16.3 Alkylation and Acylation of Aromatic Rings: The Friedel–Crafts Reaction

Friedel-Crafts Alkylation

Friedel-Crafts reactions let you attach alkyl or acyl groups directly to an aromatic ring, forming new carbon-carbon bonds. They're among the most important methods for building complexity onto benzene rings, and they rely on Lewis acid catalysts to generate the electrophile that attacks the ring.

Mechanism of Friedel-Crafts Alkylation

Friedel-Crafts alkylation is an electrophilic aromatic substitution that places an alkyl group on an aromatic ring. It requires an alkyl halide (R-X), an aromatic compound, and a Lewis acid catalyst, typically $AlCl_3$.

Here's how the mechanism works, step by step:

1. Electrophile generation: $AlCl_3$ coordinates with the alkyl halide, abstracting the halide and generating a carbocation electrophile ($R^+$) along with $AlCl_4^-$. With primary alkyl halides, the species is more accurately a polarized complex than a free carbocation, but the result is the same: a strongly electrophilic carbon.
2. Electrophilic attack: The carbocation attacks the $\pi$ electrons of the aromatic ring, forming a resonance-stabilized arenium ion (cyclohexadienyl cation) intermediate.
3. Deprotonation: A base (often $AlCl_4^-$) removes a proton from the carbon bearing the new substituent, restoring aromaticity and yielding the alkylated product. $AlCl_3$ is regenerated as catalyst.

Important Limitations

Friedel-Crafts alkylation has several complications you need to watch for:

• Carbocation rearrangements: The initially formed carbocation can undergo hydride or methyl shifts to form a more stable carbocation. This means the alkyl group that ends up on the ring may not match the one you started with. For example, reacting benzene with 1-chloropropane and $AlCl_3$ often gives isopropylbenzene (from a 1° → 2° rearrangement) alongside the expected n-propylbenzene.
• Polyalkylation: Each alkyl group you add is electron-donating, which activates the ring toward further substitution. This can lead to di- and trialkylated products (like xylenes from toluene), making it hard to stop at monoalkylation.
• Intramolecular reactions: When the alkyl halide and aromatic ring are in the same molecule, cyclization can occur, forming products like indanes.
• Deactivated rings fail: Aromatic rings bearing strong electron-withdrawing groups ($-NO_2$, $-CF_3$, etc.) are too electron-poor to react. Friedel-Crafts reactions do not work on nitrobenzene or similarly deactivated substrates.
• Regioselectivity: Existing substituents on the ring direct the incoming group to ortho/para or meta positions, following the same directing rules as other EAS reactions.

Comparison of Friedel-Crafts Alkylation and Acylation

Both reactions are electrophilic aromatic substitutions that use $AlCl_3$ as a Lewis acid catalyst and proceed through an arenium ion intermediate. But they differ in important ways.

Because acylation avoids both rearrangements and polysubstitution, it's often the more reliable reaction. You can then reduce the ketone to a $-CH_2-$ group (Clemmensen or Wolff-Kishner reduction) to get the same product as alkylation, but without the side-product headaches. This two-step sequence is a classic workaround in synthesis.

Biological Aromatic Alkylations

Nature performs Friedel-Crafts-type alkylations without metal catalysts like $AlCl_3$. Instead, enzymes handle every job the Lewis acid would do.

Enzymes catalyze biological aromatic alkylations by:

1. Activating the electrophile through formation of a reactive intermediate (often a carbocation from a diphosphate leaving group)
2. Stabilizing the transition state and the arenium ion intermediate within the active site
3. Orienting the substrates precisely so the reaction is both fast and regioselective

A well-studied example is the biosynthesis of vitamin K1 (phylloquinone). The enzyme 1,4-dihydroxy-2-naphthoate prenyltransferase catalyzes the alkylation of 1,4-dihydroxy-2-naphthoate with geranylgeranyl diphosphate. The diphosphate group departs to generate a carbocation (much like a halide leaving in the lab reaction), and the electron-rich naphthoate ring attacks it. The enzyme active site binds both substrates in the correct orientation and stabilizes the arenium ion intermediate, driving the reaction forward under mild biological conditions.

Factors Affecting Friedel-Crafts Reactions

Several factors determine whether a Friedel-Crafts reaction will work and what products you'll get:

• Substrate electronics: The aromatic ring must be sufficiently electron-rich. Rings with strong electron-withdrawing groups ($-NO_2$, $-SO_3H$, $-COR$ from a prior acylation) are too deactivated for the reaction to proceed.
• Arenium ion stability: Greater resonance stabilization of the arenium ion intermediate lowers the activation energy and speeds up the reaction. Electron-donating substituents on the ring help stabilize this intermediate.
• Carbocation rearrangement (alkylation only): Always consider whether the carbocation can rearrange to a more stable form. If a 1° carbocation can shift to 2° or 3°, it probably will.
• Choice of electrophile: Acyl halides give cleaner, more predictable results than alkyl halides because the acylium ion doesn't rearrange and the product doesn't undergo further substitution. When you need an alkyl group on the ring without rearrangement, consider acylation followed by reduction.
• Regioselectivity: Existing ring substituents direct the incoming electrophile according to standard ortho/para vs. meta directing rules, just as in other EAS reactions.