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

The tertiary hydroxyl on the diphenyl carbon leaves, giving the very stable $\mathrm{Ph}{}_2\mathrm{C}{}^{+}-\mathrm{CH}{}_2\mathrm{OH}$. The only group that can migrate from $\mathrm{CH}{}_2\mathrm{OH}$ is hydrogen, which moves to the cationic carbon to give protonated $\mathrm{Ph}{}_2\mathrm{CH}-\mathrm{CHO}$. The product is diphenylacetaldehyde, an aldehyde.