Question

Identify the major product of dehydrohalogenation of \(\text{CH}_3\text{-CH}_2\text{-CH(Br)-CH}_3\).

• A. \(\text{CH}_3\text{-CH}_2\text{-CH=CH}_2\)
• B. \(\text{H}_3\text{C-CH=CH-CH}_3\)
• C. \(\text{CH}_3\text{-(CH}_2)_2\text{CH}_3\)
• D. \(\text{CH}_3\text{-(CH}_2)_2\text{CHO}\)

Hint:

By Saytzeff's rule the more substituted alkene, but-2-ene, is the major product.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Dehydrohalogenation removes H and Br from adjacent carbons to give an alkene. When two alkenes are possible, the more substituted (more stable) alkene is the major product. This is Saytzeff's rule.

Step 2: Find the possible products:
2-Bromobutane, \(\text{CH}_3\text{CH}_2\text{CH(Br)CH}_3\), can lose H from C-1 to give but-1-ene (\(\text{CH}_3\text{CH}_2\text{CH=CH}_2\)), or from C-3 to give but-2-ene (\(\text{CH}_3\text{CH=CHCH}_3\)).

Step 3: Pick the major product:
But-2-ene has two alkyl groups on the double bond (disubstituted), while but-1-ene has only one (monosubstituted). So but-2-ene is more stable and is the major product.

Step 4: Why the other options are wrong.
Option A is the minor product. Option C, butane, has no double bond and would need reduction. Option D, butanal, has an aldehyde group and would need an oxidation, not an elimination.

Final Answer:
But-2-ene is the major product. \[ \boxed{\text{(B) }\text{H}_3\text{C-CH=CH-CH}_3} \]