Directing Influence of Substituents

Predicting the position of incoming electrophiles in benzene derivatives.

By chemca Team • Updated Jan 2026

When an electrophile attacks a monosubstituted benzene ring, the existing substituent determines two things:
1. Reactivity: Whether the reaction is faster or slower than benzene.
2. Orientation: Where the new group will attach (Ortho, Meta, or Para).

1. Ortho-Para Directors (Activators)

Activating Groups

These groups release electrons into the benzene ring (via +M, +H, or +I effects), making it electron-rich and more reactive towards electrophiles.

Mechanism: Resonance structures show increased electron density specifically at Ortho and Para positions. The incoming electrophile ($E^+$) is naturally attracted to these negative centers.

Examples (in decreasing order of strength):

Strong: $-O^-, -NH_2, -NHR, -OH$
Moderate: $-NHCOCH_3, -OCH_3$
Weak: $-CH_3, -C_2H_5$ (Alkyls)

2. Meta Directors (Deactivators)

Deactivating Groups

These groups withdraw electrons from the ring (via -M or -I effects), making it electron-deficient and less reactive than benzene.

Mechanism: Resonance withdraws electrons primarily from Ortho and Para positions, leaving positive charges there. The Meta position remains relatively electron-rich (least deactivated), so the electrophile attacks there.

Examples:

$-NO_2$ (Strongest), $-CN, -SO_3H, -CHO, -COOH, -COOR$

3. The Halogen Anomaly

Deactivating but Ortho-Para Directing

Halogens ($-F, -Cl, -Br, -I$) exhibit a unique conflict between two effects:

Inductive Effect (-I): Strong electron withdrawal due to high electronegativity. This Deactivates the ring overall.
Resonance Effect (+M): Donation of lone pairs. This increases electron density at Ortho/Para positions relative to meta.

Result: Reactivity is controlled by the stronger -I effect (Deactivating), but Orientation is controlled by the +M effect (Ortho-Para Directing).

4. Summary Table

Group Type	Effect on Rate	Directive Influence	Electronic Effects
Activators	Increases	Ortho, Para	+M, +H, +I
Deactivators	Decreases	Meta	-M, -I
Halogens	Decreases	Ortho, Para	-I (Strong) > +M (Weak)

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