formic acid. (Only ketone) H–C–R 5 R' COOC H MnO / 300 C R C OH H COO H R C H CO 2 HOH formic acid R' – C – R O ID O Carboxylic acid O O 2 Case II : When only one molecule is formic acid. 2 (6) From Grignard reagents R' – C – Cl O 2 (ii) H O / OH E3 (b) If one of the carboxylic acids is HCOOH then this acid undergoes decarboxylation because this acid is the only monobasic acid which undergoes decarboxylation even in the absence of catalyst. 3 60 O O 2 (i) R' – CN R – C – R' (ii) H O/H (3) From gem dihalides : Gem dihalides on hydrolysis give carbonyl compounds 2 O O (i) R CHX 2 R CHO HOH / O H Gemdihalide X | | O || U This method is not used much since aldehydes are affected by alkali and dihalides are usually prepared from the carbonyl compounds. (4) From alkenes ST (i) Ozonolysis : Alkenes on reductive ozonolysis give carbonyl compounds (i)O3 R CH CH R R CHO RCHO Alkene R (ii)H 2O / Zn R' CC R 2 Aldehyde HOH / O H R C R ' (ii) R C R ' X CH = CH – C – H (i) O3 O O || || R C R R ' C R ' (ii)H O / Zn 2 R – CH – CH – C – H 2 || || R ' 2 Cd R C Cl R C R ' O O || || R ' 2 CuLi R C Cl R C R ' (Only used for the preparation of ketones) In this method product is always ketone because R H and also R' H. (ii) Rosenmunds reduction : This reduction takes place in the presence of Lindlars catalyst. O O || R' 2 (7) From acid chloride (i) Acid chlorides give nucleophilic substitution reaction with dialkyl cadmium and dialkyl lithium cuprate to give ketones. This is one of the most important method for the preparation of ketones from acid chlorides. O O || H 2 / Pd BaSO 4 CaCO 3 R C Cl R C H Alkene Xylene This method is used only for aliphatic carbonyl compounds. (ii) Oxo process O || O H 2 / Pd BaSO 4 CaCO 3 Xylene CO 2 (CO )8 R CH CH 2 CO H 2 R CH 2 CH 2 CHO 150 C , 300 atm Oxo process is used only for the preparation of aldehydes. || Ar C Cl Ar C H (Only used for aldehydes) (8) From cyanides (i) Stephen aldehyde synthesis : Conversion of cyanides into aldehydes by partial reduction with SnCl 2 / HCl , followed by hydrolysis, is known as Stephens aldehyde synthesis. CHO CO/HCl /Anhy. ZnCl /Cu Cl 2 R C N R CHO (i) SnCl 2 / HCl / ether (ii)H 2O / or steam distillation Alkylcyanide | CH Benzaldehyde CH 3 CO/HCl /Anhy. ZnCl /Cu Cl O | 2 Benzene Aldehyde (Only used for aldehydes) (9) From vic diols OH OH 2 2 2 CH 3 CHO + 2 || HIO 4 R CH C R RCHO R C R H 2 O Toluene | o-methyl benzaldehyde Cl || CO/HCl /Anhy. ZnCl /Cu Cl 2 DMSO R C H R R C R Chloro benzene DMSO or (i) (CH 2 )6 N 4 C 6 H 5 CH 2 Cl C 6 H 5 CHO (ii) H 2 O / H or Cu ( NO 3 )2 or Pb( NO 3 )2 + CH CH 3 CH + 2 (ii) H O/ 2 o-methyl benzaldehyde Toluene CHO p-methyl OH OH OH || (i) NaOH R C R D YG R benzaldehyde O O CH N 3 CHO (i) Zn(CN) /HCl gas Aldehyde R benzaldehyde 3 R CHO 70 % H 2 SO 4 CHO p-Chloro (4) Gattermann formylation : This reaction is mainly given by alkyl benzenes, phenols and phenolic ethers. U (Aci form) CHO 2 o-Chloro benzaldehyde ID (11) From nitro alkanes : Nitro alkanes having at least one hydrogen atom give carbonyl compounds on treatment with conc NaOH followed by 70% H 2 SO 4. The reaction is known as Nef carbonyl synthesis. O OH O H2 NaOH R CH 2 N R C H N Tautomerisation O O 2 Cl E3 O | benzaldehyde Cl DMSO R CH 2 Cl R CHO ; Cl CHO p-methyl 60 R Pb(OCOCH 3 )4 also gives similar oxidation products. (10) From Alkyl halides and benzyl halides 3 O CHO (i) Zn(CN) /HCl gas Ketone (ii) H 2 SO 4 + 2 (ii) H O/ 2 (Iso - nitro alkane) (12) Reaction with excess of alkyl lithium : Carboxylic acids react with excess of organo lithium compound to give lithium salt of gem diols which on hydrolysis give ketones. O O || o-salicylaldehyde Phenol OCH3 || 2 2 U Preparation of only aromatic carbonyl compounds (1) From methyl arenes 2 C H CHO (ii) HOH (Etard's reaction) 3 3 6 5 2 6 C H CHO 500°C 6 Cu(NO ) / C H – CH Cl OH 2 Air/MnO 5 OH 5 (ii) H O (2) From chloro methyl CHO p-methoxy (5) Houben – Hoesch reaction : This reaction isbenzaldehyde given by di and C H CHO 3 3 Toluene 6 5 o-methoxy benzaldehyde Anisol polyhydric benzenes. (i) CrO /(CH CO) O/CH COOH ST C H – CH 6 + (ii) H O/ (ii)HOH / H 2 CHO (i) Zn(CN) /HCl gas (i) R Li (excess) R ' C OH R ' C R (i) CrO Cl OCH3 CHO p-salicylaldehyde OCH3 3 2 C H CHO 6 Pb(NO ) / 3 2 5 OH 2 6 2,4-dihydroxy OH ketone (i) RCN/HCl gas/Anhy.ZnCl 4 6 OH OH 5 C H – CHO (3) Gattermann – : This reaction is mainly given by aromatic hydrocarbons and halobenzenes. 2 COR (i) (CH ) N / (ii) formylation HO Koch 2 (ii) H O Resorcinol C H – CHO 6 2 (i) RCN/HCl gas/Anhy.ZnCl 5 2 5 (ii) H O 2 2 HO OH Phloroglucinol OH HO COR 2,4,6-trihydroxy ketone OH OH CHO (i) CHCl /Alc.KOH/ 3 + (ii) H O/H + 2 phenol (major) CHO (Minor) Physical properties of carbonyl compounds (1) Physical state : Methanal is a pungent smell gas. Ethanal is a volatile liquid, boiling points 294 K. Other aldehydes and ketones containing up to eleven carbon atoms are colourless liquids while higher members are solids. (2) Smell : With the exception of lower aldehydes which have unpleasant odours, aldehydes and ketones have generally pleasant smell. As the size of the molecule increases, the odour becomes less pungent and more fragrant. In fact, many naturally occurring aldehydes and ketones have been used in blending of perfumes and flavouring agents. (3) Solubility : Aldehydes and ketones upto four carbon atoms are miscible with water. This is due to the presence of hydrogen bonding between the polar carbonyl group and water molecules as shown below : – – C O H + O + + – H + O=C O || OH | Addition R C R ' H Nu R C R ' | Nu Adduct In addition reactions nucleophile adds on carbonyl carbon and electrophile on carbonyl oxygen to give adduct. (iv) Relative reactivity of aldehydes and ketones : Aldehydes and ketones readily undergo nucleophilic addition reactions. However, ketones are less reactive than aldehydes. This is due to electronic and stearic effects as explained below: (a) Inductive effect : The relative reactivities of aldehydes and ketones in nucleophilic addition reactions may be attributed to the amount of positive charge on the carbon. A greater positive charge means a higher reactivity. If the positive charge is dispersed throughout the molecule, the carbonyl compound becomes more stable and its reactivity decreases. Now, alkyl group is an electron releasing group (+I inductive effect). Therefore, electron releasing power of two alkyl groups in ketones is more than that of one alkyl group in aldehyde. As a result, the electron deficiency of carbon atom in the carbonyl group is satisfied more in ketones than in aldehydes. Therefore, the reduced positive charge on carbon in case of ketones discourages the attack of nucleophiles. Hence ketones are less reactive than ID (5) Reactions due to -hydrogen (6) Condensation reactions and (7) Miscellaneous reactions (1) Nucleophilic addition reactions (i) Carbonyl compounds give nucleophilic addition reaction with those reagents which on dissociation give electrophile as well as nucleophile. (ii) If nucleophile is weak then addition reaction is carried out in the presence of acid as catalyst. (iii) Product of addition reactions can be written as follows, 60 OH (1) Nucleophilic addition reactions (2) Addition followed by elimination reactions (3) Oxidation (4) Reduction E3 (6) Reimer – Tiemann reaction : Phenol gives o- and p- hydroxy benzaldehyde in this reaction. U With the increase in the size of alkyl group, the solubility decreases and the compounds with more than four carbon atom are practically insoluble in water. All aldehydes and ketones are, however, soluble in organic solvents such as ether, alcohol, etc. The ketones are good solvents themselves. D YG (4) Boiling points : The boiling points of aldehydes and ketones are higher than those of non polar compounds (hydrocarbons) or weakly polar compounds (such as ethers) of comparable molecular masses. However, their boiling points are lower than those of corresponding alcohols or carboxylic acids. This is because aldehydes and ketones are polar compounds having sufficient intermolecular dipole-dipole interactions between the opposite ends of C O dipoles. C O C O C O However, these dipole-dipole interactions are weaker than the intermolecular hydrogen