Benzene + Ethanoyl Chloride + AlCl₃: The Product, the Mechanism, and What the Exam Wants
The reaction gives acetophenone. But knowing just the product name is not going to get you marks. You need to explain how AlCl₃ generates the electrophile, why the ring attacks it, and why the product does not react again. That is what this guide walks through.
Short answer: the product is phenylethan-1-one — almost universally called acetophenone. One hydrogen on the benzene ring gets replaced by an ethanoyl group (CH₃CO–), and HCl is released. The reaction type is Friedel-Crafts acylation. Now let us get into how and why, because that is what gets you the marks.
What This Guide Covers
The Product — Name, Formula, Structure
The product is phenylethan-1-one. Common name: acetophenone. Molecular formula: C₈H₈O. Structurally, it is a benzene ring with a –COCH₃ group (an ethanoyl or acetyl group) bonded directly to the ring carbon where a hydrogen used to sit. That hydrogen leaves as HCl. The ring stays intact and aromatic.
Acetophenone is a colourless liquid at room temperature. It has a sweet, slightly floral odour — distinctive in the lab. That is not what your exam asks about, but it is a useful memory anchor for the compound.
The Overall Equation
One mole of benzene reacts with one mole of ethanoyl chloride. The AlCl₃ is a catalyst — it is regenerated at the end of the catalytic cycle and not consumed overall. In practice, you use slightly more than catalytic quantities because AlCl₃ also forms a complex with the carbonyl group of the product.
Anhydrous literally means without water. AlCl₃ reacts with moisture — even humidity in air is enough to partially destroy it. Water reacts with AlCl₃ to form aluminium hydroxide, deactivating its Lewis acid function. That is why this reaction must be set up under completely dry conditions. Exam answer to “why anhydrous?”: water reacts with AlCl₃ to form Al(OH)₃, removing the catalyst before the reaction completes.
What AlCl₃ Actually Does
AlCl₃ is a Lewis acid — it accepts electron pairs. Aluminium in AlCl₃ has only six electrons around it. That empty orbital is what makes it reactive. When AlCl₃ meets ethanoyl chloride, the chlorine lone pair on the C–Cl bond donates into that empty orbital. The C–Cl bond breaks heterolytically, generating the acylium ion [CH₃CO]⁺ and the tetrachloroaluminate anion [AlCl₄]⁻.
[CH₃CO]⁺ Is the Electrophile — Not Ethanoyl Chloride Itself
Many students write that ethanoyl chloride attacks benzene directly. That is not what happens. AlCl₃ generates the acylium ion first. That ion is what attacks the ring. The acylium ion has a very electron-deficient carbon — ideal for targeting the electron-rich benzene π system. Resonance stabilization (CH₃–C≡O⁺ ↔ CH₃–C⁺=O) makes it stable enough to persist before reacting.
Exam tip: If asked to identify the electrophile in Friedel-Crafts acylation, the answer is the acylium ion [CH₃CO]⁺ — not AlCl₃ and not ethanoyl chloride itself.The EAS Mechanism — Step by Step
Friedel-Crafts acylation is a type of electrophilic aromatic substitution (EAS). Every EAS reaction follows the same two-stage pattern: electrophilic addition to form the Wheland intermediate, then elimination to restore aromaticity. Aromaticity is the thermodynamic driver — the ring wants to return to its fully delocalized, stable state.
Electrophile Generation — AlCl₃ Activates the Acyl Chloride
AlCl₃ coordinates with the chlorine on ethanoyl chloride and breaks the C–Cl bond heterolytically, generating [CH₃CO]⁺ and [AlCl₄]⁻. Without this step, ethanoyl chloride is not electrophilic enough to attack benzene on its own.
Electrophilic Attack — The Acylium Ion Attacks the π System
The electron-deficient acylium ion [CH₃CO]⁺ approaches the electron-rich benzene ring. Two electrons from the delocalized π system attack the acylium carbon. A new C–C bond forms between a ring carbon and the acyl carbon.
Wheland Intermediate — Aromaticity Temporarily Lost
The attacked ring carbon is now sp³ hybridized — it has four bonds. The ring is no longer fully aromatic. The positive charge is delocalized around the remaining five carbons. This intermediate is less stable than benzene, driving the next step.
Proton Loss — Aromaticity Restored, HCl and AlCl₃ Released
[AlCl₄]⁻ acts as a base and removes the proton from the sp³ carbon. The full π system is restored, AlCl₃ is regenerated, and HCl is released. The aromatic product — acetophenone — is stable.
The ring does form an addition intermediate (the Wheland intermediate), but the overall reaction is substitution. The reason: aromaticity. The ring would permanently lose its stability if it stayed in the addition state. Ejecting the proton and restoring the π system is the thermodynamic payoff that drives the overall reaction. That is why EAS always ends as substitution, not addition.
Why the Reaction Stops at Monosubstitution
This is one of the most important points in Friedel-Crafts chemistry — and one of the biggest advantages of acylation over alkylation.
The Product Deactivates Itself Toward Further Electrophilic Attack
The acyl group (–COCH₃) is an electron-withdrawing group. Through induction and conjugation, the carbonyl carbon pulls electron density away from the ring. A ring with less electron density is less attractive to electrophiles. So the product is actually harder to acylate than the starting benzene. Under normal conditions, you get clean monosubstitution and nothing else.
