BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 1. STRUCTURE AND REACTIVITY OF BIOORGANIC COMPOUNDS

1.3. Selected classes of bioorganic compounds and their biological significance

1.3.2. Carbonyl compounds

Aldehydes, ketones, and carboxylic acids belong to carbonyl compounds (oxo compounds), which are hydrocarbon derivatives containing a carbonyl or oxo group in their Structure (Table 1.1). The carbon atoms of the carbonyl group are in an sp-hybridized state, meaning they possess three hybridized orbitals utilized to form three σ-bonds with adjacent carbon atoms and an oxygen atom. Another bond between the carbonyl Carbon and Oxygen is a π-bond. The high electronegativity of the carbonyl oxygen atom causes a shift of the π-bond electron density toward it, thereby generating an electrophilic center in the carbonyl group:

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Due to this electronic STRUCTURE OF THE carbonyl group, the carbon atom is susceptible to nucleophilic attack, while the oxygen is susceptible to electrophilic attack—this constitutes a nucleophilic addition reaction. An example of such reactions is The formation of cyanohydrins (α-hydroxynitriles):

The interaction of aldehydes and other carbonyl compounds with alcohols results in the formation of acetals (containing two alkoxyl groups attached to the carbon atom) and hemiacetals (possessing an alkoxyl group and an OH group at the same carbon atom):

The interaction of these substances with H₂O leads to the formation of hydrates (diols). For instance, dissolving chloral (trichloroacetaldehyde) in Water yields chloral hydrate. It exhibits narcotic properties and is used in medicine as a hypnotic.

The interaction of aldehydes and ketones with nitrogen-containing compounds of the type X-NH₂ (where X = -H, -OH, -NH₂, -C₆H₅) proceeds via a nucleophilic addition-elimination mechanism:

This yields an imine—a Schiff base—which serves as an intermediate in the biochemical reactions of Intermediate Amino acid METABOLISM, particularly Transamination and the reductive amination of α-keto acids to α-Amino Acids.

Aldehydes and ketones undergo reduction reactions to form primary (from aldehydes) and secondary (from ketones) alcohols. In living organisms, these reactions are catalyzed by dehydrogenases, with NADH and NADPH (nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate) acting as hydrogen Donors:

Oxidation reactions are characteristic only of aldehydes, yielding carboxylic acids, which are used in biochemistry for the detection of aldehydes:

Aldehydes and ketones are crucial metabolic intermediates generated in the body through the metabolism of Monosaccharides, Fatty acids, and amino acids. Formaldehyde (formic aldehyde, methanal, CH₂O) is produced in The Human Body as a product of N-dealkylation of drugs in hepatocytes. Acetone (dimethyl ketone, CH₃-CO-CH₃) is formed during Glycolysis both in a free state and as the phosphate ester dihydroxyacetone phosphate. Acetaldehyde (CH₃-CHO) is a constituent of acetyl-coenzyme A (CH₃-CO-S-CoA), a universal intermediate in Acetylation reactions and other processes.

Aldehydes and ketones are widely applied in biomedical practice:

✵ carbonyl groups serve as functional groups in numerous medications;

✵ formaldehyde, in the form of a 37-40% aqueous solution (formalin), is used for disinfection and as a preservative for anatomical specimens;

✵ acetone is employed as an organic solvent in the pharmaceutical industry, etc.