bonding in alcohols and carboxylic acids. Therefore, boiling points of aldehydes and ketones are relatively lower than the alcohols and carboxylic acids of comparable molecular masses. ST U Among the carbonyl compounds, ketones have slightly higher boiling points than the isomeric aldehydes. This is due to the presence of two electrons releasing groups around the carbonyl carbon, which makes them more polar. CH 3.. CO: H Acetaldehyde 2.52 D b.pt. 322 K CH 3.. CO: CH 3 Acetone 2.88 D b.pt 329 K aldehydes. Formaldehyde with no alkyl groups is the most reactive of the aldehydes and ketones. Thus, the order of reactivity is: H CO > H Chemical properties of carbonyl compounds Carbonyl compounds give chemical reactions due to carbonyl group and -hydrogens. Chemical reactions of carbonyl compounds can be classified into following categories. CO Formaldehy de CO > R H Aldehyde Ketone (b) Stearic effect : The size of the alkyl group is more than that of hydrogen. In aldehydes, there is one alkyl group but in ketones, there are two alkyl groups attached to the carbonyl group. The alkyl groups are larger than a hydrogen atom and these cause hindrance to the attacking group. This is called stearic hindrance. As the number and size of the alkyl groups increase, the hindrance to the attack of nucleophile also increases and the reactivity of a carbonyl decreases. The lack of hindrance in nucleophilic attack is another reason for the greater reactivity of formaldehyde. Thus, the reactivity follows the order: H CH 3 CH 3 CO > (5) Density : Density of aldehydes and ketones is less than that of water. R R CO H Formaldehy de CO > CH 3 H Acetaldehyde (CH 3 )2 CH Acetone (CH 3 )3 C CO (CH 3 )2 CH Di - isopropyl ketone CO > (CH 3 )3 C Di - tert. butyl ketone > In general, aromatic aldehydes and ketones are less reactive than the corresponding aliphatic analogues. For example, benzaldehyde is less reactive than aliphatic aldehydes. This can be easily understood from the resonating structures of benzaldehyde as shown below: Addition of sodium bisulphite All types of aldehydes give addition reaction with this reagent. OH O || O | || H or OH or R C H R C H R C H HSO 3 Na | O.. : H..– O.. : H C..– O.. : H C Adduct; white crystalline in nature C Only aliphatic methyl ketones give addition reaction with sodium bisulphite. O OH HSO 3 Na | O H Colourless crystalline product C C HCHO SO 3 Na III This reagent can be used for differentiation between aromatic and aliphatic methyl ketones, e.g. E3 H || H or OH or R C CH 3 R C CH 3 R C H ..– O.. : II O | 60 || I HCHO SO 3 Na O || CH 3 CH 2 C CH 2 CH 3 and Benzaldehyde Acetopheno ne Benzopheno ne || | D YG OH R C H HCN R C CN | H Cyanohydri n O || OH C 6 H 5 C H HCN OH | C 6 H 5 C CN | Benzaldehyde H Benzaldehyde cyanohydri n ST U Because HCN is a toxic gas, the best way to carry out this reaction is to generate hydrogen cyanide during the reaction by adding HCl to a mixture of the carbonyl compound and excess of NaCN. Benzophenone does not react with HCN. Except formaldehyde, all other aldehydes gives optically active cyanohydrin (racemic mixture). This reaction is synthetically useful reaction for the preparation of -hydroxy acids, -amino alcohols and -hydroxy aldehydes. C6 H 5 C CH 3 and CH 3 CH 2 C CH 3 This reagent can be used for the separation of aldehydes and aliphatic methyl ketones from the mixture, e.g. O || CH 3 CH 2 C CH 2 CH 3 CH 3 CH 2 CHO and These two compounds can be separated from their mixture by the use of NaHSO. Higher aliphatic ketones and aromatic ketones do not react with NaHSO. 3 3 Addition of alcohols : Carbonyl compounds give addition reaction with alcohols. This reaction is catalysed by acid and base. Nature of product depends on the catalyst. Case I : Addition catalysed by base : In the presence of a base one equivalent of an alcohol reacts with only one equivalent of the carbonyl compound. The product obtained is called hemiacetal (in case of aldehyde) and hemiketal (in case of ketone). The reaction is reversible. There is always equilibrium between reactants and product. O || CH 3 C H CH 3 O H OH | HO CH 3 C H | OCH 3 | - Hydroxy acid (If R is CH then product is lactic acid) 3 OH Hemiacetal O || CH 3 C CH 3 CH 3 O H HO OH | CH 3 C CH 3 | OCH 3 H /Pt Hemiketal 2 OH | R C(i)H SnCl CH NH 2 /HCl2 - Amino alcohol 2 (ii) HOH/ || || R C H COOH 2 R CH CN O O OH H O/H/ | || CH 3 CH 2 CH 2 C CH 3 U Some important examples of nucleophilic addition reactions Addition of HCN OH O O ID IV V It is clear from the resonating structures that due to electron releasing resonance effect of the benzene ring, the magnitude of the positive charge on the carbonyl group decreases and consequently it becomes less susceptible to the nucleophilic attack. Thus, aromatic aldehydes and ketones are less reactive than the corresponding aliphatic aldehyde and ketones. The order of reactivity of aromatic aldehydes and ketones is, C6 H 5 CHO > C6 H 5 COCH 3 > C6 H 5 COC 6 H 5 OH | R C H CHO -hydroxy aldehyde Hemiacetals and hemiketals are -alkoxy alcohols. Case II : Addition catalysed by acid : In the presence of an acid one equivalent of carbonyl compound reacts with two equivalents of alcohol. Product of the reaction is acetal (in case of aldehyde) or ketal (in case of ketone). O OCH 3 || R C H 2CH 3 OH OH O | || | H R C R' R C R ' HOH R C H H 2O | | Ketone OCH 3 OH Gemdiol Acetal Gem diols are highly unstable compounds hence equilibrium favours the backward direction. The extent to which an aldehyde or ketone is hydrated depends on the stability of gem diol. OCH 3 O | || R C R H 2O R C R 2CH 3 OH | OCH 3 Stability of gem diols depend on the following factors: Ketal (i) Steric hindrance by +I group around -carbon decreases the stability of gem diols. +I group decreases stability of gem diol and hence decreases extent of hydration. (i) Formation of acetals and ketals can be shown as follows: 60 OCH 3 H 2O C R (ii) Stability of gem diols mainly depends on the presence of –I group on -carbon. More is the –I power of the group more will be stability of gem diols. H O CH 3 R R H C O OCH 3 OCH 3 | OCH 3 Addition of terminal alkynes : This reaction is known as ethinylation. || (Excess) || | | Sod. salt of alkyne R' D YG || O || CH 3 C CH 2 CH 2 OH U This can be achieved by protection of C O group and then by deprotection Addition of Grignard reagents : Grignard reagents react with carbonyl compounds to give alcohols. Nature of alcohol depends on the nature of carbonyl compound. O (i) H – C – H ST (ii) HOH/H O RMgX Grignard reagent (i) R' – C – H (ii) HOH/H O (i) R' – C – R' | R' alkynol (2) Addition followed by elimination reactions : This reaction is given by ammonia derivatives (NH 2 Z). (i) In nucleophilic addition reactions poor nucleophile such as ammonia and ammonia derivatives requires acid as catalyst. (ii) If the attacking atom of the nucleophile has a lone pair of electrons in the addition product, water will be eliminated from the addition product. This is called a nucleophilic addition elimination. Primary amines and derivatives of ammonia react with carbonyl compounds to give adduct. In adduct nucleophilic group has lone pair of electrons. It undergoes elimination to give product known as imine. An imine is a compound with a carbon-nitrogen double bond. O 2 1°-alcohol .. H R C R H N H Z || R – CH OH | HOH / H R C C C R " This reaction is very useful reaction for the protection of carbonyl group which can be deprotected by hydrolysis. Glycol is used for this purpose. Suppose we want to carry out the given conversion by LiAlH4. LiAlH4 CH 3 C CH 2 COOC 2 H 5 OH U Ketal O ONa O R C C Na R ' C R R C C C R " O H R C R H 2 O R C R 2CH 3 OH | More is the strength of hydrogen bond more will be the stability of gem diol. ID (ii) Acetals and ketals are gem dialkoxy compounds. (iii) High yield of acetals or ketals are obtained if the water eliminated from the reaction is removed as it formed because the reaction is reversible. (iv) Acetals and ketals can be transformed back to corresponding aldehyde or ketone in the presence of excess of water. (iii) Intramolecular hydrogen bonding increases stability of gem diols. –I groups present on carbon having gem diol group increases strength of hydrogen