Reactivity of Benzylic Centers
Benzylic positions are endowed with special reactivity, as in oxidation, free radical halogenation, or hydrogenolysis. As a practical example, in the presence of suitable catalysts, p-xylene oxidizes exclusively at the benzylic positions to give terephthalic acid:
- CH3C6H4CH3 + 3 O2 → HO2CC6H4CO2H + 2 H2O
Millions of tonnes of terephthalic acid are produced annually by this method.
The enhanced reactivity of benzylic positions is attributed to the low bond dissociation energy for benzylic C-H bonds. Specifically, the bond C6H5CH2-H is about 10-15% weaker than other kinds of C-H bonds. The neighboring aromatic ring stabilizes benzyl radicals.
Read more about this topic: Benzyl
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