Question

What happens when bromine reacts with CH3-C=CH?How would you justify this reaction?

Answer 1

At room temperature, bromine reacts with an electrophilic addition mechanism to form 1,2 dibromopropane. However, at elevated temperatures, propene reacts with bromine to form 3-bromopropene. This is a allylic substitution reaction which occurs by a radical halogenation mechanism. Formation of bromine radicals is accelerated at high temperatures thus favouring this reaction at high temperatures. Also, the bond energy of C-H bond is very high and hence substitution reaction is possible only at high temperatures.

CH₃-CH=CH₂ + Br₂ $\stackrel{hightemp}{\to }$ BrCH₂CH=CH₂ (3-bromopropene ) + HBr substitution reaction

CH₃-CH=CH₂ + Br₂ $\to$ $$ CH₃CHBr-CH₂Br (1,2 dibromopropane) electrophilic addition