Ques. 1AIPMT 2015
2,3-Dimethyl-2-butene can be prepared by heating which of the following compounds with a strong acid?
- (CH3)3CCH=CH2
- (CH3)2C=CHCH2CH3
- (CH3)2CHCH2CH=CH2
- (CH3)2CH- CH3 |CH-CH=CH2
Answer
(1) (CH3)3CCH=CH2
Solution
2,3-Dimethyl-2-butene is (CH3)2C=C(CH3)2 ( C6H12). On heating with a strong acid an alkene is protonated to a carbocation, which can rearrange to a more stable one and then lose H+.
(CH3)3C-CH=CH2 H+ (CH3)3C- CH-CH3 (2°)
1,2-Methyl shift ⇒ (CH3)2 C-CH(CH3)2 (3°, more stable)
Loss of H+ ⇒ (CH3)2C=C(CH3)2 (most substituted, Saytzeff alkene)
Options (2) and (3) have the 2-methylpentane skeleton (one branch) ⇒ they give 2-methylpent-2-ene; option (4) has seven carbon atoms.
∴ only (CH3)3CCH=CH2 gives 2,3-dimethyl-2-butene.
Answer: (1)