Electrophilic aromatic substitution is a major reaction pattern of benzene and many aromatic compounds. It matters because it lets chemists add useful groups to stable aromatic rings without destroying the ring system. In these reactions, an electrophile replaces a hydrogen atom on the aromatic ring.
The key idea is that aromaticity is temporarily lost during the reaction, then restored at the end.
The general mechanism has two main steps: attack of the aromatic pi electrons on a strong electrophile, followed by loss of H+ to regain aromaticity. The intermediate is called a sigma complex or arenium ion, and it is stabilized by resonance but is not aromatic. Common examples include nitration, where benzene forms nitrobenzene, and halogenation, where benzene forms chlorobenzene or bromobenzene.
Substituents already on the ring can activate or deactivate the ring and direct new groups to ortho, meta, or para positions.
Key Facts
- General reaction: Ar-H + E+ -> Ar-E + H+
- Step 1: the aromatic ring attacks E+ to form a sigma complex, which is usually the slow, rate-determining step.
- Step 2: a base removes H+ from the sigma complex to restore aromaticity.
- Nitration electrophile: NO2+ is generated from HNO3 and H2SO4.
- Halogenation often requires a Lewis acid catalyst: Br2 + FeBr3 produces an effective Br+ electrophile.
- Activating groups usually direct ortho and para, while strong electron-withdrawing groups usually direct meta.
Vocabulary
- Electrophile
- An electron-poor species that accepts electron density from a nucleophile or pi system.
- Aromaticity
- The special stability of a cyclic, planar, conjugated system with 4n + 2 pi electrons.
- Sigma complex
- A resonance-stabilized carbocation intermediate formed after an aromatic ring bonds to an electrophile.
- Activating group
- A substituent that increases the rate of electrophilic aromatic substitution by donating electron density to the ring.
- Directing effect
- The influence of an existing substituent on where a new electrophile attaches to an aromatic ring.
Common Mistakes to Avoid
- Showing benzene permanently losing aromaticity, which is wrong because the final deprotonation step restores the aromatic pi system.
- Using E+ attack as if benzene were an ordinary alkene addition reaction, which is wrong because aromatic rings substitute H rather than add across a double bond.
- Ignoring directing effects, which is wrong because existing substituents strongly influence whether the next group enters ortho, meta, or para.
- Assuming all deactivating groups are meta directors, which is wrong because halogens deactivate the ring but still direct ortho and para.
Practice Questions
- 1 Write the two main mechanistic steps for nitration of benzene, including the electrophile formed from HNO3 and H2SO4.
- 2 Benzene reacts with Br2 and FeBr3 to form bromobenzene. If 0.50 mol of benzene reacts completely in a 1:1 stoichiometric ratio, how many moles of HBr are produced?
- 3 Toluene reacts with a nitrating mixture. Predict the major ring positions for nitration and explain why methylbenzene gives those products rather than a mostly meta product.