JEE Advanced Chemistry: General Organic Chemistry questions with solutions

Q1. Which of the following carbanions is the most stable?

1. HC≡C⁻
2. C6H5⁻
3. (CH3)3C-CH2⁻
4. (CH3)2C=CH⁻

Answer: HC≡C⁻

Carbanion stability is governed by the s-character of the carbon bearing the negative charge. HC≡C⁻ is sp hybridized (50% s-character), making it the most stable. C6H5⁻ (sp², 33%) is next, then vinyl-type (CH3)2C=CH⁻ (sp²), and the neopentyl-type carbanion (CH3)3C-CH2⁻ (sp³, 25%) is least stable.

Q2. Match each group attached to a phenyl ring in Column-I with the electronic effect(s) it shows in Column-II. Column-I (group on phenyl ring): (A) -N=O (B) -CH3 (C) -NH-C(=O)CH3 (D) -C(=O)OCH3 Column-II (effect shown): (P) +M (Q) -M (R) +H (hyperconjugation) (S) -I

1. A -> Q,S; B -> R; C -> P,S; D -> Q,S
2. A -> P,S; B -> R; C -> Q,S; D -> P,S
3. A -> Q,R; B -> P; C -> P,R; D -> Q,S
4. A -> P,R; B -> S; C -> Q,R; D -> P,R

Answer: A -> Q,S; B -> R; C -> P,S; D -> Q,S

-N=O (nitroso) is conjugated through N=O so it withdraws by resonance (-M) and is electronegative (-I). -CH3 donates by hyperconjugation (+H). -NH-C(=O)CH3 is bonded through N which has a lone pair to donate (+M) while the electronegative N gives -I. -C(=O)OCH3 is bonded through a carbonyl carbon so it withdraws by resonance (-M) and inductively (-I).

Q3. Arrange the C-N bond length (involving the amino nitrogen) in the following species in increasing order: (I) H2N-CH=CH-C(=O)-CH3, (II) H2N-CH=CH-CH=CH2, (III) H2N-CH=CH-CH2-CH3.

1. I < II < III
2. III < II < I
3. II < I < III
4. I < III < II

Answer: I < II < III

The nitrogen lone pair conjugates into the adjacent pi system. In (I) the lone pair is delocalized all the way onto the electron-withdrawing carbonyl, giving the C-N bond the most double-bond character and the shortest length. In (II) it delocalizes into an extended diene but with no strong acceptor, giving intermediate shortening. In (III) beyond the single C=C there is only a saturated chain, so the least delocalization and the longest C-N bond. Increasing bond length: I < II < III.