(i) Carboxylic acids having an α-hydrogen are halogenated at the α-position on reaction with chlorine or bromine in the presence of small amount of red phosphorus to give α-halocarboxylic acids. The reaction is known as Hell-Volhard-Zelinsky reaction. α hydrogen is the hydrogen attached to carbon which is immediately next to or attached to carbon of –COOH group of carboxylic acids.
R – CH2 – COOH R – CHX- COOH (α- halocarboxylic acid)
X = halogen
(ii) Sodium salts of carboxylic acids lose carbon dioxide to form hydrocarbons when they are heated with soda lime (NaOH and CaO in the ratio of 3 : 1) The reaction is known as decarboxylation. Sodium salts of carboxylic acids are the ones in which H of –COOH group is replaced by Na (-COONa)
R – COONa R- H + Na2CO3
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