Electrophilic Aromatic Substitution: Rate and Orientation

The mechanism

1. An electrophile is generated, usually with a catalyst.
2. The ring attacks it, giving a positively charged intermediate in which aromaticity is temporarily lost.
3. A proton is removed, restoring aromaticity.

The first step is rate-determining, because it destroys aromaticity and is therefore the high-energy step. Everything about rate and orientation follows from how a substituent affects the stability of that intermediate.

Rate: activation and deactivation

The intermediate is a cation, so anything that donates electron density stabilises it and speeds the reaction, while anything that withdraws destabilises it and slows the reaction.

Orientation

Drawing the resonance structures of the intermediate for attack at each position shows where the positive charge appears. For ortho and para attack the charge appears at the carbon bearing the substituent; for meta attack it does not.

The halogen exception

Halogens deactivate the ring but still direct ortho and para. Two effects operate in opposite directions:

• Inductive withdrawal is strong, because halogens are electronegative. This reduces electron density everywhere and slows the reaction.
• Resonance donation from a halogen lone pair reaches specifically the ortho and para positions, stabilising the intermediate there.

So induction controls the rate and resonance controls the orientation. It is the standard illustration that the two questions are separate, and being able to explain it is the mark of understanding the topic rather than memorising a table.

Ortho versus para

Both are favoured by the same substituents, so their ratio is decided by sterics and by statistics. There are two ortho positions and one para, which favours ortho statistically, but a bulky substituent hinders the adjacent positions and pushes the product toward para.

Predicting the dominant product for a bulky group is a common question, and the answer is para on steric grounds despite the statistical factor.

Multiple substituents

Where two substituents are present, the more strongly activating one usually controls orientation. Where they direct to the same position, the outcome is clear; where they conflict, the stronger activator wins, and positions between two substituents are usually avoided for steric reasons.

Practical limitations

• Friedel–Crafts reactions fail on strongly deactivated rings, so a nitro-substituted ring cannot be alkylated or acylated.
• Friedel–Crafts alkylation suffers rearrangement, because it proceeds through a carbocation. Acylation followed by reduction avoids this.
• Alkylation can substitute more than once, since the first alkyl group activates the ring toward further attack. Acylation does not, because the acyl group deactivates.

That last contrast is the standard reason for choosing acylation over alkylation, and it appears in synthesis questions constantly.

Frequently asked questions

Why do halogens deactivate but direct ortho and para?

Because inductive withdrawal reduces the rate while resonance donation specifically stabilises the ortho and para intermediates. Different effects control the two properties.

Why does Friedel–Crafts alkylation give polysubstitution?

Because the alkyl group added is activating, so the product reacts faster than the starting material. Acylation avoids this since the acyl group deactivates.

Why is meta direction described as avoiding rather than preferring?

Because a withdrawing group destabilises the ortho and para intermediates specifically. Meta is simply the position least penalised, not one that is actively favoured.

How do I predict the product with two substituents?

The stronger activator controls orientation. Where the two agree the answer is clear; where they conflict, follow the stronger, and avoid the crowded position between them.