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60 Chapter E3 29 Nitrogen Containing Compounds 2 2 2 2 2 O O D YG Corresponding to these two forms, nitrous acid gives two types of derivatives, i.e., alkyl nitrites and nitro alkanes. Alkyl nitrite O O Nitro alkane It is important to note that nitro alkanes are better regarded as nitro derivatives...
60 Chapter E3 29 Nitrogen Containing Compounds 2 2 2 2 2 O O D YG Corresponding to these two forms, nitrous acid gives two types of derivatives, i.e., alkyl nitrites and nitro alkanes. Alkyl nitrite O O Nitro alkane It is important to note that nitro alkanes are better regarded as nitro derivatives of alkanes, while alkyl nitrites are regarded as alkyl esters of nitrous acid. (1) Alkyl nitrites : The most important alkyl nitrite is ethyl nitrite. Ethyl nitrite (C H ONO) (i) General methods of preparation : It is prepared (a) By adding concentrated HCl or H SO to aqueous solution of sodium nitrite and ethyl alcohol at very low temperature (0°C). NaNO 2 HCl NaCl HNO 2 5 U 2 2 4 ST C 2 H 5 OH HNO 2 C 2 H 5 ONO H 2 O Ethyl nitrite (b) From Ethyl iodide C 2 H 5 I KONO C 2 H 5 ONO KI Ethyl iodide Pot. nitrite HCl Small amount of hydroxylamine is also formed. Nitro form R O N O ; R N Sn C2 H 5 ONO 6 H C2 H 5 OH NH 3 H 2 O U Nitrous acid exists in two tautomeric forms. Nitrite form NaOH C2 H 5 ONO H 2 O C2 H 5 OH HNO 2 (b) Reduction : Alkyl nitrites and nitro alkanes H O N O ⇌ H N (a) Hydrolysis : It is hydrolysed by aqueous alkalies or acids into ethyl alcohol. ID The important nitrogen containing organic compounds are alkyl nitrites (RONO), nitro-alkanes (RNO ), aromatic nitro compounds (ArNO ), alkyl cyanides (RCN), alkyl iso cyanides (RNC), amines (– NH ), aryl diazonium salts (ArN Cl), amides (–CONH ) and oximes (>C = N OH). Ethyl nitrite (c) By the action of N 2 O 3 on ethyl alcohol. 2C 2 H 5 OH N 2 O3 2C 2 H 5 ONO H 2 O (ii) Physical properties (a) At ordinary temperature it is a gas which can be liquified on cooling to a colourless liquid, (boiling point 17°C) having characteristic smell of apples. (b) It is insoluble in water but soluble in alcohol and ether. (iii) Chemical properties C2 H 5 ONO 4 H C2 H 5 OH NH 2 OH (iv) Uses (a) Ethyl nitrite dialates the blood vessels and thus accelerates pulse rate and lowers blood pressure, so it is used as a medicine for the treatment of asthma and heart diseases (angina pectoris). (b) Its 4% alcoholic solution (known as sweet spirit of nitre) is used in medicine as a diuretic. (c) Since it is easily hydrolysed to form nitrous acids, it is used as a source of nitrous acid in organic synthesis. Isoamyl nitrite is used as an antispasmodic in angina pectoris and as a restorative in cardiac failure. (2) Nitro alkanes or Nitroparaffins : Nitro alkanes are regarded as nitro derivatives of hydrocarbons. (i) Classification : They are classified as primary, secondary and tertiary depending on the nature of carbon atom to which nitro groups is linked. RCH 2 NO 2 ; Primary nitro alkane R CHNO 2 ; R Secondary nitro alkane R R R C NO 2 Tertiary nitro alkane (ii) General methods of preparation (a) By heating an alkyl halide with aqueous alcoholic solution of silver nitrite C 2 H 5 Br AgNO 2 C 2 H 5 NO 2 AgBr Some quantity of alkyl nitrite is also formed in this reaction. It can be removed by fractional distillation since alkyl nitrites have much lower boiling points as compared to nitro alkanes. (b) By the direct nitration of paraffins (Vapour phase nitration) With higher alkanes, a mixture of different nitro alkanes is formed which can be separated by fractional distillation. (c) By the action of sodium nitrite on -halo carboxylic acids 2 CH 2 ClOOH CH 2 NO 2 COOH NaNO NaCl – Chloro acetic acid Nitro methane (d) By the hydrolysis of –nitro alkene with water or acid or alkali (Recent method) CH 3 | | H or OH – | | NO 2 CH 3 C CH NO 2 HOH CH 3 C O CH 3 NO 2 O H2 Acetone Nitro methane Pseudo nitrol Tertiary nitro alkanes do not react with nitrous acid. (d) Thermal decomposition :. 300 C R.CH 2.CH 2 NO 2 R.CH CH 2 HNO 2 moderately On rapid heating nitro alkanes decompose with great violence. 1 3 N 2 CO 2 H 2 2 2 (e) Halogenation : Primary and secondary nitro alkanes are readily halogenated in the -position by treatment with chlorine or bromine. heat, Rapidly CH 3 NO 2 Cl 2 CH 3 NO 2 2 - Methyl,1- nitro propene NaOH 4 CH 3 CCl 3 NO 2 Chloropicrin or nitro chloroform (insecticide) E3 (e) Tertiary nitro alkanes are obtained by the oxidation of t-alkyl amines with KMnO. NaOH NO 2 Secondary α – Nitro aceticacid heat CH 3 NO 2 CO 2 CH 3 H 2O Ether or R 2 C H HON O R 2 C NO Blue colour 60 400 C CH 3 CH 3 HONO2 (fuming ) CH 3 CH 2 NO 2 H 2 O | Cl 2 NaOH R 3 CNH 2 R 3 CNO 2 H 2 O 2 Nitropropa ne CH 3 CHO CH 3 NO 2 CH 3 CH (OH )CH 2 NO 2 -Hydroxy nitropropa ne (nitro alcohol) D YG (aci- form ) (iv) Chemical properties (a) Reduction : Nitro alkanes are reduced to corresponding primary amines with Sn and HCl or Fe and HCl or catalytic hydrogenation using nickel as catalyst. RNO 2 6 H RNH 2 2 H 2 O However, when reduced with a neutral reducing agent (Zinc dust + NH Cl), nitro alkanes form substituted hydroxylamines. 4 Zn NH 4 Cl R NHOH H 2 O R – NO 2 4 H U (b) Hydrolysis : Primary nitro alkanes on hydrolysis form hydroxylamine and carboxylic acid. HCl or 80 % H 2 SO 4 RCOOH NH 2 OH RCH 2 NO 2 H 2 O ST secondary nitro alkanes on hydrolysis form ketones. HCl 2 R2 CHNO 2 2 R2 CO N 2 O H 2 O Ketone (c) Action of nitrous acid : Nitrous acid reacts with primary, secondary and tertiary nitro alkanes differently. H 2O R CH 2 O NOH R C NOH | NO 2 Primary Nitrous acid (g) Reaction with grignard reagent : The aci-form of nitroalkane reacts with Grignard reagent forming alkane. RCH N U CH 2 N OH O | NO 2 OH CH 3 MgI CH 4 RCH N O Methane 2 Acidic character :The -hydrogen atom of primary and secondary nitroalkanes are weakly acidic and thus can be abstracted by strong alkalies like aq. NaOH. Therefore, 1° and 2° nitroalkanes dissolve in aq. NaOH to form salts. For examples. CH 3 N O NaOH Na C H 2 – N O– I O H 2C N O 2 The aci-form of nitroalkanes is relatively more acidic because it produces relatively more conjugate base. (v) Uses : Nitro alkanes are used, (a) As solvents for polar substances such as cellulose acetate, synthetic rubber etc. (b) As an explosive. (c) For the preparation of amines, hydroxylamines, chloropicrin etc. R C NONa | NO 2 Red coloured sodium salt Table : 29.1 Distinction between Ethyl nitrite and Nitro ethane 2 5 ONa O Thus 1° and 2° nitroalkanes are acidic mainly due to following two reasons, (a) Strong electron withdrawing effect of the – NO group. (b) Resonance stabilisation of the carbanion (I) formed after the removal of proton. Nitrolicacid Ethyl nitrite (C H ONO) OMgI O The nitrogen of –NO carrying a positive charge exerts a powerful – I effect and thus activates the hydrogen atom of the -carbon. Thus the important reactions of nitroalkanes are those which involve -hydrogen atom of primary and secondary nitroalkanes (tertiary nitroalkanes have no -hydrogen atom and hence do not undergo such type of reactions). NaOH Test | Cl (f) Condensation with aldehyde : ID (iii) Physical properties (a) Nitro alkanes are colourless, pleasant smelling liquids. (b) These are sparingly soluble in water but readily soluble in organic solvents. (c) Their boiling points are much higher than isomeric alkyl nitrites due to polar nature. (d) Again due to polar nature, nitro alkanes are excellent solvents for polar and ionic compounds. 