Question
When 2-methylbutan-2-ol (a tertiary alcohol) is treated with concentrated at K, what is the major product? Draw the mechanism and identify the rate-determining step.
Solution — Step by Step
Tertiary alcohol + conc. + heat → acid-catalysed dehydration via E1 mechanism. The product is an alkene (Saytzeff rule: most substituted alkene predominates).
The hydroxyl oxygen is protonated by , forming a good leaving group ().
This step is fast and reversible.
Water leaves, generating a tertiary carbocation:
This is the slow, rate-determining step. Tertiary carbocation is stabilised by hyperconjugation and inductive donation from three alkyl groups.
A neighbouring β-hydrogen is removed by water (or HSO₄⁻). Two β-positions exist: the methyls vs the ethyl-CH₂. Saytzeff rule: lose H from the more substituted side to give the more substituted alkene.
Major product: 2-methylbut-2-ene, , a trisubstituted alkene.
Minor product: 2-methylbut-1-ene, — disubstituted.
Major product: 2-methylbut-2-ene. RDS: ionisation of protonated alcohol to carbocation.
Why This Works
E1 dehydration of a tertiary alcohol proceeds through a carbocation intermediate because the tertiary cation is well-stabilised. Tertiary alcohols dehydrate readily under mild acid conditions; primary alcohols need much stronger acid and higher temperature (and even then proceed via E2).
Saytzeff’s rule arises from thermodynamics: more substituted alkenes are more stable (hyperconjugation), so the lower-energy transition state to form them dominates. This is in contrast to Hofmann (least substituted alkene) which appears in special bases.
Alternative Method
For tertiary alcohols, you can predict the product without writing the full mechanism: identify all β-hydrogens, count substituents on the resulting double bond, pick the most substituted. For exam speed, this shortcut works.
Common Mistake
Students predict the less substituted product (2-methylbut-1-ene) — confusing Saytzeff with Hofmann. Saytzeff applies to E1 with a small base; Hofmann applies when base is bulky (like -butoxide) or when the leaving group is bulky (like a quaternary ammonium). Acid-catalysed dehydration always follows Saytzeff.