Following are the four molecules "P", "Q", "R" and "S". Which one among the four molecules will react with \(\mathrm{H}-\mathrm{Br}(\mathrm{aq})\) at the fastest rate ?

JEE Main · 2025 · 24 Jan 2025 · Shift 1 · Hydrocarbons

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 ?

(A)S
(B)Q✓ Correct
(C)R
(D)P

Answer: (B)

Solution

Addition of

H B r

to an alkene proceeds through protonation of the double bond to generate a carbocation, which then captures bromide; the rate of this electrophilic addition is set by how stably that carbocation forms.

Among the four alkenes, protonation of Q generates a carbocation stabilised by resonance/conjugation with the adjacent unsaturation, delocalising the positive charge and lowering the energy of the rate-determining transition state far more than the localised, non-conjugated cations available from the other three alkenes.

Q reacts with aqueous

H B r

at the fastest rate.

JEE Main · 2025 · 24 Jan 2025 · Shift 1 · Single correct option

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JEE Main · 2025 · 24 Jan 2025 · Shift 1 · Hydrocarbons

Solution

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