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 4 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