bond. E3 H O CH 3 R OH | OH R C R | R' – CH – R | | 3°-alcohol | Addition of water (ii) : Carbonyl HOH/H compounds react with water to give gem diols. This reaction is catalysed by acid. RThe reaction is reversible reaction. The overall reaction can be shown as follows R .. An imine R C O N H 2 Z H 2 O CN R H R CNZ R N HZ.. 2°-alcohol OH R' – C – R' R HOH R An imine Different Imine formation with NH 2 Z is given below O R–C– Benedict's solution and Fehling solutions are used as a reagent for the test of sugar (glucose) in blood sample. (c) Tollens reagent : Tollens reagent is ammonical silver nitrate solution. Its reacting species is Ag . It oxidises aliphatic as well as aromatic aldehydes. Redox R CHO Ag RCOOH Ag (as silver mirror) reaction This reagent has no effect on carbon-carbon multiple bond. 60 CH 2 CH CHO Ag CH 2 CH COOH Ag In this reaction the oxidation no. of Ag varies from +1 to 0. Glucose, fructose give positive test with Tollen's reagents and Fehling solution. C5 H11 O5 CHO Cu 2 O (or) Ag2 O C5 H11 O5 COOH E3 Fructose contain with Fehling solution due to presence of -hydroxyl keto group. Tollens reagent also gives positive test with terminal alkynes and HCOOH. (d) Reaction with mercuric chloride solution : R C H HgCl2 H 2 O R C OH HCl Hg2 Cl 2 () D YG O || CH 3 C C 6 H 5 C 6 H 5 C NH CH 3 || N OH (ii) H 2 O N - methyl acetamide In short product of the rearrangement can be obtained as follows: R R' C || N OH O || U Tautomerisation R ' C O H R ' C NH R || RN ST (3) Oxidation of carbonyl compounds (i) Oxidation by mild oxidising agents : Mild oxidising agents oxidise only aldehydes into carboxylic acids. They do not oxidises ketones. Main oxidising agents are: (a) Fehling solution : It is a mixture of two Fehling solution: Fehling solution No. 1 : It contains CuSO 4 solution and NaOH. Fehling solution No. 2 : It contains sodium potassium tartrate. (Roschelle salt). (b) Benedict's solution : This solution contains CuSO 4 , NaOH and sodium or potassium citrate. Reacting species of both solutions is Cu oxidation no. of Cu varies from 2 to 1. These two oxidising agents oxidise only aliphatic aldehydes and have no effect on any other functional groups (Black) || SO 2 (e) Schiff's reagent : Megenta solution colourless solution N - phenylacetamide Acetopheno xime (White) O U || (i) PCl5 O || (i) PCl5 C 6 H 5 C CH 3 CH 3 C NH C 6 H 5 N OH || O O (ii) H 2 O || O R C H Hg2 Cl 2 H 2 O R C OH HCl Hg() (PCl5 , SOCl 2 , PhSO 2 Cl, RCOCl , SO 3 , BF3 etc.) || C O (keto) group yet give positive test ID Beckmann rearrangement : Ketoxime when treated with acid at 0°C it undergoes rearrangement known as Beckmann rearrangement. Thus acid catalysed conversion of ketoximes to N-substituted amides is called Beckmann rearrangement. Acid catalyst used are proton acids (H 2 SO 4 , HCl, H 3 PO4 ) and Lewis acids Gluconic acid CH 3CHO pink colour restored (In cold). (ii) Oxidation by strong oxidising agents : Main strong oxidising agents are KMnO4 / OH / , KMnO4 / H / , K 2 Cr2 O7 / H / and conc HNO 3 / . These agents oxidise aldehydes as well as ketones. (a) Oxidation of aldehydes : Aldehydes are oxidised into corresponding acids. [O ] RCHO RCOOH C n C n KMnO 4 / O H / C6 H 5 CHO C6 H 5 COOH (b) Oxidation of ketones : Ketones undergo oxidation only in drastic conditions. During the oxidation of ketones there is breaking of carboncarbon bond between -carbon and carbonyl carbon. In this process both carbons convert into carboxylic groups. This leads to the formation of two moles of monocarboxylic acids. Case I : Oxidation of symmetrical ketones O || CH 3 CH 2 CH 2 C C=7 [O ] CH 2 CH 2 CH 3 COOH COOH CH 3 CH 2 CH 2 COOH CH 3 CH 2 COOH C 4 C 3 Total number of C 4 3 7 Thus number of carbons in any product is less than the number of carbons in ketone. Case II : Oxidation of unsymmetrical ketones : In case of unsymmetrical ketones -carbon whose bond breaks always belongs to the alkyl group which has more number of carbons. This rule is known as Popoff’s rule. O || CH 3 CH 2 CH 2 C CH 2 CH 3 COOH Example : [O ] CH 3 CO CH 2 CH 3 CH 3 COOH HOOCCH 3 (d) Baeyer- villiger oxidation : H | H C H O O C H H C OH COOH CH 3 CH 2 COOH CH 3 CH 2 COOH [O ] O Reaction will be held if the oxidising agent is performic acid. (4) Reduction of carbonyl compounds (i) Reduction of – group C – into –CH – group : Following three reagents reduce carbonyl group into CH 2 groups: (a) HI / P / (b) 2 Zn / Hg / Conc. HCl and (c) NH 2 NH 2 / OH. | COOH (CH 2 )3 CH COOH [O ] 2-Methyl cyclohaxanone O R – CH – R' (Clemmenson reduction) 2 R – CH – R' into hydroxy (ii) Reduction of carbonyl compounds (Wolff-kishner reduction)compounds : Carbonyl group converts into by CHOH group LiAlH4 , NaBH 4 , Na / C2 H 5 OH and aluminium isopropoxide. 2 U (i) X2 / OH R C CH 3 RCOOH CHX 3 2 2 (i) LiAlH4 R CHO R CH 2 OH (ii) H (b) Oxidation at -CH or CH by SeO : SeO oxidises CH 2 group into keto group and CH 3 group into aldehydic group. 2 3 In this oxidation reactivity of CH 2 is more than the CH 3 group and Oxidation is regio selective in nature. 2 CH 3 CHO CHO CHO ; SeO Glyoxal O || CH 3 C CH 3 CH 3 C CHO SeO 2 (ii) NaBH 4 (iii)Aluminium isopropoxi de 2 D YG 2 Methylglyoxal U (c) Oxidation by organic peracids : Organic peracids oxidise aldehydes into carboxylic acids and ketones into esters. This oxidation is known as Baeyer – Villiger oxidation. O O || R C R R C O R In case of aldehyde there is insertion of atomic oxygen (obtained from peracid) between carbonyl carbon and hydrogen of carbonyl carbon. In case of ketone, insertion of oxygen takes place between carbonyl carbon and -carbon. Thus the product is ester. This is one of the most important reaction for the conversion of ketones into esters. Vic dicarbonyl compound also undergo oxidation and product is anhydride. O O ST C6 H 5 COOOH Zn/Hg/Conc. HCl R – C – R' NH – NH /OH || 2 | CH 3 3 (iii) Miscellaneous oxidation (a) Haloform Reaction methyl carbonyl R – CH – R' O - Methyl adipic acid || || O E3 H || || O ID CH || || O HI/P/ O O | CH 3 C H O O C H CH 3 C OH O place between carbonyl carbon and the -carbon which has maximum number of hydrogens. H H | Adipic acid If both -carbons are not identical then bond breaking takes H || O [O ] COOH (CH 2 )4 COOH || O 60 Case III : Oxidation of cyclic ketones : Formation of dibasic acid takes place from cyclic ketones. In this case the number of carbons in ketone and dibasic carboxylic acid is always same. || O || O OH || | R C R ' R CH R ' (i) LiAlH4 (ii) NaBH 4 (iii)Aluminium isopropoxi de NaBH 4 is regioselective reducing agent because it reduced only. CHO in the presence of other reducible group. Example : NaBH 4 CH 3 CH CH CH 2 OH CH 3 CH CH CHO Crotonalde hyde Crotonyl alcohol Hydride ion of reduction. NaBH 4 attack on carbonyl carbon during OD O || | CH 3 C CH 2 CH 3 Example : CH 3 C CH 3 2-Butanone NaBD4 | D2 O D OH | NaBD CH – C – CH – CH 4 3 O 2 O D || 2 Butanone OD | || NaBH O Popoff's rule : Oxidation of unsymmetrical ketones largely take place in such a way that the smaller alkyl group remains attached to the CO group during the formation of two molecules of acids. This is known as Popoff's rule 3 CH 3 C CH 3 C6 H 5 COOOH R C C R R C O C R || 2 | HO DO 4 CH – C – CH – CH 3 2 3 | H (iii) Reductive amination : In this reduction CO group converts 2 into CH NH 2 group R R C O NH 3 R C NH CH NH 2 H 2 / Ni R (a) -hydrogen of carbonyl compounds are acidic in character due to the presence of the electron withdrawing CO group. R Imine -Hydrogen is acidic due to strong –I group; – CO –. Primary amine (iv) Reduction of ketones by Mg or Mg/Hg : In this case ketones undergo reduction via coupling reaction and product is vic cis diol. O || || | | (i) Mg / Hg R C C R R C C R || || R R | (ii)HOH R Vic cis diol (pinacol) When this reaction is carried out Mg / Hg / TiCl4 , the product is vic trans diol. (i) Hg – Mg – TiCl O 2 | R in the presence of O CH 3 C R HO OH Vic trans diol (v) Reduction of benzaldehyde by Na/C H OH : Benzaldehyde undergoes reduction via coupling reaction and product is vic diol. 2 5 O O || | | (i) Na/C 2 H 5 OH C6 H 5 C C C6 H 5 | CH 2 C R CH 2 C R Enolate ion (more stable) (c) The acidity of -hydrogen is more than ethyne. pKa value of aldehydes