(i) In Hell-Volhard-Zelinsky (HVZ) reaction, carboxylic acid having an $\alpha$-hydrogen is halogenated at $\alpha$-position on treatment with chlorine or bromine in the presence of red phosphorus to give $\alpha$-halogenated carboxylic acid. $\mathrm{RCH}_2\mathrm{COOH} \xrightarrow[(ii)\,\mathrm{H}_2\mathrm{O}]{\text{(i)}\,\mathrm{X}_2/\mathrm{Red\ P}} \underset{\mathrm{X}}{\underset{\vert}{\mathrm{RCHCOOH}}}$ (ii) In decarboxylation reaction, carboxylic acid loses $\mathrm{CO}_2$ to form hydrocarbons when their sodium salts are heated with sodalime $(\mathrm{NaOH}$ and $\mathrm{CaO}$ ) in the ratio $3:1$. $\mathrm{RCOONa} \xrightarrow[\Delta]{\mathrm{NaOH}\text{ and }\mathrm{CaO}} \mathrm{RH} + \mathrm{Na}_2\mathrm{CO}_3$