1° and 2° - Nitro alkanes are known to exist as tautomeric (nitro- form ) | CH 3 – C H – NO 2 CH 3 – C – NO 2 KMnO 4 mixture of nitro-form and aci-form. CH 3 N O O CH 3 Nitro ethane (C H NO ) 2 5 2 (Alkyl nitrite, RONO) (Nitro alkane, RNO ) 2 Boiling point Low, 17°C Reduction with metal and acid (Sn/HCl) or with LiAlH. Gives alcohol + hydroxyl amine or NH. C 2 H 5 ONO 4 H C 2 H 5 OH NH 2 OH Gives corresponding primary amine. C 2 H 5 NO 2 6 H C 2 H 5 NH 2 2 H 2 O 3 RONO 6 H ROH NH 3 H 2 O RNO 2 6 H RNH 2 2 H 2 O Readily hydrolysed to give corresponding alcohol and sodium nitrite (decomposition). Not decomposed, i.e., alcohols are not produced. But it may form soluble sodium salt, because in presence of alkali the nitro form changes into aci form, which dissolves in alkalies to form sodium salt. 4 NaOH Action of (alkalies). Much higher, 115°C C 2 H 5 ONO NaOH C 2 H 5 OH NaNO 2 RONO NaOH ROH NaNO 2 OH NaOH CH 3 CH N O Action of HNO (NaNO + HCl) No action with nitrous acid. 2 Primary nitro alkanes forms nitrolic acid, which dissolve in alkali to give red solution. Secondary nitro alkane yields pseudo-nitrol, which dissolves in alkali to give blue solution. Tertiary nitro alkanes does not react with nitrous acid. E3 2 Aromatic Nitro Compounds On the other hand, nitration of aromatic compounds having electron withdrawing groups like – NO , – SO H requires powerful nitrating agent (like fuming HNO + conc. H SO ) and a high temperature. (ii) Indirect method : The aromatic nitro compounds which can not be prepared by direct method may be prepared from the corresponding amino compound. ID Aromatic nitro compounds are the derivatives of aromatic hydrocarbons in which one or more hydrogen atom (s) of the benzene nucleus has been replaced by nitro (– NO ) group. (1) Preparation (i) Nitration (Direct method) : The number of – NO groups introduced in benzene nucleus depends upon the nature and concentration of the nitrating agent, temperature of nitration and nature of the compound to be nitrated. (a) The nature of the nitrating agent : For example, 2 3 U 2 NO NH D YG conc. HNO +conc. H SO 3 ON NO 2 2 HNO + H SO 100°C 2 NO NaNO Cu, heat 2 2 4 m-Dinitrobenzene HNO + H SO 60°C 4 3 2 4 Benzene U Nitrobenzene NO 2 NO 2 2 p-Dinitroaniline (2)p-Nitroaniline Physical properties 2 2 3 2 NaNO HBF NO NO NO 2 NO 4 4 100°C Syn-Trinitro benzene (b) Temperature of nitration : For example, NO N BF 3 2 4 Benzene 2 3 4 2 2 2 2 2 NO 2 Fuming HNO +conc. H SO 100°C ONa O 60 CH 3 CH N (i) Aromatic nitro compounds are insoluble in water but soluble in organic solvents. (ii) They are either pale yellow liquids or solids having distinct smells. For example, nitro benzene (oil of Mirabane) is a pale yellow liquid having a smell of bitter almonds. (3) Chemical properties releasing group like –OH, –NH , –CH , –OR, etc., in the nucleus facilitates nitration. Thus aromatic compounds bearing these groups (i.e. phenol, (i) Resonance in nitrobenzene imparts a partial double bond character to the bond between carbon of benzene nucleus and nitrogen of the – NO group with the result the – NO group is firmly bonded to the ring and therefore cannot be replaced other groups, i.e., it is very inert. aniline, toluene, etc.) can be nitrated readily as compared to benzene. Thus benzene is not affected by dilute HNO while phenol, aniline and toluene forms the corresponding ortho- and para-nitro compounds. O 2 m-Dinitro benzene (c) Nature of the compound to be nitrated : Presence of electron3 ST 2 2 dil. HNO O O – – N + No reaction 3 3 H SO O O – O – N + + + O O – N + – – O – N + + + + 4 OH OH OH NO OH NO 2 2 O – N conc. HNO ON 2 3 NO 2 2 + + (ii) Displacement of the – NO group : AlthoughResonance – NO group of hybrid of Resonating structures of nitrobenzene nitrobenzene cannot be replaced by other groups, but if anitrobenzene second – NO group is present on the benzene ring of nitrobenzene in the o- or pposition, it can be replaced by a nucleophile. For example, 2 2 2 2 conc. HNO3 conc. H2SO4 dil. HNO Phenol NO 2 + 3 o-Nitrophenol NO 2 Nu NO 2 2, 4, 6-Trinitrophenol p-Nitrophenol + aq. KOH, NH or C H OK 3 NO 2 2 5 NO 2 conditions are used for such reaction and the new group enters the mposition. NO NO 2 2 (a) AlCl + Cl 3 2 Cl (iii) Reduction : Aromatic nitro compounds can be reduced to a variety of product as shown below in the case of nitrobenzene. m-Chloronitrobenzene Nitrobenzene NO NO 2 2 C 6 H 5 NO 2 C 6 H 5 NO C 6 H 5 NHOH C 6 H 5 NH 2 Nitrobenzene Nitrosoben zene Phenylhydr oxylamine Aniline (b) conc. HNO conc. H SO 60 3 The nature of the final product depends mainly on the nature 2 (acidic, basic or neutral) of the reduction medium and the nature of the reducing agent. 2 NO NO 2 2 2 E3 NH 2 (c) + 6H 2 100°C + H SO (fuming) 2 + 2H O Sn + HCl NO m-Dinitrobenzene Nitrobenzene (a) Reduction in acidic medium NO 4 4 Nitrobenzene Aniline Nitrobenzene Reduction of dinitrobenzene with ammonium sulphide reduces only one – NO group (selective reduction) NO NO 2 Although nitrobenzene, itself undergoes electrophilic substitution under drastic conditions, nitrobenzene having activating groups like alkyl, – OR, – NH etc. undergoes these reactions relatively more readily. 2 2 CH 2 NH 2 m-Dinitro benzene HNO H SO 2 C 6 H 5 NO 2[ H ] C 6 H 5 NO 2 Nitroso benzene C 6 H 5 N O H 2O Nitrobenzene || C 6 H 5 NHOH C6 H 5 N Phenyl hydroxylam ine Azoxy benzene Azoxybenzene on further reduction yields azobenzene and hydrazobenzene. C6 H 5 NH Azobenzene Hydrazoben zene U C6 H 5 N ST (d) Electrolytic reduction : Weakly acidic medium of electrolytic reduction gives aniline. Strongly acidic medium gives phenylhydroxylamine which rearranges to p-aminophenol. 2 Nitrobenzene NO 2 Sym-trinitrobenzene (TNB) is preferentially prepared from easily obtainable TNT rather than the direct nitration of benzene which even under drastic conditions of nitration gives poor yields. CH COOH NO 3 NO ON ON 2 2 NO ON 2 2 2 2 Na Cr O H SO 2 2 Sodalime (–CO ) 7 2 4 2 | || electrolytic reduction in presence of conc. H SO 4 2, 4-Dinitrotoluene 2, 4, 6-Trinitrotoluene (TNT) 2[ H ] 2[ H ] C 6 H 5 – N O C 6 H 5 – N C 6 H 5 – N H 2 3 2 2 Phenylhydr oxylamine D YG Nitrosoben zene (intermediate) (c) Reduction in alkaline medium : NO 2 NO C 6 H 5 NO 2 2 H C 6 H 5 NO C 6 H 5 NHOH Azoxybenzene NO 2 HNO H SO 4 o-Nitrotoluene Zn dust NH 4 Cl || 3 ON 2 m-Nitroaniline (b) Reduction in neutral medium : C6 H 5 N NO 3 2 U NO Nitrobenzene CH 3 2 2 ( H 2 O ) CH 3 NO (NH ) S or Na S 4 3 m-Nitrobenzene sulphonic acid ID 2 SO H NH NHOH NO: Benzene is inert to nucleophiles, NO (v) NO Nucleophilic Substitution but 2, 4, in 6-Trinitro benzoicring1, 3, 5-TrinitroBenzene the benzene activates the latter(TNB) in othe presence of – NO group (TNT) and p-positions to nucleophiles. acid 2 2 2 2 2 2 2 OH + KOH 2 fuse o-Nitrophenol Nitro benzene rearrangement NO NO NO OH (vi) Effect of the – NO group on other nuclear substituents p-Nitrophenol (a) Effect on nuclear halogen : The nuclear halogen is ordinarily inert, but if it carries one or more electron-withdrawing groups (like – NO ) in o- or p-position, the halogen atom becomes active for nucleophilic substitutions and hence can be easily replaced by nucleophiles KOH, NH 3 , NaOC2 H 5 . 