Question

Explain why alcohols do not react with NaBr but when H2SO4 is added they form alkyl bromides

Answer 1

When alcohol is treated with H2SO4, it loses H2O and a double bond appears. And now NaBr reacts with the alkene and forms alkyl bromides, as halogens occupy the place of double bonds.

Answer 2

Alcohols do no react with NaBr because $\bold{B r^{-}}$ is a very weak base and cannot displace the OH in the alcohols.

On the other hand, when $\bold{\mathrm{H}_{2} \mathrm{SO}_{4}}$ which is a strong acid is added, it leads to protonation of alcohols. $B r^{-}$ now can displace $\mathrm{H}_{2} \mathrm{O}$ which is a very weak base.

When $\bold{\mathrm{H}_{2} \mathrm{O}}$ is lost or replaced, double bond appears and NaBr reacts with alkene to form alkyl bromides. The places of the double bonds are occupied by halogens.