Table of Contents
Introduction
In our day-to-day lives, we come across a wide range of substances that form the core of our food, drink, and other immediate interaction with hydroxy compounds. The hydroxy group is found all across chemistry and biology. A hydroxy or hydroxyl group is a functional group composed of one oxygen atom covalently bonded to one hydrogen atom and having the chemical formula OH. Compounds such as alcohols, phenols, and carboxylic acids contain these groups. Food items like glucose, sucrose, vinegar, and synthetic items like some plastics and other polymers also contain hydroxy groups. The variety of materials suggests that the hydroxy groups are quite versatile in their chemistry and thus constitute an integral part of organic and inorganic chemistry. The most common and abundant hydroxy compound on earth is however, water. As the most essential material that sustains life on earth it is undoubtedly the most important of the class. Many inorganic chemicals, notably sulfuric acid, the chemical compound generated on the largest industrial scale, include hydroxy groups.
In the nature, hydroxyl compounds occur in numerous forms not only on earth, but also on planets like Venus, Mars, the exoplanets, and on the surface of moon. On the earth, hydroxyl compounds are typically found in carbohydrates which form the backbone of the life process. Other hydroxy compounds mostly belonging to the alcohols or phenols are found in plants in large numbers and quantities.
Structure of hydroxy compounds
Alcohols, phenols, and carboxylic acids, the three types of hydroxy compounds have the —OH group in a crucial role as the functional group. Any organic molecule with at least one hydroxyl (OH) functional group attached to a saturated carbon atom is referred to as an alcohol. Phenols, sometimes known as phenolics, are those in which one or more hydroxyl groups (—OH) are directly linked to an aromatic hydrocarbon group. A carboxyl group (C(=O)OH) linked to an R-group makes up a carboxylic acid. A carboxylic acid's general formula is RCOOH, where R stands for an alkyl, alkenyl, aryl, or any other group.
Alcohols can be primary, secondary, or tertiary in nature depending on how many hydrogens are bonded to the carbon on which the hydroxyl group is attached.
In phenols, the functional group takes part in the resonance of the parent ring through the lone pair on the oxygen atom.
Structurally, carboxylic acids can be classified into many forms like unsaturated monocarboxylic acids, fatty acids, amino acids, keto acids, aromatic carboxylic acids, dicarboxylic acids, tricarboxylic acids, alpha hydroxy acids, beta hydroxy acids, omega hydroxy acids, and divenylether fatty acids. A carboxylate anion is formed when a carboxylic acid is deprotonated. Carboxylates have resonating structures.
Properties of hydroxy compounds
Acidity of alcohols is slightly weaker than that of water. Typically, solvation has a significant impact on the acidity of alcohols and thus they are more acidic in the gas phase than in water. They generate salts known as alkoxides when they react with strong bases such as sodium hydride or metallic sodium. More acidic than ordinary alcohols, phenols have hydroxyl group which typically has a level of acidity lying between that of aliphatic alcohols and carboxylic acids (pKa between 10 and 12). The analogous salts formed by reaction with metal hydroxides are known as phenolates or phenoxides because deprotonation of a phenol produces the equivalent negative phenolate ion or phenoxide ion. Due to their ability to donate protons (H+), carboxylic acids are Brønsted-Lowry acids. They represent the most prevalent class of organic acid. Carboxylic acids usually have mild acidity, which means that in neutral aqueous solution, they generally dissolve partly into H3O+ cations and RCOO- anions.
Thus, in terms of increasing order of acidity, these three classes of hydroxy compounds can be arranged in the increasing order as: alcohols < phenols < carboxylic acids.
Alcohols are typically highly soluble in water owing to the formation of hydrogen bonds between the hydroxyl group and the water molecules. Many sugars including glucose and sucrose are highly water soluble due to the presence of the alcoholic groups in them. In contrast, phenols are not as much soluble in water. They typically have an oily appearance and slightly miscible with water via hydrogen bonds. Carboxylic acids have polar properties. They engage in hydrogen bonding because they serve as both hydrogen-bond acceptors (-C=O) and donors (-OH). The hydroxyl and carbonyl groups combine to generate the carboxyl functional group. Because of their propensity to "self-associate," carboxylic acids typically reside as dimers in nonpolar media. Smaller carboxylic acids (1–5 carbons) are water soluble, though larger carboxylic acids have restricted solubility because of the increasing hydrophobicity of the alkyl chain. Longer chain acids are more soluble in less polar solvents like ethers and alcohols. All carboxylic acids, including the hydrophobic ones, react with aqueous sodium hydroxide to form water-soluble sodium salts.
Read more about Carbonyl Compounds
Uses of hydroxy compounds
- Alcohols, phenols, and carboxylic acids are used regularly in industrial as well as household applications.
- Ethanol is primarily used for alcoholic beverages.
- Methanol is primarily used for the production of formaldehyde and as a fuel additive.
- C6-C11 alcohols are used for plasticizers, such as in polyvinylchloride while fatty alcohols (C12-C18) are precursors to detergents.
- 1-propanol, 1-butanol, and isobutyl alcohol are used as solvents and precursors to other solvents.
- Phenol is the parent compound that is used as a disinfectant and for chemical synthesis.
- BHT (butylated hydroxytoluene) finds application as a fat-soluble antioxidant and food additive.
- Orthophenyl phenol is a fungicide that is used for waxing citrus fruits.
- Phenolphthalein is used as the most common pH indicator.
- Xylenol used in antiseptics & disinfectants.
- Phenolic compounds like L-DOPA, propofol, L-thyroxine, amoxycillin are some of the most well-known drugs used in modern times.
- Polymers, medicines, solvents, and food additives are all made with carboxylic acids.
- Acetic acid is a component of vinegar and a precursor to solvents and coatings.
- Acrylic, methacrylic, and adipic acids are precursors to polymers and adhesives.
- Citric acid is a naturally occurring acid in citrus fruits and also manufactured synthetically as a flavour and preservative in food and beverages.
- Carboxylate salts are used to manufacture soaps.