A2-Level Benzene, Aromatic Chemistry
Alkylation of benzene involves the substitution of an alkyl group onto a benzene ring.
Alkyl groups are carbon chains bonded to another carbon atom (i.e. methyl).
Halogenoalkanes are reacted with a halogen carried to produce an R group with a positive charge that can act as the required electrophile.
The reaction requires warm conditions, and HCl is formed as a product.
Alkylation of Arenes
A very important part of organic chemistry is the ability to create carbon to carbon bonds. Alkylation of arenes 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.
Remember, an alkyl group just refers to a carbon chain that is bonded to another carbon chain in a molecule (methyl, ethyl…).
New groups can be added to a benzene ring in a standard electrophilic substitution reaction. 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 used in the presence of a 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.
The carbon-halogen bond breaks.
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).
The alkyl group is now an electrophile, due to the electron deficient carbon with a positive charge.
The electrophile behaves as expected with the benzene ring, causing an electrophilic substitution reaction to occur.
The hydrogen ion released reacts with the negatively charged tetra ion to form HCl and re-generate the halogen carrier.