JEE Chemistry Amines Notes Set 01

Amines: Classes and Structure

Amines are ammonia derivatives in which one, two or three hydrogens are replaced by alkyl or aryl groups. They are called primary, secondary or tertiary according to the number of carbon groups on nitrogen (not on the carbon that carries it). The general formula of a saturated aliphatic amine is \( \mathrm{C_nH_{2n+3}N} \). Nitrogen is sp3 hybridised with a lone pair, so the shape is pyramidal. Amines with three different groups are chiral in principle but cannot be resolved because the nitrogen inverts very quickly.

Preparation of Amines

In the Hofmann reaction the amide is converted to an N-bromoamide, then to an anion, which rearranges (the R group moves from carbon to nitrogen) to an isocyanate. Water and base turn the isocyanate into the amine and carbonate.

Separating a Mixture of Amines

1. Fractional distillation: boiling points are different enough for industry use.
2. Hinsberg's method: benzenesulphonyl chloride reacts with primary amines to give a product that dissolves in KOH, with secondary amines to give a product that does not dissolve, and does not react with tertiary amines.
3. Hofmann's method: diethyl oxalate gives a solid with a primary amine, a liquid oxamic ester with a secondary amine, and no reaction with a tertiary amine.

Physical Properties

1. Methylamine, dimethylamine, trimethylamine and ethylamine are gases or very volatile liquids at room temperature; higher amines are liquids or solids with a fishy smell.
2. Primary and secondary amines form N-H...N hydrogen bonds, weaker than O-H...O. So the order of boiling points is primary above secondary above tertiary for isomers, and each is lower than the matching alcohol.
3. Tertiary amines have no N-H and boil near alkanes of similar mass.
4. Lower amines dissolve in water by hydrogen bonding with water; solubility decreases as the alkyl part gets larger. Order of solubility: 1 degree above 2 degree above 3 degree.

Basic Strength

An amine is basic because the lone pair on nitrogen accepts a proton. Alkyl groups push electrons towards nitrogen and make the ammonium ion more stable, so alkylamines are stronger bases than ammonia. In water, solvation of the ammonium ion by hydrogen bonding also matters, and a tertiary ammonium ion has only one N-H to hydrogen bond. This is why the order in water is not simply 3 degree above 2 degree above 1 degree.

Electron-donating groups on the ring (such as methyl) raise the basicity of aniline, and electron-withdrawing groups (such as nitro) lower it.

Reactions of Aliphatic Amines

1. Salt formation: \( \ce{RNH2 + HCl -> RNH3+Cl-} \). The salts are soluble in water and release the free amine with NaOH.
2. Alkylation: reaction with alkyl halides gives secondary, tertiary amines and finally a quaternary ammonium salt.
3. Acylation: with acetyl chloride or acetic anhydride, \( \ce{RNH2 -> RNHCOCH3} \). Tertiary amines cannot be acylated. Benzoylation with benzoyl chloride in aqueous NaOH is the Schotten-Baumann reaction.
4. Carbylamine test (primary amines only): \( \ce{RNH2 + CHCl3 + 3KOH -> RNC + 3KCl + 3H2O} \), foul smell of isocyanide.
5. Reaction with nitrous acid: primary amines give alcohols with release of nitrogen; secondary amines give yellow N-nitrosoamines; tertiary amines give a nitrite salt.
6. Condensation with aldehydes and ketones: primary amines give Schiff's bases \( \ce{R'CH=NR} \) (also called anils).
7. Mustard oil reaction: primary amine with \( \ce{CS2} \) and \( \ce{HgCl2} \) gives an alkyl isothiocyanate with a pungent smell.

Worked Examples

1. Which amine in the pair is the stronger base in water: \( \ce{(CH3)2NH} \) or \( \ce{(CH3)3N} \)? The secondary amine, because the trimethylammonium ion is less solvated and the three methyl groups hinder solvation.
2. Convert propanamide to ethylamine: treat with \( \ce{Br2} \) and KOH. The amine has one carbon fewer.
3. How do you obtain \( \ce{C2H5NHCH3} \) from ethyl isocyanide? Reduce with \( \ce{H2}/Ni \) or Na/alcohol; the isocyanide carbon becomes the N-methyl group.