Bromination of Aromatic Rings
Aromatic compounds react with NBS under ionic conditions to undergo bromination in the aromatic ring by substitution.In these reactions, the brominating agent could probably be the protonated NBS. Benzene when treated with NBS and a 1:1 mixture of conc. H2SO4 and water gives bromobenzene in 95% yield. Under these conditions, aromatic compounds having highly branched chains also appears to undergo bromination in the aromatic ring with selectivity.
5.1.2 Addition Reactions
NBS is often used as the source of electrophilic bromine in polar solvents. For an example, cyclohexene reacts with NBS in the presence of tetraethylammonium bromide to give trans -1,2-dibromocyclohexane in good yield (Scheme 6).
Scheme 6
A possible mechanism is represented in Scheme 7.
Scheme 7
In the presence of excess of water, alkenes undergo reaction with NBS to give bromohydrins that could be converted into epoxides in the presence of base (Scheme 8).
Scheme 8
5.1.3 Oxidation
Secondary alcohols undergo oxidation to give ketones in the presence of NBS in water. This method has found wide applications in the oxidation of steroidal alcohols. For an example, cholic acid can be selectively oxidized at C-7 using NBS in the presence of water (Scheme 9).
Scheme 9
In hot aqueous solution, α -hydroxy acids can be oxidized to give aldehydes or ketones with loss of one carbon atom (Scheme 10). For example, glycolic acid, lactic acid and mandelic acid are converted into formaldehyde, acetaldehyde and benzaldehdye. Under these conditions α -amino acids proceed decarboxylation to give aldehydes.
Scheme 10