Organic Chemistry · Chemistry of Benzene: Electrophilic Aromatic Substitution
Alkylation and Acylation of Aromatic Rings: The Friedel–Crafts Reaction
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Bromination, nitration, and sulfonation place non-carbon substituents on benzene. The Friedel–Crafts reactions are the standard EAS routes that build carbon–carbon bonds on the ring. In Friedel–Crafts alkylation EAS reaction replacing a ring H with an alkyl group using an alkyl halide + Lewis acid Full entry →, an alkyl halide reacts with the ring in the presence of a Lewis acid catalyst Electron-pair acceptor (\(\text{AlCl}_3\), \(\text{FeCl}_3\), \(\text{BF}_3\)) that generates the electrophile Full entry → (usually \(\text{AlCl}_3\)) to replace a ring hydrogen with an alkyl group:
\[\text{C}_6\text{H}_6 + \text{RCl} \xrightarrow{\text{AlCl}_3} \text{C}_6\text{H}_5\text{R} + \text{HCl} \]
In Friedel–Crafts acylation EAS reaction installing an acyl group using an acyl chloride + Lewis acid Full entry →, an acyl chloride installs an acyl group (\(\text{RCO}{-}\)), producing an aryl ketone:
\[\text{C}_6\text{H}_6 + \text{RCOCl} \xrightarrow{\text{AlCl}_3} \text{C}_6\text{H}_5\text{COR} + \text{HCl} \]
Both reactions generate a powerful electrophile — a Carbocation Carbon with only 6 valence electrons and a + charge; the alkylation electrophile Full entry → for alkylation, an Acylium ion Resonance-stabilized \(\text{RCO}^+\) cation; the acylation electrophile Full entry → for acylation — that attacks the electron-rich ring. The two are siblings with the same general mechanism but very different behavior: alkylation suffers from rearrangement and over-alkylation, while acylation is clean but consumes the Lewis acid and leaves a ketone you may have to reduce later.
Why this matters
Alkylbenzenes and aryl ketones feed some of the world's largest chemical processes. Ethylbenzene is dehydrogenated to styrene, the monomer of polystyrene; cumene (isopropylbenzene) is oxidized to phenol and acetone; toluene and xylene are solvents and fuel additives. Friedel–Crafts acylation appears in syntheses of pharmaceuticals (ibuprofen and related aryl ketones) and dyes. When a chemist wants an aryl ketone or a substituted alkylbenzene, these reactions are usually the first tool considered — and their limitations (carbocation rearrangement, failure on deactivated rings) are classic exam topics and real planning constraints.
The college version
Core Concepts
The alkylation electrophile: a carbocation
\(\text{AlCl}_3\) is a Lewis acid that coordinates to the halogen and pulls it away as \(\text{AlCl}_4^-\):
\[\text{R}{-}\text{Cl} + \text{AlCl}_3 \longrightarrow \text{R}^+ + \text{AlCl}_4^- \]
The resulting carbocation \(\text{R}^+\) is the electrophile. Because carbocation stability increases with substitution (3° > 2° > 1°), the alkyl group attaches in its most substituted form. Primary halides often give rearranged products because the 1° cation isomerizes through hydride or alkyl shifts before reaching the ring.
The acylation electrophile: an acylium ion
The Lewis acid pulls chloride off the acyl chloride to form an acylium ion, stabilized by resonance — the positive charge is shared with an oxygen lone pair:
\[\text{R}{-}\text{C}(= \text{O})\text{Cl} + \text{AlCl}_3 \longrightarrow \text{R}{-}\text{C} \equiv \text{O}^+ + \text{AlCl}_4^- \]
The acylium ion is much more stable than an ordinary carbocation, so acylium ions do not rearrange, and acylation is slower than alkylation. Acylation also needs more than one equivalent of \(\text{AlCl}_3\) because the ketone product forms a strong complex with the Lewis acid and must be freed by aqueous workup.
