Question

The major product formed in the following reaction is

• A.

  

• B.

  

• C.

  

• D.

  

Hint:

Acid protonates the OH to make it a leaving group; the resulting carbocation rearranges (hydride/alkyl shift) to the most stable, most substituted cation before losing a proton, giving the most substituted (fully conjugated, exocyclic isopropylidene) alkene as major product.

Answer 1

The Correct Option is B

Another way to pick the right structure is to work backward from alkene stability instead of forward from the mechanism step by step.

Acid treatment of an alcohol is a standard way to build an alkene by E1 dehydration: protonate the OH, lose water to make a carbocation, then lose an adjacent proton to form the double bond. Since the reaction runs under acidic, reversible conditions, the alkene that ends up as the major product is whichever one is the most stable, not necessarily the first one formed.

1. Option (A): a terminal =CH2 exocyclic double bond, disubstituted overall, the least stable choice.
2. Option (B): an exocyclic isopropylidene double bond fully flanked by ring carbons and the two methyls, a well conjugated, more substituted alkene, the most stable of the four.
3. Option (C): keeps an isopropyl group but also draws in an extra ring double bond, which would need loss of a second molecule of water or an oxidation step, not a simple single dehydration.
4. Option (D): a more substituted-looking ring alkene, but positioned so it is not conjugated with the isopropylidene fragment the way (B) is, the product of elimination toward the wrong side of the rearranged cation.

Acid-catalyzed dehydrations of polycyclic alcohols like this one are well known to proceed with skeletal (Wagner-Meerwein) rearrangement of the intermediate cation before the final proton loss, so the system can reach the most substituted, most stable alkene rather than stopping at whatever alkene sits closest to the original OH position.

Ranking the four candidates by alkene stability puts option (B) on top, which is why it is the major product formed under these acidic, thermodynamically controlled dehydration conditions.