Most of the electrophilic reactions are irreversible, but not sulphona

1.	Most of the electrophilic reactions are irreversible, but not sulphonation . Why?
Answer» Solution :Most electrophilic subsitution reactions are irreversible but sulphonation is an exception. Treatment of benzene with "oleum" (a solution of `SO_(3)` in concentrated sulphuric acid) will give the sulphonic acid, the electrophilic species being sulphur trioxide which is Lewis acid. The sulphonic acid can be converted back by treatment with hot aqueouus acid. The reason for this reversibility is the fact that the Wheland intermediate is OVERALL neutral and therefore more STABLE than other, positively charged intermediates. Hence under forcing reaction conditions, the energy difference is PROGRESSING in either the forward or backward Wheland intermediate direction for the formation of This makes the `SO_(3)H` a useful directing group if it is desired to carry our selective ortho- substitution of a monosubstiuted benzene possessing an ortholpara- activating group. Under normal circumstances, para - substitution would dominate, despite the statistical favouring of the ortho- positions due to steric HINDRANCE of the original SUBSTITUENTS. Intial sulphonation of para- gives a Disubstituted benzene in which both substituents direct to the same position.Subsequently directed electrophilic substitution and removal of the sulphonic acid group gives the ortho-disubstituted product.

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