Primary, secondary and tertiary amines all behave as Lewis bases. Which of the following is the reason?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → The nitrogen atom carries an unshared pair of electrons, which it can donate.
A Lewis base is an electron pair donor. In every amine, nitrogen is sp3 hybridised; three of its orbitals form bonds and the fourth holds an unshared pair of electrons. Nitrogen donates this pair to a proton or to another electron acceptor, for example:
\( \displaystyle \ce{R3N + H^+ -> R3NH^+} \)
Option (2) The nitrogen atom has a vacant orbital, which can accept a pair of electrons. — Incorrect. Accepting an electron pair is the behaviour of a Lewis acid, not a Lewis base.
Option (3) The hydrogen atom attached to nitrogen is easily given out as a proton. — Incorrect. Giving out a proton is acid behaviour. Tertiary amines have no hydrogen on nitrogen, yet they are bases.
Option (4) The amine molecules are held together by intermolecular hydrogen bonds. — Incorrect. Hydrogen bonding affects physical properties such as boiling point. Tertiary amines show no intermolecular hydrogen bonding, yet they are Lewis bases too.
NCERT (2026-27 edition):
9.6 Chemical Reactions – “Amines have an unshared pair of electrons on nitrogen atom due to which they behave as Lewis base.”
Summary – “Like ammonia, all the three types of amines have one unshared electron pair on nitrogen atom due to which they behave as Lewis bases.”