Aldehydes, Ketones and Carboxylic Acids PDF

Aldehydes, Ketones and Carboxylic Acid Introduction O Aldehydes || R — C— H , where R=H, alkyl or aryl group O || Ketones...

Aldehydes, Ketones and Carboxylic Acid Introduction O Aldehydes || R — C— H , where R=H, alkyl or aryl group O || Ketones R — C— R ' , where R and R' may be same or different alkyl or aryl group Carboxylic O || acids R — C— OH , where R=H or any alkyl or aryl group. Examples Aliphatic Aldehydes IUPAC Name Common Name HCHO Methanal Formaldehyde CH3CHO Ethanal Acetaldehyde CH3CH2CHO Propanal Propionaldehyde Aldehydes, Ketones and Carboxylic Acid CH3CH2CH2CHO Butanal Butyraldehyde CH3 — CH— CHO | 2-Methylpropanal Isobutyraldehyde CH3 CH2 = CHCHO Prop-2-enal Acrolein 1.  Aromatic IUPAC Name Common Name Aldehydes Benzaldehyde or Benzaldehyde or Benzenecarbaldehyde Benzenecarbaldehyde 2-Phenylethanal Phenylacetaldehyde 3-Bromobenzaldehyde m-Bromobenzaldehyde or 3-Bromobenzene- or m-Bromobenzene- carbaldehyde carbaldehyde Benzene-1, 2-dicarbaldehyde Phthaldehyde m-Methylbenzaldehyde or m-Methylbenzaldehyde or Aldehydes, Ketones and Carboxylic Acid m-Tolualdehyde m-Tolualdehyde p-Nitrobenzenecarbaldehyde 4-Nitrobenzenecarbaldehyde 2. Aliphatic Ketones IUPAC Name Common Name O || Dimethyl ketone or Propanone CH3 — C— CH3 Acetone O || Butan-2-one or Butanone Ethyl methyl ketone CH3 — C— CH2 CH3 O || Methyl n-propyl Pentan-2-one CH3 — C— CH2CH2CH3 ketone O || Pentan-3-one Diethyl ketone CH3CH2 — C— CH2 CH3 H3C O CH3 2, 4-Dimethylpentan-3- | || | Di-isopropyl ketone one CH3 — CH— C — CH — CH3 Q.1 Aldehydes, Ketones and Carboxylic Acid Write down the IUPAC name of the following compound (a) (b) A.1 (a) the IUPAC name is 3-Oxopentanal (b) the IUPAC name is Butane-1, 2, 4-tricarbaldehyde 3.  Aliphatic Ketones IUPAC Name Common Name Methyl phenyl ketone Methyl phenyl ketone or Acetophenone or Acetophenone Ethyl phenyl ketone Ethyl phenyl ketone or or Propiophenone Propiophenone Diphenyl ketone or Diphenyl ketone or Benzophenone Benzophenone 1-Phenylethan-1-one Methyl phenyl ketone Aldehydes, Ketones and Carboxylic Acid 1-Phenylpropan-1-one Ethyl phenyl ketone g-methylcyclohexane- 3-methylcyclohexane- carbaldehyde carbaldehyde 4. Orbital Structure of Aldehyde and Ketone Concept Ladder The magnitude of positive charge on the carbonyl carbon is more in aldehydes than in ketones. It is due to smaller +I effect of one alkyl group in aldehydes as compared to larger +I effect of two alkyl groups, as a One of the sp2-orbital of carbon overlap with result, nucleophilic addition a p-orbital oxygen forming a C—O, sp2-p, s-bond. reaction occur more readily in The remaining two sp2-orbitals of the carbon aldehydes than in ketones. atom form two additional s-bond either by overlappping with 1s-orbital of two H-atom as in formaldehyde, or with 1s-orbital of one hydrogen atom and one sp3-orbital of an alkyl group in aldehyde other than formaldehyde or with two sp3-orbitals of two alkyl groups in ketones. All the three s-bonds lie in the same plane and are inclined to one another at an angle of 120°. The two half-filled 2pz-orbitals, one each on Previous Year’s Questions carbon and oxygen atom, overlap sideways to form a p-bond. The electron cloud of the p-bond Carboxylic acids have higher lies both above and below the C—O, s-bond. boiling points than aldehydes, Physical properties (Aldehyde and Ketone) ketones and even alcohols of 1. Smell comparable molecular mass. It is The lower aldehydes pungent unpleasant due to their Aldehydes, Ketones and Carboxylic Acid odour, as the size of the molecule increases, [NEET] the odour becomes less pungent and more (1) formation of intramolecular fragrant. H-bonding 2. Solubility (2) formation of carboxylate ion Aldehydes and ketones upto four carbon atoms (3) more extensive association of eg. ethanal. propanone, propanal, butanal, carboxylic acid via van der Waals’ etc. are soluble in water due to H-bonding. forces of attraction As the size of the R (alkyl group) increases (4) formation of intermolecular solubility decreases. Solubility of aromatic H-bonding. aldehydes and ketones is lower than that of corresponding aliphatic aldehydes and ketones. 5.  3. Boiling point Aldehydes and ketones has weak intermolecular forces of attraction (dipole- dipole), so have low boiling point. Dipole- dipole intractions are, weaker than intermolecule H-bonding. < < < Preparation (Aldehydes and Ketones) 1. From alcohols When the vapours of alcohol are passed over Rack your Brain the copper at 300°C: Cu/300°C (a) R — CH2OH → R — CHO + H2 There is a large difference in the boiling point of butanal and Cu, 573K eg. CH3CH2OH  → CH3CHO+ H2 butan-1-ol? Ethyl alcohol Acetaldehyde Cu/300°C (b) R2CHOH  → R — CO — R + H2 eg. Aldehydes, Ketones and Carboxylic Acid Concept Ladder From oxidation of alcohols Selective oxidation of 1° y 1°-Alcohol: A primary alcohol gives aldehyde on alcohol to aldehydes are oxidation by using acidic K2Cr2O7 or pyridinium Collins’s reagent (CrO3.2C5H5N chlorochromate (CrO3.C5H5N.HCl) etc. in CH2Cl2), Sarett reagent (CrO3.2C5H5N in pyrdine), Corey’s reagent (CrO3. C5H5N.HCl) and Pyridinium dichromate (C5H5NH+)2Cr2O72-. 