. Provide a detailed mechanism for the mononitration of hydroxybenzene
(phenol) using a mixture of conc. HNO3 and conc. H2SO4, paying particular
attention to the regioselectivity of the reaction. All possible intermediates must be
shown. Explain your reasoning
Answers
The remarkable stability of the unsaturated hydrocarbon benzene has been discussed in an earlier chapter. The chemical reactivity of benzene contrasts with that of the alkenes in that substitution reactions occur in preference to addition reactions, as illustrated in the following diagram (some comparable reactions of cyclohexene are shown in the green box).
A demonstration of bromine substitution and addition reactions is helpful at this point. A virtual demonstration may be initiated by clicking here.
Many other substitution reactions of benzene have been observed, the five most useful are listed below (chlorination and bromination are the most common halogenation reactions). Since the reagents and conditions employed in these reactions are electrophilic, these reactions are commonly referred to as Electrophilic Aromatic Substitution. The catalysts and co-reagents serve to generate the strong electrophilic species needed to effect the initial step of the substitution. The specific electrophile believed to function in each type of reaction is listed in the right hand column.