Org Question- Add to ring vs. substituents.

[DonExodus]

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This question has always pissed me off. How do I know if something will add to the ring directly, or to a functional group? I know the main reactions, but halogenation is particularly pissing me off.
Take benzylic acid. What can I add to halogenate the ring vs. swap the -OH for the halogen? Catalysts?

Thanks
Don

 

[platinumcaps]

w/ benzylic acid, you just use Br/FeBr3 to add to it meta.

 

[DonExodus]

So its the catalyst that makes the difference? FeX3 adds via EAS?

 

[slayerdeus]

Do you mean benzoic acid?

To halogenate the acid group you can add SOCl2 or PBr3.

To add a halide to a benzene ring it will be hard because -COOH is moderately deactivating, adding anything will be tough but it is meta directing. Any Lewis acid such as FeCl3, FeBr3 or AlCl3 will halogenate the benzene ring.

 

[DonExodus]

And PBr3 / SOCl2 will NOT halogenate a ring with X2. Correct?

 

[slayerdeus]

And PBr3 / SOCl2 will NOT halogenate a ring with X2. Correct?

No. Along with the Lewis acid (FeBr3 or AlCl3) X2 must be added as well. The two react (ie FeBr3 and Br2) to create a highly electrophilic complex which attacks the benzene. This complex is a positively charged complex and acts as a catalyst for this reaction to add to most benzene derivatives.