2 4 Phenylhydroxylamine OH Alkaline medium of electrolytic reduction gives all pthe mono- and -Aminophenol di-nuclear reduction products mentioned above in point (c). (iv) Electrophilic substitution : Since – NO group is deactivating and m-directing, electrophilic substitution (halogenation, nitration and sulphonation) in simple aromatic nitro compounds (e.g. nitrobenzene) is very difficult as compared to that in benzene. Hence vigorous reaction 2 2 Cl Nu NO NO 2 2 + KOH, NH or C H ONa 3 NO 2 2, 4-Dinitrochlorobenzene 2 5 NO 2 (Where, Nu = OH, NH2, OC2H5) Cyanides and Isocyanides Hydrogen cyanide is known to exist as a tautomeric mixture. H–CN⇌ HN C 2 The decreasing order of the acidity of nitrophenols follows following order OH OH ON NO 2 NO 2 2 Hence, it forms two types of alkyl derivatives which are known as alkyl cyanides and alkyl isocyanides. R–N C R–C N Alkyl Cyanide Alkyl isocyanide (1) Alkyl Cyanides (i) Methods of preparation (a) From alkyl halides : The disadvantage of this method is that a mixture of nitrile and isonitrile is formed. 60 (b) Effect on phenolic –OH group : The acidity of the phenolic hydroxyl group is markedly increased by the presence of – NO group in oand p-position. RX KCN (orNaCN ) Alkyl halide P2 O5 RCN (b) From acid amides : RCONH 2 H 2O NO 2 2 2, 4, 6-Trinitro phenal CH 3 CONH 2 CH 3 CN H 2 O P2 O5 2, 4-Dinitrophenol OH RNC Isonitrile (Minor product) E3 NO RCN Nitrile (Major product) OH Acetamide Methyl cyanide Industrially, alkyl cyanides are prepared by passing a mixture of carboxylic acid and ammonia over alumina at 500°C. NO 2 Al2 O 3 RCOOH NH 3 RCOONH 4 ID Acid o- and p-Nitrophenols Phenol Increased acidity of o- and p-nitrophenols is because of the fact that the presence of electron-withdrawing – NO group in o-and p-position (s) to phenolic –OH group stabilises the phenoxide ions (recall that acidic nature of phenols is explained by resonance stabilisation of the phenoxide ion) to a greater extent. + O–N=O – O–N–O – D YG – + O O – O – Phenoxide ion Extra stabilisation of p-nitrophenate ion to –NO2 group (no –NO group) Due to 2 increased acidity of due nitrophenols, the latter react with phosphorus pentachloride to give good yields of the corresponding chloro derivative, while phenol itself when treated with PCl gives poor yield of chlorobenzene. 5 U OH Cl + POCl +HCl 5 ST Alkyl cyanide X Cl RMgX ClCN RCN Mg Alkyl cyanide Grignard reagent CH 3 MgBr ClCN CH 3 CN Mg Methyl magnesium bromide Cyanogen chloride Methylcyanide Br Cl (d) From primary amines : Primary amines are dehydrogenated at high temperature to form alkyl cyanides. This is also a commercial method. Cu or Ni RCH 2 NH 2 RCN 2H 2 Primary amine 500 C Cu or Ni CH 3 CH 2 NH 2 CH 3 CN 2H 2 Ethylamine 500 C Methyl cyanide (e) From oximes : H Aldoxime 2 + PCl 2 – H 2O P2 O5 R C NOH R CN H 2 O NO 2 NO Amide | NO 2, 4-Dinitrophenol Al2 O 3 RCONH 2 RCN (c) From Grignard reagent U 2 – H 2O Ammonium salt 3 NO 2 2, 4-Dinitrochlorobenzene (4) Uses (i) On account of their high polarity, aromatic nitro compounds are used as solvents. (ii) Nitro compounds like TNT, picric acid, TNB etc. are widely used as explosives. (iii) These are used for the synthesis of aromatic amino compounds. (iv) Nitro benzene is used in the preparation of shoe polish and scenting of cheap soaps. H 2O Alkyl cyanide (ii) Physical properties (a) Alkyl cyanides are neutral substance with pleasant odour, similar to bitter almonds. (b) Lower members containing upto 15 carbon atoms are liquids, while higher members are solids. (c) They are soluble in water. The solubility decreases with the increase in number of carbon atoms in the molecule. (d) They are soluble in organic solvents. (e) They are poisonous but less poisonous than HCN (iii) Chemical properties (a) Hydrolysis H 2O H 2O RCN RCONH 2 RCOOH NH 3 Alkyl cyanide H Amide H Acid O H 2O CH 3 CN CH 3 CONH 2 H Methyl cyanide || POCl 3 R N C H 2O R NH C H Acetamide N alkyl formamide CH 3 COOH NH 3 H 2O (ii) Physical properties (a) Alkyl isocyanides are colourless, unpleasant smelling liquids. (b) They are insoluble in water but freely soluble in organic solvents. (c) Isonitriles are much more poisonous than isomeric cyanides. (iii) Chemical properties (a) Hydrolysis : Aceticacid (b) Reduction : When reduced with hydrogen in presence of Pt or Ni, or LiAlH (Lithium aluminium hydride) or sodium and alcohol, alkyl 4 cyanides yield primary amines. RCN Alkyl cyanide 4H RCH 2 NH 2 Primary amine However, when a solution of alkyl cyanides in ether is reduced with stannous chloride and hydrochloric acid and then steam distilled, an aldehyde is formed (Stephen's reaction). H RN C 2 H 2 O RNH 2 HCOOH Alkyl isocyanide Alkyl isocyanide Aldehyde (c) Reaction with Grignard reagent : With grignard's reagent, an alkyl cyanide forms a ketone which further reacts to form a tertiary alcohol. R (c) Action of heat : When heated for sometime at 250°C, a small amount of isonitrile changes into isomeric nitrile. heat RNC RCN | R C N R ' MgX R C NMgX (d) Addition reaction : Alkyl isocyanide give addition reactions due to presence of unshared electron pair on carbon atom. R | 2 H 2O R C O NH 3 Mg R | OH X | R – C O R MgX R C OMgX RNC X 2 | R | R C OH Mg | R OH X D YG Tertiaryalcohol (d) Alcohololysis : N H2 || RCN R OH HCl R C O R Cl Alkyl Alcohol cyanide imido ester H 2O RCOO R NH 4 Cl Ester ST U (iv) Uses : Alkyl cyanides are important intermediates in the organic synthesis of a large number of compounds like acids, amides, esters, amines etc. (2) Alkyl Isocyanides (i) Methods of preparation (a) From alkyl halides : CH 3 Cl Methyl chloride RNC Isocyanide (Isonitrile) Main product AgCN RCN Cyanide (Nitrile) Minor product CH 3 NC CH 3 CN Methyl isocyanide (Main product) (b) From primary amines (Carbylamine reaction) : RNH 2 CHCl 3 3 KOH Primary amine Chloroform (c) From N-alkyl formamides : RNC 3 KCl 3 H 2 O Isocyanide RNCX 2 Alkyl iminocarbonyl halide RNC S RNCS U R R X AgCN (Halogen) H 2O Alkyl halide R : N ::: C : or R N C The following are some of the addition reactions shown by alkyl isocyanides. ID Ketone R 300 C secondary amine E3 Imine hydrochlor ide Primary amine Formic acid Ni (b) Reduction : R N C 4 H RNHCH 3 o SnCl 2 HCl H 2O R C N RCH NH.HCl RCHO NH 4 Cl [2 H ] Isocyanide 60 H Pyridine Alkyl isothiocyanate ; RNC HgO RNCO Hg Alkyl isocyanate (iv) Uses : Due to their unpleasant smell, alkyl isocyanides are used in detection of very minute leakage. Carbylamine reaction is used as a test for the detection of primary amino group. Methyl isocyanate (MIC)gas was responsible for Bhopal gas tragedy in Dec. 1984. Cyanides have more polar character than isocyanides. Hence cyanides have high boiling points and are more soluble in water. However, both isomers are more polar than alkylhalides, hence their boiling points are higher than the corresponding alkyl halides. Being less polar, isocyanides are not attacked by OH ions. Table : 29.2 Comparison of Alkyl Cyanides and Alkyl Isocyanides – Test Ethyl cyanide Ethyl isocyanide Smell Strong but pleasant Extremely unpleasant Dipole moment More ( 4D) Less ( 3D) B.P. 98°C(i.e. High) 78°C (i.e. low) Solubility in water. Soluble Insoluble Hydrolysis with acids Gives propionic acid (Acid, in general) Give ethyl amine (1° amine, in