2-Methylbut-2-ene, $(\mathrm{CH}{}_3){}_2\mathrm{C}=\mathrm{CH}-\mathrm{CH}{}_3$, undergoes hydroboration with $\mathrm{BH}{}_3\cdot \mathrm{THF}$ followed by $\mathrm{H}{}_2\mathrm{O}{}_2/\mathrm{NaOH}$. The product is

Boron adds to the less substituted alkene carbon ($\mathrm{C}{}_3$) and hydrogen to the more substituted carbon ($\mathrm{C}{}_2$). Oxidation replaces B by OH with retention, giving $(\mathrm{CH}{}_3){}_2\mathrm{CH}-\mathrm{CH}(\mathrm{OH})-\mathrm{CH}{}_3$, 3-methylbutan-2-ol. The tertiary alcohol would be the oxymercuration product.