The reaction of isopropylbenzene with bromine under light is a radical reaction. The light initially reactions with the bromine in the initiation step to form the bromine radicals:
Br₂ + hv → 2Br·
The next step is the first propagation step where a bromine radical reacts with the isopropylbenzene to abstract the hydrogen from the tertiary carbon in the isopropyl group:
CH₃ CH₃
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Br· + Ph--C--H → H-Br + Ph--C·
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CH₃ CH₃
We have now formed a radical in the benzylic position of the isopropylbenzene structure. Now this radical will react with another molecule of bromine in a second propogation step to form the final brominated product:
CH₃ CH₃
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Ph--C· + Br-Br → Ph--C--Br + Br·
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CH₃ CH₃
The bromine radical can terminate in a reaction with another bromine radical or the benzylic radical to give more of the final product. The final product is shown and is called (2-bromopropan-2-yl)benzene.