6.  (a) (b) (c) (d) y CrO3–pyridine/CH2Cl2 can also be used instead of K2Cr2O7 + H2SO4. Here due to absence of H2O aldehydrate is not formed so for their Previous Year’s Questions oxidation is not possible CrO −pyridine C2H5 − OH + [O]  3 → H3C − CHO Ketones [RCOR1] where R = R1 = CH Cl ,−H O Ethyl alcohol 2 2 2 Aldehyde(ethanal) alkyl group. It can be obtained in PCC one step by CH = CH − CH2OH → CH = CH − CHO 2 CH Cl 2 2 2 [NEET] Note : Reaction does not occur at double (1) oxidation of tertiary alcohol bond. CrO3–Pyridine is called Sarett–Collin’s (2) reaction of acid halide with Reagent or Sarett’s Reagent. alcohols y 2°-Alcohols: A secondary alcohol gives ketone (3) hydrolysis of esters on oxidation by using acidic K2Cr2O7 under (4) oxidation of primary alcohol Aldehydes, Ketones and Carboxylic Acid extreme conditions. Example 7.  y 3°-Alcohol: A tertiary alcohol gives ketone on oxidation under extreme conditions. y Previous Year’s Questions Which alkene on ozonolysis gives CH3CH2CHO and CH3COCH3? [NEET] (1) 2. From amines − (2) CH3CH2CHCHCH2CH3 − R (alkyl group) may be —CH3, —C2H5 etc. (3) CH3CH2CHCHCH3 3. From hydrocarbons (4) Ozonolysis of alkenes followed by hydrolysis give aldehydes and ketones. Examples, Ozonolysis Aldehydes, Ketones and Carboxylic Acid (a) (b) 8.  Hydroboration oxidation (c) (d) By Wacker method CuCl CH =2 CH2 + H2O + PdCl 2  2 → CH3CHO + 2HCl + Pd By the hydration of alkynes(Kucherov’s Reaction) Examples, (a) (b) Aldehydes, Ketones and Carboxylic Acid 4. From Nitriles and esters 9.  5. By hydrolysis of gem dihalides Examples, (a) (b) 6. By decarboxylation of calcium salts of carboxylic acids (a) (b) (c) (d) 7. By catalytic decomposition of fatty acids: Previous Year’s Questions Aldehydes, Ketones and Carboxylic Acid When vapours of carboxylic acid (fatty acids) are passed over thoria or magnous oxide or In the reaction, CH3CN + 2[H] heated alumina, carbonyl compounds are HCl Ether Boiling, H2O → X → Y ; the term Y is formed. [NEET] Al O RCOOH + HCOOH  2 3 → RCHO + H2O + CO2 (1) acetaldehyde 400°C (2) ethanamine CH3COOH + HCOOH  2 3Al O → CH3CHO + H2O + CO2 (3) acetone 400°C (4) dimethylamine MnO 2CH3COOH  500°C → CH3COCH3 + H2O + CO2 Acetic acid Acetone 10. Preparations for Aldehydes (Aliphatic Aldehyde) 1. Rosenmund’s reaction: Aldehydes can be formed by passing H2 gas through a boiling solution of acid chloride in xylene in the presence Pd/BaSO4. Previous Year’s Questions y Here the catalyst (Pd/BaSO4) is poisoned by sulphur or quinoline to avoid further reduction A carbonyl compound reacts of aldehydes into alcohols. with hydrogen cyanide to form O O cyanohydrin which on hydrolysis || || forms a racemic mixture of Pd/BaSO4 R − C − Cl  sulphur → R − C − H+ HCl a-hydroxy acid. The carbonyl Acidchloride Aldehyde compound is Example, [NEET] O O (1) formaldehyde || Pd/BaSO4 || (2) acetaldehyde H3C − C − Cl  sulphur → H3 C − C − H+ HCl Acidchloride Acetaldehyde (3) acetone (4) diethyl ketone Lithium tri-tert-butoxyaluminum hydride (LTBA) or SnH2 can also be used here. 1. From Grignard reagent: RMgX (Grignard reagent) gives aldehyde only with HCN as follows: 2. From Nitriles Aldehydes, Ketones and Carboxylic Acid R (alkyl group) may be —CH3, —C2H5 etc. The above reaction is also known as Stephen's reduction Example, 11.  3. From Alkenes by oxo method Previous Year’s Questions The oxidation of toluene to benzaldehyde by chromyl e.g. chloride is called [NEET] (1) Etard reaction (2) Riemer–Tiemann reaction Aromatic Aldehyde (3) Wurtz reaction 1. From side chain halogenation (4) Cannizzaro’s reaction (a) 2. From gem-diacetate (b) 3. From Toluene (c) Aldehydes, Ketones and Carboxylic Acid This reaction is also known as Etard reaction. (d) This reaction is called Gatterminann-Koch formylation. 