Carboxylic acids having atleast one `alpha`-hydrogen react with `Cl_(2) or Br_(2)` in the presence of phosphorus ( or a phosphorus halide) to give `alpha`-halo acids. This reaction is known as Hell-Volhard-Zelinsky reaction (HVZ reaction),
`R-CH_(2)COOHunderset(Delta)overset(P//Br_(2))toR-overset(Br)overset(|)"CH"-COOH`
The HVZ reaction is limited to the formation of `alpha`-chloro and a-bromo acids and it is sometimes awkward to carry out. The reagents ( `X_(2)` and P) are noxious and the reaction time is often long and the conditions of reaction are harsh.
`R- CH_(2)-CH_(2)-OHto R-CH_(2) -CH_(2) - COOH`
In above conversion, the reactant (A) follows the sequence of reactions with three reagents
A. `PBr_(3),KCN,H_(3)^(+)O`
B. `PBr_(3),H_(3)^(+)O,KCN`
C. `HCN,PBr_(3),KCN`
D. `H_(3)^(+)O,KCN,PBr_(3)`
