( ii ) From alkyl benzenes Alkyl benzene when treated with strong oxidising agent like H2CrO4 ( chromic acid ), acidic or alkaline KMnO4 gives benzoic acid .
( iii ) From acid derivatives All acid derivatives like amides ( RCONH2 ), acid halides ( RCOCl ), esters ( RCOOR ‟), acid anhydrides ( RCO-O-COR ) on hydrolysis give carboxylic acids . All acid derivatives break from RCO + .
( iv ) From nitriles and amides Nitriles are hydrolysed to amides and then to acids in the presence of H + or OH - as catalyst . Mild reaction conditions are used to stop the reaction at the amide stage .
( v ) From Grignard reagents Grignard reagents react with carbon dioxide ( dry ice ) to form salts of carboxylic acids which in turn give corresponding carboxylic acids after acidification with mineral acid .