Comprehension:
Physical and Chemical properties of Amines
Like ammonia, nitrogen atom of amines is trivalent and carries an unshared pair of electrons. Nitrogen orbitals in amines are therefore, sp3 hybridised. Each of the three sp3 hybridised orbitals of nitrogen overlap with orbitals of hydrogen or carbon depending upon the composition of the amines. The fourth orbital of nitrogen in all amines contains an unshared pair of electrons. Amines have an unshared pair of electrons on nitrogen atom due to which they behave as Lewis base as well as act as a nucleophile. Lower aliphatic amines are soluble in water because they form hydrogen bond with water molecules and also form strong intermolecular hydrogen bonding. Solvation, +I and +R groups affect the electron density over nitrogen atom. Their pKb value lies in the range of 3 to 4.22. The most widely used method for preparing amines involves the reduction of nitro group, nitriles, amides, etc. using different reagents like H2/Pd, Sn+HCl, H2/Ni, LiAlH4+H2O, Br2+NaOH etc. Primary amines can be easily detected by the carbylamine test and Hinsberg's reagent. Diazonium salt can be synthesized from amines easily for synthesis of variety of new compounds.
Trimethylamine has
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Pyramidal shape
The nitrogen of trimethylamine has three bond pairs and one lone pair, so the molecule is a pyramid with nitrogen at the apex.
Why the other options are incorrect
Option (1) Planar structure — Incorrect. A planar arrangement would need sp2 nitrogen with no lone pair in the plane; the lone pair of an amine pushes the three groups out of plane.
Option (2) Trigonal Bipyramidal structure — Incorrect. This shape needs five electron pairs (sp3d), as in \( \ce{PCl5} \); nitrogen has only four.
Option (4) Octahedral geometry — Incorrect. This shape needs six electron pairs (sp3d2), as in \( \ce{SF6} \).
NCERT (2026-27 edition):
Structure of Amines
