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 → CH3NH2 + 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.
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,
The reaction is quite simple and we can convert alkyl halide into all class of amines.
From nitrogen containing compounds
Nitro alkanes can be reduced quantitatively to their corresponding 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.
Alkyl and aryl cyanides can be reduced to their corresponding amines using LiAlH4
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:
(one of the hydrogen attached to nitrogen is substituted by a bromine atom)
(The N-bromamide anion thus formed as a result of proton by base is stabilized by resonance)
(Bromine leaves and we have an electron deficient nitrogen)
(There is a shift of alkyl group to the nitrogen)
(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.
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 →
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 + → CH3CH2CH=NCH2CH3
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.
Hydrozoic acid reacts with carboxylic acid in presence of a mineral acid to give amines.
By the reduction of an alkyl isocyanide
RNC + 4[H] → R – NH – CH3
Preparation of Tertiary Amines
3RX + NH3 R3N + 2HX