The EAS mechanism applied to both
Both reactions follow the standard three-stage EAS pathway: generate the electrophile; let it add to the ring's π system, breaking aromaticity and forming a sigma complex (arenium ion) with the positive charge delocalized over three ring carbons; then lose a proton (to \(\text{AlCl}_4^-\) or another base), restoring the aromatic sextet and giving the product plus HCl. The catalyst is regenerated in alkylation (catalytic amount suffices) but consumed in acylation (stoichiometric amount needed).
Limitations of alkylation vs. advantages of acylation
| Feature | Alkylation | Acylation |
|---|---|---|
| Electrophile | Carbocation \(\text{R}^+\) | Acylium ion \(\text{RCO}^+\) |
| Rearrangement? | Yes — 1°/2° cations rearrange | No — resonance-stabilized ion |
| Over-reaction? | Yes — alkylbenzene is more reactive → polyalkylation | No — ketone product deactivates the ring |
| Catalyst | Catalytic \(\text{AlCl}_3\) | ≥ 1 equiv \(\text{AlCl}_3\) (product complex) |
The standard strategy for an unrearranged straight-chain alkylbenzene is acylate first, then reduce: acylation gives an aryl ketone, and Clemmensen (Zn/Hg, HCl) or Wolff–Kishner (hydrazine, base) reduction converts the ketone to the alkyl group. Rings bearing strong electron-withdrawing groups (\(\text{NO}_2\), \(\text{CN}\), \(\text{SO}_3\text{H}\)) fail both reactions — nitrobenzene is inert.
How It Works / Step-by-Step Process
Predicting the Friedel–Crafts alkylation product
- Identify the alkyl group of the halide and generate the carbocation \(\text{R}^+\).
- Check whether the cation can rearrange to a more stable one (1° → 2° → 3°; hydride and alkyl shifts).
- Attach the most stable available cation to the ring at any open position (all positions are equivalent on unsubstituted benzene).
- Check the ring: if it bears a strong EWG (\(\text{NO}_2\), \(\text{CN}\), \(\text{SO}_3\text{H}\)), no reaction occurs.
Planning an acylation–reduction sequence
- Choose an acyl chloride whose acyl group matches the alkyl group you want.
- Run the acylation with ≥ 1 equiv \(\text{AlCl}_3\); the product is an aryl ketone.
- Reduce the ketone (Clemmensen: Zn/Hg + HCl; Wolff–Kishner: hydrazine + base) to the alkylbenzene.
- No rearrangement is possible anywhere in this sequence.
Common Confusions
| Do Not Confuse | With | The Difference |
|---|---|---|
| Alkylation | Acylation | Alkylation uses a carbocation (rearranges, polyalkylates); acylation uses an acylium ion (no rearrangement, stops after one substitution) |
| "Catalyst" amount of AlCl₃ | Stoichiometric amount | Alkylation regenerates AlCl₃ (catalytic works); acylation's ketone product ties it up (need ≥ 1 equiv) |
| Product of 1-chloropropane + benzene | n-propylbenzene | The 1° cation rearranges to 2°; the product is isopropylbenzene (cumene) |
| Deactivated ring | Inert ring | Rings with strong EWG (\(\text{NO}_2\)) fail Friedel–Crafts entirely; mildly deactivated rings (halogens) still react slowly |
| Acyl group | Alkyl group | Acyl = carbonyl attached to carbon (\(\text{RCO}{-}\)); alkyl = plain carbon chain (\(\text{R}{-}\)) |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Friedel–Crafts alkylation is like parking a car (the alkyl group) in a crowded lot (the benzene ring): the attendant (AlCl₃) makes a key, and the car parks. But a nervous driver's car can change into a different car before parking (rearrangement), and once one car parks, it invites more cars (polyalkylation). Acylation is like a steady delivery truck (the acylium ion) that never changes shape, delivers exactly one package, and locks the gate behind it.
