Friedel Crafts alkylation of arenes
Description: In the presence of a catalyst such as FeCl3 or AlCl3 treatment of an aromatic with an alkyl chloride provides an aromatic alkane. This reaction is called “Friedel-Crafts alkylation” .
Notes: For our purposes FeCl3 and AlCl3 are equally effective for this reaction. The reaction is also effective with alkyl bromides.
Notes: In all cases the byproduct is HCl, and FeCl3 is regenerated. Note example 3 – rearrangements can occur in this reaction with primary alkyl halides. In example 4 note that nitro groups tend to deactivate the ring towards attack. Finally note that the reaction is ineffective for alkenyl halides (example 5), because alkenyl carbocations are very unstable.
Mechanism: This is an electrophilic aromatic substitution reaction. This reaction proceeds in two phases, an activation phase and a substitution phase. Activation of the alkyl halide is achieved through attack of the chloride on FeCl3 (Step 1, arrow A) followed by cleavage of the C–Cl bond to give an alkyl carbocation (Step 2, arrow B). The carbocation is a better electrophile than the alkyl halide, hence this is referred to as “activation”.
NOTE if a rearrangement is possible to provide a more substituted carbocation, it will happen here.
The alkyl carbocation is an excellent electrophile, and is then attacked by the aromatic group (Step 3, arrow C), breaking aromaticity. Aromaticity is regained by deprotonation of the carbon, re-forming the double bond (Step 4, arrows D and E).
Notes: Note that FeCl3 is regenerated here (it is a catalyst). It is perfectly reasonable to show Cl(–) acting as a base in the fourth step.