Friedel-Crafts acylation

Friedel-Crafts acylation is the reaction that adds an acyl group, RCO, to a benzene ring. Benzene is warmed with an acyl chloride and an aluminium chloride catalyst, and the products are an aromatic ketone and hydrogen chloride. It is an electrophilic substitution: the electrophile is the acylium ion, RCO⁺, made when AlCl₃ takes a chloride ion from the acyl chloride.

The overall reaction

The standard example uses ethanoyl chloride. One hydrogen on the ring is swapped for the CH₃CO group, giving phenylethanone:

C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl (with AlCl₃ as the catalyst)

In AQA's A-level specification (3.3.10.2), Friedel-Crafts acylation is described as an important step in synthesis, and students outline its mechanism using AlCl₃ as a catalyst. OCR A (6.1.1 and 6.2.4) covers Friedel-Crafts reactions with an acyl chloride or a haloalkane and a halogen carrier, as a way of making a new C to C bond to an aromatic ring.

How the electrophile forms

Benzene's delocalised ring is stable, so it only reacts with a strong electrophile. An acyl chloride on its own isn't one. AlCl₃ fixes that: aluminium in AlCl₃ is electron deficient, with just three bonding pairs round it, so it accepts a lone pair from the chlorine of the acyl chloride and pulls the chloride ion off.

RCOCl + AlCl₃ → RCO⁺ + AlCl₄⁻

For ethanoyl chloride the electrophile is CH₃CO⁺. The positive charge sits on the carbon that was bonded to the chlorine, and that carbon is what the ring attacks. OpenStax notes the acylium ion is stabilised by the lone pairs on the neighbouring oxygen.

Friedel-Crafts acylation mechanism

Once the electrophile has formed, the mechanism has the same shape as nitration and every other electrophilic substitution of benzene: the ring attacks, then loses H⁺.

1. Make the electrophile. RCOCl + AlCl₃ → RCO⁺ + AlCl₄⁻.
2. Attack. A pair of electrons from the delocalised ring forms a bond to the positive carbon of RCO⁺. Draw the curly arrow from the circle inside the ring to that carbon.
3. The intermediate. One ring carbon now holds both an H and the acyl group, so it is out of the delocalised system. Draw a horseshoe over the other five carbons, open towards that carbon, with a + inside it.
4. Restore the ring. The C to H bond breaks and its electrons go back into the ring. Draw the curly arrow from the C to H bond into the horseshoe. H⁺ leaves and the full delocalised ring is back.
5. Regenerate the catalyst. H⁺ + AlCl₄⁻ → AlCl₃ + HCl.

AlCl₃ is used up in step 1 and made again in step 5, which is why it counts as a catalyst. The ring ends up substituted rather than added to, because keeping the delocalised ring is more stable than breaking it: the same reason benzene does substitution in preference to addition.

Naming the product

The product is always a ketone: the carbonyl carbon is bonded to the ring on one side and to the R group on the other. Take the acyl chloride, swap the Cl for a phenyl group, and name it as a ketone.

To work backwards, find the C=O next to the ring. Everything from that carbon outwards came from the acyl chloride, so put a Cl on the carbonyl carbon where the ring was.

Worked example: percentage yield

7.8 g of benzene reacts with excess ethanoyl chloride and AlCl₃, and 9.0 g of phenylethanone is collected. Find the percentage yield. Use C = 12, H = 1, O = 16.

1. Mr of benzene, C₆H₆ = 6 × 12 + 6 × 1 = 78, so moles of benzene = 7.8 ÷ 78 = 0.10 mol.
2. The equation is 1 : 1, so at most 0.10 mol of phenylethanone can form.
3. Mr of phenylethanone, C₈H₈O = 8 × 12 + 8 × 1 + 16 = 120, so the theoretical mass = 0.10 × 120 = 12.0 g.
4. Percentage yield = 9.0 ÷ 12.0 × 100 = 75%.

The stoichiometry calculator checks reacting masses like these, and the percent yield guide covers why yields fall short.

Acylation vs alkylation

Friedel-Crafts alkylation uses a haloalkane instead of an acyl chloride, with the same kind of catalyst, and adds an alkyl group. OpenStax explains why acylation is the more reliable of the two:

Why it matters in synthesis

Friedel-Crafts acylation is one of the few ways to join a carbon chain straight onto a benzene ring. The ketone it makes can then be changed. In AQA's specification, ketones are reduced to secondary alcohols by NaBH₄, so phenylethanone becomes 1-phenylethanol, C₆H₅CH(OH)CH₃. Questions on synthesis routes often start from benzene this way.

It has limits. OpenStax notes that Friedel-Crafts reactions fail on rings that carry a strongly electron-withdrawing group, such as a nitro group, or a basic amino group.

Common mistakes

• The wrong electrophile. It is RCO⁺, with the charge on the carbonyl carbon. Not RCOCl⁺, not R⁺ and not Cl⁺.
• Arrows the wrong way. The first arrow goes from the ring to the electrophile, because the ring gives the electron pair.
• A horseshoe over all six carbons. It should leave out the carbon holding H and RCO, and it needs a + inside.
• Forgetting the catalyst comes back. Finish with H⁺ + AlCl₄⁻ → AlCl₃ + HCl.
• Calling the product an aldehyde. The carbonyl carbon has two carbon neighbours, so it is a ketone.
• Mixing it up with addition-elimination. Acyl chlorides react with nucleophiles such as water, alcohols and amines by nucleophilic addition-elimination. With benzene and AlCl₃, the acyl group is the electrophile instead.

Aromatic chemistry is A-level only, so practise it with today's A-level Chemistry: Year 2 short timed paper. To brush up on drawing rings and functional groups first, the organic chemistry course starts from skeletal formulas.

Read next

Chemistry · Organic chemistryElectrophilic substitution: benzene mechanism and examplesElectrophilic substitution of benzene explained: why benzene substitutes, the nitration and acylation mechanisms, directing groups and worked examples.