Propyne is treated first with disiamylborane (a bulky borane) and then with $\mathrm{H}{}_2\mathrm{O}{}_2$ / aqueous $\mathrm{NaOH}$. The main product is

Hydroboration of a terminal alkyne with a bulky borane puts boron on the terminal carbon (anti-Markovnikov, and only one addition). Oxidation gives the enol $\mathrm{CH}{}_3\mathrm{CH}=\mathrm{CHOH}$, which tautomerises to propanal. Mercury(II) hydration would have given propanone.