Friedel-Crafts alkylation and Acylation reactions are normally in the special of the electrophilic aromatic substitution where the underlying electrophile is either a carbocation or acylium ion. The reaction is highly significant since it entail carbon-carbon bond formation thus permitting alkyl and acyl groups to be substituted onto the underlying aromatic rings. Clemmensen reduction method is normally preferable in cases where there is introduction of an n-alkyl group onto the underlying aromatic ring subsequent with the acylation. This is because clemmensen reduction route normally hinders secondary alkyl substitution that results from corresponding carbocation on rearrangement (Klein, 195-269). Nevertheless, in direct alkylation, the underlying ratio of the primary to secondary alkyl substitution can differ depending on the existing aromatic substrate and reaction conditions.
Mechanism for the Friedel-Crafts acylation of benzene
Acyl halide first reacts with the underlying Lewis acid resulting to formation of more electrophilic C, an acylium ion. The n-electrons of the underlying aromatic C=C which act as nucleophile then attack the electrophilic C+ thereby destroying aromaticity resulting to the cyclohexadienyl cation intermediate. Finally proton is removed from sp3 C bearing the acyl-group reforms the corresponding C=C and the required aromatic system resulting to the creation of HCl and regeneration of the active catalyst.
Friedel-Crafts alkylation reaction occur when an alkyl group is added to a benzene molecule through electrophile aromatic substitution reaction.
The reaction commence with the generation of methyl carbonation from the corresponding methyl bromide. Carbonation then reacts with underlying n electron system of the benzene in order to form a nonaromatic carbocation that subsequently loses a proton to in order to regenerate aromaticity of the system (Klein, 53-134). The reaction takes place in steps with an electrophile formation via reaction of the methylchlorine and aluminium chloride.
The underlying electrophile then attacks the underlying n-electron system of the existing benzene ring to result to a nonaromatic carbocation.
Positive charge on the resultant carbonation is delocalized via molecule Aromaticity is then restored through the loss of a proton from the corresponding atom that is bonded to the methyl group.
Prevailing proton the reacts with the underlying AlCl 4 − to restore the AlCl 3 catalyst thus forming HCl Carbocation then rearranges during the process to form unpredicted products.
Work Cited
Klein, David R. Organic Chemistry Ii As a Second Language: Translating the Basic Concepts. Hoboken, N.J: Wiley, 2011. Print.