When cinnamic acid react with bromine ,addition reaction rapidly occur on alkene functional group to form dibromo product
Explanation:
Phenyl ring is an aromatic hydrocarbon ,when aromatic hydrocarbons react with Cl2,Br2 or KMnO4 no reaction occur ,where as unsaturated hydrocarbon like alkene react .Aromatic hydrocarbon with these reagents undenr different conditions undergoes subtituition reaction.They react with bromine in presence of lewis acid catalyst ferric bromide.