1,1-Diphenylethane-1,2-diol, $(\mathrm{C}{}_6\mathrm{H}{}_5){}_2\mathrm{C}(\mathrm{OH})-\mathrm{CH}{}_2\mathrm{OH}$, is treated with dilute $\mathrm{H}{}_2\mathrm{SO}{}_4$. The rearranged product is

The OH on the diphenyl carbon is lost, giving the highly stabilised $(\mathrm{C}{}_6\mathrm{H}{}_5){}_2\mathrm{C}{}^{+}-\mathrm{CH}{}_2\mathrm{OH}$. The only migrating group on the neighbouring carbon is hydrogen, so H shifts and the product is $(\mathrm{C}{}_6\mathrm{H}{}_5){}_2\mathrm{CH}-\mathrm{CHO}$, 2,2-diphenylethanal.