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The nucleophilic character of amines: Mechanism of the reaction on a halogenoalkane

(The example below illustrates the reaction between iodomethane and ethylamine)

Three successive reactions can take place depending on the quantity and concentration of the reagents:

Primary Amine $ \longrightarrow $ Secondary Amine

Nucleophilic attack of ethylamine (I) on iodomethane (II): The acid-base reaction (proton exchange) between ethylamine (I) and ethylmethylammonium ion releases ethylmethylamine( III )

Secondary Amine $ \longrightarrow $ Tertiary Amine

Nucleophilic attack of ethylmethylamine (III) on iodomethane (II): The acid-base reaction (proton exchange) between ethylamine (I) and diethylmethylammonium ion releases ethyldimethylamine (IV)

Tertiary Amine $\longrightarrow$ Tetralkylammonium ion

Nucleophilic attack of ethyldimethylamine (IV) on iodomethane (II): In the absence of mobile hydrogen on the ethyltrimethylammonium ion (V) formed, an acid-base reaction (proton exchange) can no longer take place.