chlorination of aromatic compounds

Question

I was reading some notes on Electrophilic Substitution and I came across the topic chlorination of Toluene. I'm wondering why we cannot substitute chlorine in the meta position of toluene?

Answer 1

We do get meta-chlorotoluene when toluene is chlorinated by EAS, but only in minor amounts. This paper describes a process designed to selectively produce the ortho isomer over the para one, but in addition to some para-chlorotoluene still being formed the abstract reports 4.0% meta-chlorotoluene. There are also small amounts of dichlorinated products and even benzyl chloride.

This Youtube video describes a synthesis route from benzene that favors meta-chlorotoluene.

• Nitrate the benzene.

• Apply Friedel-Crafts methylation which gives substitution meta to the nitro group. The remaining steps ultimately exchange the nitro group for a chlorine atom.

• Reduce the nitro group to amino using tin plus acid.

• Convert the amino group to a diazonium ion with nitrite plus acid.

• React the diazonium ion with cuprous chloride plus HCl.

Meta-chlorotoluene is available at 98% purity from Sigma-Aldrich.