The chemical reaction starts with 1,2-dichloroethane $ClCH2-CH2Cl$ which undergoes dehydrohalogenation to form ethylene $CH2=CH2$. Ethylene then reacts with benzene $C6H6$ in a substitution reaction to form ethylbenzene $C6H5CH2-CH3$. Ethylbenzene is then oxidized to form benzoic acid $C6H5COOH$ and finally brominated to form 4-bromobenzoic acid $C6H4BrCOOH$.