12. Preparation for Ketones only 1. From organo metallics Previous Year’s Questions O O || || 2R — C— Cl + CdR'2 ∆ Dry ether → 2R — C— R' + CdCl 2 — Dialkyl cadmium CH2 — C— CH3 and CH2 = C— CH3 are || |− O O [NEET] (1) resonating structures R (alkyl group) may be —CH3, —C2H5 etc. (2) tautomers (3) geometrical isomers 2. From Grignard reagent: (4) optical isomers From Grignard Reagent and Acid Chlorides: R– CO – Cl + R′MgX  → R – CO – R′ + Mg(X)Cl R (alkyl group) may be —CH3, —C2H5 etc. Example, CH3 – CO – Cl + CH3MgCl  → CH3COCH3 + MgCl2 3. From nitriles R and R' (alkyl group) may be —CH3, —C2H5 etc. 4. From Dialkyl cuprutes O Rack your Brain || Ether R − C − Cl + R'2 CuLi  → RCOR' + LiCl + R' Cu Grignard reagent cannot be directly used in the reaction between acid chlorides/ acyl R and R' (alkyl group) may be —CH3, —C2H5 etc. chlorides with dialkylcadmium? Aldehydes, Ketones and Carboxylic Acid 5. From alkenes R' | H2C CHR'  R — C— Cl += −HCl → R — C— CH2 — C H — Cl  → R — C— CH CH — R' = || || || O O O R and R' (alkyl group) may be —CH3, —C2H5 etc. Note: It is the example of Markovnikov’s addition initiated by R–C+=O (acylium cation). 13.  Aromatic Ketones Friedel-crafts acylation Rack your Brain Why ethylbenzene not prepared by simple alkylation of benzene? R (alkyl group) may be —CH3, —C2H5, —C6H5, —Cl etc. This reaction is called Friedel-crafts acylation This reaction is the example of electrophilic substitution reaction. Concept Ladder Fries rearrangement Carbonyl group (in case of aldehydes and ketones) acquires a small positive charge and hence acts as an electrophile (Lewis acid) R (alkyl group) may be —CH3, —C2H5, —C6H5 while the carbonyl oxygen etc. carries a small negative Chemical Properties charge and hence behaves as Reactions due to carbonyl group a nucleophile (Lewis base). Nucleophilic addition reactions Thus, the carbonyl group is Due to the presence of carbonyl group polar in a nature and that is aldehydes and ketones undergo nucleophilic why aldehyde and ketones addition reactions. have large moment (2.3 –2.8D) Aldehydes, Ketones and Carboxylic Acid y Reactivity of a carbonyl compound for nucleophilic addition reaction depends upon Rack your Brain the magnitude of +ve charge present on the carbon atom of carbonyl group i.e., the Why aldehydes are more reactive reactivity is increased by electron attracting than ketones? groups like – NO2, – CX3 and decreased by 14.  electron releasing groups like methyl, alkoxy groups. y Reactivity for nucleophilic addition reaction for carbonyl compounds decreases as follows: (1) HCHO > CH3CHO > CH3CH2CHO > CH3COCH3 > CH3COC2H5 > C2H5COC2H5 > (CH3)3CCOC (CH3)3 (2) Concept Ladder In case of nucleophilic addition 1. Acetal and ketal formation reaction, nucleophile readily Aldehydes react with one equivalent of attacks the electrophilic carbon a monohydric alcohol in the presence of dry hydrogen chloride gas to yield atom of the polar carbonyl group first alkoxyalcohol intermediates called from a direction approximately hemiacetals. These then react with one more perpendicular to the plane of molecule of alcohol to give gem-dialkoxy the sp2-hybridized orbitals of the compounds known as acetals. carbonyl carbon. Aldehydes, Ketones and Carboxylic Acid 2. Addition of Grignard reagent 15.  3. Addition of Sodium bisulphite Previous Year’s Questions The product formed by the reaction of an aldehyde with a primary amine is [NEET] (1) carboxylic acid (2) aromatic acid 4. Addition of hydrogen cyanide (HCN) (3) Schiff ’s base (4) ketone. Aldehydes, Ketones and Carboxylic Acid 5. Addition of amonia derivatives 16. (a) Reaction with hydroxylamine Concept Ladder Aldehydes, Ketones and Carboxylic Acid Aldehydes and ketones is catalysed by acids. In acidic medium weak nucleophiles like ammonia derivatives readily attack the carbonyl group, if medium is to acidic ammonia derivative being basic in nature will form their respective ammonium salts. 17.  Previous Year’s Questions (b) Reaction with hydrazine. Reaction of a carbonyl compound with one of the following reagents involves nucleophilic addition followed by elimination of water. The reagent is [NEET] (1) hydrazine in presence of feebly acidic solution (2) hydrocyanic acid (3) sodium hydrogen sulphite (4) a Grignard reagent (c) Reaction with phenylhydrazine. Aldehydes, Ketones and Carboxylic Acid 18. (d) Reaction with 2, 4- dinitrophenylhydrazine (Brady's reagent) (e) Reaction with semicarbazide Important naming reactions 1. (a) Aldol condensation Aldehydes, Ketones and Carboxylic Acid Mechanism of aldol condensation Step-1 19.  Step-2 Step-3 (b) Cross aldol condensation (a) Aldehydes, Ketones and Carboxylic Acid (b) 20. (c) If benzaldehyde is used in excess, the initially formed benzalacetone further condenses with another molecules of benzaldehyde to form dibenzalacetone. Thus, Such a base-cotalysed crossed aldol condensation between an aromatic aldehyde and an aliphatic aldehyde or a ketone is called Rack your Brain Claisen Schmidt condensation or simply Claisen reaction. What type of aldehydes 2. (a) Cannizzaro reaction and ketones undergo aldol condensation? Mechanism Aldehydes, Ketones and Carboxylic Acid (b) Crossed Cannizzaro reaction 21.  