Difficulty: Intermediate to Advanced | Prerequisites: EAS directing effects, carbocation stability
Friedel-Crafts reactions are the main EAS method for attaching carbon groups to aromatic rings. This topic ties together everything from directing effects and carbocation chemistry, and it builds toward the capstone skill of multi-step aromatic synthesis, where you must choose the right order of reactions to place substituents correctly. If you are not yet solid on activating/deactivating groups and ortho/para versus meta directors, review those first.
Friedel-Crafts alkylation and acylation attach carbon chains or acyl groups to aromatic rings using a Lewis acid catalyst (typically AlCl₃). The critical complications are carbocation rearrangement in alkylation and the fact that strongly deactivated rings (those bearing –NO₂ or similar groups) will not undergo Friedel-Crafts reactions at all. Multi-step synthesis from benzene requires you to choose the right order of steps so that each new substituent directs the next one to the correct position.
Friedel-Crafts alkylation
An EAS reaction that attaches an alkyl group to an aromatic ring. Requires an alkyl halide (R–Cl, R–Br) and a Lewis acid catalyst (AlCl₃ or FeBr₃). Proceeds through a carbocation intermediate, which means rearrangement is possible.
In simple terms, you are bolting a carbon chain onto benzene using AlCl₃ to kickstart the reaction.
Friedel-Crafts acylation
An EAS reaction that attaches an acyl group (–COR) using an acyl chloride (RCOCl) and AlCl₃. Unlike alkylation, the acylium ion intermediate does not rearrange because the positive charge is stabilised by resonance with the carbonyl oxygen.
Think of it as the cleaner cousin of alkylation: you get exactly the carbon skeleton you put in.
Lewis acid catalyst (AlCl₃)
Aluminium chloride acts as a Lewis acid by accepting a lone pair from the halide of R–Cl, generating a carbocation (or a highly polarised complex) that can attack the aromatic ring.
Carbocation rearrangement
A 1,2-hydride shift or 1,2-methyl shift that converts a less stable carbocation into a more stable one. In Friedel-Crafts alkylation, a primary carbocation from a primary alkyl halide can rearrange to a secondary or tertiary carbocation before attacking the ring.
1,2-Hydride shift
A hydrogen atom with its bonding electrons migrates from an adjacent carbon to the carbocation centre, converting, for example, a secondary carbocation into a tertiary one.
1,2-Methyl shift
A methyl group with its bonding electrons migrates to an adjacent carbocation centre to produce a more stable carbocation.
Retrosynthetic analysis
Working backwards from the target molecule to benzene, deciding which bonds to disconnect and in what order the reactions should run. The key constraint: each substituent you add changes the directing effects for the next step.
Step 1: The alkyl halide (R–Cl) coordinates with AlCl₃. The Lewis acid pulls electron density away from the C–Cl bond, generating a carbocation R⁺ (or a strongly polarised complex) and AlCl₄⁻.
Step 2: The carbocation (electrophile) attacks the pi electrons of the aromatic ring, forming the arenium ion (sigma complex). The ring temporarily loses aromaticity.
Step 3: A base (often Cl⁻ from AlCl₄⁻) abstracts a proton from the carbon that was attacked, restoring aromaticity and releasing HCl. The AlCl₃ catalyst is regenerated.
If the initial carbocation is primary, it will rearrange to a more stable secondary or tertiary carbocation via a 1,2-hydride shift or 1,2-methyl shift
This means the product may have a different carbon skeleton than the starting alkyl halide
Example from HW4 Q5: benzene + 2-chloro-3-methylbutane with AlCl₃. The secondary carbocation rearranges to a tertiary carbocation (via 1,2-hydride shift), and the product has a tert-pentyl group rather than a 3-methylbutyl group on the ring
FC acylation avoids rearrangement entirely, which is why it is preferred when you need a straight-chain substituent (acylate first, then reduce with Clemmensen or Wolff-Kishner)
The existing substituent on the ring determines where the new alkyl group goes
Deactivated rings do not undergo FC reactions. If the ring bears a strong deactivating group (–NO₂, –SO₃H, –COR), the Friedel-Crafts reaction will not proceed. AlCl₃ cannot generate enough electrophilic character to overcome the ring's low electron density.
Example from HW4 Q3a: p-chloronitrobenzene + chloroethane/AlCl₃. The –NO₂ group deactivates the ring so strongly that no Friedel-Crafts product forms. However, the ethyl group does attach meta to –NO₂ in some formulations where the ring still has enough activation from the Cl
Activating and weakly deactivating groups allow the reaction to proceed, directing as expected
Example from HW4 Q3b: p-chlorophenol + chloroethane/AlCl₃. –OH is strongly activating and ortho/para directing; the ethyl group attaches ortho to –OH
Example from HW4 Q3c: p-toluenesulfonic acid + chloroethane/AlCl₃. –SO₃H is meta-directing and deactivating; the ethyl group goes meta to –SO₃H
The general strategy:
Work backwards from the target. Identify all the substituents and their positions on the ring.
Determine the order of installation. The directing effect of each group must steer the next group to its correct position.
If you need a meta relationship between two groups, install a meta director first
If you need an ortho/para relationship, install the ortho/para director first
Remember that some groups can be converted. A nitro group (–NO₂, meta director) can be reduced to an amine (–NH₂, ortho/para director). An acyl group can be reduced to an alkyl group (Clemmensen reduction). This lets you change the directing character partway through.
FC reactions fail on deactivated rings. If you need both a –NO₂ and an alkyl group, install the alkyl group first (FC works on the undeactivated ring), then nitrate.
Target: a trisubstituted benzene with Cl, Br, tert-butyl, and NO₂.
