sorry... excuse my ignorance... but for what purpose you you require propionic acid? just as a substitute for GAA?
Propionic acid is used to pickle several analgesic precursors (to obtain for example the reversed ester of phenoperidine
http://webhome.idirect.com/~schapman/g616.gif (http://webhome.idirect.com/~schapman/g616.gif)
<-- phenoperidine
In the reversed ester, the -CO-O-Et group is replaced by a -O-CO-Et group, which increases analgesic activity exceedingly). Phenoperidine itself is 25 - 50 x morphine.
A haloform reaction on MEK would indeed bee nice. However, a possible side reaction is the alpha halogenation of the R group in R-CO-CH3. Sometimes, these groups are even cleaved (JACS (1950) vol 72 p 1642).
Hence, an accurate procedure would bee wellcome.