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NEET CHEMISTRYHydrocarbonsMedium

Question

2,3-dimethyl-2-butene can be prepared by heating which of the following compounds with a strong acid?

A

(CH3)2CH−CH(CH3)−CH=CH2(CH_3)_2CH-CH(CH_3)-CH=CH_2

B

(CH3)3C−CH=CH2(CH_3)_3C-CH=CH_2

C

(CH3)2C=CH−CH2−CH3(CH_3)_2C=CH-CH_2-CH_3

D

(CH3)2CH−CH2−CH=CH2(CH_3)_2CH-CH_2-CH=CH_2

Step-by-Step Solution

  1. Reaction Type: The reaction involves the treatment of an alkene with a strong acid, leading to the formation of a carbocation intermediate, followed by rearrangement and elimination to form a more stable alkene (isomerisation).
  2. Mechanism for Option B ((CH3)3C−CH=CH2(CH_3)_3C-CH=CH_2):
  • Protonation: The acid (H+H^+) adds to the double bond according to Markovnikov's rule to form a secondary carbocation: (CH3)3C−CH=CH2+H+→(CH3)3C−C+H−CH3(CH_3)_3C-CH=CH_2 + H^+ \rightarrow (CH_3)_3C-C^+H-CH_3
  • Rearrangement: The secondary carbocation undergoes a 1,2-methyl shift to form a more stable tertiary carbocation: (CH3)3C−C+H−CH3→1,2-Me shift(CH3)2C+−CH(CH3)2(CH_3)_3C-C^+H-CH_3 \xrightarrow{\text{1,2-Me shift}} (CH_3)_2C^+-CH(CH_3)_2
  • Elimination: Loss of a proton (H+H^+) from the adjacent carbons yields the most substituted alkene (Zaitsev product). Removing a proton from the adjacent secondary carbons gives tetrasubstituted alkene: (CH3)2C+−CH(CH3)2→−H+(CH3)2C=C(CH3)2(CH_3)_2C^+-CH(CH_3)_2 \xrightarrow{-H^+} (CH_3)_2C=C(CH_3)_2
  1. Product: The final product is 2,3-dimethyl-2-butene.
  2. Other Options: The other options do not yield the specific carbon skeleton of 2,3-dimethyl-2-butene through simple acid-catalyzed rearrangement.

Exam Context & Concepts Covered

This question aligns with the NEET CHEMISTRY syllabus, specifically targeting concepts from Hydrocarbons. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

CHEMISTRYHydrocarbonsdimethylbutenepreparedheatingfollowingcompounds

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