general) Hydrolysis with alkalies Same as above No action Reduction Gives propylamine (1° amine, in general) Gives ethylmethyl amine (2° amine, in general) Stephen's reaction Gives propionaldehyde (Aldehyde, in general) Does not occur Heating (250°C) No effect Ethyl cyanide is formed (a) Reduction of nitro compounds Amines Sn HCl or R NO 2 6[H ] RNH 2 2 H 2 O Amines are regarded as derivatives of ammonia in which one, two or all three hydrogen atoms are replaced by alkyl or aryl group. –H +R C 2 H 5 NO 2 6[H ] C 2 H 5 NH 2 2 H 2 O (b) Reduction of nitriles (Mendius reaction) 3 –2H + 2R R C N 4[H ] R CH 2 NH 2 –3H + 3R CH 3 C N 4[H ] CH 3 CH 2 NH 2 R NHsecondary or tertiary R N depending on AminesRNH are classified as primary, (Secondary) the number (Primary) of alkyl groups attached to nitrogen atom. (Tertiary) 2 2 3 Methyl cyanide The start can be made from alcohol or alkyl halide. The characteristic groups in primary, secondary and tertiary amines | | (imino) Alcohol | (tert nitrogen) Quaternary ammonium iodide Tetramethyl ammonium iodide (c) By reduction of amides with LiAlH RCONH 2 RCH 2 NH 2 | | Ethylamine (d) By reduction of oximes : The start can be made from an aldehyde or ketone. H 2 NOH LiAlH4 RCHO RCH NOH RCH 2 NH 2 CH 3 D YG Ethylmethylamine Acetamide Aldehyde Mixed amines : C 2 H 5 N H ; C 6 H 5 N H CH 3 LiAlH4 CH 3 CONH 2 CH 3 CH 2 NH 2 U Triethylamine R R Methylaniline Oxime C O H 2 NOH R R Ketone orH 2 Ni C NOH LiAlH4 (2) General methods of preparation (i) Methods yielding mixture of amines (Primary, secondary and tertiary) (a) Hofmann's method :The mixture of amines (1°, 2° and 3°) is formed by the alkylation of ammonia with alkyl halides. U CH 3 I CH 3 I NH 3 CH 3 NH 2 (CH 3 )2 NH Methyliodide Methylamine (1) ST Trimethylamine (3) CH 3 I Tetramethyl ammonium iodide The primary amine may be obtained in a good yield by using a large excess of ammonia. The process is also termed as ammonolysis of alkyl halides. It is a nucleophilic substitution reaction. (b) Ammonolysis of alcohols : 3 CH 3 OH NH 3 2 CH 3 NH 2 Al O 350 C (ii) Methods yielding primary amines CH NH 2 (e) Hofmann's bromamide reaction or degradation (Laboratory method) : By this method the amide (–CONH ) group is converted into primary amino (– NH ) group. 2 2 R CO NH 2 Br2 4 KOH R NH 2 2 KBr K2 CO 3 2 H 2O Amide Pri - amine This is the most convenient method for preparing primary amines. This method gives an amine containing one carbon atom less than amide. (f) Gabriel phthalimide synthesis : This method involves the following three steps. Phthalimide is reacted with KOH to form potassium phthalimide. The potassium salt is treated with an alkyl halide. The product N-alkyl phthalimide is put to hydrolysis with hydrochloric acid when primary amine is formed. CO CH 3 OH CH 3 OH (CH 3 )2 NH (CH 3 )3 N Primary amine may be obtained in a good yield by using a excess of ammonia. R R Primary amine Dimethylamine (2) (CH 3 )3 N (CH 3 )4 NI CH 3 I Primary amine Oxime The aliphatic amines have pyramidal shape with one electron pair. In amines, N undergoes sp hybridisation. 3 4 LiAlH4 (1) Simple and mixed amines : Secondary and tertiary amines may be classified as simple or mixed amines according as all the alkyl or aryl groups attached to the nitrogen atom are same or different. For example, Simple amines : (CH 3 )2 NH ; (CH 3 CH 2 )3 N Primary amine This sequence gives an amine containing one more carbon atom than alcohol. R | or R N R X | R Tetra- alkyl ammonium salt Dimethylamine Alkyl nitrile Na C 2 H 5 OH ID ; (CH 3 ) 4 NI R 4 NI R CN 4 RCH 2 NH 2 LiAlH or In addition to above amines, tetra-alkyl derivatives similar to ammonium salts also exist which are called quaternary ammonium compounds. NH 4 I ; Alkyl chloride E3 (amino) SOCl 2 KCN R OH R Cl N are: – NH 2 ; – NH ; Ethylamine 60 NH Zn HCl or Ni or LiAlH4 CO KOH –H O NH NK CHX 2 5 2 CO CO Phthalimide Potassium phthalimide CO COOH NC H 2 CO N-Ethyl phthalimide 5 HOH HCl C H NH + 2 5 2 COOH Phthalic acid When hydrolysis is difficult, the N-alkyl phthalimide can be treated with hydrazine to give the required amine. CO NH | NR + NH 2 CO heat 2 CO –NH | + RNH 2 CO –NH ST U D YG U ID E3 60 Hydrazine (g) By decarboxylation of -amino acids directly converted to the primary amine without the necessity of isolating alkyl azide. R C HC OOH RCH 2 NH 2 Ba(OH )2 | O heat NH 2 || CH 2 COOH CH 3 NH 2 heat heat Methyl amine NH 2 (NaN 3 H 2 SO 4 N 3 H NaHSO 4 ) - amino acetic acid (k) By Ritter reaction : It is a good method for preparing primary amines having -tertiary alkyl group. (Glycine) (h) By means of a Grignard reagent and chloramine : RMgX ClNH 2 RNH 2 MgXCl (i) By hydrolysis of Isocyanides or Isocyanates H OH ( HCl ) R N C 2 H 2 O R NH 2 HCOOH Alkyl amine H OH (CH 3 )3 C OH H 2 SO 4 HCN (CH 3 )3 C NH 2 60 H HCN H 2 O R 3 C N CH R3 C R 3 C OH Tert-carboniumion Alkyl isocyanide H 2O OH CHO R 3 CNH R 3 C NH 2 HCOO Pri -amine (l) Reductive amination of aldehydes and ketones : O H CH 3 NC 2 HOH CH 3 NH 2 HCOOH methyl isonitile H OH CH 3 N C O 2 KOH CH 3 NH 2 K 2 CO 3 H OH || Methyl isocyanate Ni,150 C R C H NH 3 H 2 R CH 2 NH 2 H 2 O R NCO 2 KOH R NH 2 K 2 CO 3 Aldehyde Alkyl isocyanate (j) By Schmidt reaction : Hydrazoic acid Alkyl amine In this reaction the acyl azide (R – CON ) and alkyl isocyanate (R – NCO) are formed as an intermediate. R – COOH N 3 H RCON 3 H 2O RCON 3 R N C O N 2 D YG R N C O H 2O R NH 2 CO 2 Alkyl amine The overall reaction which proceeds by the elimination of nitrogen from acyl azide followed by acidic or alkaline hydrolysis to yield primary amine containing one carbonless, is called Curtius Degradation. The method uses acid chloride to prepare primary amine through acyl azide. O O O || 2 RCH 2 NH 2 Ni H CH 3 O || | Ni,150 C R C CH 3 NH 3 H 2 R CH NH 2 Alkyl isocyanate Acyl azide || 300 atm Ketone This reaction probably takes place through the formation of an imine (Schiff's base). The primary amine can also be converted into sec. or tert. amines by the following steps H 2 Ni R CHO RNH 2 RCH 2 NH R Sec. amine RNH 2 2 H 2 C O 2 HCOOH RN (CH 3 )2 2 H 2 O 2CO 2 || 2 3 R C OH R C Cl R C N3 SOCl NaN Acyl chloride O || N2 Acyl azide Tert.- amine (m) By reduction of azide with NaBH U 2 NaOH Alkyl halide (1or2 ) heat ST The mechanism of curtius rearrangement is very similar to Hofmann degradation. N=N=N – C | | O 2 Alkyl azide H 2O 1 amine (n) By Leuckart reaction : Aldehydes or ketones react with ammonium formate or with formamide to give formyl derivative of primary amine. O || Amm.formate 2 H 2 O CO 2 NH 3 O || C O 2 HCONH 2 CHNH C H CO 2 NH 3 R Sodium azide C O 2 HCOONH 4 CHNH – C – H C O –N + N–N N R 4 NaBH 4 R X NaN 3 RN 3 RNH 2 R C N 3 R N C O R NH 2 Na 2 CO 3 R Primary amine H H | | ( H 2 O ) R C O H HN [ R C NH ] 2 Imine U 3 Acyl azide 300 atm ID R COOH N 3 H R NH 2 N 2 CO 2 Conc. H 2 SO 4 Acid Tert butylamine (1 amine) Tert- butyl alcohol E3 | NaN H SO (conc.) 3 4 R C OH 2 RNH 2 N 2 CO 2 Ba(OH )2 N Formamide C Intramolecular alkyl shift R–N=C=O | Schmidt reaction converts R – COOH to R–NH , which is a O modification of curtius degradation. In this reaction a carboxylic acid is warmed with sodium azide (Na N ) and conc. H SO. The carboxylic acid is These formyl derivatives are readily hydrolysed by acid to yield primary amine. O 2 + – 3 2 4 R R || H CHNH C H HOH R R R 3 N H X NaOH R 3 N NaX H 2 O 180 200 C C O HCOONH 4 (b) Reduction of N, N-disubstituted amides : The carbonyl group is converted into – CH group. Amm. formate R3 N H X Trialkylammonium salt This is called Leuckart reaction, i.e., Ketone 2 R R RCON R 2 N , N -disubstituted amide CHNH 2 H 2 O CO 2 Primary amine On commercial scale, ethylamine is obtained by heating a mixture of ethylene and ammonia at 450°C under 20 atmospheric pressure in presence of cobalt catalyst. 