AIIMS 2005 Chemistry Elimination Reactions MCQ Question

To determine the major product obtained from the reaction of 1-bromo-2-methylbutane (CH₃CH₂CH(Br)CH₃) with methoxide ion (CH₃O⁻) in methanol (CH₃OH), we need to analyze the reaction mechanism involved.

Reaction Overview

The compound CH₃CH₂CH(Br)CH₃ is a secondary alkyl halide. When treated with a strong base like methoxide ion, we can expect either an elimination reaction (E2 or E1) or a substitution reaction (S_N2 or S_N1). However, in this case, the base is strong enough to favor elimination over substitution.

Elimination Reaction Mechanism

1. Base-Induced Elimination: The methoxide ion (CH₃O⁻) abstracts a proton (H⁺) from a β-carbon (the carbon adjacent to the carbon bearing the bromine). This leads to the formation of a double bond and the elimination of bromide ion (Br⁻).

2. Possible β-Hydrogens: In CH₃CH₂CH(Br)CH₃, the β-carbons are:

   • The CH₃ and CH₂ groups.

   The structure can be visualized as follows:

       CH3
        |
   CH3-CH-CH-CH2-Br
        |
       CH2
   

3. Product Formation: The elimination can occur at two sites:

   • Elimination at the CH₂ group results in:
     • Product A: CH₃CH₂CH=CH₂ (but-2-ene).
   • Elimination at the CH₃ group results in:
     • Product B: CH₃CH=CH-CH₃ (but-1-ene).

Since both products are alkenes, we should consider the stability of the alkenes. The more substituted alkene (more stable due to hyperconjugation and the number of alkyl groups attached to the double bond) is favored according to Zaitsev's rule.

Major Product Determination

Among the possible products formed:

• Product B (CH₃CH=CHCH₃) is a more substituted alkene (but-2-ene) compared to Product A (CH₃CH₂CH=CH₂) (but-1-ene).

Final Product and Answer

Thus, the major product obtained from the reaction of CH₃CH₂CH(Br)CH₃ with CH₃O⁻/CH₃OH is:

Correct Answer: B) CH₃CH=CHCH₃ (but-2-ene).

Clarification of Incorrect Options:

• Option A (CH₃CH₂CH(OCH₃)CH₃): This option suggests a substitution product where the bromine is replaced by a methoxy group. While possible, the presence of the strong base and the structure of the substrate favor elimination over substitution.

• Option C (CH₃CH=CHCH₃): This is but-1-ene, which is less substituted and therefore less stable compared to the major product (but-2-ene).

• Option D (CH₃CH₂CH₂CH₂OCH₃): This option implies a long-chain ether formation, which is not a plausible product from the elimination reaction under the given conditions.

Conclusion

The elimination reaction of CH₃CH₂CH(Br)CH₃ with CH₃O⁻/CH₃OH predominantly yields but-2-ene (CH₃CH=CHCH₃) as the major product due to the preference for more substituted alkenes, following Zaitsev's rule.