Electrophilic aromatic substitution is a major reaction pattern of benzene and substituted aromatic rings. This cheat sheet helps students predict products, choose reagents, and explain why aromatic rings react by substitution instead of addition. It is especially useful for organizing nitration, halogenation, sulfonation, and Friedel-Crafts reactions in one reference.
Students also need it to compare activating and deactivating groups when predicting major products.
The core idea is that an aromatic ring attacks an electrophile, forms a resonance-stabilized arenium ion, and then loses to restore aromaticity. Substituents change the ring’s reactivity and direct new groups to ortho, meta, or para positions. Electron-donating groups usually activate the ring and direct ortho and para, while most electron-withdrawing groups deactivate the ring and direct meta.
Important exceptions include halogens, which deactivate but direct ortho and para.
Key Facts
- The general electrophilic aromatic substitution pattern is .
- The rate-determining step is usually formation of the arenium ion, also called the sigma complex, because aromaticity is temporarily lost.
- Nitration of benzene uses and to generate the electrophile .
- Halogenation uses or to generate a stronger electrophilic halogen source.
- Sulfonation uses in to add the sulfonic acid group, giving .
- Friedel-Crafts alkylation commonly uses and to form , but carbocation rearrangements can occur.
- Friedel-Crafts acylation uses and to form and usually avoids rearrangement.
- Strong activating groups such as , , , and are generally ortho-para directors, while strong deactivating groups such as , , , , , , and are meta directors.
Vocabulary
- Electrophilic aromatic substitution
- A reaction in which an aromatic ring replaces a hydrogen atom with an electrophile while regaining aromatic stability.
- Electrophile
- An electron-poor species, written generally as , that accepts electron density from the aromatic ring.
- Arenium ion
- A resonance-stabilized carbocation intermediate formed when an aromatic ring bonds to an electrophile and temporarily loses aromaticity.
- Activating group
- A substituent that increases the reaction rate of an aromatic ring by donating electron density to the ring.
- Deactivating group
- A substituent that decreases the reaction rate of an aromatic ring by withdrawing electron density from the ring.
- Ortho, meta, and para
- The relative positions on a disubstituted benzene ring, where ortho is , meta is , and para is .
Common Mistakes to Avoid
- Forgetting that aromaticity must be restored is wrong because the final step removes to reform the aromatic ring.
- Treating all deactivating groups as meta directors is wrong because halogens are deactivating but direct incoming electrophiles to ortho and para positions.
- Predicting only one product for monosubstituted benzene is often wrong because ortho and para directors can form both ortho and para products, with para often favored by sterics.
- Using Friedel-Crafts reactions on strongly deactivated rings is wrong because groups like make the ring too unreactive for normal Friedel-Crafts alkylation or acylation.
- Ignoring rearrangements in Friedel-Crafts alkylation is wrong because carbocation-like intermediates can shift to form more stable carbocations before substitution.
Practice Questions
- 1 Predict the major product when benzene reacts with and .
- 2 Toluene reacts with . Identify the major directing positions for bromination and name the two main products.
- 3 Nitrobenzene reacts with . Predict whether the major product is ortho, meta, or para chloronitrobenzene.
- 4 Explain why benzene usually undergoes substitution with instead of addition across the ring.