[spirochete]

I'm curious about ways to replace a hydroxyl group with a halogen that don't effect a carbonyl group.  Here are my possibilities, although other suggestions are welcome:

1) acid catalyzed (HI, HBr etc)  I know this won't work because X2 adds electrophilically to the conjugate acid produced by protonation of the ketone. 

2) PBr3, PCl3 etc.  I'm not sure about this one.  Could the catalyst or the halide ion interact with the carbonyl group?

3)  Tosyl chloride.  Also not sure about this one.  Same general issues as PX3.

Edit: I know I could just protect the carbonyl with an acetal, but I'm curious of ways to avoid that

[zq]

I would use 48% aqueous HBr to make the transformation (OH to Br). I think this will not affect carbonyl. I have used it in the presence of an amide. For a prep go to pmgb.wordpress.com.

[g-bones]

as long as your carbonyl is not a carboxylic acid, you can use thionyl chloride (SOCl2) to replace an alcohol with a halogen (chlorine).  you can also used triphenyl phosphine, I2 and a weak proton sponge (triethyl amine or pryridine) to do the trick.

[venkatjohnny]

hi,
for replacing OH group by Halo( ex:Cl)group, u can confidently use SOCl2, Oxalyl chloride, POCl3 or Isocyanuryl chloride. These are the efficient reagents suitable for the replacing OH group as per my knowledge. Among these SOCl2 is the better choice coz work up is very easy due to the formation of gaseous by products ( SO2, HCl).. have a nice time..

[vhpk]

I think to replace hydroxyl group without affecting carbonyl group, you should change carbonyl group to acetal.
Ex: CH3CHO + CH3OH --(HCl)--> CH3CH(OH)OCH3.
After that, you can do whatever you want because acetal group is passive in reactions, when finishing, you hydrolyzed in acidic solution to regenerate carbonyl group, I think this is important because:
CH3CHO + PCl5 --> CH3CHCl2 + POCl3