Friedel–Crafts Alkylation

Introduction

Friedel–Crafts alkylation makes a new bond between an aromatic ring and an alkyl group. An alkyl halide activated by a Lewis acid, commonly AlCl₃, supplies a carbon electrophile. The ring attacks, forms a sigma complex, and loses H to regain aromaticity. This useful carbon–carbon bond-forming method has two important complications: the alkyl electrophile can rearrange, and the first alkyl group can activate the ring toward further substitution.

Core explanation

In a common scheme, R–Cl interacts with AlCl₃, which accepts electron density from chlorine and promotes cleavage or strong polarisation of the C–Cl bond. The resulting reactive species has carbocation-like character, often drawn as R⁺ together with AlCl₄⁻. For a suitable secondary or tertiary alkyl chloride, this representation can be helpful. The aromatic pi system bonds to the electrophilic carbon of R, giving a nonaromatic positively charged sigma complex. A base removes H from the ring carbon that acquired R, the C–H electrons restore aromaticity, and an alkylbenzene forms.

For benzene plus 2-chloropropane with AlCl₃, the electrophilic isopropyl fragment adds to the ring and deprotonation gives isopropylbenzene, commonly called cumene. The net change is ring C–H replaced by ring C–CH(CH₃)₂. The chlorine is not incorporated into the organic product. The new bond is between the ring and the carbon that bore Cl, unless a rearrangement changes the electrophilic carbon skeleton before ring attack.

Rearrangement risk arises because a carbocation-like alkyl electrophile can undergo a hydride or alkyl shift to a more stable cation. A primary halide may not deliver the unrearranged primary group: 1-chlorobutane, for example, can give a substantial amount of sec-butylbenzene after a shift rather than only n-butylbenzene. To solve a problem, draw the initially expected cation-like structure, inspect adjacent shifts, and then let the ring attack the more plausible resulting electrophile. Do not relocate a carbon group without a connected migration path.

Polyalkylation risk arises because an alkyl group usually donates electron density and activates the aromatic ring toward another electrophilic substitution. The first product can react again with the alkylating mixture. A large excess of benzene may help favour monoalkylation by making unalkylated ring molecules more abundant, but it does not change the underlying activation of the product. If a single clean ring alkyl group is essential, Friedel–Crafts acylation followed by carbonyl reduction can be a more controlled alternative for some targets.

There are substrate limitations. An aromatic ring bearing a strong electron-withdrawing group may be too deactivated for ordinary Friedel–Crafts alkylation. Basic amino substituents can bind the Lewis acid or become protonated, disrupting the intended catalysis. Aryl halides and vinyl halides are not suitable ordinary alkylating electrophiles because making aryl or vinylic carbocations is energetically unfavourable. These restrictions are as important as the textbook benzene example when planning a real synthesis.

The precise structure of the activated electrophile may be an ion pair or strongly polarised complex rather than a completely free carbocation in every case. Nevertheless, carbocation-like stability and rearrangement are useful predictive ideas. The EAS ring portion remains the same: C–C bond formation causes temporary loss of aromaticity, and ring deprotonation restores it.

Step-by-step reasoning

Identify the aromatic substrate, alkyl halide and Lewis acid. Draw the alkyl electrophile after activation and inspect whether adjacent hydride or alkyl shifts could produce a more stable carbon centre. Choose the ring position using existing substituent effects. Draw ring pi attack to form a C–C bond and a sigma complex, then remove the ring H and restore aromaticity. Finally consider whether the alkylated product is activated toward a second substitution.

Visual explanation

Draw R–Cl coordinated to AlCl₃, with the C–Cl bond polarised toward Cl. An arrow carries the activated carbon fragment to benzene, making one ring C–R bond and a sigma complex with H still attached. A final arrow removes H and restores the ring circle. Beside the product, draw a second arrow to a dialkylbenzene to illustrate why monoalkylation may be difficult to stop.

Real-world analogy

An established circle admits a new member only when a coordinator prepares that member to enter. Once inside, the new member makes the circle more welcoming, so a second entrant may arrive even faster. The coordinator is AlCl₃, and the new member is the alkyl electrophile. If that entrant rearranges its belongings before entering, the installed group differs from the intended starting fragment.

Real-world example

Cumene can be made conceptually by alkylating benzene with an isopropyl electrophile. In synthetic planning, a chemist seeking a straight butylbenzene must be cautious about direct alkylation with 1-chlorobutane because rearranged sec-butylbenzene may form. An acylation–reduction route can avoid that particular cation shift and often suppress repeated ring substitution.

Why?

Why does polyalkylation occur despite using only one initial alkylation step in the mechanism drawing? The first attached alkyl group donates electron density to the ring and stabilises future sigma complexes, making the product ring more reactive than benzene toward another electrophile. Molecules in the reaction mixture can therefore undergo a second EAS event before all reagent is consumed.

Common misconception

"The alkyl group from an alkyl chloride always appears unchanged on the ring." Carbocation-like intermediates can shift before attack, especially when a less stable initially formed cation can become secondary or tertiary. Also, the first alkylated ring may undergo further alkylation. Predict these limitations before naming a single clean product.

Worked example

Question: Benzene reacts with 2-chloropropane and AlCl₃. What is the main monoalkylation product, and why might more than one alkyl group eventually appear on some molecules?

Reasoning: Activation produces an isopropyl electrophile; the ring forms a C–C bond to its central carbon and rearomatises. The resulting isopropyl group activates the ring toward another EAS reaction.

Answer: Isopropylbenzene, or cumene, is the monoalkylation product; polyalkylated products can also appear because cumene is more reactive than benzene toward further electrophilic substitution.

Quick check

1. Why is chlorobenzene not a standard Friedel–Crafts alkylating reagent for installing a phenyl group? Answer: An aryl C–Cl bond does not generate the ordinary carbocation-like electrophile needed under these conditions.

Exam focus

Draw AlCl₃ activation, aromatic attack on carbon and sigma-complex deprotonation. Check the alkyl electrophile for adjacent hydride or alkyl shifts before attaching it to the ring. Mention polyalkylation and ring deactivation when relevant. Distinguish the method from Friedel–Crafts acylation, whose electrophile is an acylium ion.

Advanced insight

An alkylated ring's increased reactivity and an initially unstable cation's tendency to rearrange are kinetic effects that can be managed but not wished away. Using excess benzene changes collision probabilities, while choosing acylation followed by reduction changes the electrophile pathway entirely. The best route depends on whether monoalkylation, carbon skeleton fidelity or reagent economy matters most.

Summary

Friedel–Crafts alkylation uses a Lewis acid to activate an alkyl halide, allowing an aromatic ring to form a C–C bond and then rearomatise. It can make alkylbenzenes such as cumene. Carbocation-like electrophiles may rearrange, and the installed alkyl group may activate the ring toward polyalkylation. Strongly deactivated rings, basic amino-bearing rings and aryl or vinyl halide electrophiles are poor standard candidates.

Practice questions

1. What is the main monoalkylation product of benzene and 2-chloropropane/AlCl₃? Answer: Isopropylbenzene, also called cumene. 2. Why can 1-chlorobutane yield a rearranged ring alkyl group? Answer: Its initially primary carbocation-like electrophile can undergo a hydride shift to a more stable secondary centre before benzene attacks. 3. Why is polyalkylation a risk? Answer: The first alkyl group activates the aromatic ring, making further electrophilic substitution easier. 4. Which type of halide is unsuitable as an ordinary Friedel–Crafts alkylating electrophile: alkyl, aryl or vinyl? Answer: Aryl and vinyl halides are unsuitable under ordinary conditions because the required cation-like species are too high in energy.