and ketones are generally 19 – 20 where as pKa value of ethyne is 25. (d) Compounds having active methylene or methyne group are even more acidic than simple aldehydes and ketones. O || pKa 15.9 C6 H 5 CH 2 C CH 3 (ii)HOH ID H H O || Carbanion (less stable) 4 (ii) HOH O Base || Cyclohexanone || O | –C–C– (b) Thus| carbonyl compounds having -hydrogen convert into Carbon carbanions -in the presence of base. This carbanion is stabilised by delocalisation of negative charge. E3 O H | OH OH 60 R - phenyl acetone OH OH | O | C6 H 5 CH C H C6 H 5 vic diol || O || pKa 8.5 C6 H 5 C CH 2 C CH 3 (Bouveault-blanc reaction) - benzoyl acetone (ii) Halogenation : Carbonyl compounds having -hydrogens undergo halogenation reactions. This reaction is catalysed by acid as well as base. (a) Acid catalysed halogenation : This gives only monohalo derivative. O O D YG U Aldehydes are reduced to 1° alcohols whereas ketones to 2° alcohols. If carbon – carbon double bond is also present in the carbonyl compound, it is also reduced alongwith. However, the use of the reagent 9BBN (9–borabicyclo (3, 3, 1) nonane) prevents this and thus only the carbonyl group is reduced Example : HOCH 2CH 2 NH 2 9 BBN CH CH CHO Cinnamaldehyde CH CHCH 2OH || Br / CH COOH Cinnamyl alcohol U O || CH 3 CH 2 C CH 2 CH 3 can also use LiAlH in this reaction. If reducing agent is aluminium iso propoxide (CH 3 C H O )3 Al. Product will be alcohol. This reaction is called | bromo acetone Acetone (b) Base catalysed halogenation : In the presence of base all hydrogens of the same carbon is replaced by halogens. If reducing agent is NaH, reaction is called Darzen's reaction, we 4 || CH 3 C CH 3 23 CH 3 C CH 2 Br – X2/OH CH 3 Excess O X O || || | CH 3 CH 2 C C CH 3 X X X Carbonyl compounds having three -hydrogens give haloform reaction. (vi) Hydrazones when treated with base like alkoxide give hydrocarbon (Wolf – Kishner reduction). N. NH 2 O R C CH 3 2 R C CX 3 OH RCOO CHX 3 ST Meerwein – pondorff verley reduction (MPV reduction). The percentage yield of alkanes can be increased by using diethylene glycol in Wolf Kishner reduction. Then reaction is called Huang – Millan conversion. || || NH 2 NH 2 RONa R C R ' R C R ' R CH 2 R Hydrazone (vii) Schiff's base on reduction gives secondary amines. R ' NH 2 H 2 / Ni R CH O R CH NR ' R CH 2 NHR Aldehyde Schiff's base (5) Reactions due to -hydrogen (i) Acidity of -hydrogens : Secondary amine CH 3 CH C CH CH 3 | | | O || X / OH O || (iii) Deuterium exchange reaction : Deuterium exchange reaction is catalysed by acid (D ) as well as base (OD). In both the cases all the hydrogens on only one -carbon is replaced by D. O || O || R C CH 2 R R C CD2 R ; D 2 O / OD If substrate and reagent both are carbonyl compounds then one should have at least one -hydrogen and other may or may not have hydrogen. O || || R C CH 2 R R C CD 2 R D2 O / D (iv) Racemisation : Ketones whose -carbon is chiral undergo Racemisation in the presence of acid as well as base. O CH 3 O CH 3 || H or | || Condensation reaction always takes place in the presence of acid or base as catalyst. Best result is obtained with base at lower temp. OH O | OH H H 2 methyl-1 - phenyl -1 - one Racemic mixture CH 3 O | || C 2 H 5 C C C6 H 5 | H (v) Alkylation : Carbonyl compounds having -hydrogens undergo alkylation reaction with RX in the presence of base. This reaction is S N 2 reaction. The best result is obtained with CH 3 X. Other halides undergo elimination in the presence of strong base. O O CH 3 CH 3 || NaH CH 3 C C (Small base) CH 3 CH 3 I Carbanion CH 3 LDA (Bulky base) O CH 3 || | CH 3 C C CH 3 | O CH 3 CH 2 C CH CH 3 || CH 3 CH 2 C CH CH 3 I || CH 3 CH 3 D YG O (Main product) Ethyl- isopropyl ketone (vi) Wittig reaction : Aldehyde and ketones undergo the wittig reaction to form alkenes. Ph3 P CH 2 C O C CH 2 Aldehyde or ketone alkene Condensation is carried out at lower temperature ( 20C) because product of the reaction is alcohol which has strong –I group at carbon. Such type of alcohols are highly reactive for dehydration. They undergo dehydration in the presence of acid as well as base even at 25°C. They also undergo elimination even on strong heating. OH | Ph3 P O Triphenyl Phosphoniu m oxide | Dehydratio n β R | O CHR 2 O Ph3 P CHR 1 Ph3 P CHR 1 U | | || || ST O CHR 2 O CHR 2 (6) Condensation reaction of carbonyl compounds : Nucleophilic addition reaction of compounds having carbonyl group with those compounds which have at least one acidic hydrogen at -carbon is known as condensation reaction. In this addition reaction : Substrate is always an organic compound having a carbonyl group, O || || Both molecule are the same or One should have no -hydrogen atom and other should have at least two -hydrogens. (b) These reactions are practical when base is NaOH and reaction temperature is high ( 100). (c) The reaction is two step reaction. First step is aldol formation and second step is dehydration of aldol. OH | CH 3 CHO CH 3 CHO CH 3 CH CH 2 CHO NaOH / OH Dehydratio n CH 3 CH CH CHO a, unsaturate d aldehyde Due to hyper-conjugation in crotonaldehyde further condensation give conjugated alkene carbonyl compound. CH – CH = CH – CHO + CH – CH = CH – CHO 3 3 NaOH OH | CH – CH = CH – CH – CH – CH = CH – CHO 3 2 –H O 2 O O || || H C H , C6 H 5 C H , R C H , R C R etc. Addition always takes place on the carbonyl group. Reagents of the condensation reaction are also organic compounds having at least one hydrogen on -carbon and -carbon should have –I group, e.g. CH 3 NO 2 , CH 3 CH CHO , | CH 3 R (a) This reaction takes place between two molecules of carbonyl compounds; one molecule should have at least two -hydrogen atoms. In this reaction best result is obtained when e.g. O C CH Z (i) Aldol condensation Ph3 P CHR 1 CHR 2 Ph3 P CHR 1 || R HO / R C CH 2 Z U CH 3 (Main product) R ID || CH 3 C CH | E3 OH | H or R C R CH 3 Z R C C H 2 Z C6 H 5 C C C 2 H 5 C6 H 5 C C C 2 H 5 | || | 60 O CH – CH = CH – CH = CH – CH = CH – CHO 3 CH – (CH = CH –) – CHO 3 3 Condensed compound OH / Mechanism : C 6 H 5 CHO CH 3 CHO CH 3 CH 2 CN C6 H 5 CH CH CHO HOH Step I : HO H CH 2 CHO O || Step II : C 6 H 5 C CH 2 CHO | H OH O | Test of aldehydes and Ketones (Distinction) Table : 27.1 Test No colour. With Fehling's solution Give red precipitate. No precipitate is formed. With Tollen's reagent Black precipitate or silver mirror is formed. No black precipitate or silver mirror is formed. With saturated sodium bisulphite solution in water Crystalline compound (colourless) is formed. Crystalline compound (colourless) is formed. With 2, 4-dinitrophenyl hydrazine Orange-yellow or red well defined crystals with melting points characteristic of individual aldehydes. Orange-yellow or red well defined crystals with melting points characteristic of individual ketones. With sodium hydroxide Give brown resinous mass (formaldehyde does not give this test). No reaction. A deep red colour (formaldehyde does not respond to this test). Red colour which changes to orange. | | H Step III : OH | C6 H 5 CH CH CHO C6 H 5 CH CH CHO HOH | H With sodium nitroprusside and few drops of sodium hydroxide Some commercially important aliphatic carbonyl compounds ID OH In aldol condensation, dehydration occurs readily because the double bond that forms is conjugated, both with the carbonyl group and with the benzene ring. The conjugation system is thereby extended. E3 | H Ketones Give pink colour. C 6 H 5 C CH 2 CHO C 6 H 5 C CH 2 CHO OH HOH Aldehydes With Schiff's reagent 60 O O || | HOH CH 2 C H CH 2 C H Ethanal Platinised asbestos HCHO (i) 2CH 3 OH O 2 D YG dil NaOH (a) CH 3 CHO CH 3 CH 2 CHO U Crossed aldol condensation : Aldol condensation between two different aldehydes or two different ketones or one aldehyde and another ketone provided at least one of the components have -hydrogen atom gives different possible product Propanal OH CH 3 | | CH 3 CH CH CHO CH 3 CH 2 CHOH CH 2 CHO However crossed aldol condensation is important when only it the components has -hydrogen atom. CH 2 O CH 3 CHO CH 2 CH 2 CHO CH 2 CH CHO H 2O | (Acrolein) OH U (3 - hydroxy propanal) 300 400 C Formaldehy de CH 3 OH [O] HCHO H 2 O K 2 Cr2 O7 H 2 SO 4 HCHO (ii) CH 3 OH Cu or Ag 300 400 C Formaldehy de (iii) Ca(HCOO) 2 HCHO Heat