Decreasing order of dehydration of the following alcohols is

Solution

Chemistry ma’am · 3 steps

Acid dehydration goes through a carbocation, so rank them by how stable that carbocation is.

1. What decides the rate. In acid, the -OH is protonated and leaves as water, giving a carbocation (E1). The more stable that carbocation, the faster the alcohol dehydrates.
2. Double bond position. A C=C right next to the cationic carbon (allylic) spreads the positive charge by resonance. A C=C one carbon further away cannot do this.

   Alcohol	Carbocation formed	Why	Rank
   b, both C=C next to C-OH	doubly allylic (pentadienyl)	charge spread over 3 carbons, and losing H⁺ gives benzene, an aromatic product	1
   d, one C=C next to C-OH	allylic	charge spread over 2 carbons by resonance	2
   c, C=C one carbon away	plain secondary	no resonance with the double bond	3
   a, phenol	aryl cation, practically not formed	C-O bond has partial double-bond character, and the cation would sit on an sp² carbon	4

3. So. The order is b > d > c > a, which is option (b).

The idea to keep. For acid dehydration, picture the carbocation. Allylic beats plain secondary, doubly allylic beats allylic, an aromatic product is a big bonus, and phenols do not dehydrate at all.

Answer from the NTA final answer key. Solution by PrepWiser.