Organic chemistry study guide · Reaction Mechanism MCAT Tutor
Electrophilic aromatic substitution: directing groups and Friedel–Crafts
Benzene resists addition and instead undergoes electrophilic aromatic substitution (EAS): H is replaced by E while the ring stays aromatic. Substituents already on the ring control where the next group goes — ortho/para directors are usually activating; meta directors are deactivating. Friedel–Crafts reactions install alkyl or acyl groups but fail when the ring is too deactivated.
Written for undergraduate orgo, pre-med, and MCAT organic chemistry review — United States exam-style framing.
Short lesson
EAS mechanism in brief
Electrophile (E⁺) attacks the ring to give a resonance-stabilized σ complex (arenium ion). Loss of H⁺ restores aromaticity.
The slow step is electrophile attack; electron-donating groups speed the step and favor ortho/para products.
Directing and activating effects
Ortho/para directors: alkyl, OH, OR, NR₂, halogens (weakly deactivating but ortho/para).
Meta directors: NO₂, CN, SO₃H, carbonyl-related groups (CF₃, COR, COOH, esters).
When multiple substituents disagree, the stronger activator wins placement — practice multi-step synthesis order.
Friedel–Crafts alkylation and acylation
Alkylation: R–Cl + AlCl₃ generates R⁺ (or equivalent) for EAS. Watch carbocation rearrangement in the alkyl chain.
Acylation: acid chloride gives ketone on the ring; acylium ion does not rearrange like simple alkyl cations — often preferred for clean product.
Friedel–Crafts fails on strongly deactivated rings (e.g., nitrobenzene) and can polyalkylate because product is more activated than benzene.
Common traps on exams
Treating halogens as meta directors
Cl, Br, I are deactivating but ortho/para directors through resonance — a frequent multiple-choice split.
Friedel–Crafts on nitrobenzene
Strongly deactivated rings do not undergo Friedel–Crafts. Plan synthesis order: install alkyl before nitro, or use other routes.
Ignoring rearrangement in FC alkylation
n-propyl chloride can give isopropylbenzene via hydride shift. Acylation avoids that rearrangement.
Wrong order in disubstituted synthesis
Putting a meta director on first blocks ortho/para placement later — retrosynthesis from the target substituent pattern fixes order.
Practice-style prompts
Try these on paper, then check product and mechanism in OrgoMech.
- Nitration of toluene — which positions are favored and why?
- Why does Friedel–Crafts alkylation of benzene often give polyalkylation?
- Starting from benzene, outline steps to meta-bromonitrobenzene (order matters).
- Classify COOH on benzene: activating or deactivating? ortho/para or meta?
- Compare Friedel–Crafts alkylation vs acylation for installing a propyl group without rearrangement.
Student questions
Common questions undergraduates ask about this topic before exams.
- What is electrophilic aromatic substitution (EAS)?
- An electrophile replaces a hydrogen on benzene while the ring stays aromatic. A σ complex (arenium ion) forms, then H⁺ leaves to restore aromaticity.
- What are ortho/para directors?
- Substituents that donate electron density and direct the next group to ortho and para positions — e.g., alkyl, –OH, –OR, –NR₂. Halogens are deactivating but still ortho/para directors.
- What are meta directors?
- Strongly electron-withdrawing groups such as –NO₂, –CN, –SO₃H, and carbonyl-related groups that direct substitution to the meta position.
- Why does Friedel–Crafts fail on nitrobenzene?
- The ring is too deactivated for electrophilic attack under Friedel–Crafts conditions. Plan synthesis order: install activating groups before strong deactivators when possible.
- Does Friedel–Crafts alkylation rearrange?
- Primary alkyl chlorides can rearrange to more stable carbocations (e.g., n-propyl → isopropyl). Acylation avoids that alkyl cation rearrangement.
- Why can Friedel–Crafts polyalkylate?
- The product ring is more activated than benzene, so further alkylation can occur until the ring is deactivated enough to stop.
- How is EAS tested alongside orgo 2 carbonyls?
- Synthesis order problems often combine EAS with acylation, reduction, or side-chain chemistry — map directing effects before drawing a multi-step route.
Further reading (external)
Open textbooks and reference sites for the same topics covered above. We link out to established science education publishers — not affiliate pages.
- Electrophilic aromatic substitution (LibreTexts) — LibreTexts
- OpenStax — Aromaticity and reactions of benzene — OpenStax
- PubChem — NIH NLM
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Drill this topic in OrgoMech
Predict the major product for your structure, then order the curved-arrow mechanism — the same skills tested on homework, finals, and the MCAT orgo section. Predict the major product, then order the curved arrows. Organic chemistry practice for reactions, mechanisms, and the days left before your final.