Question

Which of the following accounts for the anti-Markovnikov regiochemistry of the reaction of an alkene with HBr in the presence of peroxides? A) The alkene reacts with HBr to give a carbocation B) The alkene reacts with a bromine radical to give an alkyl radical C) The alkene reacts with peroxide to give an vinylic radical D) The alkene reacts with peroxide to give an allylic radical

Answer 1

B) The alkene reacts with a bromine radical to give an alkyl radical.

Theory : Anti-markovnikov rule

"Anti-markovnikov's rule means the hydrogen will be add to the carbon atom which has less number of hydrogen atom."

Mechanism of anti- markovnikov rule

— hv R-ö-ö-r Peroxide initiator 2R-ö. Alkoxy radical Alkoxy R-0 + ROH + Br Halide radical radical H-für: - Hydrogen halide -cc- Alkene Halide Br* + radical H Alkyl radical H + Br_c_OCR Alkyl radical H + H- Br: - Hydrogen halide Br C-CH HH Alkyl halide Br Halide radical

Step 1: Generation of free radical through homolytic cleavage of peroxide compound.

Step -2 : Attack of generated free radical on hydrogen halide to form halide radical through homolysis.

Step 3: Attack of generated halide radical on alkene molecule to form alkyl radical through homolysis.

Step 4: Attack of a generated alkyl radical on hydrogen halide to form alkyl halide through homolytic cleavage of hydrogen halide bond.