Formaldehy de Calcium formate HCHO (iv) CH 2 CH 2 O 3 H2 Pd Formaldehy de (v) CH 4 O 2 HCHO Mo -oxide Methane Catalyst Formaldehy de HCHO (vi) CO H 2 Elec.discharge Intra molecular aldol condensation : One molecule Intramolecular condensation give aldol compounds Example : ST Formaldehyde : Formaldehyde is the first member of the aldehyde series. It is present in green leaves of plants where its presence is supposed to be due to the reaction of CO 2 with water in presence of sunlight and chlorophyll. (1) Preparation OH di. NaOH O CH (CH 2 )5 CHO CHO (ii) Claisen – Schmidt reaction : Crossed aldol condensation between aromatic aldehyde and aliphatic ketone or mixed ketone is known as Claisen – Schmidt reaction. Claisen – Schmidt reactions are useful when bases such as sodium hydroxide are used because under these conditions ketones do not undergo self condensation. Some examples of this reaction are : O O || OH || C6 H 5 CHO CH 3 C CH 3 C6 H 5 CH CH C CH 3 100 C 4 Phenyl 3 buten - 2 - one Formaldehy de (2) Physical properties (i) It is a colourless, pungent smelling gas. (ii) It is extremely soluble in water. Its solubility in water may be due to hydrogen bonding between water molecules and its hydrate. (iii) It can easily be condensed into liquid. The liquid formaldehyde boils at – 21°C. (iv) It causes irritation to skin, eyes, nose and throat. (v) Its solution acts as antiseptic and disinfectant. (3) Uses (i) The 40% solution of formaldehyde (formalin) is used as disinfectant, germicide and antiseptic. It is used for the preservation of biological specimens. (ii) It is used in the preparation of hexamethylene tetramine (urotropine) which is used as an antiseptic and germicide. K 2 SO 4 Cr2 (SO 4 )3 3 CH 3 CHO 7 H 2 O (iii) It is used in silvering of mirror. (iv) It is employed in manufacture of synthetic dyes such as pararosaniline, indigo, etc. (v) It is used in the manufacture of formamint (by mixing formaldehyde with lactose) – a throat lozenges. Potassium sulphate | H 2 SO 4 CH 3 CHO NH 3 CH 3 CH NH 2 Acetaldehyde ammonia 60 CH 3 CHO (NH 4 ) 2 SO 4 Acetaldehyde (x) Manufacture : Acetaldehyde can be manufactured by one of the following methods: (a) By air oxidation of ethyl alcohol E3 Ag 2CH 3 CH 2 OH O 2 2CH 3 CHO 2 H 2 O 300 C (1) Preparation : It may be prepared by any of the general methods. The summary of the methods is given below (b) By dehydrogenation of alcohol Cu CH 3 CH 2 OH CH 3 CHO (i) By oxidation of ethyl alcohol with acidified potassium dichromate or with air in presence of a catalyst like silver at 300°C. 300 C (c) By hydration of acetylene ID (ii) By dehydrogenation of ethyl alcohol. The vapours of ethyl alcohol are passed over copper at 300°C. HgSO 4 ,(1%), 60 C CH CH H 2 O CH 3 CHO (iii) By heating the mixture of calcium acetate and calcium formate. PdCl2 ,CuCl 2 H 2 C CH 2 O 2 H 3 C CHO U D YG (vii) By hydration of acetylene with dil. H 2 SO 4 and HgSO 4 at 60°C. (i) Acetaldehyde is a colourless volatile liquid. It boils at 21°C. (ii) It has a characteristic pungent smell. (iii) It is soluble in water, chloroform, ethyl alcohol and ether. Its aqueous solution has a pleasant odour. In water, it is hydrated to a considerable extent to form ethylidene diol. CH 3 CHO H 2 O CH 3 CH (OH )2 (viii) By ozonolysis of butene-2 and subsequent breaking of ozonide. U (ix) Laboratory preparation : Acetaldehyde is prepared in the laboratory by oxidation of ethyl alcohol with acidified potassium dichromate or acidified sodium dichromate. K 2 Cr2 O7 4 H 2 SO 4 K 2 SO 4 Cr2 (SO 4 )3 4 H 2 O 3[O] ST [CH 3 CH 2 OH O CH 3 CHO H 2 O] 3 H 2O (2) Physical properties (v) By the reduction of acetyl chloride with hydrogen in presence of a catalyst palladium suspended in barium sulphate (Rosenmund's reaction). in ether and hydrolysis (Stephen's method). H 2 SO 4 (40 %) (d) From ethylene (Wacker process) (iv) By heating ethylidene chloride with caustic soda or caustic potash solution. (vi) By the reduction of CH 3 CN with stannous chloride and HCl Water OH (vii) Rongalite – a product obtained by reducing formaldehyde sodium bisulphite derivative with zinc dust and ammonia and is used as a reducing agent in vat dyeing. Acetaldehyde is the second member of the aldehyde series. It occurs in certain fruits. It was first prepared by Scheele in 1774 by oxidation of ethyl alcohol. Acetaldehyde To recover acetaldehyde, the distillate is treated with dry ammonia when crystallised product, acetaldehyde ammonia, is formed. It is filtered and washed with dry ether. The dried crystals are then distilled with dilute sulphuric acid when pure acetaldehyde is collected. (vi) It is used for making synthetic plastics like bakelite, ureaformaldehyde resin, etc. Acetaldehyde Chromic sulphate (3) Uses : Acetaldehyde is used : (i) In the preparation of acetic acid, acetic anhydride, ethyl acetate, chloral, 1,3-butadiene (used in rubbers), dyes and drugs. (ii) As an antiseptic inhalent in nose troubles. (iii) In the preparation of paraldehyde (hypnotic and sporofic) and metaldehyde (solid fuel). (iv) In the preparation of acetaldehyde ammonia (a rubber accelerator). K 2 Cr2 O 7 3 CH 3 CH 2 OH 4 H 2 SO 4 Potassium dichromate S.No. Ethyl alcohol Sulphuric acid Table : 27.2 Comparative study of formaldehyde and acetaldehyde Reaction Similarities 1. Addition of hydrogen (a) H in presence of catalyst, Ni, Pd or Pt (b) LiAlH4 (ether) (c) Amalgamated zinc + conc. HCl (Clemmenson reduction) 2 Formaldehyde HCHO Acetaldehyde CH CHO Forms methyl alcohol Forms ethyl alcohol HCHO H 2 CH 3 OH Forms methyl alcohol Forms methane CH 3 CHO H 2 CH 3 CH 2OH Forms ethyl alcohol Forms ethane HCHO 4 H CH 4 H 2O CH 3 CHO 4 H C2 H 6 H 2O 3 3. 4. Addition of NaHSO 3 solution Forms bisulphite addition product Forms bisulphite addition product HCHO NaHSO 3 CH 2 (OH )SO 3 Na CH 3 CHO NaHSO 3 Addition of HCN Forms formaldehyde cyanohydrin CH 3 CH (OH )SO 3 Na Forms acetaldehyde cyanohydrin HCHO HCN CH 2 (OH )CN CH 3 CHO HCN Forms ethyl alcohol CH 3 CH (OH )CN Forms isopropyl alcohol Addition of Grignard reagent followed by hydrolysis OMgI HCHO CH 3 MgI CH 2 CH 3 CH 3 CH 2 OH H 2O CH 3 CHO CH 3 MgI 60 2. CH 3 C HOMgI 2 H O Mg (OH )I | CH 3 Mg (OH )I E3 CH 3 CH OH | 7. 8. H 2O CH 2 O H 2 NOH H 2O CH 3 CH O H 2 NOH With hydrazine (NH 2 NH 2 ) CH 2 NOH Forms formaldehyde hydrazone CH 3 CH NOH Forms acetaldehyde hydrazone H 2O CH 2 O H 2 N NH 2 H 2O CH 3 CH O H 2 NNH 2 CH 2 NNH 2 CH 3 CH NNH 2 With phenyl hydrazine (C 6 H 5 NHNH 2 ) ID Forms formaldoxime U 6. With hydroxylamine NH 2 OH Forms formaldehyde phenyl hydrazone With semicarbazide (H 2 NNHCONH 2 ) H 2O CH 2 O H 2 NNHC 6 H 5 D YG 5. CH 3 Forms acetaldoxime H 2O CH 3 CH NNHC 6 H 5 CH 2 NNHC 6 H 5 Forms formaldehyde semicarbazone H 2O CH 2 O H 2 NNHCONH 2 CH 2 NNHCONH 2 9. With alcohol (C2 H 5 OH ) in presence of acid Forms ethylal U HCl H 2 C O 2C 2 H 5 OH With thioalcohols (C 2 H 5 SH ) in presence of acid ST 10. 11. Oxidation with acidified K2Cr2O7 12. 13. With Schiff's reagent With Tollen's reagent Forms acetaldehyde phenyl hydrazone CH 3 CH O H 2 NNHC 6 H 5 Forms acetaldehyde semicarbazone CH 3 CH O H 2 NNHCONH 2 H 2O CH 3 CH NNHCONH 2 Forms acetaldehyde diethyl acetal HCl CH 3 CHO 2C 2 H 5 OH OC2 H 5 CH 2 OC2 H 5 CH 3 CH OC2 H 5 OC2 H 5 Forms thio ethylal Forms acetaldehyde diethyl thioacetal H 2 C O 2C 2 H 5 SH CH 3 CH O 2C 2 H 5 SH SC 2 H 5 SC 2 H 5 CH 2 CH 3 CH SC 2 H 5 Forms formic acid HCHO O HCOOH Restores pink colour of Schiff's reagent Gives black precipitate of Ag or silver mirror Ag2O HCHO 2 Ag HCOOH SC 2 H 5 Forms acetic acid CH 3 CHO O CH 3 COOH Restores pink colour of Schiff's reagent Gives black precipitate of Ag or silver mirror Ag2O CH 3 CHO 2 Ag CH 3 COOH 14. With Fehling's solution or Benedict's solution 15. Polymerisation Gives red precipitate of cuprous oxide Gives red precipitate of cuprous oxide 2CuO HCHO Cu 2O HCOOH 2CuO CH 3 CHO Undergoes polymerisation Cu 2O CH 3 COOH Undergoes polymerisation nHCHO Evaporation 3CH 3 CHO (HCHO )n Paraformal dehyde H SO Conc. 2 4 dil. H SO. distill 2 3 HCHO Room temp. (HCHO )3 Metaformaldehyde heat (CH 3 CHO )3 Paraldehyd e H2SO4Conc. dil. H2SO4. distill 60 4 CH 3 CHO 4 (CH 3 CHO )4 Metaldehyde Dissimilarities 16. With PCl No reaction 5 Forms ethylidene chloride Cl E3 CH 3 CHO PCl5 CH 3 CH POCl3 Forms chloral CH 3 CHO 3Cl 2 CCl 3 CHO With chlorine No reaction 18. With SeO No reaction 19. Iodoform reaction (I +NaOH) 20. With dil. alkali (Aldol condensation) No reaction 21. With conc. NaOH (Cannizzaro's reaction) Forms sodium formate and methyl alcohol 2 HCHO NaOH HCOONa 22. With ammonia ID 17. No reaction U NH 2 Forms bakelite plastic Forms urea-formaldehyde plastic Form formose (a mixuture of sugars) Condensation in presence of Ca(OH )2 H 2 / Ni PCl5 Alc. HCHO CH 3 OH CH 3 Cl Methyl alcohol Methyl chloride KCN NaNO 2 Na / Alcohol CH 3 CN CH 3 CH 2 NH 2 Methyl cyanide acetaldehyde OH With phenol With urea Formaldehy de Forms addition product, ammonia CH 3 CHO NH 3 CH 3 CH Inter conversion of formaldehyde and acetaldehyde (1) Ascent of series : Conversion of formaldehyde into acetaldehyde (i) Se H 2 O Forms iodoform CH 3 CHO 3 I2 4 NaOH CH 3 CH (OH )CH 2 CHO Forms a brown resinous mass CH 3 OH Forms hexamethylene tetramine (urotropine) 6 HCHO 4 NH 3 (CH 2 )6 N 4 6 H 2 O ST 23. 