Note: All aldehydes (with or without a-hydrogens) Rack your Brain can be made to undergo Cannizzaro reaction on treatment with aluminium ethoxide. Formaldehyde and benzaldehyde However, under these conditions, the alcohol and the acid produced as a result give cannizzaro reaction but of Cannizzaro reaction, combine together to acetaldehyde does not. Explain? form esters. For example. Some important reduction reactions of aldehydes and ketones Previous Year’s Questions Aldehydes and ketones can be reduced to a variety of compounds under different conditions. 1. Reduction to hydrocarbons Reaction between benzaldehyde The carbonly group in aldehydes and ketones and acetophenone in presence of can be reduced to methylene group to form dilute NaOH is known as [NEET] hydrocarbons. The following methods are (1) Aldol condensation generally empolyed. (2) Cannizzaro’s reaction (i) Wolff-Kishner reduction. (3) Cross Cannizzaro’s reaction In this method, the aldehyde or the ketone is heated with hydrazine and (4) Cross Aldol condensation. KOH or potassium tert butoxide in a high boiling solvent such as ethylene glycol. NH.NH KOH,glycol R − CH= O  2 2 → R − CH= NNH2  → R − CH3 + N2 Aldehydes, Ketones and Carboxylic Acid −H O2 453−473K Hydrazone (ii) With HI + P (Red.) On heating an aldehyde or a ketone with hydriodic acid and red phosphorus to 423K (150°C), it is reduced to the corresponding alkane, e.g., 22.  Re d P, 423K CH3CHO+ 4HI  → CH3 — CH3 + H2O + 2I2 Acetaldehyde Ethane Re d P, 423K CH3COCH3 + 4HI  → CH3CH2CH3 + H2O + 2I2 Acetone Pr opane (iii) Clemmensen reduction. This involves the reduction of an aldehyde of a ketone with zinc amalgam and conc. hydrochloric acid. For example, Previous Year’s Questions Zn−Hg/HCl RCHO + 4[H]  → R − CH3 + H2O Which of the following reactions Alkane will not result in the formation of carbon-carbon bonds? [NEET] (1) Reimer–Tiemann reaction Clemmensen reduction is widely used for (2) Cannizzaro reaction the reduction of aldehydes or ketones (3) Wurtz reaction which are sensitive to alkalies. (4) Friedel–Crafts acylation 2. Reduction to pinacols Ketones on reduction with magnesium amalgam and water form pinacols. 3. Reduction to alcohol Aldehydes and ketones on reduction give primary and secondary alcohol respectively. Reduction is carried out either catalystically Rack your Brain with H2 in presence of Ni, Pt or Pd or chemically Aldehydes, Ketones and Carboxylic Acid with lithium aluminium hybride (LiAlH4) or Name one reagent to distinguish simply diborane (B2H6). between 2-pentaone and 3-pentanone. For example, 23.  For example, Previous Year’s Questions RCOR (Ketones) can also be reduced to A strong base can abstract an the corresponding secondary alcohols with a-hydrogen from aluminium isopropoxide in isopropyl alcohol. For example, [NEET] (1) ketone (2) alkane (3) alkene (4) amine This reaction is called Meerwein-Ponndorf- Verley Reduction. It involves transfer of hydride ion from isopropyl alcohol to the ketone and thus, this reduction can be regarded as reverse of oppenauer oxidation. Some important oxidation reactions of aldehydes and ketones 1. Oxidation of aldehydes Concept Ladder Aldehydes are easily oxidised to carboxylic acids containing the same number of carbon Ammoniacal silver nitrate atom. solution is called Tollen’s reagent. It is used to test aldehydes. both aliphatic and aromatic aldehydes reduce Tollens’ reagent to shining silver mirror. It is Reason: Presence of a hydrogen atom on the also used to distinguish carbonyl group which can be converted into aldehydes from ketones. —OH group without involving the cleavage of any other bond. Therefore, they are oxidized Aldehydes, Ketones and Carboxylic Acid not only by strong oxidizing agent like HNO3, KMnO4 and K2Cr2O7 but also by weak oxidizing agents like bromine water, Ag+, Cu2+ ions etc. As a result, aldehydes act as strong reducing agent. They reduce (i) Tollens' reagent to metallic silver (silver Rack your Brain mirror) and (ii) Fehling's solution or Benedict's solution to What is the function of Roschelle a red precipitate of cuprous oxide (Cu2O). salt in Fehling’s solution? (i) Reduction of Tollen's reagent When an aldehyde is heated with Tollens' reagent the latter is reduced to metallic silver 24.  