Aldehydes can be oxidized to carboxylic acids with mild oxidizing agents such as `Ag(NH_3)_2^+OH^(-)`. See fig.1.

Example : See fig.2.

Alkenes can be oxidized to carboxylic acids with hot alkaline `KMnO_4`.

Nitriles can be prepared by nucleophilic substitution reactions of al kyl halides with sodium cyanide. Hydrolysis of nitriles yields a carboxylic acid with a chain one carbon atom more than the original alkyl halide.

Example: `Br-CH_2-CH_2-CH_2-Br overset(NaCN)-> NC-CH_2-CH_2-CH_2-CN overset(H_2O^(+))-> HO_2C-CH_2-CH_2-CH_2-CO_2H`

This synthetic method is generally limited to the use of primary alkyl halides. Aryl halides (except for those with onho and para nitro groups) do not react with sodium cyanide.

Grignard reagents react with carbon dioxide to yield magnesium carboxylate which on acidification produces carboxylic acids.

`R-X + Mg oversettext(Diethylether) -> RMgX overset(CO_2) -> RCOOMgX overset(H_3O^+) -> RCOOH`

`Ar-Br + Mg oversettext(Diethylether) -> ArMgBr overset(CO_2) -> ArCOOMgBr overset(H_3O^+) -> ArCOOH`

Trihalogen derivatives in which the three halogen atoms are all attached to the same carbon atom yields carboxylic acid on hydrolysis.

`RCX_3 overset(KOH) -> R-C(OH)_3 -> RCOOH + H_2O`

Methyl ketone can be converted to carboxylic acids via the haloform reaction.

A recent method for manufacturing fatty acids is to heat an olefin with carbon monoxide and steam under pressure at `300-400^(o)C` in the presence of a catalyst, e.g. phosphoric acid.

`CH_2 =CH_2 + CO + H_2O -> CH_3CH_2COOH`

The most convenient preparation for formic acid is to heat glycerol with oxalic acid at `1 00 - 110 ^(o)C`.