3,4-Dichloronitrobenzene (1,2-dichloro-4-nitrobenzene) reacts with one equivalent of sodium methoxide in methanol on warming. The major product is

Only a chlorine ortho or para to the nitro group is activated for SNAr. In 1,2-dichloro-4-nitrobenzene the Cl on C-1 is para to $\mathrm{NO}{}_2$ (C-4), while the Cl on C-2 is meta. Methoxide therefore replaces the C-1 chlorine through a Meisenheimer complex, leaving the meta chlorine untouched.