JEE Chemistry Amines Notes Set 06

Classes of Nitrogen Compounds

Amines are derivatives of ammonia in which hydrogens are replaced by alkyl or aryl groups: one replaced gives a primary amine \( \ce{RNH_2} \), two a secondary amine \( \ce{R_2NH} \), three a tertiary amine \( \ce{R_3N} \). The nitrogen is \( \ce{sp^3} \) hybridised with one lone pair and a pyramidal shape. Related nitrogen compounds are nitroalkanes and nitroarenes \( \ce{R-NO_2} \) (carbon bonded to N), alkyl nitrites \( \ce{R-ONO} \) (carbon bonded to O), cyanides \( \ce{R-C#N} \) and isocyanides \( \ce{R-N#C} \). For example \( \ce{C_3H_9N} \) has four amine isomers: propylamine, isopropylamine, ethylmethylamine and trimethylamine, and \( \ce{C_4H_{11}N} \) has four primary amine isomers.

Preparation of Amines

1. Reduction of nitro compounds: \( \ce{RNO_2 + 6[H] -> RNH_2 + 2H_2O} \) with Sn/HCl, Fe/HCl or catalytic hydrogen. Nitrobenzene gives aniline this way.
2. Reduction of nitriles and isocyanides: \( \ce{RCN} \) with \( \ce{LiAlH_4} \) or Na/ethanol gives \( \ce{RCH_2NH_2} \) (one more carbon than the halide used to make it), while \( \ce{RNC} \) gives the secondary amine \( \ce{RNHCH_3} \). Amides are reduced to amines of the same carbon count by \( \ce{LiAlH_4} \).
3. Ammonolysis of alkyl halides with alcoholic ammonia gives a mixture of primary, secondary, tertiary amines and the quaternary salt, because each product is a better nucleophile than the one before.
4. Gabriel phthalimide synthesis: potassium phthalimide with an alkyl halide, then hydrolysis, gives only pure primary aliphatic amines. Aryl halides do not react, so aromatic amines cannot be made this way.
5. Hofmann bromamide degradation: \( \ce{RCONH_2 + Br_2 + 4NaOH -> RNH_2 + Na_2CO_3 + 2NaBr + 2H_2O} \). The amine has one carbon less than the amide, the reaction needs a primary amide, and the intermediates are the N-bromoamide and the isocyanate \( \ce{RN=C=O} \). The Curtius and Schmidt reactions are related and also lose the carbonyl carbon.
6. Aromatic amines: reduction of nitroarenes, and ammonolysis of chlorobenzene with \( \ce{NH_3} \) and \( \ce{Cu_2O} \) at about 570 K.

Basicity

Amines are basic because the nitrogen lone pair accepts a proton to give an alkylammonium ion. In water the order for the methyl series is \( \ce{(CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3} \) (pKb about 3.3, 3.4, 4.2 and 4.8). For the ethyl series it is \( \ce{(C_2H_5)_2NH > (C_2H_5)_3N > C_2H_5NH_2 > NH_3} \). The pattern comes from a balance of three effects: the +I effect of alkyl groups increases electron density on N, the ammonium ion is stabilised by hydrogen bonding to water (more N-H bonds, more stabilisation) and bulky alkyl groups cause steric hindrance. In the gas phase or in a non-polar solvent without solvation the order is simply tertiary > secondary > primary.
Aromatic amines are much weaker (aniline pKb 9.4) because the lone pair is delocalised into the ring. Electron-withdrawing groups such as \( \ce{-NO_2} \) lower basicity further and electron-donating groups (for example \( \ce{-OCH_3} \), \( \ce{-CH_3} \)) raise it slightly. Benzylamine behaves like an alkylamine because the lone pair is not conjugated. Amides, with the lone pair delocalised onto the carbonyl oxygen, are practically neutral, while amidines are strongly basic. Primary amines have the highest boiling points among the amine isomers because they hydrogen-bond most, but their boiling points are still lower than those of alcohols.

