CH2=CHCH2CH=CH2NBS−−−→ A, A is:
CH2=CHCHCH=CH2 | Br
CH2=CHC=CHCH2Br
CH2=CHCH2CH=CHBr
CH2=CHCH2C=CH2 | Br
N-bromosaccinamide (NBS) follows allylic substitution of bromine to give alkene.