16.1 Aliphatic Amines

Amines are considered derivatives of ammonia where 1, 2 or all 3 of the hydrogen atoms are replaced by alkyl groups.

Based on the number of alkyl groups attached to the nitrogen atom, amines can be classified as primary, secondary, and tertiary amines. The primary, secondary, and tertiary amines are named in the IUPAC system as Aminoalkane, N-alkylaminoalkane, and N,N-dialkylaminoalkane, respectively. e.g.,

• H₃C–NH₂ — Aminomethane (1° amine)
• CH₃CH₂CH₂–NH–CH₃ — N-methyl-1-aminopropane (2° amine)
• CH₃CH₂–N(CH₃)₂ — N,N-dimethylaminoethane (3° amine)

It is a simple and convenient method to separate the three classes of amines from their mixture. It is based on the different reactivity of the amines with diethyl oxalate.

The mixture of primary, secondary and tertiary amines is first of all treated with diethyl oxalate.

1. Primary amines react with diethyl oxalate, giving solid dialkyloxamide.
   Diethyl oxalate + 2 R–NH₂1° amine → RNH–CO–CO–NHRDialkyloxamide (solid) + 2 C₂H₅OHEthanol
2. Secondary amines react with diethyl oxalate, giving liquid dialkyloxamic ester.
   Diethyl oxalate + R₂NH2° amine → C₂H₅OOC–CO–NR₂Dialkyloxamic ester (liquid) + C₂H₅OHEthanol
3. Tertiary amines do not react.

Now the mixture is subjected to filtration, where solid dialkyloxamide separates as residue. This is then treated with KOH to recover the primary amine.

The filtrate, which contains dialkyloxamic ester, tertiary amine, and ethanol, is then subjected to fractional distillation. The tertiary amine, since it has the lowest boiling point, gets distilled first. Then, ethanol follows. The remaining dialkyloxamic ester is treated with KOH to recover the secondary amine.

The mixture of potassium oxalate, secondary amine and ethanol is further distilled to obtain pure secondary amine.

1. By Hoffmann’s ammonolysis of alkyl halides
   (refer to properties of haloalkanes)
2. By Hoffmann’s bromoamide degradation or Hoffmann’s hypobromide rearrangement reaction
   (refer to properties of amides)
3. By reduction of amides (studied in derivatives of carboxylic acids)
4. By reduction of nitroalkanes (refer to properties of nitroalkanes)
5. By reduction of alkanenitriles (cyanides)
   R–C≡NAlkanenitrile (alkyl cyanide) + 4[H] LiAlH₄ → R–CH₂–NH₂Aminoalkane (alkyl amine)

1. Lower members are gases with an ammoniacal smell. Higher members are liquids with a fishy smell.
2. Lower members are soluble in water due to the formation of intermolecular H-bonds. Higher amines containing six or more carbon atoms are insoluble.
3. Amines, except tertiary, are capable of forming H-bonds; therefore, their boiling points are relatively higher than corresponding nonpolar organic compounds. But their boiling points are lower than those of alcohols and carboxylic acids of comparable molecular weight.

Among isomeric amines, primary amines have the highest boiling point because they can form H-bonds, and tertiary amines have the lowest because they cannot form H-bonds.

Chemical Reactions of Primary Amines

1. With conc. HCl

   Amines, being bases, react with acids, giving an ammonium salt.

   R–NH₂ + HCl → R–NH₃⁺ Cl⁻Alkyl ammonium chloride
2. Alkylation

   Primary amines react with alkyl halides to give secondary amine, tertiary amine, and quaternary ammonium salt successively.

   R–NH₂Aminoalkane + R’–XHaloalkane − HX → R–NH–R’N-alkylaminoalkane (2° amine)
   R–NH–R’2° amine + R’–X − HX → R–N(R’)₂N,N-dialkylaminoalkane (3° amine)
   R–N(R’)₂3° amine + R’–X → [R–N(R’)₃]⁺ X⁻N,N,N-tetraalkylammonium halide (4° ammonium salt)

   Revise Hoffmann’s ammonolysis of haloalkanes in the haloalkanes chapter.

3. Acylation with acid chloride
   (refer to ammonolysis of acid chloride in derivatives of carboxylic acids)
4. Carbylamine reaction

   Both aliphatic and aromatic primary amines react with chloroform in the presence of alcoholic KOH to form isocyanide (carbylamine), with a very offensive, pungent smell like that of rotten onion. This reaction is called the carbylamine reaction.

   This reaction can act as a test for both primary amines and chloroform. As a test, this reaction is called the carbylamine test.

   R–NH₂1° amine + CHCl₃Chloroform + 3 KOH (alc.) warm → R–NCAlkyl isocyanide (carbylamine) — offensive smell + 3 KCl + 3 H₂O
5. Test of 1°, 2°, 3° amines with nitrous acid

   Reaction of primary amines with nitrous acid

   (studied in the preparation of alcohols)

   R–NH₂ + HNO₂ NaNO₂ / HCl → ice-cold R–OH + N₂ + H₂O

   e.g.,

   CH₃NH₂Methanamine + HNO₂ NaNO₂ / HCl → 0–5°C CH₃OHMethanol + N₂ + H₂O

   Reaction of secondary amines with nitrous acid

   Secondary amines, both aliphatic and aromatic, react with nitrous acid to yield N-nitrosoamines, which are yellowish or greenish oils in water.

   R₂NH2° amine + HNO₂ NaNO₂ / HCl → R₂N–NON-alkyl-N-nitrosoaminoalkane (greenish/yellowish oily liquid) + H₂O

   Reaction of a tertiary aliphatic amine with nitrous acid

   These give N-nitrosoammonium salts.

   R₃N3° amine + HNO₂ NaNO₂ / HCl → 0–5°C R₃N·HNO₂Trialkylammonium nitrite (3° ammonium salt, water-soluble)