1. Reaction Orientation (ortho/meta/para)
Examine mechanism of the substitution reaction. For benzene + Br2 :
Intermediate: in the delocalized carbocation intermediate, charge is always on ortho and para carbons
The site of reaction will depend on the effect the substituent has on the carbocation intermediate.

2. Alkyl Groups: Ortho/Para Directors
Alkyl groups direct substitution to positions ortho and para to themselves. (They also are activators.)
meta not stabilized

3. -OH and -NH2 Groups: ortho/para
Alkoxyl, and amino groups have a strong, electron-donating resonance effect. They are strong activators.
Direct substitution to the ortho/para positions.
meta not stabilized

4. Activating Substituents – o/p directors
For activating sustituents, cation intermediates formed from ortho or para attack are stabilized, so reaction there is favored.
Meta attack gives a delocalized carbocation without any special stabilization. x

5. Meta Directing Deactivators - Carbonyl
meta - no problem

6. Deactivating Substituents – meta directors
For deactivating sustituents, cation intermediates formed from ortho or para attack are destabilized.
Meta attack gives a delocalized carbocation without any special destabilization, so meta is favored. x

7. Deactivating Substituents – meta directors
Ortho/para attack is bad, meta attack is no problem.
Deactivating substituents such as carbonyl or -CF3 are therefore generally meta directors.
reaction sites x

8. Halogens: Contradictory Effects
A halogen substituent -X (-F, -Cl, -Br, -I) will withdraw electron density via an inductive effect due to high electronegativity.
deactivates ring by induction, slows reaction rate
Halogens can also donate electron density via a resonance effect of the lone pair of electrons.
resonance suggests halogens may be ortho-para directors

9. Substituent Effects in Aromatic Rings
Electron donating goups increase rate of reaction.
Electron withdrawing groups decrease rate of reaction.
Due to inductive effects (based on electronegativity), halogens make aromatic rings slightly less reactive.
least reactive
most reactive
But resonance effects cause halogens to be ortho/para directors.

10. Halogens: o/p directing deactivators
Electron-withdrawing inductive effect controls reactivity, and makes halogens deactivators.
“Induction controls rate of reaction”
Resonance effect controls the orientation of reaction, makes halogens ortho/para directors.
“Resonance controls position of reaction”

11. Halogens: o/p Directing Deactivators

12. Summary: Effect of Substituents
All activating groups (make reactions faster) are ortho/para directors.
Deactivating groups (slow the reaction rate) are meta directors.
Exception: Halogens are deactivating groups and ortho/para directors.