450 C , 20 atm (CH 3 )4 NOH (CH 3 )3 N CH 3 OH R2 NH Secondary amine (R)4 NOH (R)3 N olefin H 2 O H 2 O NaX (3) Separation of mixture of amines : When the mixture consists of salts of primary, secondary and tertiary amines along with quaternary salt, it is first distilled with KOH solution. The mixture of three amines distils over leaving behind non-volatile quaternary salt. ID R2 N H 2 X NaOH Pt (b) Reduction of isonitriles : R NC 4[H ] RNHCH 3 Alkyl isonitrile Sec. amine NR heat 2 HNO 2 Dialkyl aniline OH H ON 2 NR NaOH 2 ON OH + R NH 2 Sec. amine p-Nitroso-dialkyl aniline p-Nitroso phenol This is one of the best method for preparing pure secondary amines. (d) Hydrolysis of dialkyl cyanamide U CaN CN 2 NaOH 2 RX Na 2 N CN R 2 N CN Calcium Sodium Dialkyl cyanamide cyanamide cyanamide H or R 2 N CN 2 HOH R 2 NH CO 2 NH 3 Dialkylamine ST OH RNH 2.HI or RN H 3 I K O H RNH 2 KI H 2 O Primary amine (Volatile), Distillate R 2 NH.HI or R 2 N H 2 I K O H R 2 NH KI H 2 O D YG Aniline RX 3 U Secondary amine formed by this method always possesses one –CH group linked directly to nitrogen. (c) Reaction of p-nitroso-dialkyl aniline with strong alkali solution : NH E3 R2 N H 2 X dialkyl ammonium salt The hydroxides thus formed on heating decompose into tertiary amines. Tetramethyl ammonium hydroxide gives methyl alcohol as one of the products while all other tetra-alkyl ammonium hydroxides give an olefin and water besides tertiary amines. (iii) Methods yielding secondary amines (a) Reaction of primary amines with alkyl halides ter. amine R 4 N I AgOH R 4 N O H AgI CH 2 CH 2 NH 3 CH 3 CH 2 NH 2 R NH 2 R X R2 NH HX 4[H ] (c) Decomposition of tetra-ammonium hydroxides : The tetra-alkyl ammonium hydroxides are formed when corresponding halides are treated with moist silver oxide. Cobalt catalyst Ethylene LiAlH4 RCH 2 N R 2 H 2 O 60 R R 3 RX NH 3 R 3 N 3 HX CHNH 2 H 2 O CO 2 (e) Reduction of N-substituted amides : Reduction of N-substituted amides with LiAlH yields secondary amines. 4 Alkyl -amino ketones are formed by the action of ketone with formaldehyde and NH (or primary or secondary amines). The product is referred to as Mannich base and the reaction is called Mannich Reaction. R 3 N.HI or R 3 N H I K O H R 3 N KI H 2 O R 4 N I (non-volatile tetra-alkyl ammonium salt) has no reaction with KOH, however remains as residue. This mixture is separated into primary, secondary and tertiary amines by the application of following methods. (i) Fractional distillation : The boiling points of primary, secondary and tertiary amines are quite different, i.e., the boiling point of C H NH is 17°C, (C H ) NH is 56°C and (C2 H 5 )3 N is 95°C and thus, these can be separated by fractional distillation. This method is used satisfactorily in industry. (ii) Hofmann's method : The mixture of three amines is treated with diethyl oxalate. The primary amine forms a solid oxamide, a secondary amine gives a liquid oxamic ester while tertiary amine does not react. CO OC 2 H 5 H NHR 2 C 2 H 5 OH | C ONHR | CO OC 2 H 5 H NHR CONHR Pr imary Diethyl oxalate 2 2 5 5 2 2 Dialkyl oxamide (Solid) amine 3 CH 3 COCH 3 HCHO RNH 2 CH 3 COCH 2 CH 2 NHR heat Which can be reduced to alkyl amines. LiAlH4 R CONH R 4[H ] RCH 2 NH R H 2 O N - Alkyl acid amide (iv) Methods yielding tertiary amines (a) Reaction of alkylhalides with ammonia Sec.amine C 2 H 5 OH COOC 2 H 5 HNR 2 CONR 2 | COOC2 H 5 Diethyl oxalate Secondary amine | COOC2 H 5 Dialkyloxamic ester (liquid) Primary amine is recovered when solid oxamide is heated with caustic potash solution and collected as distillate on distilling the reaction mixture. (iii) Amines are soluble in water. This is due to hydrogen bonding between amine and water molecules. Amines are also soluble in benzene and ether. H OK COOK | 2 RNH 2 H OK COOK Primary amine Pot.oxalat e (Distillate) The liquid (mixture of oxamic ester+ tertiary amine) is subjected to fractional distillation when tertiary amine distils over. The remaining liquid is distilled with KOH to recover secondary amine. CONR 2 HOK COOK | R 2 NH | C 2 H 5 OH Secondary COOK COOC 2 H 5 HOK amine Pot. oxalate (iii) Hinsberg's method : It involves the treatment of the mixture with benzene sulphonyl chloride, i.e., Hinsberg's reagent (C H SO Cl). The solution is then made alkaline with aqueous alkali to form sodium or potassium salt of monoalkyl benzene sulphonamide (soluble in water). C6 H 5 SO 2 Cl HNHR C6 H 5 SO 2 NHR 6 Primary amine 5 2 N - Alkyl benzene sulphonami de NaOH C6 H 5 SO 2 N ( Na)R Soluble salt H H | | H – O : H – N : H – O : H – N : | | | | H R H R Hydrogen bonding b etween amine and water molecules Solubility decreases with increase of molecular mass. (5) Chemical properties : The main reactions of amines are due to the presence of a lone pair of electrons on nitrogen atom. Amines are electrophilic reagents as the lone pair of electrons can be donated to electron seeking reagents, (i.e., electrophiles). 60 CO NHR Except the amines containing tertiary butyl group, all lower aliphatic amines are stronger bases than ammonia because of + I (inductive) effect. The alkyl groups, which are electron releasing groups, increase the electron density around the nitrogen thereby increasing the availability of the lone pair of electrons to proton or Lewis acids and making the amine more basic (larger K ). Thus, it is expected that the basic nature of amines should be in the order tertiary > secondary > primary, but the observed order in the case of lower members is found to be as secondary > primary > tertiary. This anomalous behaviour of tertiary amines is due to steric factors, i.e., crowding of alkyl groups cover nitrogen atom from all sides and thus makes the approach and bonding by a proton relatively difficult which results the maximum steric strain in tertiary amines. The electrons are there but the path is blocked, resulting the reduced in its basicity. E3 CO NHR | b The secondary amine forms N,N-dialkyl benzene sulphonamide which does not form any salt with NaOH and remains as insoluble in alkali solution. C 6 H 5 SO 2 Cl HNR 2 C 6 H 5 SO 2 NR 2 ID Sec. amine NaOH No reaction (Insoluble in water, soluble in ether) (i) The order of basic nature of various amines has been found to vary with nature of alkyl groups. Alkyl group Relative strength CH – R NH > RNH > R N > NH CH – R NH > RNH > NH > R N (CH ) CH – RNH > NH > R NH > R N NH > RNH > R NH > R N (CH ) C – (ii) Basic nature of aromatic amines : In aniline or other aromatic amines, the lone pair present on nitrogen atom is delocalized with benzene ring by resonance. :NH NH NH NH D YG U Tertiary amine does not react. The above alkaline mixture of the amines is extracted with ether. Two distinct layers are formed. Lower layer, the aqueous layer consists of sodium salt of N-alkyl benzene sulphonamide (primary amine) and upper layer, the ether layer consists of N,N-dialkyl