Why don't Friedel-Crafts reactions work with a deactivating group on the benzene ring? Why is it...

Why don't Friedel-Crafts reactions work with a deactivating group on the benzene ring?

Why is it hard to put one alkyl group on a benzene ring with the Friedel-Crafts reaction?

Expert Solution

For the Friedel-Craft reaction to occur, the reactivity benzene needs to be similar to or more than that of chloro benzene. However, when a deactivating group such as nitro is present, the electrophile is not strong enough to attack the benzene ring. Consider the following resonance structures.

The electron withdrawing group deactivates the benzene ring by taking a pair of electrons from the pi system.There is no negative charge on the benzene ring, but only a neutral meta position. Only a strong nucleophile can attack this position. the friedel craft electrophile is not strong for this attack, beacuse it has a partial positive carbon which is not attracted to the neutral meta position.

Friedel craft reactions result in polyalkylations beause the product formed is more reactive than benzene. The product has a alkyl activating group, which attracts electrophiles more than benzene. Therefore, instead of attacking benzene, the electrophiles attack the para and ortho positions which have partial negative charges.

queen_honey_blossom answered 2 years ago

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