JEE Main · 2021 · 17 Mar 2021 · Shift 2 · General Organic Chemistry

Addition of $\mathrm{HBr}$ to but-1-ene begins with protonation of the double bond. Adding the proton to the terminal carbon leaves the positive charge on C-2 and gives a secondary carbocation; adding it to C-2 leaves the charge on the terminal carbon and gives a primary one. The secondary ion carries alkyl groups on both sides of the charged carbon, so five $\alpha$ hydrogens hyperconjugate with the empty orbital and two chains release electrons inductively, against two such hydrogens and one chain in the primary ion; the secondary ion is therefore the more stable of the two. By Hammond's postulate the transition state leading to the more stable ion resembles it and lies lower in energy, so its activation energy is the smaller and it is reached the faster. Carbocation B is more stable and is formed at the faster rate.