Read the following passage and answer questions
Greater is the stability of the substituted ammonium cation, stronger should be the corresponding amine as a base. Thus, the order of basicity of aliphatic amines should be : primary > secondary > tertiary, which is opposite to the inductive effect based order. Secondly, when the alkyl group is small, like –CH3 group, there is no steric hinderance to H-bonding. In case the alkyl group is bigger than –CH3 group, there will be steric hinderance to H-bonding. Therefore, the change of nature of the alkyl group, e.g., from –CH3 to –C2H5 results in change of the order of basic strength. Thus, there is a subtle interplay of the inductive effect, solvation effect and steric hinderance of the alkyl group which decides the basic strength of alkyl amines in the aqueous medium.
Which factor is not responsible for basic strength of amines ?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Conjugation factor
As the passage says, the basic strength of alkyl amines in water is decided by the interplay of three factors: the inductive effect of the alkyl groups, the solvation effect (hydrogen bonding of the protonated amine with water) and the steric hindrance of the alkyl groups to that hydrogen bonding. A conjugation factor is not one of them, so the answer is conjugation factor.
Option (1) Conjugation factor — Not responsible (for alkyl amines).
Option (2) Inductive effect — Responsible for the basic strength. Alkyl groups push electrons to nitrogen.
Option (3) Solvation effect — Responsible for the basic strength. The more N–H bonds the cation has, the better water stabilises it.
Option (4) Steric hinderance — Responsible for the basic strength. Bulky groups hinder solvation.