Chemical Reactions of Amines

1. Salt formation: \( \ce{RNH_2 + HCl -> RNH_3^+Cl^-} \). Amines dissolve in acids, so benzene and aniline can be separated by dilute HCl.
2. Acylation: \( \ce{RNH_2 + (CH_3CO)_2O -> RNHCOCH_3 + CH_3COOH} \) for primary and secondary amines (tertiary amines have no N-H). Acetylation of aniline protects the amino group during nitration and bromination, because the acetamido group is a weaker activator and is not oxidised.
3. Carbylamine reaction (Hofmann isocyanide test): \( \ce{RNH_2 + CHCl_3 + 3KOH -> RNC + 3KCl + 3H_2O} \) gives a very foul-smelling isocyanide and is specific for primary amines, aliphatic or aromatic.
4. Hofmann mustard oil reaction: a primary amine with \( \ce{CS_2} \) and \( \ce{HgCl_2} \) gives an alkyl isothiocyanate \( \ce{RN=C=S} \) with a mustard-oil smell.
5. Hinsberg's reagent, benzenesulphonyl chloride: a primary amine gives a sulphonamide that dissolves in alkali, a secondary amine gives a sulphonamide that does not dissolve, and a tertiary amine does not react. This distinguishes and separates the three classes.
6. Reaction with nitrous acid (\( \ce{NaNO_2 + HCl} \)): a primary aliphatic amine gives an alcohol and nitrogen gas, a primary aromatic amine gives a diazonium salt at 0 to 5°C, a secondary amine gives a yellow oily N-nitrosoamine (Liebermann test), and a tertiary aliphatic amine only forms a salt.
7. Reaction with aldehydes: primary amines condense to give Schiff bases (imines), for example aniline and benzaldehyde give \( \ce{C_6H_5N=CHC_6H_5} \). Secondary amines with ketones give enamines.
8. Electrophilic substitution of aniline: the amino group is strongly activating and ortho/para directing. Bromine water gives 2,4,6-tribromoaniline at once. Direct nitration with the acid mixture gives oxidation products and a meta product because aniline is protonated to the meta-directing anilinium ion; the acetanilide route is used to get para-nitroaniline. Sulphonation by baking aniline hydrogen sulphate gives sulphanilic acid, which exists as a zwitterion.

Benzenediazonium Salts

Diazotisation, \( \ce{C_6H_5NH_2 + NaNO_2 + 2HCl -> C_6H_5N_2^+Cl^- + NaCl + 2H_2O} \), is carried out at 0 to 5°C because aryl diazonium salts decompose at higher temperature, and excess acid is used to stop coupling of the diazonium ion with unreacted amine. The salt is a useful intermediate:

Removing the amino group with \( \ce{H_3PO_2} \) is how a compound such as 3,4,5-tribromoaniline gives 1,2,3-tribromobenzene, and 2,4,6-tribromoaniline gives 1,3,5-tribromobenzene. The coupling reaction gives the azo dyes, which contain the chromophore \( \ce{-N=N-} \).

Nitro Compounds

1. Nitrobenzene: from benzene with the nitrating mixture of conc. \( \ce{HNO_3} \) and conc. \( \ce{H_2SO_4} \). Sulphuric acid is the stronger acid and protonates nitric acid, which loses water to give the electrophile \( \ce{NO_2^+} \). It is an oily pale-yellow liquid with the smell of bitter almonds (oil of mirbane). The nitro group is deactivating and meta-directing, so further nitration gives m-dinitrobenzene and then 1,3,5-trinitrobenzene, and nitrobenzene does not undergo Friedel-Crafts reactions (it is itself used as a solvent for them).
2. Reduction of nitrobenzene depends on the medium: in strongly acidic medium (Sn/HCl) it gives aniline, in neutral medium (\( \ce{Zn/NH_4Cl} \)) N-phenylhydroxylamine, in alkaline medium (\( \ce{Zn/NaOH} \)) azoxybenzene, azobenzene and finally hydrazobenzene, and electrolytic reduction in strong acid gives p-aminophenol by rearrangement. Mild reduction of m-dinitrobenzene with \( \ce{NH_4HS} \) changes only one nitro group, giving m-nitroaniline.
3. Nitroalkanes: primary and secondary nitroalkanes react with nitrous acid (primary give nitrolic acids that form a red solution with alkali, secondary give blue pseudonitroles), tertiary ones do not react because they have no alpha hydrogen. Chlorination of nitromethane gives chloropicrin, \( \ce{CCl_3NO_2} \), a tear gas. Do not confuse nitroalkanes with the isomeric alkyl nitrites \( \ce{R-ONO} \), which are esters of nitrous acid.
4. TNT and picric acid are explosive polynitro compounds, and amatol is TNT mixed with ammonium nitrate.

Cyanides and Isocyanides

1. Alkyl cyanides (nitriles) are made from alkyl halide and KCN (the cyanide ion is an ambident nucleophile that attacks through carbon with alkyl halides), by dehydration of amides with \( \ce{P_2O_5} \) or \( \ce{POCl_3} \), and from diazonium salts by the Sandmeyer reaction. Aryl halides do not react with KCN directly.
2. Hydrolysis gives the acid and ammonia; reduction gives the primary amine; a Grignard reagent gives a ketone after hydrolysis; and the Stephen reaction (\( \ce{SnCl_2/HCl} \) then water) gives an aldehyde.
3. Isocyanides \( \ce{R-N^+#C^-} \) are made only by the carbylamine reaction (or from alkyl halide and silver cyanide). They have a very offensive smell, hydrolyse with dilute acid to a primary amine and formic acid but are stable to alkali, and are reduced to secondary amines \( \ce{RNHCH_3} \).
4. Methyl isocyanate, \( \ce{CH_3N=C=O} \), is made from methylamine and phosgene and was the gas released in the Bhopal disaster.