Preparation Of Amines

Amines of Class 12

Now, that we have learnt something about amines, we shall now get into the methods of their preparation.

From Alkyl halides

Many organic halogen compounds are converted into amines by treatment with aqueous (or) alcoholic solution of ammonia. This reaction is generally carried out either by allowing the reactants to stand together at room temperature (or) by heating them under pressure. Displacement of halogen by NH3 yields the amine salt, from which free amine can be liberated with hydroxide ion.

CH3Cl + NH3 Preparation Of AminesCH3NH2 + Cl– + H2O

The above reaction is a class of substitution reaction, which we know as nucleophilic substitution.

Ammonia can act as a nucelophile and it can also act as a base.

If ammonia acts a nucleophile substitution takes place,

CH3CH2CH2Br + NH3 → CH3CH2CH2NH2 + HBr

And, if ammonia acts as a base, elimination takes place.

Preparation Of Amines

It is very evident that primary alkyl halides under go substitution very easily than tertiary alkyl halides, which undergo elimination very easily.

Look at the sequence of reactions below,

RX Preparation Of Amines

The reaction is quite simple and we can convert alkyl halide into all class of amines.

From nitrogen containing compounds

Nitro compounds

Nitro alkanes can be reduced quantitatively to their corresponding amines.

Preparation Of Amines

Nitro compound can be reduced in two general ways: (A) by catalytic hydrogenation using molecular hydrogen, or (B) by chemical reduction, usually by a metal and acid.

This method cannot be used when the molecule also contains some other easily hydrogenated group, such as a Carbon carbon double bond. Chemical reduction is most often carried out by adding hydrochloric acid to a mixture of the nitro compound and metal, usually granulated tin or iron.

Preparation Of Amines

Nitriles

Alkyl and aryl cyanides can be reduced to their corresponding amines using LiAlH4

Preparation Of Amines

Amides

Amides can directly be converted into their corresponding amines. This reaction is carried out by treating the amide with a mixture of base and bromine (KOH + Br2). This reaction is called as Hofmann Bromamide reaction.

The reaction is as follows,

RCONH2 + Br2 + 4KOH → RNH2 + K2CO3 + 2KBr + 2H2O

Here we can see that the amine formed has one carbon less than that of the corresponding amide. Due to the loss of carbon atom, this reaction is also called as Hofmann degradation of amides.

The mechanism of the reaction is as follows:

Step 1

 Preparation Of Amines

(one of the hydrogen attached to nitrogen is substituted by a bromine atom)





Step 2

Preparation Of Amines

(The N-bromamide anion thus formed as a result of proton by base is stabilized by resonance)


Step 3

Preparation Of Amines

(Bromine leaves and we have an electron deficient nitrogen)




Step 4

Preparation Of Amines

(There is a shift of alkyl group to the nitrogen)

Step 5

Preparation Of Amines

(A simple hydrolysis of an imine gives us the amine)

Apart from this, amides can be dehydrated by P2O5 to their corresponding nitriles and nitriles can then be reduced.

Preparation Of Amines  R—CH2NH2

By this method you are retaining the number of carbon atoms in both amide and the amine.

From carbonyl compounds

While studying carbonyl compounds we have seen that carbonyl compounds can be converted into any other functional group. How are we converting carbonyl group into amino group?

See, the following sequence,

CH3CH = O + NH3 Preparation Of Amines

The reactions are clear and simple so that, we can get an amine from carbonyl compound just by reductive amination (amination and reduction).

Using this reductive amination we can go from 1° amine to 2° amines. How?

Look at the following reaction.

CH3CH2CH = O + Preparation Of Amines → CH3CH2CH=NCH2CH3

↓H2/Ni

Preparation Of Amines

Curtius reaction

Amines can be prepared by treating acid chloride with sodium azides the isocyanate thus formed is decomposed with treatment of water and amines are obtained.

Preparation Of Amines

Schmidt reaction

Hydrozoic acid reacts with carboxylic acid in presence of a mineral acid to give amines.

Preparation Of Amines

By the reduction of an alkyl isocyanide

RNC + 4[H] → R – NH – CH3

Preparation of Tertiary Amines

3RX + NH3 Preparation Of Amines R3N + 2HX

(in excess)

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