This section covers the nucleophilic substitution of halogenoalkanes by ammonia and the two-stage mechanism that forms a primary amine.
Ammonia as a nucleophile
Ammonia and amines contain a lone pair on nitrogen, so they can act as nucleophiles. When ammonia reacts with a halogenoalkane, the halogen is replaced and a primary amine can be formed.
The process has two stages. Ammonia first attacks as a nucleophile, forming a positively charged intermediate. A second ammonia molecule then acts as a base and removes a proton.
For example, bromoethane forms ethylamine:
CHX3CHX2Br+2NHX3CHX3CHX2NHX2+NHX4Br
Mechanism
In the first stage, the nitrogen lone pair attacks the carbon attached to bromine as the carbon–bromine bond breaks.
The second ammonia molecule accepts one of the protons from the intermediate. The electrons in that N–H bond return to nitrogen, producing the neutral primary amine.
Exam Tip: In the overall primary-amine reaction, account for two ammonia molecules: one supplies the attacking lone pair and the other accepts a proton from the intermediate.
02•Reactions of Amines
Further substitution
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Successive reactions
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Secondary and tertiary amines
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