Quick Notes Alkylation of Benzene

  • Alkylation of benzene involves the substitution of an alkyl group onto a benzene ring.
    • Alkyl groups are carbon chains that bond onto another carbon chain (e.g. methyl).
    alkylation of benzene halogenoalkane electrophilic substitution halogen carrier a-level chemistry
  • Halogenoalkanes are reacted with a halogen carrier to produce an R group with a positive charge, which can act as the required electrophile.
  • The reaction requires warm conditions and HCl is formed as a product.

Full Notes Alkylation of Benzene

A very important part of organic chemistry is the ability to create carbon to carbon bonds. Alkylation of benzene refers to the process of adding an alkyl group to a benzene ring. This creates a bond between a carbon atom in a benzene ring and a carbon atom in the alkyl group.

alkylation of benzene halogenoalkane electrophilic substitution halogen carrier a-level chemistry

Remember, an alkyl group just refers to a carbon chain that is bonded to another carbon chain in a molecule (methyl, ethyl…).

alkyl chains ethyl methyl propyl

New groups can be added to a benzene ring in a standard electrophilic substitution reaction. (See Benzene Reactions.) However, the group to be added must be turned into an electrophile before it can be substituted onto the benzene ring.

To produce an electrophile of an alkyl group, a chloroalkane is usually used in the presence of a halogen carrier.

forming carbocatio from halogenoalkane and halogen carrier

The chlorine-carbon bond in the chloroalkane is already polar, due to chlorine’s high electronegativity compared to the carbon it’s bonded with. The halogen carrier increases this degree of polarisation sufficiently for a co-ordinate bond to form between the halogen and the aluminium, allowing the formation of the positively charged alkyl ion.

Forming Alkyl Electrophile

halogenoalkane with halogen carrier mechanism step 1

The carbon-halogen bond breaks.

halogenoalkane with halogen carrier mechanism step 2

The chlorine forms a co-ordinate bond with the halogen carrier (producing a negatively charged ion), and the carbon it broke its bond with is left with a positive charge (carbocation).

halogenoalkane with halogen carrier mechanism product

The alkyl group is now an electrophile, due to the electron deficient carbon with a positive charge.

Alkylation Reaction

alkylation of benzene mechanism electrophilic substitution a-level chemistry

The electrophile behaves as expected with the benzene ring, causing an electrophilic substitution reaction to occur. See Benzene Reactions.

reforming halogen carrier catalyst

The hydrogen ion released reacts with the negatively charged tetra ion to form HCl and re-generate the halogen carrier.