Bromination occurs on alkene functional groups, but not on alkenes found within aromatic functional group, such as the phenyl ring of cinnamic acid. chemists would describe this difference in reactivity as an example of

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.

the ratio of the conjugate base to the acid required to attain the desired ph.