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Chemistry · JEE Advanced only

Rearrangement Reactions of Alkyl Carbocations (JEE Advanced)

Not in the JEE Main syllabus, but it is in JEE Advanced. Study time: about 3 to 4 hours for concepts and practice problems.

Last updated: 26 October 2026

Direct answer

Carbocations are unstable species that can rearrange themselves to become more stable before reacting further.

Carbocations are unstable species that can rearrange themselves to become more stable before reacting further. In rearrangement reactions, a hydrogen atom or an alkyl group shifts from an adjacent carbon to the positively charged carbon, converting a less stable carbocation into a more stable one. This often leads to unexpected products in substitution and elimination reactions.

Official syllabus

What the JEE Advanced syllabus says

“Rearrangement reactions of alkyl carbocation”

Source: JEE Advanced 2026 Information Bulletin, Annexure-I. Closest JEE Main topic: mechanisms of substitution reactions.

JEE Main covers basic SN1 and SN2 mechanisms but does not test carbocation rearrangements in depth. For Advanced, you must predict when and how rearrangements occur, and identify the actual product formed after the shift.

Key ideas

What you must understand

  • Carbocation stability order: 3° > 2° > 1° > methyl. Rearrangements happen to reach a more stable carbocation.
  • Hydride shift (1,2-H shift): A hydrogen with its bonding electrons moves from an adjacent carbon to the carbocation center.
  • Methyl or alkyl shift (1,2-alkyl shift): An alkyl group migrates with its bonding electrons when no hydride shift can increase stability.
  • Rearrangements are common in SN1 and E1 reactions because a free carbocation intermediate is formed.
  • Ring expansion can occur when a carbocation is adjacent to a small ring, relieving ring strain.
  • Always draw the carbocation intermediate and check if a neighboring carbon has a hydrogen or alkyl group that can migrate to give a more stable cation.
Formulas

Formulas and key facts

  • Stability: 3° carbocation > 2° carbocation > 1° carbocation > CH₃⁺
  • 1,2-Hydride shift: R₂CH–C⁺R₂ → R₂C⁺–CHR₂ (H migrates with 2 electrons)
  • 1,2-Methyl shift: (CH₃)₂C–C⁺H₂ → (CH₃)C⁺–CH(CH₃) (CH₃ migrates)
  • Driving force: ΔG < 0 when product carbocation is more stable
  • Ring expansion: cyclobutyl cation → cyclopentyl cation (relieves strain)
  • Rearrangement rate depends on: stability gain and migration aptitude of the group
Avoid

Common mistakes

  • Forgetting to check for rearrangement and writing the product from the original carbocation directly.
  • Confusing which group migrates: always the group on the carbon adjacent to the positive charge, not on the same carbon.
  • Assuming rearrangement always happens. It only occurs if a more stable carbocation can form.
  • Ignoring ring expansion possibilities when a carbocation is next to a strained ring like cyclopropane or cyclobutane.
Worked example

A JEE Advanced-level example

Question. 3,3-dimethylbutan-2-ol is treated with concentrated HBr. Identify the major product and explain the mechanism.

Show solution
  1. Step 1: Protonation of the alcohol by HBr gives (CH₃)₃C–CH(OH₂⁺)–CH₃, which loses water to form a 2° carbocation at C2: (CH₃)₃C–C⁺H–CH₃.
  2. Step 2: Check for rearrangement. The adjacent carbon (C3) has three methyl groups. A methyl group can migrate from C3 to C2.
  3. Step 3: After 1,2-methyl shift, the carbocation moves to C3, giving a 3° carbocation: (CH₃)₂C⁺–CH(CH₃)–CH₃, which is more stable.
  4. Step 4: Bromide ion attacks the 3° carbocation to form the product.
  5. Step 5: The major product is 2-bromo-2,3-dimethylbutane: (CH₃)₂CBr–CH(CH₃)–CH₃.

Answer: 2-bromo-2,3-dimethylbutane (rearranged product via 1,2-methyl shift from 2° to 3° carbocation)

Solved twice independently; both solutions agree.

FAQs

Frequently asked questions

Is Rearrangement Reactions of Alkyl Carbocations in the JEE Main syllabus?

No. It is in the JEE Advanced 2026 syllabus only. The closest JEE Main topic is mechanisms of substitution reactions.

How long does Rearrangement Reactions of Alkyl Carbocations take to prepare for JEE Advanced?

About 3 to 4 hours for concepts and practice problems, if you already know the related JEE Main topic.

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