$-\mathrm{OH}$ (hydroxyl groups).
$C_{n} H_{2 n+2} (Alkanes) \xrightarrow[{ + OH}]{{ - H}} C_{n} H_{2 n+1} O H$ or $C_{n} H_{2 n+2} O$



$\begin{array}{*{20}{c}}
{C{H_2} - C = CH - C - N{H_2}} \\
{|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,||\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \\
{N{H_2}\,\,\,\,\,\,\,\,\,\,OC{H_3}\,\,\,\,\,\,\,\,\,\,\,\,\,\,O\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}
\end{array}$
