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Advanced Level Chemistry: Organic nitrogen compounds - Preparation of Amines

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.


(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.

See section 3.6 The substitution reaction between halogenoalkanes (haloalkanes) and ammonia and amines forming primary, secondary and tertiary amines 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).

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

(a) Nitriles from halogenoalkanes

R-CN  +  4[H]  ===>  R-CH2NH2

R = H, alkyl or aryl

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

RCH(OH)CN + 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


INDEX of Organic Nitrogen Compound NOTES

All Advanced A Level Organic Chemistry Notes

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Doc Brown's Advanced Level Chemistry Revision Notes

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INDEX of Organic Nitrogen Compound NOTES

 All Advanced Organic Chemistry Notes


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