(a) Write a note on Hell-Volhard-Zelinsky (HVZ) reaction.
(b) Write a short note on Koch reaction.
(c) Write a short note on Arndt-Eistert reaction

Explanation

(a) When a carboxylic acid that contains a-hydrogen atoms is treated with chlorine or bromine in presence of phosphorus, the a-hydrogen atoms are replaced by halogen atoms. This reaction is called Hell-Volhard-Zelinsky (HVZ) reaction. This reaction is used for the synthesis of `alpha`-substituted carboxylic acids.
`CH_(3)COOH overset(Cl_(2))underset((P))rarr ClCH_(2)COOH+HCl`
`R overset(alpha)(C )H_(2)COOH+Cl_(2)overset(hv)underset((P))rarr RCH(Cl)COOH rarr RC(Cl_(2))COOH`
(b) This reaction qis used for the conversion of alkenes into monocarboxylic acids. When a mixture of alkene carbon monoxide and steam is heated under pressure at 623K in presence of ortho phosphoric acid `(H_(3)PO_(4))`, monocarboxylic acid is formed.
`underset("Ethene")(H_(2)C=CH_(2))+CO+H_(2)Ounderset("Pressure,623K")overset(H_(3)PO_(4))tounderset("Propionic acid")(CH_(3)CH_(2)COOH)`
( c) The reaction consist of increasing the carbon chain length by one methylene `(- CH_(2))` group in carboxylic acid through carbene mechanism. For example