Re: the reaction of CH3COOH and Cl2

The reaction you have been looking at is generally called 
alpha-halogenation of carbonyl compounds. These reactions occur under 
acidic (like acetic acid) or basic conditions. However the carbonyl (C=O) 
compounds that undergo this reaction are almost always ketones or aldehydes 
(see diagram below). The alpha protons of other carbonyl compounds such as 
acids are not as acidic as those of ketones and aldehydes so this reaction 
rarely occurs.




The key to this reaction is however the fact that in either acid or basic 
solutions a carbonyl group may exist in its enol form for a greater time 
than in neutral conditions. When the carbonyl group is in its enol form the 
double bond of the enol can attack the chlorine molecule and add a chlorine 
atom at the alpha carbon (the carbon which is one carbon removed from the 
C=O bond). The reason carbonyl compounds other than ketones and aldehydes 
don't react readily in this way is that they don't enolise sufficiently to 
allow the reaction to occur.

The reaction that would be used to alpha-brominate a carboxylic acid is 
called the Hell-Volhard-Zelinskil reaction and involves the use of 
phosphorus tribromide as well as bromine and water. Most standard organic 
texts should give you plenty of detail on this reaction.