which deposits on the walls of the test tube as bright silver mirror. During this reduction, Concept Ladder the following reactions occur: Acetaldehyde reduces AgNO3 + NH4OH  → NH4NO3 + AgOH Tollen’s to produce shining silver mirror and produces red ppt. of Cu2O with → [Ag(NH3 )2 ]+ OH− + 2H2O AgOH + 2NH4OH  Fehling’s solution. ( Tollen' s reagent ) RCHO + 2[Ag(NH3 )2 OH  → RCOOH + H2O + 4NH3 + 2Ag ↓ Aldehyde ( Tollen' s reagent ) Carboxylic acid silver mirror Previous Year’s Questions or RCHO + Ag 2O  → RCOH + 2Ag ↓ Which of the following is This test also known as silver mirror test. incorrect? Note: [NEET] Aromatic and aliphatic aldehydes reduce (1) FeCl3 is used in detection of Tollens' reagent. phenol. (ii) Reduction of Fehlling's solution (2) Fehling solution is used in When an aliphatic aldehyde is heated with detection of glucose Fehling's solution, the latter is reduced to (3) Tollens’ reagent is used in give a red ppt. of cupprous oxide. During this reduction the following reaction occur: detection of unsaturation. (4) NaHSO3 is used in detection CuSO4 + 2NaOH  → Cu(OH)2 + Na2SO4 of carbonyl compound. ∆ Cu(OH)2 → CuO + H2O RCHO + 2CuO  → Cu2O ↓ + RCOOH Concept Ladder Aldehyde Aldehydes, Ketones and Carboxylic Acid Cuprousoxide Carboxylic (Redppt.) acid Due to presence of a H-atom or on the carbonyl group, aldehydes can be more RCHO + 2Cu+2 + 3OH  → RCOO− + 2Cu+ + H2O easily oxidised than ketones. (Red ppt.) As a result, aldehydes act (iii) Reduction of Benedict's solution as reducing agent and thus Benedict's solution, is the alkaline solution of reduce Tollen’s reagent, Cu2+ ions complexed with citrate ions. Fehling’s solution etc. Redppt. RCHO + 2Cu+2 + 3OH−  → RCOO− + 3H2O + Cu2O ↓ 25.  (i) Oxidation with strong oxidising agents. Strong oxidising agents like conc. HNO3, Rack your Brain KMnO4/H2SO4, K2Cr2O7/H2SO4, etc. oxidises ketones. During these oxidations rupture of Benzaldehyde reduces Tollens’ the carbon-carbon bond occurs on either reagent but not the Fehling’s or side of the keto group giving a mixture of the Benedict’s solution? carboxylic acids, each containing lesser number of carbon atoms than the original ketone. For example If case of unsymmertical ketones such as butan-2-one, pentan-2-one, etc. the keto group stays perferentially with the smaller alkyl group (Popoff's rule). For example, in case of pentan-2-one, the major mode of cleavage is path 'a' in which the keto group stays with the methyl group. (ii) Oxidation of methyl ketones with sodium hypohalite (NaOX or X2 + NaOH)—Halogorm Aldehydes, Ketones and Carboxylic Acid reaction. Aldehydes and ketones containing CH3CO— Previous Year’s Questions group, on treatment with an excess of halogen in presence of alkali produce a haloform (chloroform, bromoform or iodoform). In CH3CHO and C6H5CH2CHO can be this reaction all the three H-atoms of the distinguished chemically by methyl group are first replaced by halogen [NEET] atoms to form either a trihaloaldehyde or a (1) Benedict’s test trihaloketone which subsequently reacts with (2) iodoform test alkali to yield a haloform and the salt of a (3) Tollens’ reagent test carboxylic acid containing one carbon atom less than the starting aldehyde/ketone, for (4) Fehling’s solution test 26.  example, (i) (ii) When this reaction is carried out with sodium hypoiodite, NaOI or NaOH/I2, yellow precipitate of iodoform is produced. For example, Due to the formation of yellow ppt. of iodoform in this reaction, it is known as iodoform test and is used for characterising compounds Previous Year’s Questions containing CH3CO—group or any group such as CH3CH(OH)—which can be easily oxidised Following compounds are given to CH3CO— group by halogens. (i) CH3CH2OH Halogenation (ii) CH3COCH3 Aldehydes and ketones containing a-hydrogen (iii) CH3 — CHOH atoms undergo halogenation when treated | with halogen in presence of an acid or a base. CH3 Aldehydes, Ketones and Carboxylic Acid However, in presence of a base, polyhalogenation (iv) CH3OH occurs (see halogorm reaction) but in presence Which of the above compound(s), of acids, the reaction can be stopped at the on being warmed with iodine monohalogenation stage by using one mole of solution and NaOH, will give the halogen. iodoform? [NEET] (1) (i), (iii) and (iv) (2) Only (ii) (3) (i), (ii) and (iii) (4) (i) and (ii) 27.  With excess of halogen, di- and tri-halogen derivatives are formed Rack your Brain Formaldehyde, however, does not undergo this reaction since it does not have a-halogen atoms. What type of ketones undergo Carboxylic Acids iodoform test? Concept Ladder Carboxylic acids are much stronger acids (approx. 1011– 1012 times stronger) than alcohols. This is due to the reason that both carboxylic y Organic compounds having carboxylic group acid and carboxylate anion —COOH are called carboxylic acids. are stabilized by resonance y The name carboxyl is derived from carbonyl but neither the alcohol (ROH) nor their alkoxide and hydroxyl (–OH). ions (RO–) are stabilized by y Monocarboxylic acid (–COOH) of aliphatic resonance. series are also known as fatty acids. y The general formula is CnH2nO2 or CnH2n+1 COOH. Structures IUPAC Name Common Name CH3 — CH — CH2 — C OOH 3-Methylbutanoic | Aldehydes, Ketones and Carboxylic Acid Isovaleric acid CH3 acid CH3 — CH = CH — C OOH But-2-enoic acid Crotonic acid Propane-1, 3-dioic HOO C— CH2 — C OOH Malonic acid acid 28.  