SN2 vs. Nucleophilic aromatic substitution

[lol105]

I'm getting some stuff mixed up, so I need clarification.

Aromatic rings cannot go under SN2, but can go through nucleophilic aromatic substitution if there is a halogen ortho or para to a strongly deactivating group?

Oh, and would I need to worry about this on DAT?

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[smthngintheway]

Yeah I know what you're talking about. The "Nucleophilic Aromatic Substitution" is basically a SN2 reaction that's made possible due to the stabilizing effect of a e- withdrawing group either ortho or para (or both) to the Leaving group. The nucleophile can be a OH- or a NH2.

You should realize however that Nucleophilic Aromatic Substitution can also happen without EWGs on the ortho or para position. This will lead to the "Benzyne Mechanism." Look it up if your interested.

I've been doing a lot of practice problems and I've never seen this asked however. I would guess that there's probably a very small chance of being asked this on the actual DATs but I guess it doesn't hurt to know it.

Oh and a methyl group on a benzene ring (in Toluene for instance) can go through normal SN1 and SN2. Not sure about E1 or E2 though....

 

[TheDentist16]

Yeah I know what you're talking about. The "Nucleophilic Aromatic Substitution" is basically a SN2 reaction that's made possible due to the stabilizing effect of a e- withdrawing group either ortho or para (or both) to the Leaving group. The nucleophile can be a OH- or a NH2.

You should realize however that Nucleophilic Aromatic Substitution can also happen without EWGs on the ortho or para position. This will lead to the "Benzyne Mechanism." Look it up if your interested.

I've been doing a lot of practice problems and I've never seen this asked however. I would guess that there's probably a very small chance of being asked this on the actual DATs but I guess it doesn't hurt to know it.

Oh and a methyl group on a benzene ring (in Toluene for instance) can go through normal SN1 and SN2. Not sure about E1 or E2 though....

You can do an E1 or E2 on a benzene ring substituent that is at least 2 carbons long with an appropriate leaving group and a strong base, such as t-butoxide. E1 and E2 won't work on a 1-carbon substituent such as the methyl group on toluene. There is already a conjugated system within the benzene ring. However from what I've seen, you should know all the EAS reagents/products formed (the roadmaps on OChem Destroyer are a nice at-a-glance review). A tricky one is the radical addition of bromine in the presence of light, such as with toluene.

Also, on a sidenote, HBr in the presence of a peroxide (ROOR) will do an anti-markovnikov addition across an alkene.