What Is Benzylic Hydrogen?
When a hydrogen molecule is attached to a carbon molecule on a benzene ring, the hydrogen is called a “benzylic hydrogen.” The benzylic hydrogen is always attached to the carbon molecule on the side chain.
The term “benzylic” is used to describe the position of the first carbon bonded to a benzene or other aromatic ring. The molecule is referred to as a “benzylic carbocation,” and the benzyl-free radical has the formula “C6H5CH2.” The benzyl carbocation has the formula “C6H5CH2+,” and the carbanion has the formula “C6H5CH2-.” The benzylic hydrogens on a benzene ring are activated toward a free radical attack and are useful for discussion of reaction mechanisms. Further, reactions of benzylic halides show enhanced reactivity. The possibility that these observations reflect a general benzylic activation is supported by the susceptibility of alkyl side chains to oxidative degradation. Such oxidations are normally affected by hot acidic pemanganate solutions, but for large scale industrial operations, catalyzed air-oxidations are preferred. If the benzylic position is completely substituted, this oxidative degradation does not occur.
The prefix “benzyl” refers to a C6H5CH2 substituent like benzyl benzoate, benzyl hydrogenate or benzyl chloride. None of these species can be formed in significant amounts under normal conditions, but they are visible reaction mechanisms.