Electrophilic Aromatic Substitution – Directing Effects

What determines ortho/para vs meta direction in EAS?

The electron-donating or electron-withdrawing ability of the substituent already on the ring.

What do electron-withdrawing groups (EWGs) do to benzene reactivity?

They deactivate the ring (make it less reactive toward electrophiles).

What do electron-donating groups (EDGs) do to benzene reactivity?

They activate the ring (make it more reactive toward electrophiles).

Strong deactivators direct where?

Meta

NO2 (Nitro) directing effect

Strongly deactivating, meta-directing

Inductive effect: Strong withdrawing

Resonance effect: Strong withdrawing

SO3H (sulfonic acid) directing effect

Strongly deactivating, meta-directing

NR3+ (tertiary amine) directing effect

Strongly deactivating, meta-directing

CN (nitrile) directing effect

Strongly deactivating, meta-directing

Inductive effect: Strong withdrawing

Resonance effect: Strong withdrawing

COOH (carboxylic acid) directing effect

Strongly deactivating, meta-directing

Inductive effect: Strong withdrawing

Resonance effect: Strong withdrawing

CHO (aldehyde) directing effect

Strongly deactivating, meta-directing

Inductive effect: Strong withdrawing

Resonance effect: Strong withdrawing

COR (acyl group) directing effect

Strongly deactivating, meta-directing

Inductive effect: Strong withdrawing

Resonance effect: Strong withdrawing

COCH3 (acetyl) directing effect

Strongly deactivating, meta-directing

Inductive effect: Strong withdrawing

Resonance effect: Strong withdrawing

Why are carbonyl-containing groups meta directors?

They withdraw electron density by resonance and induction, destabilizing ortho/para carbocation intermediates.

Halogens directing effect

Deactivating but ortho/para-directing

Why are halogens ortho/para directors even though they are deactivating?

They withdraw by induction but donate by resonance.

F directing effect

Weakly deactivating, ortho/para-directing

Inductive effect: Strong withdrawing

Resonance effect: Weak donating

Cl directing effect

Weakly deactivating, ortho/para-directing

Inductive effect: Strong withdrawing

Resonance effect: Weak donating

Br directing effect

Weakly deactivating, ortho/para-directing

Inductive effect: Strong withdrawing

Resonance effect: Weak donating

I directing effect

Weakly deactivating, ortho/para-directing

Inductive effect: Strong withdrawing

Resonance effect: Weak donating

Hydrogen substituent effect

Neutral (benzene baseline reactivity)

Alkyl (CH3) directing effect

Weakly activating, ortho/para-directing

Inductive effect: Weak donating

Resonance effect: n/a

Why are alkyl groups activating?

Hyperconjugation and weak electron donation stabilize the carbocation intermediate.

OCH3 directing effect

Strongly activating, ortho/para-directing

NH2 directing effect

Strongly activating, ortho/para-directing

Inductive effect: Weak withdrawing

Resonance effect: Strong donating

OH directing effect

Strongly activating, ortho/para-directing

Inductive effect: Weak withdrawing

Resonance effect: Strong donating

NHCOR (amide) directing effect

Moderately activating, ortho/para-directing

Why are lone-pair donors strongly activating?

They donate electron density by resonance into the ring.

Order of reactivity (least to most reactive)

Strong deactivators < Halogens < Benzene < Alkyl < Amide < OH < OCH3 < NH2

Which groups are meta directors?

Strong electron-withdrawing groups (NO2, SO3H, CN, carbonyls, NR3+)

Which groups are ortho/para directors?

Electron-donating groups and halogens

What does EAS stand for?

Electrophilic Aromatic Substitution

What does NAS stand for?

Nucleophilic Aromatic Substitution

What attacks the ring in EAS?

An electrophile (E+)

What attacks the ring in NAS?

A nucleophile (Nu−)

Which intermediate forms in EAS?

Carbocation (arenium ion)

Which intermediate forms in NAS?

Carbanion (Meisenheimer complex)

What stabilizes EAS intermediates?

Electron-donating groups (EDGs)

What stabilizes NAS intermediates?

Electron-withdrawing groups (EWGs)

Which groups activate EAS?

Electron-donating groups

Which groups activate NAS?

Electron-withdrawing groups

Which groups deactivate EAS but activate NAS?

Nitro, carbonyl, cyano, and other EWGs

Why do EWGs help NAS?

They stabilize the negative charge of the carbanion intermediate

Why do EDGs help EAS?

They stabilize the positive charge of the carbocation intermediate

Directing effect of EWGs in EAS

Meta-directing

Directing effect of EWGs in NAS

Ortho/para-directing

Directing effect of EDGs in EAS

Ortho/para-directing

Do EDGs help NAS?

No, they usually hinder NAS

What gets replaced in EAS?

Hydrogen on the ring

What gets replaced in NAS?

A leaving group (usually halide)

Which reaction needs a leaving group on the ring?

Nucleophilic aromatic substitution

Does EAS require a leaving group?

No, it replaces H

Main goal of EAS

Add a substituent to benzene

Main goal of NAS

Replace a halogen with a nucleophile

Classic example of NAS substrate

Nitro-substituted aryl halide

Why are nitro groups important in NAS?

They strongly activate the ring toward nucleophilic attack

One-word difference: EAS vs NAS

Electrophile vs Nucleophile

Quick rule: EDG helps what?

EAS

Quick rule: EWG helps what?

NAS