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

1.3.1. Hydrocarbons and Hydroxy Compounds

Hydrocarbons (and their derivatives) consist of hydrogen and carbon atoms. Their carbon chains can be open, forming acyclic (aliphatic) compounds, or closed, forming cyclic hydrocarbons. Saturated aliphatic hydrocarbons are called alkanes, while unsaturated ones include alkenes (containing one double bond), alkadienes (two double bonds), and alkynes (containing a triple bond). Cyclic hydrocarbons (also referred to as homocyclic or carbocyclic to distinguish them from heterocyclic compounds) are subdivided into alicyclic (saturated) and aromatic hydrocarbons (arenes).

Alkanes and their derivatives have long been used in medicine. These include liquid paraffin (a mixture of alkanes with carbon chains up to C₁₅), which acts as a laxative; petroleum jelly or petrolatum (a mixture of liquid and solid alkanes up to C₂₅), used as an ointment base; paraffin wax (C₁₈-C₃₅), employed as a physiotherapeutic agent due to its high heat capacity; and ozokerite (a natural mixture of higher alkanes), used similarly to paraffin. Chloroform (CHCl₃) and halothane (CF₃CHBrCl) are used for inhalation anesthesia, whereas ethyl chloride (C₂H₅Cl), owing to its high volatility, promotes Skin cooling and local anesthesia.

Monocyclic aromatic structures are integral components of numerous Biomolecules, including Amino Acids, Vitamins, Coenzymes, Hormones, Neurotransmitters, and Pharmaceuticals. The same applies to polycyclic condensed arenes. For instance, 1,4-naphthoquinone (a naphthalene derivative) is a structural component of vitamin K; phenanthrene hydrogenation products are found in various Alkaloids, including the morphine group; Tetracyclines (Antibiotics) are derivatives of the polycyclic aromatic hydrocarbon tetracene; and many condensed-ring arenes are known carcinogens present in coal tar, internal combustion engine emissions, and tobacco smoke.

Hydroxy compounds of hydrocarbon nature are substances in which hydrogen atoms are replaced by an -OH group. These include alcohols and phenols. Thiols are structurally and chemically similar to alcohols and phenols, differing in that the hydrogen atoms are replaced by an -SH group. Depending on THE POSITION OF the -OH group, alcohols are classified as primary (RCH₂-OH), secondary (R₂CH-OH), and tertiary (R₃C-OH). Polyhydric alcohols (polyols) contain multiple -OH groups. In biochemistry, Ethylene glycol and glycerol are of particular interest. Ethylene glycol is produced in micromolar amounts during metabolic processes, whereas high concentrations lead to nephropathy. Glycerol serves as the backbone for all phosphoglycerides, which are essential components of Introduction/36.html">Biological Membranes. Among cyclic polyols, significant attention is given to Inositol (cyclohexane-1,2,3,4,5,6-hexol), a key structural component of membrane lipid molecules such as phosphatidylinositol 4,5-bisphosphate.

Depending on the number of -OH groups, phenols are classified as monohydric, dihydric, or trihydric (phenols, arenediols, and arenetriols, respectively). The hydroxyl group is attached to a benzene carbon atom in an sp² Hybridization state:

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The chemical properties of phenols are governed by the mutual Influence of the -OH group and the benzene ring, which involves the conjugation of the lone pair of p-electrons from the oxygen atom of the hydroxyl group with the π-electrons of the aromatic ring (p,π-conjugation). As a result:

✵ electron density on the oxygen atom decreases, leading to increased polarization of the hydroxyl group;

✵ the polarized -OH group readily dissociates, which accounts for the acidic properties of phenols;

✵ the hydroxyl group increases the electron density within the benzene ring, with maxima at the ortho and para positions relative to the -OH group, thereby facilitating electrophilic substitution reactions.

The acidic properties of phenols enable their reactions with metals and metal hydroxides to form phenolates (phenoxides, such as C₆H₅ONa). Examples of electrophilic substitution reactions include the bromination and nitration of phenol, yielding 2,4,6-tribromophenol and 2,4,6-trinitrophenol (picric acid), respectively. These reactions are frequently utilized in laboratory practice for investigating biomolecules and BIOLOGICALLY ACTIVE SUBSTANCES whose molecules contain a phenolic ring.

Thiols (mercaptans) are hydrocarbon derivatives in which -OH groups are replaced by a thio (mercapto) group. Examples include thiols such as methanethiol (CH₃-SH) and ethanethiol (C₂H₅-SH, also referred to as methyl and ethyl mercaptans) as well as thiophenols:

As evident from their molecular Structure, thiols act as acids and react with metals to form thiolates (mercaptides): soluble ones like sodium ethanethiolate (CH₃CH₂-S-Na⁺), and insoluble ones such as mercury(II) ethanethiolate ((CH₃CH₂-S)₂Hg). The latter are used in medicine as antidotes for heavy metal poisoning.