meta-director
/ MEH-tuh /
A meta-director is a substituent that steers the next incoming electrophile to the meta positions of the ring — the carbons two away from itself, neither adjacent nor directly across. If an ortho/para-director welcomes its new neighbor close by, a meta-director pushes it as far away as the substitution pattern allows.
The mechanism is the mirror image of ortho/para-direction, and again the arenium ion tells the story. Meta-directors are electron-withdrawing groups — -NO2, -C(=O)R, -COOH, -SO3H, -C(triple)N, -CF3, -NR3(+) — that pull electron density out of the ring. When the electrophile attacks ortho or para to such a group, one resonance structure of the cation puts the positive charge directly on the ring carbon bearing the electron-withdrawing group. That is the worst possible place for positive charge to sit, because the group is already greedy for electrons; it makes that intermediate badly destabilized. Attack at the meta position avoids ever placing the charge on that carbon, so the meta intermediate, while still not great, is the least bad — and meta product wins.
Notice the tidy correlation: meta-directors are also (with no exceptions) deactivating groups, because the same electron-withdrawal that destabilizes the intermediate also slows the whole reaction. So a meta-director both makes the ring sluggish and sends the new group to meta. This pairing is the flip side of the activator/ortho-para-director pairing, and together the two patterns let you predict almost any disubstituted product at a glance — keeping in mind the halogens, which sit awkwardly outside both clean groupings.
Brominating nitrobenzene gives almost entirely meta-bromonitrobenzene (1-bromo-3-nitrobenzene). The -NO2 group is a meta-director, so the new bromine lands two carbons away, in the meta position.
Electron-withdrawing groups push the next substituent to meta.
Meta is not preferred because anything stabilizes the meta intermediate; it is preferred because the ortho and para intermediates are especially bad (they put the positive charge right on the carbon next to the electron-grabbing group). Meta wins by being the lesser evil. Also: all meta-directors are deactivators, with no exceptions.