|Acids and Bases|
An organic acid is an organic compound with acidic properties. The relative stability of the conjugate base of the acid determines its acidity. In biological systems, organic compounds containing these groups are generally referred to as organic acids.
Common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are stronger acids. Phenols, with aromatic –OH, are usually much weaker. Phosphoric acid derivatives are also moderately strong. Other groups, such as the thiol group –SH and the enol group, can also confer acidity, usually weakly.
In general, organic acids are weak acids, i.e. they do not fully dissociate in water. Lower molecular mass organic acids such as formic and lactic acids are miscible in water, but higher molecular mass organic acids, such as benzoic acid and oleic acid, are less soluble. Organic acids are very soluble in organic solvents. p-Toluenesulfonic acid is a comparatively strong acid that dissolves in many organic solvents.
The pKa, a logarithmic measure of the acid dissociation constant, categorizes the strength of an acid: the lower, or more negative, the number, the stronger the acid. A few examples include: (COOH is the carboxyl group)
|Formic acid (methanoic acid)||HCOOH||3.8|
|Acetic acid (ethanoic acid)||CH3COOH||4.7|
|Propionic acid (propanoic acid)||CH3CH2COOH||4.9|
|Butyric acid (butanoic acid)||CH3CH2CH2COOH||4.8|
|Valeric acid (pentanoic acid)||CH3CH2CH2CH2COOH||4.8|
|Caproic acid (hexanoic acid)||CH3CH2CH2CH2CH2COOH||4.9|
|Oxalic acid (ethanedioic acid)||(COOH)(COOH)||1.2|
|Lactic acid (2-hydroxypropanoic acid)||CH3CHOHCOOH||3.9|
|Malic acid (2-hydroxybutanedioic acid)||(COOH)CH2CHOH(COOH)||3.4|
|Citric acid (2-hydroxypropane-1,2,3-tricarboxylic acid)||CH2(COOH)COH(COOH)CH2(COOH)||3.1||Abundant in citrus fruits, especially in lemon|
|Benzoic acid (Benzenecarboxylic acid (IUPAC) or phenylmethanoic acid (not IUPAC name))||C6H5COOH||4.2|
|Carbonic acid (hydroxymethanoic acid, not an IUPAC name)||OHCOOH or H2CO3||3.6||Carbonic acid, a weak acid formed by CO2 gas dissolving in water, is considered inorganic for historical reasons as its dissociation products are carbonate and bicarbonate ions.|
|Phenol (carbolic acid or hydroxybenzene, not IUPAC names)||C6H5OH||9.9||The 19th century name carbolic acid came from the original German name karbolsäure, or coal-oil-acid. It is a weak, non-carboxylic acid, but is corrosive and causes chemical burns on the skin due to a protein-degenerating effect.|
|Uric acid (7,9-Dihydro-1H-purine-2,6,8(3H)-trione)||C5H4N4O3||5.6||Uric acid is a heterocyclic purine derivative which is a diprotic acid but not a carboxylic one; it loses a hydrogen ion at the location of a nitrogen atom.|
|Taurine (2-aminoethanesulfonic acid)||C2H7NO3S||9.0||One of the few natural sulfonic acids, discovered in bile.|
|p-Toluenesulfonic acid (4-methylbenzenesulfonic acid)||CH3C6H4SO3H||-2.8||A strong acid that, unlike some strong mineral acids, is non-oxidizing.|
|Trifluoromethanesulfonic acid (triflic acid)||CF3SO3H||-12||One of the strongest acids in existence, organic or inorganic; about a thousand times stronger than sulfuric acid; known as a superacid. First synthesized in 1954, it also resists oxidation/reduction reactions and does not sulfonate substrates like sulfuric or chlorosulfonic acid.|
|Aminomethylphosphonic acid||CH6NO3P||0.4||A strong phosphonic acid|
In terms of production, the dominant organic acid is acetic acid. It is heavily used for the synthesis of polymers, e.g. polyvinyl acetate. It is also a precursor to myriad compounds such as pharmaceuticals, agrichemicals and fragrances.
The global organic acid market was $6.94 billion in 2016, and is projected to grow at a CAGR of 6.11% in value terms to reach $12.54 billion by the end of 2026, on the back of rising demand for natural and environmentally acceptable additives and rapid growth in food & beverage processing industry across the globe.
Application in food
Organic acids are used in food preservation because they have bacteriostatic properties. The key principle on the mode of action of organic acids on bacteria is that non-dissociated (non-ionized) organic acids can penetrate the bacteria cell wall and disrupt the normal physiology of certain types of bacteria that we call pH-sensitive, meaning that they cannot tolerate a wide internal and external pH gradient. Among those bacteria are Escherichia coli, Salmonella spp., C. perfringens, Listeria monocytogenes, and Campylobacter species.
Upon passive diffusion of organic acids into the bacteria, where the pH is near or above neutrality, the acids will dissociate and lower the bacteria internal pH, leading to conditions that will impair or stop the growth of bacteria. Thereafter, the anionic part of the organic acids, which cannot escape the bacteria in its dissociated form, will accumulate within the bacteria and disrupt many metabolic functions, leading to osmotic pressure increase, incompatible with the survival of the bacteria.
Carboxylic acids are pervasive in nature. Major classes of compounds include the amino acids and fatty acids, which comprise building blocks of life. Citric, lactic, pyruvic, oxaloacetic and other acids are continuously recycled in energy producing systems such as the citric acid cycle.
For commercial applications, almost all organic acids are produced industrially. Illustrative of the scale of the industrial production, 5.5 billion kilograms of acetic acid were produced in 1996. Dominant acids include acetic acid produced by carbonylation]] of methanol, propionic acid produced by hydrocarboxylation of ethylene, and terephthalic acid produced by aerobic oxidation of para-xylene. Sulfonic acids are produced by treatment of organic compounds with sulfur trioxide or sulfuric acid.
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