Question

Explain why (CH₃)₂NH is more basic than (CH₃)₃N in aqueous solution.

Answer 1

The substituted ammonium cation is stabilised in the aqueous state not only by the alkyl group (+I), which releases electrons but also by solvation with water molecules. Less solvation will occur and the ion will be less stabilised as its size increases. Ions are stable in the following order:

\[\begin{array}{cc} \ce{OH2}\phantom{.....}\\ \ce{⫶}\phantom{........}\\ \ce{H}\phantom{........}\\ |\phantom{........}\\ \ce{R-N^+-N...OH2}\\ |\phantom{........}\\ \ce{H}\phantom{........}\\ \ce{⫶}\phantom{........}\\ \ce{OH2}\phantom{.....} \end{array}\]

>

\[\begin{array}{cc} \phantom{......}\ce{R}\phantom{....}\ce{H...OH2}\phantom{}\\ \phantom{}\backslash\phantom{..}/\phantom{}\\ \ce{\overset{⊕}{N}}\\ \phantom{}/\phantom{..}\backslash\phantom{}\\ \phantom{......}\ce{R}\phantom{....}\ce{H...OH2}\phantom{} \end{array}\]

>

\[\begin{array}{cc} \ce{R}\phantom{............}\\ \backslash\phantom{..........}\\ \ce{R - \overset{+}{N}-H...OH2}\\ /\phantom{.........}\\ \ce{R}\phantom{............} \end{array}\]

The stronger the matching amine as a base, the more stable the substituted \[\ce{\overset{+}{N}H4}\] cation.

Thus the order of basicity, Primary > Secondary > Tertiary.