benzene sulphonamide (secondary amine) and tertiary amine. Two layers are separated. The upper layer is fractionally distilled. One fraction obtained is tertiary amine and the other fraction is treated with concentrated HCl to recover secondary amine hydrochloride which gives free secondary amine on distillation with NaOH. C6 H 5 SO 2 NR 2 HCl H 2 O C6 H 5 SO 2.OH R2 NH.HCl R 2 NH.HCl NaOH R 2 NH NaCl H 2 O 3 2 2 5 2 2 3 2 3 3 3 2 2 3 + 3 3 3 3 2 3 2 2 3 + + 2 2 2 2 Sec. amine – – The aqueous layer is acidified and hydrolysed with dilute HCl. The hydrochloride formed is then distilled with NaOH when primary amine distils over. – U C6 H 5 SO 2 N ( Na)R HCl C6 H 5 SO 2 NHR NaCl – Sulphonami de of primary amine ST C6 H 5 SO 2 NHR HCl H 2 O C6 H 5 SO 2.OH RNH 2.HCl Primary amine hydrochlor ide RNH 2.HCl NaOH RNH 2 NaCl H 2 O (4) Physical properties (i) Lower amines are gases or low boiling point liquids and possess a characteristic ammonia like smell (fishy odour). Higher members are solids. (ii) The boiling points rise gradually with increase of molecular mass. Amines are polar compounds like NH and have comparatively higher boiling points than non-polar compounds of similar molecular masses. This is due to the presence of intermolecular hydrogen bonding. 3 N–H | | H H Resonance hybrid But anilinium ion is less resonance stabilized than aniline. + NH + NH 3 3 No other resonating structure possible Thus, electron density is less on N atom due to which aniline or other aromatic amines are less basic than aliphatic amines. However, any group which when present on benzene ring has electron withdrawing effect (– NO , – CN, – SO H, – COOH – Cl, C H , etc.) decreases basicity of aniline (Nitroaniline is less basic than aniline as nitro group is electron withdrawing group (– I group) and aniline is more basic than diphenyl amine), while a group which has electron repelling effect (– NH , – OR, R –, etc.) increases basicity of aniline. Toluidine is more basic than aniline as – CH group is electron repelling group (+ I group). 2 H H H | | | H – N : H – N : H – N : | R | | R R Hydrogen bonding in amines + N–H 2 3 3 6 5 Further greater the value of K or lower the value of pK , stronger will be the base. The basic character of some amines have the following b R2 NH HOH ⇌ R2 N H 2 OH – ⇌ [R2 NH 2 ] OH – b order, R 2 NH RNH 2 C6 H 5 CH 2 NH 2 NH 3 C6 H 5 NH 2 R3 N HOH ⇌ R3 N HOH – ⇌ [R3 NH ] OH – N-alkylated anilines are stronger bases than aniline because of steric effect. Ethyl group being bigger than methyl has more steric effect, so Nethyl aniline is stronger base than N-methyl aniline. Thus, basic character is, The aqueous solutions of amines behaves like NH OH and give ferric hydroxide precipitate with ferric chloride and blue solution with copper sulphate. 3 RNH 3OH FeCl3 Fe(OH)3 3 RNH 3Cl C6 H 5 NHCH 3 C6 H 5 NH 2 NH 3 C6 H 5 NHC 2 H 5 C6 H 5 NHCH 3 C6 H 5 NH 2 C6 H 5 NHC 6 H 5 RNH 2 Pri. amine Nitroanilines–m-isomer > p- > o Aniline is less basic than ammonia. The phenyl group exerts –I Quaternary sa lt – HCl ClOCCH 3 RNHOCCH 3 N - Alkyl acetamide – HCl R2 NH ClOCCH 3 R2 NOCCH 3 Sec. amine N , N - Dialkyl acetamide Tertiary amines do not react since they do not have replaceable hydrogen on nitrogen. Therefore, all these above reactions are used to distinguish between 1 o , 2 o and 3 o -amines. (vii) Action of sodium 1o amine 3 D YG basic and with more ‘s’ character (sp-hybridized) is least basic. Examples in decreasing order of basicity are, H CH – N CHC H CH – C N CH 3 N 2 3 3 3 2 (sp ) (sp ) CH 3 CH 2CH 2 NH 2 H 2C CHCH 2 NH 2 HC CCH 2 NH 2 (CH 3 )2 NH CH 3 NH 2 NH 3 C 6 H 5 NH 2 Electron withdrawing (C H –) groups decrease electron density on nitrogen atom and thereby decreasing basicity. Sod. salt 2 R 2 NH 2 Na 2[R 2 N ]– Na H 2 U H CH C N H CH 3 – C N 2 3 2 The compounds with least 's' character (sp -hybridized) is most 2o amine Sod.sa lt (viii) Action of halogens RNH 2 Alkyl amine X2 X2 RNHX RNX 2 R 2 NH Dialkyl amine NaOH NaOH Dihalo-alkyl amine X2 R 2 NX NaOH Halo -dialkyl amine (ix) Reaction with Grignard reagent RNH 2 Mg CH 3 CH 4 RNH Mg I I 5 (CH 3 )2 NH CH 3 NH 2 C 6 H 5 NHCH 3 C6 H 5 NH 2 U CH 3 CH 2 NH 2 HO(CH 2 )3 NH 2 HO(CH 2 )2 NH 2 Electron withdrawing inductive effect of the –OH group decreases the electron density on nitrogen. This effect diminishes with distance from the amino group. ST Tert. amine 2 RNH 2 2 Na 2[RNH ]– Na H 2 Not available due to delocalization 6 HX ID (inductive) effect, i.e., it withdraws electrons. This results to the lower availability of electrons on nitrogen for protonation. Ethylamine and acetamide both contain an amino group but acetamide does not show basic nature. This is because lone pair of electrons on nitrogen is delocalised by resonance with the carbonyl group which makes it less available for protonation. (sp 3 ) Sec.amine E3 Phenylene diamines –p-isomer > m- > o- | HX (vi) Reaction with acetyl chloride (Acylation) Chloroanilines–p-isomer>m-> o- || R X R X R X RNH 2 RNH R R – NR 2 (R – N R 3 )X – Pri.amine In Toluidines –p-isomer > m- > o- O– (v) Reaction with alkyl halides (Alkylation) 60 C6 H 5 N (C 2 H 5 )2 C6 H 5 NHC 2 H 5 C6 H 5 N (CH 3 )2 O 4 CH 3 CH 2 NH 2 C 6 H 5 CONH 2 CH 3 CONH 2 (iii) Salt formation : Amines being basic in nature, combine with mineral acids to form salts. R NH 2 HCl R N H 3 C l Alkylammonium chloride 2 R – NH 2 H 2 SO 4 (R N H 3 )2 SO 4– Alkylammonium sulphate (iv) Nature of aqueous solution : Solutions of amines are alkaline in nature. R 2 NH CH 3 – Mg – I CH 4 R 2 N – Mg – I (x) Carbylamine reaction : This reaction is shown by only primary amines. This is a test of primary amines and is used to distinguish primary amines from secondary and tertiary amines. RNH 2 CHCl 3 3 KOH (Alc.) RNC Alkylisocyanide (carbyl amine) 3 KCl 3 H 2 O Isocyanides are bad smelling compounds and can be easily detected. (xi) Reaction with nitrous acid (a) Primary amines form alcohols with nitrous acid (NaNO + HCl). Nitrogen is eliminated. 2 RNH 2 HONO ROH N 2 H 2 O Pri. amine Alcohol Methyl amine is an exception to this reaction, i.e., CH 3 NH 2 2 HONO CH 3 – O – N O N 2 2 H 2 O Methyl nitrite 2CH 3 NH 2 2 HONO CH 3 – O – CH 3 2 N 2 3 H 2 O Dimethyl ether RNH 2 HOH ⇌ R N H 3 OH – ⇌ [RNH 3 ] OH – (b) Secondary amines form nitrosoamines which are water insoluble yellow oily liquids. (xv) Ring substitution in aromatic amines : Aniline is more reactive than benzene. The presence of amino group activates the aromatic ring and R 2 NH HONO R 2 NNO H 2 O Sec. amine Dialkyl nitrosoami ne Nitrosoamine on warming with phenol and conc. H SO give a brown or red colour which soon changes to blue green. The colour changes to red on dilution and further changes to blue or violet with alkali. This colour change is referred to Liebermann's nitroso reaction and is used for the test of secondary amines. (c) Tertiary amines react nitrous acid to form nitrite salts which are soluble in water. These salts on heating give alcohols and nitrosoamines. 