Write the sulphonation reactions of benzene.

(N/A) Sulphonation of benzene involves the replacement of a hydrogen atom by a sulphonic acid group $(-SO_3H)$.
$1$. When benzene is heated with fuming sulphuric acid ($H_2SO_4 + SO_3$ or oleum) at $353 \ K$,it gives benzene sulphonic acid.
$2$. Further heating of benzene sulphonic acid with oleum at $573 \ K$ leads to the formation of benzene $m$-disulphonic acid.