Question

Methyl amine reacts with methyl iodide. For completion of reaction, how many moles of methyl iodide are required?
A.1
B.2
C.3
D.4

Answer 1

Hint: Substituted alkyl amine keep on reacting with alkyl halide present in the reaction mixture until the quaternary salt is formed. For this completion of reaction, alkyl halide must be present in excess than alkyl amine.

Complete step Answer:
When alkyl amine reacts with the alkyl halide, it results in the formation of quaternary salt of alkyl amine halide. For this, alkyl halide keeps on reacting with alkyl amine until all the hydrogen of amine are substituted by the alkyl group. Here alkyl amine acts as nucleophile and with an increasing alkyl group attached to amine, the nucleophilicity of amine will increase due to $$+\text{I}$$ effect of the alkyl group. Substituted alkyl amine keep on reacting with alkyl halide if alkyl halide is in excess in the reaction mixture. The reaction will tend to stop at tertiary amine stage
Reaction of methyl amine with methyl iodide is as follow:
$$\text{C}{\text{H}}_3\text{N}{\text{H}}_2+\text{C}{\text{H}}_3\text{I}\to {\left(\text{C}{\text{H}}_3\right)}_2\text{NH}+\text{HI}$$ , the formed N methyl methylamine will react with another methyl iodide in the reaction mixture.
$$\left(\text{C}{\text{H}}_3\right)\text{NH}+\text{C}{\text{H}}_3\text{I}\to {\left(\text{C}{\text{H}}_3\right)}_3\text{N}+\text{HI}$$ , similarly N,N dimethyl methylamine will react with another methyl iodide which is present in the reaction mixture.
$${\left(\text{C}{\text{H}}_3\right)}_3\text{N}+\text{C}{\text{H}}_3\text{I}\to {\left(\text{C}{\text{H}}_3\right)}_4{\text{N}}^{+}{\text{I}}^{-}$$ , as there is no possibilities of further more reaction as nitrogen of amino group cannot react with methyl iodide any further. This is the final product at the end of the reaction.
Hence, total 3 moles of methyl halide will be required to react with methyl amine for the completion of reaction as the end product of this reaction is quaternary salt of methylamine iodide.

Thus, the correct option is C.

Note: The nucleophilicity of trialkyl amine is less than that of diethyl amine largely because the trialkyl amine nitrogen is tertiary, which will increase steric hindrance and slow down the reaction rate.