The compound shown is subjected to Birch reduction ($\mathrm{Na}$, liquid $\mathrm{NH}{}_3$, ethanol). Which structure is the major product?

The compound is anisole. In the Birch reduction an electron-donating group such as $\mathrm{OCH}{}_3$ stays on an sp2 carbon of the 1,4-cyclohexadiene, giving 1-methoxycyclohexa-1,4-diene. The isomer with the donor on an sp3 carbon is the pattern for electron-withdrawing groups, the conjugated 1,3-diene is not the kinetic product, and cyclohexane is over-reduction.