Q. Acetic acid can be halogenated in presence of phosphorous and chlorine. Formic acid cannot be halogenated with same way because of

 1951  227 Aldehydes Ketones and Carboxylic Acids Report Error

Solution:

HVZ reaction is given by only those carboxylic acids which have atom.
but formic acid does not have - hydrogen atom hence formic acid cannot be halogenated.