2 directs the incoming group preferably to ortho and para positions. (a) Halogenation NH NH 2 2 Br + 3Br Br + 3HBr 2 heat [R3 NH ] NO 2– R – OH R 2 N –N O Nitrosoami ne This reaction (nitrous acid test) is used to make distinction between primary, secondary and tertiary amines. (xii) Reaction with carbon di sulphide : This Hofmann’s mustard oil reaction is used as a test for primary amines. NHR HgCl 2 SH S C S RNH 2 S C heat 1 Br 2, 4, 6-Tri Bromoaniline (white ppt.) This reaction is used as a test for aniline. However, if monosubstituted derivative is desired, aniline is first acetylated with acetic anhydride and then halogenation is carried out. After halogenation, the acetyl group is removed by hydrolysis and only monosubstituted halogen derivative is obtained. It may be noted that – NH group directs the attacking group at oand p-positions and therefore, both o- and p-derivatives are obtained. 60 Alcohol Trialkylammoniumni trite E3 R3 N HONO Tert.amine 4 2 Alkyl dithiocarbamic acid RNC S HgS 2HCl Alkyl isothiocyanate (Mustard oil smell) 2 2 No reaction HgCl NHCOCH 2 (CH CO) O –CH COOH 3 Dialkyl dithiocarbamic acid 2 NHCOCH NH R 2 C NH R 2 CO NH 3 R 2 CHNH 2 H 2O [O ] KMnO 4 Ketimine Ketone (b) Oxidation of secondary amines [O ] R 2 NH R 2 N – NR 2 Sec. amine KMnO 4 Tetra-alkyl hydrazine [O ] ; R 2 NH NH 2 Br + + 2 3 Aldehyde D YG Aldimine (minor) 2 H O, H , –CH COOH + H 2O p-Bromoacetanilide Acetanilide 3 RCH NH RCHO NH 3 RCH 2 NH 2 KMnO 4 Br 2 Aniline U (a) Oxidation of primary amines Pri. amine 3 3 (xiii) Oxidation : All the three types of amines undergo oxidation. The product depends upon the nature of oxidising agent, class of amine and the nature of the alkyl group. [O ] NHCOCH 3 Br ID NR 2 SH S C S R 2 NH S C NH Black ppt. o-Bromoaniline Br Br (minor) p-Bromoacetanilide p-Bromoaniline (major) (major) Acetylation deactivates the ring and controls the reaction to monosubstitution stage only because acetyl group is electron withdrawing R 2 NOH Dialkyl hydroxylam ine group and therefore, the electron pair of N-atom is withdrawn towards the carbonyl group. (c) Oxidation of tertiary amines : Tertiary amines are not oxidised by potassium permanganate but are oxidised by Caro's acid or Fenton's reagent to amine oxides. (b) Nitration : Aromatic amines cannot be nitrated directly because they are readily oxidized. This is because, HNO is a strong oxidising agent and results in partial oxidation of the ring to form a black mass. H 2 SO 5 3 R 3 N [O] [R 3 N O] Amine oxide U Tert. amine 2 (xiv) Reaction with other electrophilic reagents RNH 2 O CH R RN CH R Aldehyde ST Pri. amine Therefore, to solve this problem, nitration is carried out by protecting the –NH group by acetylation. The acetylation deactivates the ring and therefore, controls the reaction. The hydrolysis of nitroacetanilides removes the protecting acyl group and gives back amines. Schiff's base NH NHCOCH O O || || O Carbonyl chloride Dialkyl urea (Symmetrical) + Cl – C – CH 2 2 RNH 2 Cl – C – Cl RNH – C – NHR 2 HCl O || RNHH O C N – R RNH – C – HNR Isocyanate 2 | Isothiocyanate 3 3 Aniline 2 4 o-Nitroacetanilide Acetanilide NHCOCH NH 3 NH 2 2 NO S RNHH S C N – R RNH – C – NH R HNO , H SO 288 K Acetyl chloride Dialkyl urea (Unsymmetr ical) || NHCOCH NO 3 2 –CH COOH H O, H + + 3 + 2 Dialkyl thiourea NO 2 p-Nitroacetanilide o-Nitroaniline (minor) NO 2 p-Nitroaniline (major) 3 (c) Sulphonation NH NH HSO + 2 3 + H SO 2 – 4 The sulphanilic acid exists as a dipolar ion (structure II) which has acidic and basic groups in the same molecule. Such ions are called Zwitter ions or inner salts. Heat 453-473 K 4 Aniline (6) Uses Anilinium hydrogen sulphate NH (i) Ethylamine is used in solvent extraction processes in petroleum NH 2 refining and as a stabiliser for rubber latex. + 3 60 (ii) The quaternary ammonium salts derived from long chain aliphatic tertiary amines are widely used as detergents. (iii) Aliphatic amines of low molecular mass are used as solvents. SO H SO : 29.3 Distinction between primary, secondary and tertiary amines Table – 3 3 Zwitter ion structure (II) amine Primary Secondary amine Tertiary amine E3 Sulphanilic acid (I) Test Bad smelling carbylamine (Isocyanide) is formed. No action. No action. Action of CS and HgCl. (Mustard oil test) Alkyl isothiocyanate is formed which has pungent smell like mustard oil. No action. No action Action of nitrous acid. Alcohol is formed with evolution of nitrogen. Forms nitrosoamine which gives green colour with phenol and conc. H SO (Liebermann's test). Forms nitrite in cold which on heating gives nitrosoa- mine which responds to Liebermann's test. Acetyl derivative is formed. No action. 3 2 2 ID Action of CHCl and alcoholic KOH. (Carbylamine test) 2 4 Acetyl derivative is formed. Action of Hinsberg's reagent. Monoalkyl sulphonamide is formed which is soluble in KOH. Dialkyl sulphonamide is formed which is insoluble in KOH. No action. Action of methyl iodide. 3 molecules (moles) of CH I to form quaternary salt with one mole of primary amine. 2 moles of CH I to form quaternary salt with one mole of secondary amine. One mole of CH I to form quaternary salt with one mole of tertiary amine. U Action of acetyl chloride. D YG 3 Aniline does not form alcohol with nitrous acid but it forms benzene diazonium chloride which shows dye test. Aniline Aniline was first prepared by Unverdorben (1826) by dry distillation of indigo. In the laboratory, it can be prepared by the reduction of nitrobenzene with tin and hydrochloric acid. C 6 H 5 NO 2 6 H C 6 H 5 NH 2 2 H 2 O Sn , HCl Aniline U Nitrobenzene Aniline produced combines with H 2 SnCl 6 (SnCl 4 2 HCl) to form a double salt. ST 2C 6 H 5 NH 2 SnCl 4 2 HCl (C 6 H 5 NH 3 )2 SnCl 6 Double salt From double salt, aniline is obtained by treating with conc. caustic soda solution. (C6 H5 NH 3 )2 SnCl 6 8 NaOH 2C6 H5 NH 2 6 NaCl Na2 SnO 3 5 H 2O On a commercial scale, aniline is obtained by reducing nitrobenzene with iron filings and hydrochloric acid. 3 Aniline is also obtained on a large scale by the action of amine on chlorobenzene at 200°C under 300-400 atm pressure in presence of cuprous catalyst. 200 C 2C 6 H 5 Cl 2 NH 3 Cu 2 O 2C 6 H 5 NH 2 Cu 2 Cl 2 H 2 O 300 400 atm Properties Aniline when freshly prepared is a colourless oily liquid (b.p. 184°C). It has a characteristic unpleasant odour and is not poisonous in nature. It is heavier than water and is only slightly soluble. It is soluble in alcohol, ether and benzene. Its colour changes to dark brown on standing. It shows all the characteristic reactions discussed earlier. Uses : (1) It is used in the preparation of diazonium compounds which are used in dye industry. (2) Anils (Schiff's bases from aniline) are used as antioxidants in rubber industry. (3) It is used for the manufacture of its some derivatives such as acetamide, sulphanilic acid and sulpha drugs, etc. (4) It is used as an accelerator in vulcanizing rubber. Some important conversions (1) Conversion of methylamine to ethylamine (Ascent) 2 3 CH 3 NH 2 CH 3 OH CH 3 I HNO NO NH Cl + 2 NH – 3 3 2 Methylamine PI Methyl alcohol Fe /HCl 30% 3 Na CO 2 3 Methyl iodide CH 3 CN CH 3 CH 2 NH 2 LiAlH4 NaCN Methyl cyanide (2) Conversion of ethylamine to methylamine (Descent) Ethylamine 2 CH 3 CH 2 NH 2 CH 3 CH 2 OH HNO Ethylamine Ethanol (2) Preparation of diazonium salts : [O ] CH 3 CHO K 2 Cr2 O7 H 2 SO 4 Acetaldehyde NaNO 2 HCl NaCl HONO NH CH 3 COOH CH 3 COCl SOCl 2 [O ] Aceticacid N Cl + 2 Acetyl chloride CH 3 CONH 2 CH 3 NH 2 NH 3 Br2 KOH Acetamide NaNO HCl, 273 K Aniline K Cr O H 2 SO 4 Ethyl alcohol K Cr O Aceticacid 60 Ca (OH ) H 2 SO 4 (i) Diazonium salts are generally colourless, crystalline solids. Calcium