24. 25. 3 HCl Forms glyoxal CH 3 CHO SeO 2 CHO.CHO CHl 3 HCOONa 3 NaI 3 H 2 O Forms aldol CH 3 CHO HCH 2 CHO D YG 2 U 2 Cl Ethyl amine No reaction No reaction No reaction H 2 SO 4 (dil.) CH 3 CH 2 OH CH 3 CHO Ethyl alcohol (ii) K 2 Cr2 O7 Acetaldehyde CH 3 MgI H 3O HCHO CH 3 CH 2OMgI Formaldehy de Ether HCl Cu CH 3 CH 2 OH CH 3 CHO Ethyl alcohol 300 C Acetaldehyde (iii) Reduction H2, Ni, Pd CH 3 CHOHCH 3 or LiAlH4 Isopropyl alcohol Amalgamated Zn CH 3 CH 2 CH 3 + conc. HCl Ca (OH )2 7 HCHO 2 2 HCOOH K Cr O Formaldehy de Formic acid H 2 SO 4 3 2 (HCOO )2 Ca CH 3 CHO (CH COO ) Ca heat Calcium formate Propane Acetaldehyde NaHSO3 (2) Descent of series : Conversion of acetaldehyde into formaldehyde Acetone sodium bisulphite derivative HCN (i) CH 3 CHO CH 3 COOH K 2 Cr2 O7 NH 3 H 2 SO 4 Acetaldehyde (CH 3 )2 C(OH )SO 3 Na CH3MgI Br2 / KOH Heat CH 3 COONH 4 CH 3 CONH 2 Amm. acetate OH CN (CH 3 )2 C Aceticacid Acetone cyanohydri n (CH 3 )3 COH Ether Tertiary butyl alcohol Acetamide NH2OH (CH 3 )2 C NOH Formaldehy de NH2NH2 (CH 3 )2 C NNH 2 NaOH 7 (ii) CH 3 CHO 2 2 CH 3 COOH CH 3 COONa C6H5NHNH2 Cu HCl 300 C Methyl amine K Cr O Acetaldehyde H 2 SO 4 Aceticacid Sod.acetat e hv Methane Acetone phenyl hydrazone (CH 3 )2 C NNHCONH 2 Acetone semicarbazone PCl5 CH 3 OH HCHO Cu 300 C Formaldehy de Acetone It is a symmetrical (simple) ketone and is the first member of the homologous series of ketones. In traces, it is present in blood and urine. (1) Preparation : (CH 3 )2 C Isopropyli dene chloride Cl2 CH 3 COCCl 3 Trichloroacetone I2 +NaOH CH COCH (i) (CH 3 COO )2 Ca C Conc. H2SO4 500 C (ii) 2CH 3 CHOHCH 3 O2 Cu (iii) CH 3 CHOHCH 3 300 C 2 propanol (iv) (a) CH 3 CH CH 2 PdCl2 H 2 O CH COCH (b) CH 3 CH CH 2 H 2 SO 4 3 3 CH 3 CH (HSO 4 )CH 3 Ba(OH)2 H2O Cu CH 3 CH (OH )CH 3 HNO2 300 C 2 (v) 2C3 H 5 OH H 3 O Zn (CrO ) U ST (ii) It is inflammable liquid. It boils at 56 C. (iii) It is highly miscible with water, alcohol and ether. (3) Chemical properties 3 Mesitylene (1, 3, 5-trimethyl benzene) CHCl 3 Chloroform CH 3 COOH CO 2 H 2 O (CH 3 )2 C(OH )CCl 3 Chloretone (CH 3 )2 C(OH )CH 2 COCH 3 Diacetone alcohol CH 3 COCH NOH (CH 3 )2 C(NH 2 )CH 2 COCH 3 OH Mg–Hg + H2O 420 C o CH 3 Diacetone amine catalyst (vi) 2CH CH 3 H 2O The acetone thus obtained is purified with the help of sodium bisulphite. (2) Physical properties : (i) It is a colourless liquid with characteristic pleasant odour. C CH (Oximino acetone) NH3 500 C (vii) From pyroligneous acid : Pyroligneous acid containing acetic acid, acetone and methyl alcohol is distilled in copper vessel and the vapours are passed through hot milk of lime. Acetic acid combines to form nonvolatile calcium acetate. The unabsorbed vapours of methanol and acetone are condensed and fractionally distilled. Acetone distills at 56 o C. CH CHCl propene Isopropyl alcohol C CaOCl2 3 CH CH (Bleaching powder) heat K2Cr2O7 + H2SO4 D YG propene U Isopropyl alcohol 3 3 ID 3 CHI 3 Iodoform CH (Acetone) calciumacetate Cl Cl E3 heat Acetone hydrazone (CH 3 )2 C NNHC 6 H 5 H2NNHCONH2 AgOH CH 4 CH 3 Cl Cl 2 Sodalime Acetoxime 60 CH 3 NH 2 CH 3 OH HCHO NaNO 2 Schiff's reagent Tollen's reagent Fehling's solution OH | | No reaction No reaction No reaction Pinacol (CH 3 )2 C — C (CH 3 )2 If acetone would be in excess in ketal condensation or catalyst (ZnCl 2 / dry HCl) is used then three moles of acetone undergoes condensation polymerisation and form a compound called ‘Phorone’. CH CH 3 | CH 3 C O 3 | CH 3 C CH H CH H CH C=O 3 | CH 3 C O H CH H Acetone (3 molecule) CO ZnCl 2 dry. HCl CH 3 C CH | CH 3 phoron (i) As a solvent for cellulose acetate, cellulose nitrate, celluloid, lacquers, resins, etc. Molecular mass of phorone = 3 mole of acetone – 2 mole of H 2 O (ii) For storing acetylene. Reformatsky reaction: This reaction involves the treatment of aldehyde and ketone with a bromo acid ester in presence of metallic zinc to form - (iii) In the manufacture of cordite – a smoke less powder explosive. hydroxy ester, which can be easily dehydrated into , -unsaturated ester. (iv) In the preparation of chloroform, iodoform, sulphonal and chloretone. Benzene (a) BrCH 2 COOC 2 H 5 Zn Br Zn CH 2 COOC 2 H 5 (v) As a nailpolish remover. (vi) In the preparation of an artificial scent (ionone), plexiglass (unbreakable glass) and synthetic rubber. (5) Tests (i) Legal's test : When a few drops of freshly prepared sodium nitroprusside and sodium hydroxide solution are added to an aqueous solution of acetone, a wine colour is obtained which changes to yellow on standing. (ii) Indigo test : A small amount of orthonitrobenzaldehyde is added to about 2 ml. of acetone and it is diluted with KOH solution and stirred. A blue colour of indigotin is produced. (iii) Iodoform test : Acetone gives iodoform test with iodine and sodium hydroxide or iodine and ammonium hydroxide. Organo zinccompound (b) Addition to carbonyl group Zn Br CH 3 | 60 CH 3 | C O CH 2 COOC 2 H 5 CH 3 C CH 2 CH 2 COOC 2 H 5 | CH 3 OZn Br CH 3 E3 | HOH / H CH 3 C CH 2 Br Zn OH | | OH COOC 2 H 5 - hydroxyest ers CH 3 | ID CH 3 C CH COOC 2 H 5 (4) Uses Table : 27.3 Comparison between Acetaldehyde and Acetone Acetaldehyde U Reaction Similarities 1. Reduction with H 2 and Ni or Forms ethyl alcohol LiAlH4 CH 3 CHO H 2 CH 3 CH 2 OH D YG Ni Acetone Forms isopropyl alcohol CH 3 COCH 3 H 2 CH 3 CHOHCH 3 2. Clemmensen's reduction (Zn/Hg and conc. HCl) Forms ethane (an alkane) Forms propane (an alkane) CH 3 CHO 4 H CH 3 CH 3 H 2 O CH 3 COCH 3 4 H CH 3 CH 2 CH 3 H 2 O 3. Addition of HCN Forms acetaldehyde cyanohydrin Forms acetone cyanohydrin OH OH CH 3 CHO HCN CH 3 CH (CH 3 ) 2 CO HCN (CH 3 ) 2 C CN White crystalline derivative U 4. Addition of NaHSO 3 CN White crystalline derivative OH ST CH 3 CHO NaHSO 3 CH 3 CH 5. Grignard reagent followed by hydrolysis 6. With hydroxylamine (NH 2OH ) 7. With hydrazine (NH 2 NH 2 ) 8. With phenyl (C6 H 5 NHNH 2 ) hydrazine OH (CH 3 ) 2 CO NaHSO 3 (CH 3 ) 2 C SO 3 Na Forms isopropyl alcohol SO 3 Na Forms tertiary butyl alcohol CH 3 CHO CH 3 MgI (CH 3 )2 CH OMgI (CH 3 )2 CO CH 3 MgI (CH 3 )3 COMgI CH 3 CHOHCH 3 H 2O H 2O (CH 3 )3 COH Forms acetaldoxime (an oxime) Forms acetoxime (an oxime) CH 3 CHO H 2 NOH CH 3 CH NOH (CH 3 )2 CO H 2 NOH (CH 3 )2 C NOH Forms acetaldehyde hydrazone Forms acetone hydrazone CH 3 CHO H 2 NNH 2 CH 3 CH NNH 2 (CH 3 )2 CO H 2 NNH 2 (CH 3 )2 C NNH 2 Forms acetaldehyde phenylhydrazone Forms acetone phenyl hydrazone CH 3 CHO H 2 NNHC 6 H 5 (CH 3 )2 CO H 2 NNHC 6 H 5 CH 3 CH NNHC 6 H 5 9. With semicarbazide Forms acetaldehyde semicarbazone (H 2 NNHCONH 2 ) CH 3 CHO H 2 NNHCONH 2 (CH 3 )2 C NNHC 6 H 5 Forms acetone semicarbazone (CH 3 )2 CO H 2 NNHCONH 2 CH 3 CH NNHCONH 2 Forms ethylidene chloride (Gem dihalide) 10. With PCl5 (CH 3 )2 C NNHCONH 2 Forms isopropylidene chloride (Gem dihalide) Cl Cl CH 3 CHO PCl5 CH 3 CH (CH 3 )2 CO PCl5 (CH 3 )2 C Cl 12. With alcohols Forms chloral (Gem trihalide) Forms trichloro acetone (Gem trihalide) CH 3 CHO Cl 2 CCl 3 CHO CH 3 COCH 3 Cl 2 CCl 3 COCH 3 Forms acetal (a diether) Forms ketal (a diether) 60 11. With chlorine Cl OC2 H 5 OC 2 H 5 (CH 3 )2 CO 2C 2 H 5 OH (CH 3 )2 C E3 CH 3 CHO 2C2 H 5 OH CH 3 CH OC2 H 5 Forms glyoxal 14. Iodoform