A force $\overrightarrow{F}$ = $\alpha$ $\hat{l}$ + 3$\hat{j}$ + 6$\hat{k}$ is acting at a point $\overrightarrow{r}$ = 2$\hat{l}$ − 6$\hat{j}$ − 12$\hat{k}$. The value of $\alpha$ for which angular momentum about origin is conserved is: