ortho/para-director
/ OR-thoh PAIR-uh /
When a benzene ring already carries one substituent and you run a second EAS reaction, the new group does not land randomly. The existing group decides where its new neighbor goes. A group that sends the incoming electrophile to the positions next to itself (ortho) and directly across the ring from itself (para) is called an ortho/para-director. The two ortho and one para positions are favored; the meta positions are largely avoided.
Why these positions? The answer lives in the arenium ion. When the electrophile attacks ortho or para to an ortho/para-directing group, one of the resonance structures of the resulting cation places the positive charge on the very carbon that bears the substituent. If that substituent has a lone pair (like -OH, -NH2, -OR, or a halogen), it can then donate into that positive carbon, creating an extra, especially stable resonance structure that fully satisfies every atom's octet. Attack at meta gives no such bonus. The more stable intermediate forms faster, so ortho/para products dominate. Alkyl groups direct ortho/para too, because they best stabilize the cation when the charge sits adjacent to them.
The practical payoff is enormous: ortho/para-directors include the strong activators (-OH, -NH2, -OR), the weak activators (alkyl, aryl), and — the famous exception — the deactivating halogens. So a director's name tells you position, while activating-versus-deactivating tells you rate; you need both to plan a synthesis. A common real-world wrinkle is that the para product often outnumbers the ortho simply because the ortho position is crowded by the existing group (steric hindrance).
Toluene (methylbenzene) nitrates to give mainly ortho- and para-nitrotoluene, with very little meta. The methyl group is a weak activator and an ortho/para-director, so the second group lands next to it or across from it.
The group already on the ring picks the spot — ortho and para, not meta.
Directing is about which arenium ion is more stable, not a memorized list. The decisive clue is whether the existing group has a lone pair or pi system that can donate into the positively charged intermediate when attack happens ortho or para — if so, it directs there. This is why even the rate-slowing halogens still direct ortho/para.