The dehydration of alcohol follows the order:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Tertiary > Secondary > Primary
Acid-catalysed dehydration of an alcohol goes through a carbocation. The alcohol is protonated, water leaves (the slow, rate-determining step) and a proton is lost from the carbocation to give the alkene. The more stable the carbocation, the more easily the alcohol dehydrates. Alkyl groups stabilise a carbocation by their +I effect and hyperconjugation, so tertiary > secondary > primary in carbocation stability.
Tertiary — first. It gives a tertiary carbocation, the most stable one, so it dehydrates under the mildest conditions (20% \( \ce{H3PO4} \), 358 K).
\( \ce{(CH3)3C-OH} \xrightarrow[\text{358 K}]{\text{20\% }\ce{H3PO4}} \ce{(CH3)2C=CH2 + H2O} \)
Secondary — second. A secondary carbocation is less stable, so a stronger acid at a higher temperature is needed (85% \( \ce{H3PO4} \), 440 K).
\( \ce{CH3CH(OH)CH3} \xrightarrow[\text{440 K}]{\text{85\% }\ce{H3PO4}} \ce{CH3-CH=CH2 + H2O} \)
Primary — last. A primary carbocation is the least stable, so ethanol needs concentrated \( \ce{H2SO4} \) at 443 K.
\( \ce{C2H5OH} \xrightarrow[\text{443 K}]{\text{conc. }\ce{H2SO4}} \ce{CH2=CH2 + H2O} \)
Shortcut: Primary alcohols dehydrate least easily, and only Option (3) ends with Primary.
NCERT (2026-27 edition):
Dehydration