Start with benzene
Friedel-Crafts alkylation with tert-butyl chloride / AlCl₃ to install tert-butyl (an ortho/para director)
Nitrate (HNO₃/H₂SO₄) to place –NO₂ (goes para or ortho to tert-butyl)
Halogenate (FeBr₃/Br₂ and Cl₂/AlCl₃) to install Br and Cl at the remaining positions directed by the existing groups
The exact order depends on which positions you need occupied; draw it out and verify each step directs correctly
Friedel-Crafts alkylation is how ethylbenzene is made industrially (benzene + ethylene + AlCl₃ or zeolite catalyst), and ethylbenzene is the precursor to styrene (polystyrene production). Multi-step aromatic synthesis logic is the foundation of pharmaceutical manufacturing, where complex ring-substitution patterns must be built up in the right sequence.
Students forget that FC reactions do not work on strongly deactivated rings. If the ring has –NO₂, –SO₃H, or a similar group, FC alkylation and acylation will not proceed. This is one of the most common synthesis errors: trying to do FC on a nitrobenzene.
Students draw the product with the original carbon skeleton, ignoring rearrangement. If the alkyl halide can form a primary carbocation, always check whether it will rearrange to a more stable secondary or tertiary carbocation before attacking.
Students install substituents in the wrong order. In multi-step synthesis, the order matters enormously. Installing –NO₂ before an alkyl group means FC alkylation fails. Think about directing effects and reactivity requirements at each step.
Students confuse FC alkylation and acylation products. Alkylation gives an alkyl group (–R) on the ring. Acylation gives an acyl group (–COR), which is a ketone. Acylation products do not rearrange, but they deactivate the ring (which prevents polyalkylation, an advantage in some situations).
⚠️ Drawing the full FC alkylation mechanism (Lewis acid activation, carbocation formation, electrophilic attack, deprotonation) is a very common exam question. Be ready to draw all four steps with curved arrows.
⚠️ Carbocation rearrangement questions appear frequently. If a question gives you a primary alkyl halide with AlCl₃, the product almost certainly involves rearrangement.
⚠️ Multi-step synthesis from benzene is a staple of CHM 26200 exams. You will be given a target molecule and asked to propose a route. The grading typically rewards correct ordering of steps and correct directing-effect reasoning.
⚠️ Know the limitation: FC reactions do not work on rings bearing –NO₂, –CN, –SO₃H, or –COR groups (nor on amine-bearing rings, because the amine coordinates with AlCl₃).
True or False: Friedel-Crafts alkylation can be performed on nitrobenzene.
False. The –NO₂ group deactivates the ring too strongly for FC reactions.
Fill in the blank: Friedel-Crafts acylation does not undergo carbocation rearrangement because the acylium ion is stabilised by ______.
Resonance with the carbonyl oxygen.
True or False: If you need an alkyl group and a nitro group on a ring, you should nitrate first, then alkylate.
False. You must alkylate first (FC works on the activated ring), then nitrate.
Fill in the blank: The Lewis acid catalyst in Friedel-Crafts reactions is typically ______ or FeBr₃.
AlCl₃ (aluminium chloride).
True or False: A 1,2-hydride shift converts a secondary carbocation into a primary one.
False. It converts a less stable carbocation into a more stable one (e.g. secondary to tertiary).
Q: Benzene is treated with 2-chloro-3-methylbutane and AlCl₃. Predict the product and explain any rearrangement.
A: The secondary carbocation formed initially (on C2) rearranges via a 1,2-hydride shift to a more stable tertiary carbocation (on C3). The product is tert-pentylbenzene (2-methyl-2-butylbenzene), not 3-methylbutylbenzene.
Q: Phenol (C₆H₅OH) is treated with chloroethane and AlCl₃. Where does the ethyl group attach?
A: –OH is a strong activating ortho/para director. The ethyl group goes to the ortho and para positions. Due to steric considerations, the para product is often the major one, but both form.
Q: You need to synthesise 4-bromo-2-nitrotoluene starting from benzene. Propose a synthesis route.
A: (1) Friedel-Crafts alkylation with CH₃Cl/AlCl₃ to give toluene. (2) Brominate with Br₂/FeBr₃; the –CH₃ group directs ortho/para, giving a mixture; take the para-bromotoluene. (3) Nitrate with HNO₃/H₂SO₄; the –CH₃ is ortho/para directing and –Br is ortho/para directing. The NO₂ lands ortho to –CH₃ and meta to –Br (position 2), giving 4-bromo-2-nitrotoluene.
Q: Why can you not do Friedel-Crafts alkylation on a ring bearing an –NH₂ group?
A: The nitrogen lone pair on –NH₂ coordinates with the Lewis acid AlCl₃, forming a complex that ties up both the catalyst and the amine. The amine effectively becomes an ammonium salt, which is strongly deactivating and meta-directing, and the catalyst is no longer available to activate the alkyl halide.
Q: What advantage does Friedel-Crafts acylation have over alkylation?
A: Two advantages. First, acylation does not suffer from carbocation rearrangement, so the carbon skeleton of the product matches the acyl chloride exactly. Second, the product ketone is a deactivating group, which prevents further acylation (no polysubstitution), whereas in alkylation the product is an activated ring that can undergo multiple alkylations.
Friedel-Crafts reactions connect backwards to carbocation chemistry (from your study of SN1 reactions and alkene additions) and forwards to aromatic synthesis design. The rearrangement behaviour is the same 1,2-shift logic you learned with secondary and tertiary carbocations in earlier chapters.
Multi-step synthesis ties together every EAS reaction type: nitration, halogenation, sulfonation, Friedel-Crafts alkylation and acylation. Being comfortable with all of them and their directing effects is what lets you propose a viable route from benzene to a complex target.
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