Question

Which of the following is a major product obtained in the reaction of isobutylene with hydrogen bromide?

• A. Iso butyl bromide

Answer 1

Major product: 2‑bromo‑2‑methylpropane (tert‑butyl bromide).

Answer 2

Isobutylene is 2‐methylpropene,

$$(CH_3)_2C{=}CH_2.$$

When HBr adds across the double bond according to Markovnikov’s rule, the proton attaches to the terminal carbon and the bromide attaches to the more substituted carbon. This gives:

$$(CH_3)_3C{-}Br,$$

which is 2‐bromo‑2‑methylpropane (tert‑butyl bromide). Note that “isobutyl bromide” would be the product of anti‑Markovnikov addition (or a rearranged product) but is not the major product under normal conditions.