Reactions of Carboxylic Acids

Carboxylic acids undergo reactions to produce derivatives of the acid. The most common derivatives formed are esters, acid halides, acid anhydrides, and amides.

Esters are compounds formed by the reaction of carboxylic acids with alcohols, and they have a general structural formula of:

The simplest method of preparation is the Fischer method, in which an alcohol and an acid are reacted in an acidic medium. The reaction exists in an equilibrium condition and does not go to completion unless a product is removed as fast as it forms.

The Fischer esterification proceeds via a carbocation mechanism. In this mechanism, an alcohol is added to a carboxylic acid by the following steps:

1. The carboxyl carbon of the carboxylic acid is protonated.

2. An alcohol molecule adds to the carbocation produced in Step 1.

3. A proton is lost from the oxonium ion generated in Step 2.

4. A proton is picked up from solution by a hydroxyl group.

5. A pair of unshared electrons from the remaining hydroxyl group helps the water molecule leave.

6. The oxonium ion loses a proton to generate the ester.

7. Esters can also be prepared in a nonreversible reaction of an acid with an alkoxide ion.

Nonreversible ester formation

The nonreversible esterification reaction proceeds via a nucleophilic substitution reaction.

1. Acting as a nucleophile, the alkoxide ion is attracted to the carbon atom of the carboxyl group.

2. The oxonium loses a proton.

3. An unshared electron pair from the alkoxide ion moves toward the carbonyl carbon, assisting the hydroxyl group's exit.

Methyl ester formation

Methyl esters are often prepared by the reaction of carboxylic acids with diazomethane.

Amide formation

Amides are compounds that contain the following group:

Substituted amides can contain the following groups:

An amide name is based on the name of the carboxylic acid of the same number of carbon atoms, but the ‐oic ending is changed to amide. Amides with alkyl groups on the nitrogen are substituted amides and are named the same as N‐substituted amides, except the parent name is preceded by the name of the alkyl substituent and a capital N precedes the substituent name.

Amides are ordinarily prepared by a reaction of acid chlorides with ammonia or amines.

An amide is prepared by reacting an acid halide with ammonia.

An N‐substituted amide is prepared by reacting an acid halide with a primary amine.

An N,N‐disubstituted amide is prepared by reacting an acid halide with a secondary amine.

You can also react ammonia with esters to prepare primary amides.

The mechanism for amide formation proceeds via attack by the ammonia molecule, which acts as a nucleophile, on the carboxyl carbon of the acid chloride or ester. The alkoxide ion that forms assists with the displacement of the chloride ion or alkoxy group.

1. The ammonia molecule attacks the carboxyl carbon, which leads to the formation of an alkoxide ion.

2. The ammonium ion loses a proton to form an —NH 2 group.

3. An unshared electron pair on the alkoxide ion oxygen moves in to help displace the leaving group.

Acid halide formation

Carboxylic acids react with phosphorous trichloride (PCl 3), phosphorous pentachloride (PCl 5), thionyl chloride (SOC l 2), and phosphorous tribromide (PBr 3) to form acyl halides.

Acid anhydride formation

Following is the anhydride group:

This group forms by reacting the salt of a carboxylic acid with an acyl halide.

Decarboxylation reaction

Decarboxylation is the loss of the acid functional group as carbon dioxide from a carboxylic acid. The reaction product is usually a halocompound or an aliphatic or aromatic hydrocarbon.

The following illustration shows the sodalime method:

Alipathic and aromatic acids can be decarboxylated using simple copper salts.

Hunsdiecker reaction

In a Hunsdiecker reaction, the silver salt of an aromatic carboxylic acid is converted by bromine treatment to an acyl halide.

Kolbe electrolysis

In Kolbe electrolysis, electrochemical oxidation occurs in aqueous sodium hydroxide solution, leading to the formation of a hydrocarbon.