Chlorination of Arenes to give Chloroarenes

by James

Description: Treatment of an aromatic (such as benzene) with chlorine (Cl2) and a Lewis acid such as AlCl3 or FeCl3 leads to formation of the chlorinated aromatic.

Notes: AlCl3 is a catalyst for this reaction. The byproduct is hydrochloric acid (HCl). FeCl3 (among others) can also be used as a catalyst for this reaction.


Notes:  Note that the byproduct for each of these reactions is HCl

Mechanism: This is an electrophilic aromatic substitution reaction and proceeds in two phases, an activation phase (Steps 1 and 2) and a substitution phase (Steps 3 and 4). Attack of chlorine on the Lewis acid (Step 1, arrow A) makes Cl2 more electron poor and thus a better electrophile. Cleavage of Cl–Cl gives Cl(+) which is very reactive (Step 2, arrrow B). Attack of an aromatic PI bond on Cl(+) leads to a carbocation (Step 3, arrow C) after which deprotonation (Step 4, arrows D and E) regenerates aromaticity.

Notes: It’s also reasonable to show arrow B happening at the same time as arrow C

It’s also reasonable to show cleavage of the Cl–Al bond to give Cl(–) followed by deprotonation of the carbocation with Cl(–).


{ 3 comments… read them below or add one }

Mohammed Sharif

Aren’t the examples reactions para-attacks, not ortho?



Yes they are – there’s a note next to the example saying (+ ortho) meaning that they also give the ortho product, but it’s not shown.


Andrew thibodeau

In the reaction of 3-cyclohexenyl prop-1-ol…..with bromine (Br2, electrophille).. ……….i NOW correctly understand in the first step of mechanism the nucleophille is the pi-bond, NOT one of the the lone pairs on the hydroxyl group. A professor explained the pi bond trumps an oxygen lone pair in nucleophicity. The story is a 3 membered bromonium bridge cation is formed and then an oxygen lone pair orbital comes around to attack (forming an oxonium ion). This effects a cyclohexane ring closing. My question is: Why doesnt (or maybee it -is- a minor product) the Br- anion….(remember him… created from the ‘inital’ attack?) stereo anti-attack one of the carbon bridge heads of the bromonium ion (+) created? Like in typical Br2 anti-add stereo of an alkene? “They’re” always saying how the large radii halogens are great nucleophiles because their chrages are polarized/spread out over the atom (e.g. Iodine being best nucleophile). Why is the oxygen lone pair orbital the better nucleophile here?
Or do i -still- have the mech wrong? Damn. A halohydrin being formed here right?

Andrew- seeking understanding, aspiring orgo chemist


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