Worked example
Example 1: Benzene + 1-chloropropane + AlCl₃
Removing \(\text{Cl}^-\) from \(\text{CH}_3\text{CH}_2\text{CH}_2\text{Cl}\) gives the 1° carbocation \(\text{CH}_3\text{CH}_2\text{CH}_2^+\). A hydride shift moves \(\text{H}^-\) from C2 to C1, converting it into the more stable 2° cation \(\text{CH}_3\text{CH}^+\text{CH}_3\). The ring attacks this cation, so the product is isopropylbenzene (cumene), not n-propylbenzene:
\[\text{C}_6\text{H}_6 + \text{CH}_3\text{CH}_2\text{CH}_2\text{Cl} \xrightarrow{\text{AlCl}_3} \text{C}_6\text{H}_5\text{CH}(\text{CH}_3)_2 + \text{HCl} \]
Lesson: with a 1° or 2° alkyl halide, expect a rearranged product.
Example 2: How much ethylbenzene can 10.0 g of benzene make?
Balanced reaction: \(\text{C}_6\text{H}_6 + \text{CH}_3\text{CH}_2\text{Cl} \rightarrow \text{C}_6\text{H}_5\text{CH}_2\text{CH}_3 + \text{HCl}\). Molar masses: benzene \(M = 6(12.01) + 6(1.008) = 78.11\) g/mol; ethylbenzene \(M = 8(12.01) + 10(1.008) = 106.17\) g/mol.
Step 1 — moles of benzene. Show the formula, then substitute:
\[\text{mol C}_6\text{H}_6 = \frac{\text{mass}}{M} = \frac{10.0\ \text{g}}{78.11\ \text{g/mol}} = 0.128\ \text{mol} \]
Step 2 — moles of product. The stoichiometry is 1:1:
\[\text{mol C}_6\text{H}_5\text{C}_2\text{H}_5 = 0.128\ \text{mol C}_6\text{H}_6 \times \frac{1\ \text{mol C}_6\text{H}_5\text{C}_2\text{H}_5}{1\ \text{mol C}_6\text{H}_6} = 0.128\ \text{mol} \]
Step 3 — mass of product (dimensional analysis in one chain):
\[ \text{mass C}_6\text{H}_5\text{C}_2\text{H}_5 = 10.0\ \text{g C}_6\text{H}_6 \times \frac{1\ \text{mol C}_6\text{H}_6}{78.11\ \text{g}} \times \frac{1\ \text{mol C}_6\text{H}_5\text{C}_2\text{H}_5}{1\ \text{mol C}_6\text{H}_6} \times \frac{106.17\ \text{g}}{1\ \text{mol C}_6\text{H}_5\text{C}_2\text{H}_5} = 13.6\ \text{g} \]
The theoretical yield is 13.6 g of ethylbenzene; real yields are lower because of polyalkylation and workup losses.
Example 3: Acetyl chloride → acetophenone, and the product–catalyst complex
Acetylation of benzene gives acetophenone:
\[\text{C}_6\text{H}_6 + \text{CH}_3\text{COCl} \xrightarrow{\text{AlCl}_3} \text{C}_6\text{H}_5\text{COCH}_3 + \text{HCl} \]
The acylium ion \(\text{CH}_3\text{C} \equiv \text{O}^+\) does not rearrange, and the ketone product stops further substitution. But the carbonyl oxygen coordinates strongly to \(\text{AlCl}_3\), forming a \(\text{C}_6\text{H}_5\text{COCH}_3\cdot\text{AlCl}_3\) complex — which is why acylation needs a full equivalent (or more) of Lewis acid and an aqueous workup to release the free ketone.
How much acetophenone comes from 5.00 g of acetyl chloride (\(M = 2(12.01)+3(1.008)+35.45+16.00 = 78.50\) g/mol; acetophenone \(M = 8(12.01)+8(1.008)+16.00 = 120.15\) g/mol)?
\[ \text{mass C}_6\text{H}_5\text{COCH}_3 = 5.00\ \text{g CH}_3\text{COCl} \times \frac{1\ \text{mol}}{78.50\ \text{g}} \times \frac{1\ \text{mol C}_6\text{H}_5\text{COCH}_3}{1\ \text{mol CH}_3\text{COCl}} \times \frac{120.15\ \text{g}}{1\ \text{mol}} = 7.65\ \text{g} \]
Key takeaways
- Alkylation: benzene + \(\text{RCl}\) + catalytic \(\text{AlCl}_3\) → alkylbenzene + HCl; the electrophile is a carbocation.