reaction Forms methyl glyoxal CH 3 CHO SeO 2 CHOCHO Se H 2 O (CH 3 )2 CO SeO 2 CH 3 COCHO Se H 2 O Forms iodoform Forms iodoform ID 13. With SeO 2 OC 2 H 5 (I2 NaOH ) 15. Bleaching powder Forms chloroform 16. Aldol condensation with mild alkali Forms aldol Forms chloroform 2CH 3 CHO CH 3 CHOHCH 2CHO 2CH 3 COCH 3 (CH 3 )2 C(OH )CH 2COCH 3 17. Polymerisation Undergoes polymerisation Does not undergo polymerisation but gives condensation reaction Forms diacetone ammonia D YG U Forms diacetone alcohol Forms acetaldehyde ammonia 18. With NH 3 OH CH 3 CHO NH 3 CH 3 CH (CH 3 )2 CO NH 3 OC(CH 3 )2 (CH 3 )2 C(NH 2 )CH 2COCH 3 NH 2 19. With conc. NaOH Forms brownish resinous mass No reaction 20. With HNO 2 No reaction Forms oximino acetone No reaction U 21. With chloroform alk. sodium Forms chloretone OH (CH 3 )2 CO CHCl 3 (CH 3 )2 C CCl 3 Deep red colour Red colour changes to yellow on standing 23. With sodium nitroprusside + Pyridine Blue colour No effect 24. Boiling point 21o C 56 o C 25. With Schiff's reagent Pink colour Does not give pink colour 26. With Fehling's solution Gives red precipitate No reaction 27. With Tollen's reagent Gives silver mirror No reaction 28. Oxidation with acidified Easily oxidised to acetic acid Oxidation occurs with difficulty to form acetic acid K2Cr2O7 CH 3 CHO O CH 3 COOH ST 22. With nitroprusside CH 3 COCH 3 HNO 2 CH 3 COCH NOH Dissimilarities CH 3 COCH 3 O CH 3 COOH CO 2 H 2 O Aromatic Carbonyl Compounds [O ] CH 2 OH Aromatic aldehydes are of two types : Those in which aldehyde (CHO ) group is attached to side chain, This method is used for commercial production of benzaldehyde. (v) By hydrolysis of benzal chloride : CHCl 2 e.g., phenyl acetaldehyde, C6 H 5 CH 2 CHO. They closely resemble with aliphatic aldehydes. Aromatic ketones are compounds in which a carbonyl group ( C O) is attached to either two aryl groups or one aryl group and one alkyl group. Examples are : CHO COC 6 H 5 COCH 3 OH Benzal Chloride ( H 2 O ) Intermedia te (unstable) Benzaldehyde This is also an industrial method. (vi) By oxidation of Toluene CH 3 CHO O 2 H 2O E3 350 o C toluene benzaldehyde Commercially the oxidation of toluene is done with air and diluted with nitrogen (to prevent complete oxidation) at 500 o C in the presence of oxides of Mn, Mo or Zr as catalyst. C6 H5CHO or Partial oxidation of toluene with manganese dioxide and dilute ID Benzaldehyde, CHO Ca (OH )2 V2 O 5 CHO OH OH CH NaOH CHO Benzaldehyde Acetophenone Benzopheno ne Salicylaldehyde (Methyl phenyl (Diphenyl ketone) ketone) CHO Benzaldehyde 60 The compounds in which CHO group is attached directly to an aromatic ring, e.g., benzaldehyde, C 6 H 5 CHO. Benzyl alcohol sulphuric acid at 35 o C , also forms benzaldehyde. CrO Toluene (CH 3 CO )2 O Benzylidene acetate C6 H 5 CHO 2CH 3 COOH (vii) Etard's reaction : C6 H 5 CH 3 2CrO2 Cl 2 CN | C6 H 5 CH 3 2CrO2 Cl 2 2 C6 H 5 CHO H O C 6 H 5 CHOC 12 H 21 O10 2 H 2 O C 6 H 5 CHO Brown addition product Benzaldehyde D YG Amygdalin 2C 6 H 12 O 6 HCN Glucose Benzaldehyde is also known as oil of bitter almonds. (1) Method of preparation (i) Laboratory method : It is conveniently prepared by boiling benzyl chloride with copper nitrate or lead nitrate solution in a current of carbon dioxide. heat 2C 6 H 5 CH 2 Cl Cu ( NO 3 )2 2C 6 H 5 CHO CuCl 2 2 HNO 2 Benzyl chloride H /H O 3 2 C6 H 5 CH 3 C6 H 5 CH (OCOCH 3 )2 U Benzaldehyde is the simplest aromatic aldehyde. It occurs in bitter almonds in the form of its glucoside, amygdalin (C 20 H 27 O11 N ). When amygdalin is boiled with dilute acids, it hydrolyses into benzaldehyde, glucose and HCN CO 2 or ST Benzaldehyde (ix) Gattermann reaction HC N HCl AlCl3 H C NH AlCl4 ; C 6 H 5 H HC NH C 6 H 5 CH NH 2 Benzene (ii) Rosenmund reaction : Pd / BaSO 4 C6 H 5 COCl H 2 C6 H 5 CHO HCl xylene Benzene HCl [2 HNO 2 NO NO 2 H 2 O] Benzyl chloride AlCl3 CO HCl Benzaldehyde U Pb( NO3 )2 Benzaldehyde (viii) Gattermann-koch aldehyde synthesis : Benzene is converted into benzaldehyde by passing a mixture of carbon monoxide and HCl gas under high pressure into the ether solution of benzene in presence of anhydrous aluminium chloride and cuprous chloride. CHO C6 H 5 CH NH 2 H 2 O AlCl4 C6 H 5 CHO NH 3 AlCl3 HCl Benzaldehyde (iii) By dry distillation of a mixture of calcium benzoate and calcium CHO formate Thus, O HCN HCl H 2 O AlCl3 NH 4 Cl || C 6 H 5 COO Ca Ca C 6 H 5 COO Calcium benzoate O CH CH || O O heat 2C 6 H 5 CHO 2CaCO 3 Benzaldehyde (Major product) Calcium formate (iv) By oxidation of benzyl alcohol : This involves the treatment of benzyl alcohol with dil. HNO 3 or acidic potassium dichromate or chromic anhydride in acetic anhydride or with copper catalyst at 350 o C. (x) Stephen's reaction : Benzaldehyde is obtained by partial reduction of phenyl cyanide with stannous chloride and passing dry HCl gas in ether solution followed by hydrolysis of the aldimine stannic chloride with water. HCl / SnCl 2 C6 H 5 C N [C6 H 5 CH NH ]2 H 2 SnCl 6 Phenyl cyanide Ether Aldimine complex H 2O 2C6 H 5 CHO H2NNH2 C6 H 5 CH NNH 2 H 2 O Benzaldehyde hydrazone H2N.NHC6H5 C6 H 5 CH N. NHC 6 H 5 H 2 O (xi) By ozonolysis of styrene CHO O O3 C6 H 5 CH CH 2 C6 H 5 – CH Benzaldehyde phenyl hydrazone H2NOH H 2O CH 2 C6 H 5 CH NOH H 2 O Benzaldoxime Vinyl benzene O O C6 H 5 CHO HCHO H 2 O2 (xii) Grignard reaction O H N.NHCONH2 C6 H 5 CH NNHCONH 2 H 2 O 2 Benzaldehyde semicarbazone Br O || (Benzaldehyde) H NC H 2 6 C6 H 5 CH NC 6 H 5 H 2 O 5 || Benzylidine aniline(Schiff's base) HCOC 2 H 5 BrMgC6 H 5 C 6 H 5 C H Mg Benzaldehyde PCl OC 2 H 5 Other reagents like carbon monoxide or HCN can also be used in place of ethyl formate. (xiii) From Diazonium salt CH NOH + HCl + N Formaldoxi me Benzal chloride 2CH OH 3 2 Benzaldoxime C 6 H 5 CHCl 2 POCl3 HCl OCH 3 | C 6 H 5 CH H 2O | OCH 3 Methyl acetal of benzaldehyde HO 2 E3 N N Cl HCH NOH 5 60 Ethyl formate (iii) Oxidation : Benzaldehyde is readily oxidised to benzoic acid even on exposure to air. CHO Benzaldehyde [O ] C 6 H 5 CHO C 6 H 5 COOH Acidified K 2 Cr2 O7 , alkaline KMnO 4 and dilute HNO 3 can be ID (2) Physical properties (i) Benzaldehyde is a colourless oily liquid. Its boiling point is 179 o C. (ii) It has smell of bitter almonds. (iii) It is sparingly soluble in water but highly soluble in organic solvents. (iv) It is steam volatile. (v) It is heavier than water (sp. gr. 1.0504 at 15 o C ). (vi) It is poisonous in nature. (3) Chemical properties (i) Addition reaction: The carbonyl group is polar as oxygen is more electronegative than carbon, used as oxidising agents for oxidation. (iv) Reducing properties : Benzaldehyde is a weak reducing agent. It reduces ammonical silver nitrate solution (Tollen's reagent) to give silver mirror but does not reduce Fehling's solution. D YG U C 6 H 5 CHO Ag 2 O 2 Ag C 6 H 5 COOH C O Thus, The positive part of the polar reagent always goes to the carbonyl oxygen and negative part goes to carbonyl carbon. OH OH HCN H C 6 H 5 CH C 6 H 5 CH H 2O CN COOH Benzaldehyde cyanohydri n NaHSO3 CHO Mandelic acid OH C 6 H 5 CH U SO 3 Na Benzaldehyde sodium bisulphite (White solid) OMgI CH3MgI 2[H] LiAlH4 H C 6 H 5 CH C 6 H 5 CH ST (Benzaldehyde) OH CH 3 H 2O 1-Phenyl -1-ethanol (2o alcohol) C 6 H 5 CH 2 OH Benzyl alcohol However on reduction with sodium amalgam and water, it gives hydrobenzoin, Na Hg C6 H 5 CH O 2 H O CHC 6 H 5 H 2O C6 H 5 CH CH C6 H 5 | OH OH Hydrobenzo in (ii) Reactions involving replacement of carbonyl oxygen Benzoic acid Zn Hg C 6 H 5 CHO 4 H C 6 H 5 CH 3 H 2 O HCl (vi) Schiff's reaction: It restores pink colour to Schiff's reagent (aqueous solution of p-rosaniline hydrochloride decolourised by passing sulphur dioxide). (vii) Tischenko reaction : On heating benzaldehyde with aluminium alkoxide (ethoxide) and a little of anhydrous AlCl3 or ZnCl 2 , it undergoes an intermolecular oxidation and reduction (like aliphatic aldehydes) to form acid and alcohol respectively as such and react to produce benzyl benzoate (an ester). Al(OC 2 H 5 )3 2C 6 H 5 CHO C 6 H 5 CH 2 OOCC 6 H 5 Benzaldehyde Benzyl benzoate (ester) (viii) Reactions in which benzaldehyde differs from aliphatic CH 3 | Benzaldehyde (v) Clemmensen's reduction : With amalgamated zinc and conc. HCl, benzaldehyde is reduced to