[SVXX]

This is a little tricky one. I'm supposed to convert ethanoic acid to methyl ethanoate with a bromine atom at the alpha carbon of C=O (alpha-bromo methylethanoate, if you please).
I'll list the methods with which I have doubts on whether the product will be obtained in high yields -:

1. Fischer Esterification - First I use the Hell-Volhard-Zelinsky reaction to alpha-brominate ethanoic acid. Now when I do Fischer esterification, won't the alcohol group's lone pair prefer to perform an S_N2 reaction on C-Br instead?

2. Trans-esterification - This one I am sure won't work. It requires the usage of -COO^- as a nucleophile. There would be an intramolecular nucleophilic substitution instead, as bromine would act as a neighbouring group participator.

3. Diazomethane-induced esterification - This is another one which uses the carboxylate ion as a nucleophile, and an alkyl diazonium ion to introduce -CH₃ on -O^-. Again bromine would interfere as an NGP.

And why NGP? Because kinetically, the intramolecular formation of a 3-membered ring is much more feasible(which occurs when bromine is knocked out) even though the 3-member ring may not be stable in the long run.
Now could anyone please suggest a way to get around this?
PS : The purpose of preparing an alpha-bromoester is for the Reformatsky reaction.

[Doc Oc]

Method 1 should work fine.  Two things work in your favor; acid halides are incredibly reactive (much more so than an alkyl halide) and alcohols are weak nucleophiles.

I don't see much point to the other two routes since you would likely use the Hell-Vollhardt-Zelinski reaction to install the alpha bromine anyways.  But carboxylates aren't awesome nucleophiles either, that anion can delocalize between the two oxygens.  What you would more likely see in this reaction is an E2 elimination of the alpha bromine to give you an a,b-unsaturated carboxylic acid.

[SVXX]

Right...and Fischer esterification is done in an acidic medium, is it not? That would work against S_N(we need a base to make it a carboxylate anion). Well thanks...that should do!

[Doc Oc]

Right...and Fischer esterification is done in an acidic medium, is it not?

Yes, that's correct.

[uvcyclotron]

I am not sure about the diazomethane-induced esterification method, but the first method sparks some interest..
Could you not use the Fischer esterification first, and the HVZ reaction later..? Would this result in a different product?

[AnkurGel]

@uvcyclotron: No. You can't do fischer esterification first and then HVZ. HVZ can't occur in esters at all. The HVZ reactions requires highly acidic hydrocarbon functional group to induce it's effect which is COOH while ester groups retains more electron-rich character.. let alone acidity.

[uvcyclotron]

@AnkurGel ah..sorry for that, i looked up the mechanism of HVZ later, and noticed that the -OH group of -COOH has an active participation in this reaction (it being substituted by a -Br and back later on from a water molecule), so definitely an ester instead would goof up the entire process.
Thanks.

[g-bones]

quenching your HVZ with the alcohol you wish to esterify with alone should do the trick, especially with an acyl bromide.  thats its specific purpose (or quenching with water to return the acid), you'll knock this one out of the park