1. The ammonium salt formation is reversible. Upon heating, a slower but thermodynamically more favorable reaction between the carboxylic acid and the amine takes place.

2. Dicarboxylic acids react with amines to give imides. Imides, or the nitrogen analogs of cyclic anhydrides, can be formed through the reaction of a dicarboxylic acid and ammonia or a primary amine.

3. Amino acids cyclize to lactams. Some amino acids can undergo an intramolecular cyclization to the corresponding cyclic amide, or lactam. The penicillin class of antibodies derives its biological activity from a lactam function.

4. Reduction of Carboxylic Acids by Lithium Aluminum Hydride 19-11 Carboxylic acids are converted to their corresponding primary alcohols by reduction with LiAlH 4.

5. Bromination Next to the Carboxy Group: the Hell-Volhard-Zelinsky Reaction 19-12 Monobromination of alkanoic acids at the  -carbon can be accomplished by treatment with Br 2 in the presence of catalytic amounts of PBr 3. The PBr 3 is usually generated in situ by adding a little red phosphorous to the starting materials, which is then converted into PBr 3 by the bromine present.

7. Halogenation at the  -carbon of a carboxylic acid is a useful intermediate in the synthesis of 2-hydroxy- and 2-amino acids.

8. Biological Activity of Carboxylic Acids 19-13 Fatty acids are derived from the coupling of acetic acid. Acetic acid is the primary building block for the biosynthesis of more naturally occurring compounds than any other single precursor substance. The substance 3-methyl-3-butenyl pyrophosphate is the crucial intermediate in the synthesis of terpenes:

9. Fats are esters of long-chain carboxylic acids. Hydrolysis of fats, or saponification, yields the corresponding fatty acids. The most important fatty acids are between 12 and 22 carbons long and may contain cis carbon-carbon double bonds. Most fatty acids contain an even number of carbon atoms since they are derived directly from the polymerization of acetic acid units.

10. Reduction of the ketone to a methylene follows. The resulting acid replaces the acetic acid in step 1 and the process repeats until the carbon chain is 16 carbons long.

11. Arachidonic acid is a biologically important unsaturated fatty acid. Arachidonic acid is an essential fatty acid which is enzymatically converted into many important biological molecules:

12. Nature also produces complex polycyclic carboxylic acids. Many complex, biologically active natural products contain carboxylic acid groups. The carboxylic acid may not contribute directly to the biological activity, but may: Confer water solubility Allow for salt formation or ion transport Enable micellar-type aggregation