toluene. aldehydes (a) With fehling's solution : No reaction (b) Action of chlorine : Benzoyl chloride is formed when chlorine is passed through benzaldehyde at its boiling point in absence of halogen carrier. This is because in benzaldehyde there is no -hydrogen atom present which could be replaced by chlorine. o 170 C C 6 H 5 CHO Cl 2 C 6 H 5 COCl HCl KOH (c) Cannizzaro's reaction : 2C6 H 5 CHO Benzaldehyde C6 H 5 CH 2OH C6 H 5 COOK Benzyl alcohol Potassium benzoate The possible Mechanism is First step is the reversible addition of hydroxide ion to carbonyl Benzoin can be readily oxidised to a diketone, i.e, benzil. group. H C 6 H 5 C O OH | (Fast) C6 H 5 C O | CuSO 4 C 6 H 5 CH C C 6 H 5 [O] C6 H 5 C C C 6 H 5 | H Benzil CH COONa 3 C 6 H 5 CH O H 2 CHCOOCOCH 3 C 6 H 5 CH CHCOOCOCH 3 | C 6 H 5 C O C 6 H 5 C O C 6 H 5 C O C 6 H 5 C O | ion transfer (slow) OH | | H OH Cinnamic acid acid Alkoxide ion (H exchange) + | H 2 C CO H | C 6 H 5 C OH C 6 H 5 C O | | H O Benzyl alcohol C6 H 5 CH O Propionic anhydride ID Two different aldehydes each having no -hydrogen atom, exhibit crossed Cannizzaro's reaction when heated in alkaline solution. C6 H 5 CHO HCHO C6 H 5 CH 2 OH HCOONa D YG Formaldehy de heat CHO | - Methyl cinnamic acid (f) Claisen condensation [Claisen-schmidt reaction] | CH 3 NaOH C 6 H 5 CHO H 2 C CHO Propionald ehyde CH OH CH 3 | C6 H 5 CH C CHO H 2 O -Methyl cinnamic aldehyde NaOH (Dil.) C6 H 5 CHO H 2CHCOCH 3 Acetone COOH 2 C6 H 5 CH CHCOCH 3 H 2O NaOH / 100 C H / H 2O CHO U CHO O | || || | O H H O | || C C OH Two moleculesof benzaldehyde Benzoin (An aldol) hydroxy ketone Benzoin can also be reduced to a number of product i.e., Na-Hg/C H OH 2 Pyridine COOH Malonic acid ( hydroxy ketone) C6 H 5 CHOH CHOH C6 H 5 Hydrobenzoin Cinnamic acid (h) Reaction with aniline : Benzaldehyde reacts with aniline and forms Schiff's base Warm C6 H 5 CH O H 2 NC 6 H 5 C6 H 5 CH NC 6 H 5 Aniline 5 Reaction with Dimethylaniline OH O | OH || C6 H 5 C C C6 H 5 [H] | Zn-Hg/HCl H Benzoin | H | H 2O C 6 H 5 CH CH C 6 H 5 C 6 H 5 CH CHC 6 H 5 Stilbene H 2 H /Raney Ni 2 C6 H 5 CH 2 CH 2 C6 H 5 2 H 2 O Dibenzyl C6 H 5 CH CHCOOH CO 2 H 2O | [H] COOH 2 C6 H 5 CH O H 2 C Alc. KCN ST C C Benzylidene acetone (g) Knoevenagel reaction CH OH COOH (d) Benzoin Condensation H (Dil.) Sod. formate Benzyl alcohol Aldehyde which do not have hydrogen ( C6 H 5 CHO, CCl 3 CHO, (CH 3 )3 C CHO, CH 2 O etc.) undergoes Cannizzaro’s reaction. Intramolecular cannizzaro reaction CHO CH 3 C6 H 5 CH C COOH CH 3 CH 2 COONa U So one molecule of aldehyde acts as hydride donor and the other acts as hydride acceptor. In other words, Cannizzaro's reaction is an example of self reduction and oxidation. NaOH CH CH COONa 3 O 2 CH 3 CH 2 CO Benzoate ion Third Step is exchange of protons to give most stable pair alcohol and acid anion. Benzaldehyde Acetic acid CH 3 –H + H H O 2 C6 H 5 CH CHCOOH CH 3 COOH E3 Hydride H 2O Acetic anhydride 60 Benzaldehyde H H | || O O (e) Perkin's reaction Second step is the transfer of hydride ion directly to the another aldehyde molecule, the latter is thus reduced to alkoxide ion and the former (ion I) is oxidised to an acid. | || Pyridine H 2O O Benzoin OH Anion (I) H || OH | ( H 2 O ) Benzylidene aniline (Schiff's base) N (CH 3 )2 H CH O Conc. H SO 4 2 H AlCl 3 C 6 H 5 H Cl COCH 3 C 6 H 5 COCH 3 HCl N (CH 3 )2 Benzene CH ( H 2 O ) N (CH 3 )2 N (CH 3 )2 Dimethyl aniline Acetyl chloride acetate. Tetramethy l diamino triphenyl methane (Malachite green) (i) Reaction with Ammonia : Benzaldehyde reacts with ammonia to form hydrobenzamide aldehyde other than CH 2O give aldehyde ammonia while CH 2 O forms urotropine. O || C 6 H 5 COO O CCH 3 C 6 H 5 COO Calcium benzoate || O || Bromo ethylaceta te C 6 H 5 CHCH 2 COOC 2 H 5 C 6 H 5 CH CH 2 COOC 2 H 5 H 2O | OH (k) Reaction of benzene ring CHO 3 H SO (conc.) NO2 CHO H SO 4 fuming Benzaldehyde CHO D YG 2 SO3H m Benzaldehyde Sulphonic acid Cl (a) CH 3 C N C 6 H 5 MgBr CH 3 C NMgBr H2O C6 H 5 COCH 3 NH 3 Mg(OH)Br O || Ethyl acetate O || Br OC 2 H 5 (vi) Commercial preparation : Ethylbenzene is oxidised with air at C 6 H 5 C CH 3 Mg 126 o C under pressure in presence of a catalyst manganese acetate. CH 2 CH 3 COCH 3 Catalyst O2 CHO 2 | C6 H 5 (b) C 6 H 5 MgBr H 5 C 2 O C CH 3 U m Nitrobenza ldehyde 2C6 H5 COCl (CH 3 )2 Cd 2C6 H5 COCH 3 CdCl 2 (v) By Grignard reagent ID -hydroxy ester E3 C 6 H 5 CHO CH 2 N 2 C 6 H 5 COCH 3 N 2 (iv) By treating benzoyl chloride with dimethyl cadmium. C6 H 5 CH O Zn Br C H 2 COOC 2 H 5 4 Acetopheno ne (iii) By methylation of benzaldehyde with diazomethane. 2 2C6 H 5 CCH 3 2CaCO 3 CH C6 H 5 (j) Reformatsky reaction HNO (conc.) 60 O Hydrobenza mide | Calcium acetate C6 H 5 CH N C6 H 5 CH N OZnBr O CCH 3 Ca Ca C6 H 5 CHO H 2 NH O CH C 6 H 5 C6 H 5 CHO H 2 NH Benzaldehyde Acetopheno ne (ii) By distillation of a mixture of calcium benzoate and calcium H 2O 126 o C pressure FeCl (4) Uses : Benzaldehyde is used, (i) In perfumery 3 Cl m Chlorobenz aldehyde (ii) In manufacture of dyes U (iii) In manufacture of benzoic acid, cinnamic acid, cinnamaldehyde, Schiff's base, etc. (2) Physical properties : It is a colourless crystalline solid with melting point 20 o C and boiling point 202 o C. It has characteristic pleasant odour. It is slightly soluble in water. Chemically, It is similar to acetone. (3) Chemical properties : OH ST (5) Tests : (i) Benzaldehyde forms a white precipitate with NaHSO 3 solution. | HCN | CN (ii) Benzaldehyde forms a yellow precipitate with 2 : 4 dinitrophenyl hydrazine. Acetopheno ne cyanohydri ne CH 3 (iii) Benzaldehyde gives pink colour with Schiff's reagent. (iv) Benzaldehyde forms black precipitate Tollen's reagent. H2NOH or silver mirror with (v) Benzaldehyde on treatment with alkaline KMnO4 and subsequent acidification gives a white precipitate of benzoic acid on cooling. C 6 H 5 C CH 3 | Rearrangem ent C 6 H 5 C NOH H 2 SO 4 Acetopheno ne oxime or (Methylphenyl ketoxime) C 6 H 5 NHCOCH C6H5COCH3 (Acetophenone) Clemmensen reduction Acetophenone, C6H5COCH3, Acetyl Benzene Zn(Hg)/HCl (1) Method of preparation (i) Friedel-Craft's reaction : Acetyl chloride reacts with benzene in presence of anhydrous aluminium chloride to form acetophenone. Reduction 3 Acetanilide C6 H 5 CH 2 CH 3 Ethyl benzene C 6 H 5 CH OH Na/C2H5OH Oxidation Cold KMnO4 | CH 3 Methyl phenyl carbinol (2 o alcohol) [O] C6 H 5 COCOOH C6 H 5 COOH Phenyl glyoxylicacid Benzoic acid Oxidation PCl5 C 6 H 5 CCl 2 CH 3 2, 2-Dichloroethylbenzene Cl2 C 6 H 5 COCH 2 Cl powerful lachrymator or tear (It is relatively harmless but C6H5COCH3 Iodoform reaction I2/NaOH C 6 H 5 COONa CHI 3 Iodoform Nitration O | || C 6 H 5 C CH C C 6 H 5 Dypnone (It is used as hypnotic) NO 2 C 6 H 4 COCH 3 m -Nitroacetophenone HNO /H SO 2 CH 3 60 Aldol type condensation Al-ter-butoxide 3 gas and is used by police to disperse mobs.) Phenacyl chloride (Used as a tear gas) 4 HSO 3 C 6 H 4 COCH conc. H2SO4 3 Acetopheno ne m sulphonic acid (4) Uses : It is used in perfumery and as a sleep producing drug. Benzophenone, C6H5COC6H5 Diphenyl carbinol HCl Diphenyl methane U (ii) Clemmenson reduction : Zn / Hg C6 H 5 COC 6 H 5 C6 H 5 CH 2 C6 H 5 H 2 O ID (1) Method of preparation (i) From alkyl benzenes HNO 3 C 6 H 5 CH 2 C 6 H 5 2O C 6 H 5 COC 6 H 5 (2) Physical properties : It is a colourless, pleasant smelling solid. (3) Chemical properties : It shows the characteristic properties of keto group but does not give bisulphite compounds. (i) Reduction : Na Hg C 6 H 5 COC 6 H 5 2 H C 6 H 5.CHOHC 6 H 5 D YG (iii) Fusion with KOH : Fuse C6 H 5 COC6 H 5 KOH C6 H 5 COOK C6 H 6 Pot. tert.Butoxide C6 H 5 COC 6 H 5 H 2 O C6 H 5 COOH C6 H 6 Ether Benzoic acid U Acidified K Cr O i.e., chromic acid sulphuric acid mixture is known 2 2 7 as Jone’s reagent. When used as an oxidising agent unlike acidified KMnO it does not diffect a double bond. 4 CH =CHCH OH 2 2 K Cr O /H SO 2 2 7 2 4 CH =CHCHO 2 ST Vilsmeyer reaction : this reaction involves the conversion of aromatic compounds to aldehydes in the presence of a 2 amino and formic CHOacid. (CH ) NH + HCOOH+POCl 3 2 Benzane o 3 Benzaldehyde although reduces Tollen’s reagent. It does not reduce Fehling or Benedict solution. E3 (Acetophenone) C 6 H 5 COCHO Phenyl glyoxal SeO2
