[rleung]

Hi,

I have a quick question.  Strong nucleophiles can attack sp2 carbons that are part of benzene rings and substitute the halogen substituent of the benzene ring, right (under heat as a substitution rxn)?  My book shows an example in the solutions manual, but we have not covered this reaction yet.  Thank you.

[Yggdrasil]

Usually nucleophiles cannot react with an aromatic system, since the conjgated pi system acts like a nucleophile in most aromatic substitution reactions.  However, there are two cases in which one can react an nucleophile with a benzene ring:

1) In the presence of a strong base, a halogenated benzene ring can be deprotonated in an elimination reaction to form benzyne in situ.  Benzyne is a very reactive electrophile which can react with the base.

2) A strong nucleophile can react with an halogenated benzene ring if the halogen is ortho- or para- to a strongly electron withdrawing group such as a nitro group.  If a strongly electron withdrawing group is not present, the reaction will not occur.

[movies]

2) A strong nucleophile can react with an halogenated benzene ring if the halogen is ortho- or para- to a strongly electron withdrawing group such as a nitro group.  If a strongly electron withdrawing group is not present, the reaction will not occur.

This is usually called S_NAr chemistry, by the way.  It proceeds through an addition-elimination mechanism similar to esterification.

[plu]

This is usually called S_NAr chemistry, by the way.  It proceeds through an addition-elimination mechanism similar to esterification.

I believe the intermediate you get here is known as a Meisenheimer complex.

[movies]

I believe the intermediate you get here is known as a Meisenheimer complex.

Isn't the Meisenheimer complex the other way around?  Like in a Freidel-Crafts reaction when you form the delocallized cation?

[plu]

Isn't the Meisenheimer complex the other way around?  Like in a Freidel-Crafts reaction when you form the delocallized cation?

As I recall, the Meisenheimer complex is the intermediate formed when the nucleophile attacks the halogen-substituted carbon on the benzene ring to form an anion