Also worth noting: AlCl₃ forms a stable complex with the carbonyl group of acetophenone. This ties up some of the catalyst and further suppresses any second substitution. It is also why you often need a slight excess of AlCl₃ in practice.Acylation vs. Alkylation — Key Differences
Both reactions use AlCl₃ and proceed through EAS. Both attach a carbon group to benzene. But they differ in ways that come up repeatedly in organic chemistry courses.
| Feature | Acylation | Alkylation |
|---|---|---|
| Reagent | Acyl chloride (e.g., CH₃COCl) | Alkyl halide (e.g., CH₃CH₂Cl) |
| Electrophile | Acylium ion [RCO]⁺ | Carbocation [R]⁺ |
| Product group on ring | Ketone (–COR) | Alkyl group (–R) |
| Polysubstitution? | No — acyl group deactivates ring | Yes — alkyl group activates ring |
| Carbocation rearrangements? | No — acylium ion is resonance-stabilized | Yes — carbocations can rearrange |
| Selectivity | High — clean monosubstitution | Lower — mixture of products common |
This selectivity advantage is why acylation is often the first step in a multi-step aromatic synthesis. Acylate to attach a carbon chain cleanly. Then reduce the ketone to a methylene group (Clemmensen or Wolff-Kishner reduction) if you need an alkyl group. Two steps, but far more predictable than direct Friedel-Crafts alkylation.
Conditions and Practical Notes
What the Reaction Requires
- Benzene — as solvent and reactant
- Ethanoyl chloride (acetyl chloride) — the acylating agent
- Anhydrous AlCl₃ — Lewis acid catalyst; must be kept dry throughout
- Dry conditions — drying tube or inert gas atmosphere to exclude moisture
- Reflux or mild warming — temperature conditions depend on the specific substrate
What Comes Out
- Acetophenone — the product; separated by distillation after aqueous workup
- HCl gas — acidic fumes released during the reaction; requires venting or scrubbing
- AlCl₃ complex — the catalyst complexes with the product carbonyl and is released on careful aqueous workup
Students sometimes write that water is released. It is not. The leaving group from the electrophile is Cl⁻ (from [AlCl₄]⁻), and the proton removed from the ring combines with it to form HCl. No oxygen is involved in that step. Water as a by-product would mean a condensation reaction. This is a substitution reaction. The by-product from Friedel-Crafts acylation using an acyl chloride is always HCl.
Exam Mistakes to Avoid
Writing that ethanoyl chloride directly attacks benzene
It does not. AlCl₃ must generate the acylium ion first. The electrophile is [CH₃CO]⁺, not CH₃COCl. Missing this step in a mechanism answer loses marks.
Draw acylium ion formation as your first step
Show CH₃COCl + AlCl₃ → [CH₃CO]⁺ + [AlCl₄]⁻ before the ring gets involved. That is where the mechanism starts.
Stopping the mechanism at the Wheland intermediate
The Wheland intermediate is not the product. If you leave the mechanism there, you have drawn addition, not substitution. The ring must lose a proton to restore aromaticity.
Always show the deprotonation step
Draw [AlCl₄]⁻ removing H⁺ from the Wheland intermediate. Show curly arrows from the C–H bond to form HCl and regenerate AlCl₃. That completes the mechanism.
Calling AlCl₃ a Bronsted acid
AlCl₃ does not donate protons. It is a Lewis acid — an electron pair acceptor. Calling it a Bronsted acid is a factual error that exam markers will catch.
State AlCl₃ is a Lewis acid with an empty orbital
Aluminium has only six electrons — an empty 3p orbital. It accepts the lone pair from the chlorine in C–Cl. That is Lewis acid behaviour. Be explicit about this in your answer.
Writing the product name without checking the IUPAC format
“Phenylethanone” is incomplete. The correct IUPAC name is phenylethan-1-one. If the question asks for the IUPAC name, include the locant.
Know both names and draw the structure
Phenylethan-1-one = acetophenone = C₆H₅COCH₃. You should be able to draw the benzene ring with a C=O and CH₃ attached from any of those names.
Frequently Asked Questions
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Benzene + ethanoyl chloride + anhydrous AlCl₃ gives acetophenone (phenylethan-1-one) + HCl. That is the answer. But the understanding that matters is in the mechanism: AlCl₃ generates the acylium ion, the acylium ion attacks the π system, the Wheland intermediate loses a proton to restore aromaticity, and AlCl₃ is regenerated.
Two features make acylation cleaner than alkylation. First, acylium ions do not rearrange (unlike carbocations in alkylation). Second, the product deactivates the ring, so you get one substitution and it stops. No messy polysubstitution. No rearranged products. That predictability is why Friedel-Crafts acylation is one of the most useful reactions in aromatic synthesis.
When your exam shows benzene + an acyl chloride + AlCl₃, you know: reaction type is EAS / Friedel-Crafts acylation; the electrophile is the acylium ion; the product is an aryl ketone; the by-product is HCl. Draw the mechanism step by step — curly arrows from π system to electrophile, Wheland intermediate, proton removal, ring restored. That sequence, drawn carefully, is what earns full marks.