Amines
Amines can be considered as derivatives of ammonia, obtained by replacement of one, two or all the three hydrogen atoms by alkyl and/or aryl groups. They are classified as primary (1°), secondary (2°) and tertiary (3°) depending upon the number of hydrogen atoms replaced by alkyl or aryl groups in ammonia molecule. In aryl amines, –NH2 group is directly attached to the benzene ring. C6H5NH2 is the simplest example of aryl amine. Amines can be prepared by various synthetic pathways. For example the reduction of nitro compounds with metals in acidic medium produces amines. Reduction with iron scrap and hydrochloric acid is preferred because FeCl2 formed gets hydrolyzed to release hydrochloric acid during the reaction. Thus, only a small amount of hydrochloric acid is required to initiate the reaction.
Similarly, an alkyl or benzyl halide on reaction with an ethanolic solution of ammonia undergoes nucleophilic substitution reaction in which the halogen atom is replaced by an amino (–NH2) group. The primary amine thus obtained behaves as a nucleophile and can further react with alkyl halide to form secondary and tertiary amines, and finally quaternary ammonium salt. Reduction of nitriles and amides also yield amines.
Moreover, amines behave as nucleophiles due to the presence of unshared electron pair. Amines, being basic in nature, react with acids to form salts. Basic character of amines can be better understood in terms of their Kb and pKb values. Larger the value of Kb or smaller the value of pKb, stronger is the base. Basicity of amines is related to their structure. Basic character of an amine depends upon the ease of formation of the cation by accepting a proton from the acid. The more stable the cation is relative to the amine, more basic is the amine.
Which of the following factors does not affect the basicity of amines?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Molecular weight
An amine is basic because the lone pair on nitrogen accepts a proton. As the passage says, the more stable the protonated cation compared with the amine, the stronger the base. In water, three structural factors decide this:
Inductive effect (+I): alkyl groups push electrons towards nitrogen, making the lone pair more available and stabilising the cation.
Solvation: the cation is stabilised by hydrogen bonding with water; the more N–H hydrogens it has, the better it is solvated.
Steric effect: bulky alkyl groups crowd the nitrogen and hinder both protonation and hydrogen bonding of the cation.
Option (1) Inductive effect — Affects basicity. The +I effect of alkyl groups makes aliphatic amines stronger bases than ammonia.
Option (2) Steric effect — Affects basicity. Crowding by three alkyl groups is one reason tertiary amines are weaker bases in water than expected from the +I effect.
Option (3) Molecular weight — Does not affect basicity. Basicity depends on how electrons and groups are arranged around nitrogen, not on mass: in water \( \ce{(CH3)3N} \) (59 g mol−1) is a weaker base than \( \ce{(CH3)2NH} \) (45 g mol−1), while \( \ce{C6H5NH2} \) (93 g mol−1) is weaker than \( \ce{NH3} \).
Option (4) Solvation — Affects basicity. Better hydration of \( \ce{RNH3^{+}} \) and \( \ce{R2NH2^{+}} \) than of \( \ce{R3NH^{+}} \) changes the order of basicity in water.
NCERT (2026-27 edition):
Chemical Reactions – “… You may find some discrepancies while trying to interpret the Kb values of amines on the basis of +I or –I effect of the substituents present in amines. Besides inductive effect, there are other effects like solvation effect, steric hinderance, etc., which affect the basic strength of amines.”