JEE Main · 2005 · 08 May 2005 · Offline · Hydrocarbons

Adding one $\mathrm{HBr}$ to 1,3-butadiene can give either the 1,2-product (3-bromobut-1-ene) or the 1,4-product (1-bromobut-2-ene), and temperature decides which wins. Low temperature favours the faster-forming 1,2-product under kinetic control, but at a warmer $40^{\circ }$C the addition becomes reversible and the system settles into the more stable product under thermodynamic control. That more stable product is 1-bromo-2-butene, whose internal double bond is sturdier than the terminal one, so it predominates under thermodynamically controlled conditions.