COCH3 2-(1-Oxoethyl) | a-Acetylsuccinic butane-1, 4-dioic HOO C— CH2 — CH — C OOH acid acid Isomerism Shown by Acids Carboxylic acids show chain isomerism and functional isomerism with esters, hydroxy Concept Ladder carbonyl compounds and hydroxy oxiranes or cyclic ethers. In the case of C2H4O2, the various Carboxylic acid and carboxylate possible arrangements are, ion, both are stabilized by 1. CH3–COOH 2. HCOOCH3 resonance but the carboxylate 3. CHO 4. ion is better resonance | stabilized because it consists CH2 =OH of two equivalent and identical canonical forms. It is character Acids also show optical and geomatrical of carboxylic acids. isomerisms. e.g., Valeric Acid (optically active) Rack your Brain Why are carboxylic acids called fatty acids? and Physical Properties Aldehydes, Ketones and Carboxylic Acid Physical state: Carboxylic acids upto C3 – atoms are colourless liquids and pungent smelling & from C4 – C9 are rotten butter smelling colourless liquids. Concept Ladder Solubility: Formic acid is a stronger acid Due to H-bonding lower acids upto C4 – atom than acetic acid Due to +I effect are completely soluble in H2O. With the increase of the alkyl group of molecular weight the solubility decreases. Acidic strength CH3COOH > Example : CH3CH2COOH > (CH3)2CHCOOH HCOOH > CH3COOH > C2H5COOH > C3H7COOH > (CH3)2CCOOH 29.  y Carboxylic acids get dimerise i.e., exits as cyclic dimmers due to hydrogen bonding Concept Ladder Due to strong H-bonding, the boiling points of carboxylic Boiling points acids are higher than those The boiling points of carboxylic acids are of alcohols with comparable more than that of corresponding alcohols, acid molecular masses. Most of the carboxylic acids in the derivatives or carbonyl compounds due to higher solid, liquid or even in the extent of hydrogen bonding. vapour state exist as cyclic y Boiling point of acids ∝ Molecular weight dimers. Example, HCOOH < CH3COOH < C2H5COOH < C3H7COOH Melting point The melting point of an acid with even number Rack your Brain of carbon atoms is more than the acid having next odd number of carbon atoms. Boiling point of acetic acid is Example, higher than that of n-propanol? C4H9COOH > C5H11COOH y The —COOH group and R (alkyl group) in acids with even number of carbons, lie on opposite sides and hence provide a closer packing in the lattice and have high melting points. Previous Year’s Questions Preparation Which of the following represents 1. By the hydrolysis of acid derivatives the correct order of the acidity in Aldehydes, Ketones and Carboxylic Acid Acid derivatives can be easily hydrolyzed into the given compounds? acids by using dilute acids or dilute alkalines as [NEET] follows (1) FCH2COOH > CH3COOH > O BrCH2COOH > ClCH2COOH || (2) BrCH2COOH > ClCH2COOH > y R − C − Cl + H2O  → R − COOH + HCl Acid chloride FCH2COOH > CH3COOH (3) FCH2COOH > ClCH2COOH > NaOH HO R − COCl  −NaCl → R − COONa → 2 R − COOH BrCH2COOH > CH3COOH (4) CH3COOH > BrCH2COOH > ClCH2COOH > FCH2COOH. 30.  O || y R − C − NH2 + H2O  → R − COOH + NH3 Concept Ladder Amide The nature of substituents O || affects the stability of the y R − C − OR '+ H2O  → R − COOH + R '− OH carboxylate ion and hence Ester affects the acidity of the y carboxylic acids. Electron withdrawing groups (EWG) R and R' (alkyl group) may be —CH3, —C2H5 etc. increase the stability of the 2. By the hydrolysis of cyanides (R–CN) carboxylate ion (conjugate Hydrolysis of cyanides by dilute HCl give acids base) by dispersing the negative charge and hence increases the R − C ≡ N + 2H2O  → R − COOH + NH3 ↑ acidity. Example, HCl CH3 − C ≡ N + 2H2O → CH3COOH + NH3 ↑ If hydrolysis is done by H2O amide is the final product. 3. From Grignard reagent and carbon dioxide Rack your Brain RMgX (Grignard reagent) on reaction with carbon dioxide followed by hydrolysis gives CH3COOH is highly soluble in acids as follows: water but hexanoic acid is only slightly soluble? Example, Aldehydes, Ketones and Carboxylic Acid R–Li (R may be –CH3, –C2H5) also gives R– COOH with CO2 as follows. 31.  