Oxidation of Alkyl Side Chains

Alkyl side chains are activated towards oxidation at the benzylic position, because free radicals are stabilized at that position through resonance.  When heated with aqueous potassium permanganate (KMnO₄) under acidic conditions, alkylbenzenes are oxidized to benzoic acids, as long as the alkyl group contains at least one hydrogen at the benzylic position.  This means that tert-butylbenzene (shown below) does not react.  Aryl methyl ketones (acetophenones), made by the Friedel-Crafts reaction, can also be oxidized to benzoic acids using hot aqueous permanganate.

Bromination of the benzylic Carbon

The benzylic C-H bonds weaker than most sp³ hybridized C-H. 

Because of the weak C-H bonds, benzylic hydrogens can form benzylic halides under radical conditions.

NBS as a Bromine Source

NBS (N-bromosuccinimide) is the most commonly used reagent to produce low concentrations of bromine. When suspended in tetrachloride (CCl₄), NBS reacts with trace amounts of HBr to produce a low enough concentration of bromine to facilitate the benzylic bromination reaction via a radical process.