Question
Write the factors influencing the basicity of amines.
✓
Answer
Factors influencing the basicity of amines are as follows:
- Influence of +I effect:
1. Basicity of amines is related to the structural effects which influence the stabilization of various species. Greater is the stabilization of the protonated amine, that is, the conjugate acid, greater is the basicity of the amine.
2. An alkyl group exerts an electron releasing inductive effect (+I) which stabilizes the positive charge on the atom bonded to it. As we move from conjugate acid of ammonia
$NH _4^{+}$
to that of tertiary amine $(R_3NH^+)$, the number of alkyl groups (R) bonded to nitrogen goes on increasing steadily. This results in increasing stabilization of the conjugate acids and thereby an increasing order of basic strength is expected. - Order of stabilization:
Expected order of basic strength: $NH_3 < R – NH_2 < R_2NH < R_3N$
3. The expected order of basic strength on the basis of the +I effect differs from the observed order. However, decreased basic strength of 3° amine implies that the conjugate acid of 3° amine is less stabilized even though the +I effect of three alkyl groups in $R_3NH^+$ is expected to be large. Decreased basic strength of 3° amine implies that the conjugate acid of 3° amine is less stabilized even though the +I effect of three alkyl groups in $R_3N^+H$ is expected to be large. This is suggestive of the existence of another influencing factor in the stabilization of conjugate acids of amines. - Influence of solvation by water: The solvent water stabilizes the conjugate acid by hydrogen bonding through the ‘H’ bonded to the ‘$N^+$’.
The number of ‘H’ atoms bonded to the ‘$N^+$’ decreases from 4 in
$NH _4^{+}$
to 1 in $R_3NH^+$. As a result,
$NH _4^{+}$
is best stabilized by solvation while the stabilization by solvation is very poor in $R_3NH^+$. Thus, the basic strength of 3° amines is decreased. - Combined influence of +I effect and solvation:
1. The combined influence of both, +I effect and solvation on stabilization if conjugate acids of aliphatic amines decide the observed basic strength and $pK_b$ value. These two influencing factors operate in opposite directions.
2. The net result is that as we move from $NH_3$ to $RNH_2$ to $R_2NH$, the basic strength increases due to better stabilization of the corresponding conjugate acids.
3. But, 3° amine is a weaker base than 2° amine because the stabilization of conjugate acid of 3° amine by solvation is very poor.
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