4. From the Hydrolysis of Haloforms Hydrolysis of haloforms give acids. Concept Ladder Formic acid contains a H-atom attached to a >C=O group and hence can be regarded as an aldehyde. Therefore, it acts as a reducing agent and hence decolourises pink violet Note: colour of KMnO4 solution. y KOH (strong base) can also be used in place acetic acid, on the other of NaOH (weak base). hand, contains an alkyl (i.e., 5. By oxidation of carbonyl compounds CH3) group attached to >C=O By using strong oxidising agent like K2Cr2O7 or group. Therefore, it does not KMnO4 in acidic medium carbonyl compounds act as a reducing agent and undergo oxidation. hence does not decolourise KMnO4 solution. R–CHO + [O]  → R–COOH Example, CH3CHO + [O]  → CH3COOH y During oxidation of ketones the carbonyl group goes with smaller alkyl group according to Popoff ’s rule. Example, Rack your Brain Aldehydes, Ketones and Carboxylic Acid Formic acid reduces Tollens’ reagent but acetic acid does not. Explain? From dicarboxylic acid or esters 32. 6. From amide (R–CONH2) Amides on reaction with HONO nitrous acid) give acids. O || R − C − NH2 + HNO2  → R − COOH + N2 + H2O Concept Ladder Example, Olefins on heating with CO and steam under pressure at 573– O || 673K in presence of H3PO4 as C6H5 − C − NH2 + HNO2  → C6H5COOH + N2 + H2O catalyst gives monocarboxylic 7. From hydrocarbon acid. Heating of alkenes with carbon monoxide and water at high temperature and pressure gives acids H PO R—CH = CH2 + CO + H2O  3 4 350°C → R—CH2—CH2—COOH H PO CH3—CH = CH2 + CO + H2O  3 4 350°C → CH3—CH2—CH2—COOH + (CH3)2CHCOOH Butanoic acid 2-Methyl propanoic acid (a) From higher alkanes Higher alkanes like C6H14, C7H16 undergo oxidation to give acids as follows: ∆ ,120°C 2R — CH3 + 3O2  Mnacetateor stearate → 2R — COOH + 2H2O Example, ∆ ,120°C 2C6H13 — CH3 + 3O2  Mnacetateor stearate → 2C6H13 — COOH + 2H2O Normal hep tane Hep tanoic acid (b) From alkenes Aldehydes, Ketones and Carboxylic Acid On oxidative cleavage of alkenes by alkaline KMnO4 (hot) gives acids. O O | | Hot[O] || || — = C C—  Alk.KMnO → — C— OH + — C— OH Rack your Brain 4 Example, How can you distinguish between Hot[O] CH =3 — CH CH — CH3  Alk.KMnO → 2CH3COOH an alcohol and a carboxylic acid. 4 y If the double bond in alkenes at terminal position formic acid (HCOOH) is formed which further oxidizes into CO2 and H2O. 33.  Hot [O] CH3 — CH — CH2 → KMnO CH3COOH + HCOOH  → H2O + CO2 4 (c) From alkynes On ozonolysis alkynes followed by hydrolysis give acids. 1.O3 /CCl 4 R — C ≡ C — R ' → 2.H O RCOOH + R ' COOH 2 Example, 1.O3 /CCl 4 CH3 — C ≡ C— C2H5 → 2.H O CH3COOH + C2H5COOH 2 Pentyne −2 8. From R–ONa with CO Sodium alkoxide (RONa) on reaction with CO followed by reaction with HCl gives acids. ∆ HCl R — ONa + CO  NaOH → R — COONa → R — COOH Example, ∆ HCl C2H5ONa + CO  NaOH → C2H5COONa → C2H5COOH Sod.ethoxide Note : For HCOOH preparation CO and NaOH is used. ∆ HCl CO + NaOH  473K → HCOONa  −NaCl → HCOOH 9. From Ketones Methyl Ketones on oxidation by NaOX or X2/NaOH give acids as follows: Aldehydes, Ketones and Carboxylic Acid Note: Rack your Brain This reaction is also known as haloform reaction. Why formaldehyde HCHO does not give idoform test? 34. Chemical Properties Reactions due to –COOH group 1. Salt formation Concept Ladder Acid reacts with Na, NaOH, NaHCO3, Na2CO3 to form acid salts. Carboxylic acids like alcohols Na react with active metals like R — COOH → NaOH,NaHCO3 or R — COONa + H2 Na2CO3 Na, K, Ca, Mg, Zn etc. to form their respective salts Example, liberating H2 gas. Na CH3COOH  NaOH → CH3COONa + H2 Here CO2 evolved is from sodium bicarbonate (NaHCO3) or sodium carbonate (Na2CO3) and not of R–COOH. e.g., C6 H5 C OOH + NaHCO3  → C6H5 C OONa + CO2 + H2O Reaction with PCl5 or SOCl2 Rack your Brain R — COOH + PCl5  → R − COCl + HCl + POCl 3 Esterification of carboxylic acids Example, with alcohols is a nucleophilic CH3COOH + PCl5  → CH3COCl + HCl + POCl 3 acyl substitution reaction. Why? Pyridine R — COOH + SOCl 2  → R − COCl + SO2 + HCl Example, Pyridine CH3COOH + SOCl 2  → CH3COCl + SO2 + HCl 2. Ester formation or esterfication Acid on reaction with alcohols (ROH) or Aldehydes, Ketones and Carboxylic Acid diazomethane (CH2N2) in presence of dilute Concept Ladder acid or base gives esters. When carboxylic acids are R — COOH + HOR ' dil.  →R − COOR '+ H2O heated with alcohols or Acid phenols in presence of conc. Example, H2SO4 or dry HCl gas (Fischer- dil. CH3COOH + HOC2H5  → CH3COOC2H5 + H2O Speier esterification), esters acid are formed. The reaction is dil. R — COOH + CH2.N2 → R − COOCH3 + NH3 reversible in nature and is HCl known as esterificiation. Example, dil.HCl C2H5COOH + CH2N2 → C2H5COOCH3 + NH3 Methyl propanoate 35.  3. Reaction with NH3 Acid reacts with ammonia (NH3) to give Rack your Brain ammonium salt which on heating gives amide as shown: What happen when acetic acid is NH3 ∆ R — COOH → RCOONH4  −H O → R — CONH2 treated with soda-lime? 2 Example, NH ∆ CH3COOH → 3 CH3COONH4  −H O → CH3CONH2 2 Acetamide 4. Decarboxylation Acids undergo decarboxylation with soda lime (CaO + NaOH) to give alkane. NaOH CaO, ∆ R — COOH  −H O → R − COONa → +NaOH R — H + Na2CO3 2 Example, NaOH CaO, ∆ CH3COOH  −H O → CH3COONa → +NaOH CH4 + Na2CO3 2 CaO, ∆ HCOONa + NaOH → Na2CO3 + H2 5. Dehydration Carboxylic acids ndergo dehydration with conc. H2SO4 or P2O5 to give acid anhydrides. 