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JEE Main levelSingle correct optionDifficulty 2 of 3o4261

This primary alcohol is dehydrated only by concentrated sulfuric acid at about 170 degrees C, whereas its tertiary isomer dehydrates in warm dilute acid. What is the best explanation?

  1. AThe tertiary alcohol has more beta-hydrogens and so is a stronger base
  2. BA free primary carbocation is very unstable, so the reaction needs forcing conditions, while the tertiary cation is far more stable
  3. CThe primary alcohol cannot form a double bond because it has too few carbons
  4. DPrimary alcohols do not accept a proton from sulfuric acid

Answer: B

The alcohol is butan-1-ol. Acid-catalysed dehydration needs a carbocation (or a very carbocation-like transition state) after protonation of the OH group. Stability follows tertiary > secondary > primary, so the primary substrate needs far more vigorous conditions than the tertiary alcohol. Protonation itself occurs readily for all alcohols.
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