- Acylation: benzene + \(\text{RCOCl}\) + ≥ 1 equiv \(\text{AlCl}_3\) → aryl ketone + HCl; the electrophile is a resonance-stabilized acylium ion.
- Both go through the classic EAS sequence: form electrophile → attack ring to make a sigma complex (aromaticity lost) → deprotonate to restore aromaticity.
- Rearrangement is an alkylation problem only — acylium ions never rearrange.
- Polyalkylation is an alkylation problem only — acyl groups deactivate the ring, stopping the reaction after one substitution.
- No Friedel–Crafts on strongly deactivated rings — nitrobenzene and similar rings do not react.
- Synthesis shortcut: acylate → reduce (Clemmensen or Wolff–Kishner) to make unrearranged alkylbenzenes.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Write the general equations for Friedel–Crafts alkylation and acylation of benzene.
Show answer
Alkylation: \(\text{C}_6\text{H}_6 + \text{RCl} \xrightarrow{\text{AlCl}_3} \text{C}_6\text{H}_5\text{R} + \text{HCl}\). Acylation: \(\text{C}_6\text{H}_6 + \text{RCOCl} \xrightarrow{\text{AlCl}_3} \text{C}_6\text{H}_5\text{COR} + \text{HCl}\).
What electrophile does each reaction generate, and which one can rearrange?
Show answer
Alkylation uses a carbocation \(\text{R}^+\) — it rearranges. Acylation uses a resonance-stabilized acylium ion \(\text{RCO}^+\) — it does not.
Why does benzene + 1-chloropropane + AlCl₃ give cumene rather than n-propylbenzene?
Show answer
The initially formed 1° propyl cation \(\text{CH}_3\text{CH}_2\text{CH}_2^+\) undergoes a hydride shift to the more stable 2° cation \(\text{CH}_3\text{CH}^+\text{CH}_3\), which then attacks the ring.
Why is acylation self-limiting (no polyacylation) while alkylation is not?
Show answer
Alkyl groups are electron-donating (activating), so the monoalkylated product is attacked again (polyalkylation). Acyl groups withdraw electron density (deactivating), so the ketone product stops reacting.
Why does acylation require a full equivalent of AlCl₃?
Show answer
The ketone product forms a strong Lewis acid–base complex with AlCl₃ (\(\text{C}_6\text{H}_5\text{COR}\cdot\text{AlCl}_3\)), consuming the catalyst; aqueous workup releases the free ketone.
What two-step route gives an unrearranged straight-chain alkylbenzene, and why is it needed?
Show answer
Friedel–Crafts acylation, then reduction of the aryl ketone (Clemmensen: Zn/Hg + HCl; or Wolff–Kishner: hydrazine + base). Direct alkylation with a 1° or 2° halide would rearrange.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Friedel–Crafts alkylation
- EAS reaction replacing a ring H with an alkyl group using an alkyl halide + Lewis acid
- Friedel–Crafts acylation
- EAS reaction installing an acyl group using an acyl chloride + Lewis acid
- Carbocation
- Carbon with only 6 valence electrons and a + charge; the alkylation electrophile
- Acylium ion
- Resonance-stabilized \(\text{RCO}^+\) cation; the acylation electrophile
- Sigma complex (arenium ion)
- Cationic intermediate formed when the electrophile bonds to the ring, breaking aromaticity
- Lewis acid catalyst
- Electron-pair acceptor (\(\text{AlCl}_3\), \(\text{FeCl}_3\), \(\text{BF}_3\)) that generates the electrophile
- Clemmensen / Wolff–Kishner reduction
- Methods that convert an aryl ketone (C=O) into a CH₂ group
Sources & references
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