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JEE Main 2025 Jan 24 Shift 1, Chemistry Q2: Mechanism of Some Important Reactions of Alkenes

JEE Main2025Jan 24, Shift 1Chemistry
Q.

Following are the four molecules "P", "Q", "R" and "S".

Which one among the four molecules will react with H-Br(aq) at the fastest rate ?

  1. A

    S

  2. B

    Q

  3. C

    R

  4. D

    P

Solution

Electrophilic addition of HBr to an alkene proceeds through a carbocation intermediate; the rate is governed by the stability of that cation.

In molecule Q the oxygen is directly bonded to the alkene carbon (a vinyl ether). On protonation, the resulting carbocation is stabilised by powerful resonance donation from the adjacent oxygen lone pair, producing an oxocarbenium-type intermediate.

The cations from P (oxygen separated from the double bond by an sp carbon), R (only hyperconjugation from a methyl) and S (alkyl substitution alone) are markedly less stabilised than the oxygen-conjugated cation from Q.

Q reacts fastest with HBr(aq). Option (2).

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