The correct order of the basic strength of methyl substituted amines in aqueous solution is [2019]

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Solution

Q.
The correct order of the basic strength of methyl substituted amines in aqueous solution is [2019]

1 ${CH}_{3} {NH}_{2} > {({CH}_{3})}_{2} NH > {({CH}_{3})}_{3} N$

2 ${({CH}_{3})}_{2} NH > {CH}_{3} {NH}_{2} > {({CH}_{3})}_{3} N$

3 ${({CH}_{3})}_{3} N > {CH}_{3} {NH}_{2} > {({CH}_{3})}_{2} NH$

4 ${({CH}_{3})}_{3} N > {({CH}_{3})}_{2} NH > {CH}_{3} {NH}_{2}$

Ans.
(2)

The basicity of amines in aqueous solution depends on the stability of the ammonium cation or conjugate acid formed by accepting a proton from water which in turn depends on the $+ I$ -effect of alkyl group, extent of hydrogen bonding and steric factor. All these factors are favourable for $2 °$ amines. Therefore, $2 °$ amines are the strongest bases. If the alkyl group is small i.e., ${CH}_{3}$ then there is no steric hindrance to H-bonding. Thus, the stability due to hydrogen bonding predominates over the stability due to $+ I$ -effect of — ${CH}_{3}$ group and hence primary amine is a stronger base than $3 °$ amine. Hence, overall decreasing basic strength for methylamines in aqueous solution is

${({CH}_{3})}_{2} NH > {CH}_{3} {NH}_{2} > {({CH}_{3})}_{3} N$

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