Why is it necessary to maintain excess bromine in the reaction mixture with trans-cinnamic acid and bromine

It is necessary because Trans-cinnamic is a limiting reagent in the mixture reaction while Bromine is the excess reagent

Explanation:

It is necessary to maintain excess bromine in the reaction mixture because Bromine is the excess reagent in the reaction mixture and if it's quantity is less it would consume the limiting reagent ( Trans-cinnamic ) completely . hence Bromine should maintain excess quantity in the reaction mixture