JEE Main 2021 · 27 August · morning · Single correct · +4 / −1
Which of the following is not a correct statement for primary aliphatic amines?
- (a) The intermolecular association in primary amines is less than the intermolecular association in secondary amines.
- (b) Primary amines on treating with nitrous acid solution from corresponding alcohols except mythyl amine.
- (c) Primary amines are less basic than the secondary amines.
- (d) Primary amines can be prepared by the Gabriel phthalimide synthesis.
Answer and solution
Answer (a) The intermolecular association in primary amines is less than the…
Solution
Chemistry ma’am · 3 steps
Count the N–H hydrogens: a primary amine has two, so it hydrogen bonds more, not less.
- Association means H-bonding. Amine molecules stick together through N-H … N hydrogen bonds. Each N–H hydrogen can form one such bond, so the more N–H hydrogens, the stronger the association.
- Compare the two. A primary amine R-NH₂ has two N–H hydrogens, while a secondary amine R₂NH has only one. So association in primary amines is greater, and the order is 1° > 2° > 3°. This is also why primary amines boil higher than isomeric secondary amines.
Option What the chemistry says Statement is (a) Two N–H in 1° against one in 2°, so 1° associates more incorrect (b) RNH₂ + HNO₂ gives an unstable diazonium ion that loses N₂ to give ROH. CH₃NH₂ is the known exception correct (c) In water, the extra alkyl group makes 2° amines more basic than 1° amines correct (d) Gabriel synthesis alkylates phthalimide once and then frees R-NH₂, so it gives only primary amines correct - So. The statement that is not correct is (a). Primary amines associate more than secondary amines, not less.
The idea to keep. Hydrogen bonding in amines follows the number of N–H bonds: 1° > 2° > 3°. Basicity in water is a separate question, and there 2° amines usually come out on top.
Answer from the NTA final answer key. Solution by PrepWiser.