Part 8. The chemistry of organic nitrogen (organonitrogen) compounds

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Part 8.3 Methods of preparing aliphatic and aromatic amines

Introduction

Various methods of synthesising aliphatic amines and aromatic amines are described using a variety of starting molecules and employing different reagents.

You are essentially changing one functional group to another.

All had already been described on other previously written pages of organic chemistry, so this page is essentially a summary of links below a general equation to illustrate the method.

Sub-index for this page

(1) The reaction between halogenoalkanes and ammonia and amines

(2) The synthesis of primary aromatic amines by reduction of nitro-aromatic compounds

(3) The synthesis of amines by reducing nitriles

(4) The preparation of amines from the reduction of amides

(1) The reaction between halogenoalkanes and ammonia and amines

A method of synthesising primary aliphatic amines

R-X  +  2NH₃  ===>  RNH₂  +  NH₄⁺X^-

A secondary aliphatic amine is made by reacting an amine with a halogenoalkane

R'-X  +  2RNH₂  ===>  RNHR'  +  RNH₃⁺X^-

A tertiary aliphatic amine is made by reacting a secondary amine with a halogenoalkane

R"X  +  2RNHR'   ===>  NRR'R"  +  RR'NH₂⁺X^-

With excess of the amine you can form a quaternary ammonium salts via the tertiary amine

e.g RX  +  NR₃  ===>  [NR₄]⁺  +  X^-

R, R' and R" groups are usually alkyl, but can sometimes be aryl and they may be all the same or all different.

X = halogen e.g. Cl or Br.

Uses of quaternary ammonium salts

See section 3.6 The substitution reaction between halogenoalkanes (haloalkanes) and ammonia and amines forming primary, secondary and tertiary amines and quaternary ammonium salts for all the details and introduction to the mechanism see Nucleophilic substitution by ammonia/primary amine to give primary/secondary amines

(2) The synthesis of primary aromatic amines by reduction of nitro-aromatic compounds

The basic reaction is:  R-NO₂  +  6[H]  ===>  R-NH₂  +  2H₂O

where R = aryl (aromatic ring, un-substituted or substituted)

This can be achieved in three ways

Refluxing the nitro-aromatic compound with tin and concentrated hydrochloric acid (Sn/HCl) to yield an aromatic amine like phenylamine

In the laboratory reduction using lithium tetrahydridoaluminate(III) (LiAlH₄)

In industry reduction by passing the nitro-aromatic and hydrogen over a nickel catalyst (H₂/Ni)

See section 7.10 Preparation of phenylamine and other aromatic amines for all the details

(3) The synthesis of amines by reducing nitriles

The nitrile can be aliphatic or aromatic

(a) Reducing nitriles synthesised from halogenoalkanes

R-C≡N  +  4[H]  ===>  R-CH₂NH₂

R = H, alkyl or aryl

(b) Reducing hydroxy-nitriles obtained hydrogen cyanide addition to aldehydes and ketones

RCH(OH)C≡N + 4[H] ==> RCH(OH)CH₂NH₂

R = H, alkyl or aryl

This is achieved:

In the laboratory reduction using lithium tetrahydridoaluminate(III) (LiAlH₄)

In industry reduction by passing the nitro-aromatic and hydrogen over a nickel catalyst (H₂/Ni)

See section 6.12 Nitriles - molecular structure, preparations and reactions for all the details

(4) The preparation of amines from the reduction of amides

R-CONH₂  +  4[H]  ===>  R-CH₂NH₂  +  H₂O

This can be achieved using lithium tetrahydridoaluminate(III) (LiAlH₄)

See section 6.11 Amides - molecular structure, preparations and reactions for all the details

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