Which of the following compounds does not dissolve in conc. $${H_2}S{O_4}$$  even on warming ?

Solution :
$$\eqalign{ & C{H_2} = C{H_2} + {H_2}S{O_4} \to C{H_3}C{H_2}OS{O_3}H \cr & {C_6}{H_6} + {H_2}S{O_4} \to {C_6}{H_5}S{O_3}H + {H_2}O \cr & {C_6}{H_{14}} + {H_2}S{O_4} \to {\text{No reaction}} \cr & {C_6}{H_5}N{H_2} + {H_2}S{O_4} \to {C_6}{H_5}\mathop N\limits^ + {H_3}HSO_4^ - \cr} $$
Only hexane does not dissolve in conc. $${H_2}S{O_4}$$  even on warming.