Alcohols - Acidic Dehydration

Acid-catalysed dehydration of alcohols follows an E1 mechanism in which the protonated hydroxyl leaves as water to give a carbocation, and that carbocation then loses a proton to form an alkene. Since carbocation formation is rate-determining, the alcohol giving the most stable cation dehydrates fastest. 2-methyl-2-propanol is tertiary and yields a tertiary carbocation, so it loses water most readily and is the correct choice. 1-butanol and 2-methyl-1-propanol are primary and would require an unstable primary cation, so they react slowly and often only at higher temperatures, eliminating both. 2-butanol is secondary and dehydrates faster than the primary isomers but still slower than the tertiary alcohol, so it is not the best answer. The overall reactivity order tertiary > secondary > primary parallels carbocation stability, exactly as taught in the NCERT treatment of alcohol reactions and as tested repeatedly in JEE. It is also worth noting that the reaction obeys Saytzeff orientation, so when a choice exists the more substituted and thermodynamically more stable alkene predominates, and rearrangement of less stable cations to more stable ones can further accelerate the tertiary pathway. As a consistency check, the same stability ordering that governs the Lucas test also governs dehydration, confirming the tertiary alcohol as fastest.