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Q.
Acetic acid can be halogenated in presence of phosphorous and chlorine. Formic acid cannot be halogenated with same way because of
Aldehydes Ketones and Carboxylic Acids
Solution:
HVZ reaction is given by only those carboxylic acids which have $\alpha- H$ atom.
but formic acid does not have $\alpha$ - hydrogen atom hence formic acid cannot be halogenated.