6. Reduction Concept Ladder Aldehydes, Ketones and Carboxylic Acid with use of LiAlH4 carboxylic acids can be reduced into alcohols. Carboxylic acids containing an electron withdrawing O || group such as >CO or — LiAlH4 R — C— OH + 4[H]  → R — CH2OH + H2O COOH or —NO2 at the b-carbon atom w.r.t. the — 7. Oxidation or burning COOH group readily undergo Except HCOOH, all other mono carboxylic decarboxylation on heating. acids are resistant to oxidation. 3-Oxobutanoic acid  ∆ → Propanone It can be oxidized only by prolong heating using strong oxidizing agents and get converted to CO2 and H2O. 36.  O || ∆ R — C— OH  O → CO2 + H2O Concept Ladder 2 Formic acid may vbe rearded Example, both as an aldehyde as well O as a carboxylic acid. Like || ∆ CH3 — C— OH  → CO2 + H2O aldehydes, it can be easily O 2 oxidised to carbonic acid 8. Reaction with urea which decomposes to give CO2 and H2O. Acid reacts with urea to give amides. R—COOH + NH2CONH2  → R—CONH2 + NH3 + CO2 Example, CH3COOH + NH2CONH2  → CH3CONH2 + NH3 + CO2 9. Heating effect of acid salts When calcium salts of carboxylic acids (RCOO)2Ca are heated, ketones are formed, and when calcium formate is heated an aldehyde is formed. ∆ (R — COO)2 Ca + (R ' COO)2 Ca → R — CO — R + R '— CO — R '+ R — COR ' Example, Concept Ladder Dry distillation of calcium salt Aldehydes, Ketones and Carboxylic Acid of fatty acids gives aldehydes or ketones. For example, dry distillation of calcium formate gives formaldehyde, that of calcium acetate gives acetone while that of 10. Reducing properties of HCOOH a mixture of calcium acetate HCOOH shows reducing properties and and calcium formate gives reduces Fehling’s solution, Tollen’s reagent, acetaldehyde. mercuric chloride and KMnO4. 37.  Ag 2O HCOOH  ( Tollen reagent ) → H2O + CO2 + 2Ag ↓ Silver mirror Rack your Brain 2CuO HCOOH  (Fehling solution) → H2O + CO2 + Cu2O ↓ (Red. ppt ) Carboxylic acids are acidic than alcohol or phenols. Why? 2HgCl HCOOH → 2 2HCl + CO2 + Hg 2Cl 2 ↓ Hg Cl HCOOH → 2 2 2HCl + CO2 + 2Hg ↓ Black 2KMnO4 + 3H2SO4 + 5CH2O  → K2SO4 + 3MnSO4 + 5CO2 + 8H2O 11. Reaction with organometallics Acids react with organo-metallics to give alkanes. R '— CH2MgX + R — COOH → R ' CH3 + R — COOMgX Example, CH3CH2MgX + CH3COOH  → CH3 — CH3 + CH3COOMgX R — COOH + RLi  → R '— H + R — COOLi Example, CH3COOH + C2H5Li  → C2H6 + CH3COOLi Concept Ladder Distinction between R–COOH and phenol: Formic acid is used as a coagulating agent for latex in rubber industry, acetic acid is used a solvent in the manufacutre of plastic, rayon and silk and Higher fatty acids are used for the manufacture of soaps and detergents. Aldehydes, Ketones and Carboxylic Acid Some Important Naming Reactions 1. Schmidt reaction: Acid reacts with hydrazoic acid (N3H) in presence of conc. H2SO4 to give a primary amine. ∆ R — COOH + N3H  Conc.H SO → R — NH2 + N2 + CO2 2 4 38. Example, ∆ C2H5COOH + N3H  conc.H SO → C2H5NH2 + N2 + CO2 2 4 Hydrazoic acid Ethyl amine 2. Hell–Volhard Zelinsky reaction Concept Ladder Carboxylic acids having a-H atom react with Cl2 or Br2 in presence of catalysts like red P4, iron to give α-chloro or bromo substituted Chloroacetic acid is stronger acids. than that of acetic acid, Chlorine atom possess –I effect which withdraws electrons from O–H bond. Which results decrease in R (alkyl group) may be —CH3, —C2H5 etc. electron density in the O–H Some Important Compounds bond and increases the Benzoic Acid acidity Benzoic acid is present in nature in combined Rack your Brain state as esters in Balsam and in the urine of horse as benzoyl glycine (Hippuric acid). Benzoic acid do not undergo Preparation friedel craft reaction? 1. By the hydrolysis of cyanobenzene Concept Ladder 2. By the hydrolysis of benzamide Aromatic acids undergo the Aldehydes, Ketones and Carboxylic Acid electrophilic substitution reactions of the benzene 3. By the hydrolysis of benzoyl chloride ring such as halogenation, nitration and sulphonation. Since the —COOH group is electron-withdrawing, therefore, it is metal 4. By the hydrolysis of ethyl benzoate directing. 39.  5. By the hydrolysis of benzoic anhydride Concept Ladder Oxidation of alkylbenzenes with alkaline KMnO4 or acidified K2Cr2O7 gives benzoic acid. During these oxidation, the 6. By oxidation reactions aromatic nucleus remains intact Toluene on oxidation by alkaline KMnO4 or but each side chain is oxidised CrO3 gives benzoic acid. to –COOH group irrespective of its length.