acetate heat CH 3 COCH 3 (ii) These are readily soluble in water but less soluble in alcohol. Acetone (iii) They are unstable and explode in dry state. Therefore, they are generally used in solution state. E3 (4) Conversion of propionic acid to (i) Ethylamine, (ii) n-Butylamine. (iv) Their aqueous solutions are neutral to litmus and conduct electricity due to the presence of ions. SOCl 2 NH 3 (i) CH 3 CH 2COOH CH 3 CH 2COCl Propionic aicd Propionyl chloride (4) Chemical properties of diazonium salts 2 CH 3 CH 2 CONH 2 CH 3 CH 2 NH 2 Br KOH (i) Substitution reaction : In substitution or replacement reactions, nitrogen of diazonium salts is lost as N and different groups are introduced in its place. (a) Replacement by –OH group Ethylamine 2 N3H or C 2 H 5 COOH C 2 H 5 NH 2 H 2 SO 4 (conc.) ID Propionami de 4 5 (ii) CH 3 CH 2COOH CH 3 CH 2CH 2OH LiAlH PBr Ether Propionic acid Benzene diazonium (3) Physical properties of diazonium salts 7 2 CH 3 CHO 2 2 CH 3 COOH (CH 3 COO )2 Ca Acetaldehyde n - Propyl alcohol N Cl + Propyl cyanide Na C 2 H 5 OH CH 3 CH 2 CH 2 CH 2 NH 2 o r LiAlH4 D YG NaCN 2 CH 2 CH 2 CH 2 Br.CH 2 Br Br CCl 4 + – 2 Ethylene bromide Ethylene cyanide + H PO + H O 3 The diazonium salts have the general formula ArN 2 X – , where X N 2 ( N 2 3 3 ST U – N NCl – Benzenediazonium chloride CH N NCl + 3 HO (c) Replacement by–Cl group N Cl + Cl – 2 Cu Cl 2 +N 2 2 Chlorobenzene This reaction is called Sandmeyer reaction. When the diazonium salt solution is warmed with copper powder and the corresponding halogen acid, the respective halogen is introduced. The reaction is a modified form of Sandmeyer reaction and is known as Gattermann reaction. N Cl + Cl – 2 N NCl N NBr – o-chlorobenzenediazonium chloride – p-Toluenediazonium chloride Cl Benzene may be an anion like Cl , Br etc. and the group N ) is called diazonium ion group. (1) Nomenclature : The diazonium salts are named by adding the word diazonium to the name of the parent aromatic compound to which they are related followed by the name of the anion. For example, + – Cu HCl m-Hydroxybenzenediazonium bromide The diazonium salt may contain other anions also such as NO 3– , HSO 4– , BF4 etc. 2 + N + H PO + HCl 2 Benzene diazonium chloride – ON 2 Hypophosphoric acid 1,4 - Diaminobut ane Diazonium salts + Phenol (b) Replacement by hydrogen LiAlH + 2 N Cl 4 NCCH 2CH 2CN NH 2CH 2 CH 2 CH 2CH 2 NH 2 – + N + HCl 2 Benzene diazonium chloride n-Butylamine (5) Conversion of ethylene to 1,4-diaminobutane Warm + HO U Propyl bro mide OH – 2 KCN CH 3 CH 2CH 2 Br CH 3 CH 2 CH 2CN Ethylene 2 chlorideprimary amine to diazonium The reaction of converting aromatic salt is called diazotisation. 7 2 C2 H 5 NH 2 C2 H 5 OH 2 2 Ethylamine + NaCl + H O 2 Methylamine (3) Conversion of ethylamine to acetone HNO – 2 + I – 2 N NHSO 2 (d) Replacement by iodo (–I) group N Cl + + N – 4 p-Nitrobenzenediazonium hydrogen sulphate + KI + N + KCl Heat 2 Iodobenzene (e) Replacement by – F group N Cl N BF – + F – + 2 2 4 N NCl + + + HBF Heat 4 Fluoroboric acid Benzene diazonium fluoroborate This reaction is called Balz Schiemann reaction. H (pH 4.5) 273-278 K 2 3 CH 3 N N,N-dimethyl-p-aminoagobenzene 3 CH 3 (orange) important dye obtained by coupling the diazonium salt of sulphanilic acid with N, N-dimethylaniline. CuCN + N Na O S 2 + NH – 3 Cyanobenzene NaNO , HCl 273-278 K 2 2 E3 Sod. Salt of sulphanilic acid The nitriles can be hydrolysed to acids. N NCl Na O S + – 3 CN CH Coupling occurs para to hydroxy or amino group. All azo compounds are strongly coloured and are used as dyes. Methyl orange is an CN – CH 60 N Cl N=N + Fluorobenzene (f) Replacement by Cyano (– CN) group + N – COOH NN Na O S + – 3 Hydrolysis N(CH ) Cl + H 3 2 N, N-Dimethylaniline ID OH – Benzoic acid This method of preparing carboxylic acids is more useful than carbonation of Grignard reagents. (g) Replacement by – NO group 2 + + – 2 2 2 NaNO Cu + NaBF + N 2 4 4 2 D YG HBF NO 4 Diazonium fluoro borate + 2 Nitrobenzene NH NH 2 SH – 2 3 Methyl orange Diazonium salts are highly useful intermediates in the synthesis of large variety of aromatic compounds. These can be used to prepare many classes of organic compounds especially aryl halides in pure state. For example,. 1, 2, 3-tribromo benzene is not formed in the pure state by direct bromination of benzene. However, it can be prepared by the following sequence of reaction starting from p-nitroaniline through the formation of diazonium salts as : (h) Replacement by thio (–SH) group N Cl N(CH ) N=N – 3 U N BF N Cl – Na O S + 273-278 K + KSH 2 Br Br Br Diazotisation 2 + N + KCl Potassium hydro sulphide 2 NO N Cl + Thiophenol – 2 Brp-Nitroaniline (ii) Coupling reactions : The diazonium ion acts as an electrophile because there is positive charge on terminal nitrogen. It can react with nucleophilic aromatic compounds (Ar–H) activated by electron donating groups (– OH and – NH ), which as strong nucleophiles react with aromatic diazonium salts. Therefore, benzene diazonium chloride couples with electron rich aromatic compounds like phenols and anilines to give azo compounds. The azo compounds contain –N = N– bond and the reaction is called coupling reaction. Br NO Br 2 2 Br Br U CuBr Sn, HCl ST 2 Br 2 Br NO 2 Br Br Br Diazotisation N NCl + + NO Br OH – NH Phenol Base (pH 9-10) 273-278 K N=N Br 2 Br OH N Cl + – 2 Br H PO 3 2 p-Hydroxyazobenzene (yellow) N NCl + + H (pH 4.5) 273-278 K + NH – 1, 2, 3-Tribromo benzene (5) Uses of diazonium salts (i) For the manufacture of azo dyes. (ii) For the industrial preparation of important organic compounds like m-bromotoluene, m-bromophenol, etc. 2 N=N NH 2 (iii) For the preparation of a variety of useful halogen substituted p-Aminoazobenzene (orange) arenes. Alkyl nitrites are the esters of nitrous acid. Nitroparaffins are used as solvents for oils, fats, resins, esters, rubbers and cellulose etc. nitromethane is used as high power fuel in racing automobiles. 60 Nitrobenzene is good solvent in friedel crafts reaction because it dissolves AlCl 3 tendency of primary, secondary and tertiary amines to bind a proton, is due to the unshared pair of electrons on the nitrogen. When a proton is bound, positive ion is formed and originally electrically neutral amine takes on the charge of the proton. When ions are formed in this way, they are called onium ions. The ion formed in case of amines are substituted ammonium ions. The hydronium ion, H O is also the onium ion, which belongs to the class of oxonium ions. + 3 Some derivatives of ammonia arranged in order of deecreasing are (CH ) N OH , (CH ) NH, CH NH , (CH ) N, NH , C H NH , C H NHCH , C H NH , (C H ) NH, CH CONH. 6 5 + 3 3 6 5 2 – 4 3 6 5 2 2 3 3 2 3 3 3 2 6 5 2 ID basicity E3 All amines have basic properties. The basic property, that is, the In water the basicity follows the order : Primary < Tertiary < Secondary amine, with reference to hydronium ion, H O. In this case solvation factor and steric effect alter, to some extent, the order of basicity because of inductive effect alone. + 3 U In a non-polar solvent such as benzene, using trichloroacetic acid as D YG the reference acid, the basicity follows the order Tertiary < Secondary < Primary amines. The solvation factor is absent but steric effect upsets the inductive effect of alkyl groups. ST U Carylamine test is specific for primary amines.
