Carboxylic acids or organic acids are the compounds containing in the molecule the carboxyl functional group attached to the hydrocarbon radical. They are largely distributed in nature and are intermediates in the degradation pathways of amino acids, fats, and carbohydrates.
The carboxyl group consisting of a carbonyl (C=O) with a hydroxyl group (O–H) attached to the same carbon atom and is usually written as –COOH or CO2H. The compounds presenting two or more carboxylic groups are called dicarboxylic, tricarboxylic acids, while their salts and esters are called carboxylates. By the nature of the radical, they can be classified into saturated, unsaturated, or aromatic acids. In the International Union of Pure and Applied Chemistry (IUPAC) nomenclature, carboxylic acids have an “-oic acid” suffix added to hydrocarbons having the same number of carbon atoms. Still, some organic acids are called by their common name, for example, formic acid and acetic acid.
The molecular weight of organic acids varies widely from relatively small compounds such as formic and acetic acids too much larger compounds (fatty acids) with higher numbers of carboxylic and phenolic functional groups. Monocarboxylic acids with 5–10 carbon atoms in the chain are colorless liquids with unpleasant smells. As the carbon chain length increases (>10 carbon atoms) the acids are waxlike solids, and their smell diminishes with increasing molar mass and decreasing volatility.
Organic acids are weak acids with pKa values ranging from 3 (carboxylic) to 9 (phenolic) meaning that they do not dissociate totally in a neutral aqueous solution to produce H+ cations. The representative low molecular weight organic acids (formic, oxalic, and malic) have a relatively low pKa (<4.0).
Due to the presence of both hydroxyl and the carbonyl groups in the molecule, the carboxylic acids can exhibit hydrogen bonding with themselves leading to increased stabilization of the compounds and show elevated boiling points. They are polar molecules soluble in polar solvents, but as the alkyl chain increases their solubility decreases due to the hydrophobic nature of the carbon chain. In non-polar media, carboxylic acids exist as dimeric pairs due to their capacity to form hydrogen bonds .
2. Applications in Life Sciences
Carboxylic acids are compounds occurring naturally in different stages of life cycles (living organism-Krebs cycle; fermentation processes, and geological processes) or can be produced in the laboratories or at large scale (synthesis) from oxidation reactions of aldehydes, primary alcohols, and hydrocarbons, oxidative cleavage of olefins, base catalyzed dehydrogenation of alcohols or through the hydrolysis of nitriles, esters, or amides. The organic acids play significant and varied roles in our contemporary society as evidenced by multiple applications in the field of medicine, agriculture, pharmaceuticals, food, and other industries.
Carboxylic acids and their derivatives are used in the production of polymers, biopolymers, coatings, adhesives, and pharmaceutical drugs. They also can be used as solvents, food additives, antimicrobials, and flavorings.
Organic acids have important roles in the
Carboxylic acids also play significant roles in the
Solubilizer acting in modulating solubility, lipophilicity, and cell permeation (e.g. antibiotic or antihistaminic drug classes);
Prodrug and/or bioprecursor acting as compounds not biologically active but converted into active ones in specific conditions (e.g. drugs from antihypertensive, antithrombotic, or antiviral classes);
Pharmacophore providing specific interactions with an enzyme, triggering, or blocking its biological response (e.g. blood cholesterol-reducing drugs, nonsteroidal anti-inflammatory drugs).
Carboxylic acid-containing drugs play a major role in the medical treatment of pain and diseases .
They are also used in a wide variety of applications as ingredients in cosmetics. A class of organic acids with an important contribution in the cosmetic field is the alpha hydroxy acids (AHAs). Citric, malic, tartaric, and lactic and glycolic acids are part of this category and are extensively used in cosmetics for purposes such unblock/clean pores, improve the skin texture, whitening, anti-wrinkle, or acne treatment. Also, carboxylic acids represented by aldobionic acids (ABAs), retinoic acids, vitamin C, and azelaic acid are most effective in providing antioxidant and anti-aging protection, as well as improving moisture-retention [5, 6]. The carboxylic acid-based esters are the derivatives most well-known for their flavors and fragrances and are widely used in various applications including perfumes, deodorant, and air fresheners.
Fatty acids represent the class of carboxylic acids recognized for its utility in the cosmetic industry since their water-soluble salts (soaps) have been used as cleansers, since antiquity and are the most useful surfactants known.
Although is a controversy issue about the role of organic acids in
One chapter of this book offers, the detailed discussion of mechanisms of organic acid on the acquisition of soil phosphate in the fields of plant physiology, plant nutrition, and soil chemistry. Some plant species strongly mobilize soil phosphate by carboxylates improving this macronutrient acquisition.
The carboxylic acid compounds still may find applications that cannot be fully covered in this chapter. As conclusion, starting from food to medicine, from the human body to earth and environment, the production, destruction, absorption, or release of these compounds show a strong impact on all the processes/reactions that take place.
As a final conclusion, this subject is an endless one and the classes of compounds that contain the carboxyl functional group, along with all their derivatives, are inseparable from everything that life means on this earth.
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