Acetic acid can be halogenated in the presence of red P and `Cl_2`, but formic acid cannot be halogenated int eh same way. Explain.

Explanation

Halogenation of carboxylic acids with red P and halogen (`Cl_(2)` or `Br_(2)`) one of the `alpha`-hydrogen is replaced by halogen. For example
`underset("Acetic acid")(CH_(3)COOH) + Cl_(2) overset("Red P")rarr underset("Monochloroacetic acid")(CH_(2)ClCOOH) + HCl`
As formic acid (HCOOH) has no `alpha`-hydrogen atom, if cannot be halogenated in this manner.