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An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, sulfonic acids, sulfinic acid and thionothiolic acid. In this review, we mainly talked about the low molecular weight carboxylic acids.
Fig. 1 Structure of carboxylic acids
Low molecular weight carboxylic acids include oxalic, succinic, tartaric, formic, malic, acetic, butyric, lactic, fumaric, maleic, and citric acids, all of which can derive from the decomposition of soil matter, plant root exudates, microbial metabolites, etc.
Many carboxylic acids are colorless liquids with disagreeable odors. The carboxylic acids with 5 to 10 carbon atoms all have special odors. The acids with more than 10 carbon atoms are waxlike solids, and their odor diminishes with increasing molar mass and resultant decreasing volatility.
Carboxylic acids exhibit strong hydrogen bonding between molecules. They therefore have high boiling points compared to other substances of comparable molar mass.
The carboxyl group readily engages in hydrogen bonding with water molecules. The acids with one to four carbon atoms are completely miscible with water. Solubility decreases as the carbon chain length increases because dipole forces become less important and dispersion forces become more predominant.
The carboxylic acids are weak in the sense that this ionization is very incomplete. At any one time, most of the acid will be present in the solution as un-ionized molecules. Acid strength is strongly correlated to stability of the conjugate base that will form by removing a proton. In order to analyze how acidic a molecule is likely to be, then you need to estimate the stability of its conjugate base.
The inductive effects of alkyl groups cause a significant variation in the acidities of different carboxylic acids. Notice that the inductive effect drops off after the alkyl chain is about three carbons long.
|HCOOH (Methanoic acid)||3.75|
|CH3COOH (Ethanoic Acid)||4.76|
|CH3CH2COOH (Propanoic Acid)||4.87|
|CH3CH2CH2COOH (Butanoic Acid)||4.82|
Table 1. pKa of different carboxylic acids
Due to their unique sensory indicators, as well as their physiological and chemical properties, low molecular weight carboxylic acids are widely used in all walks of life.
Low molecular weight carboxylic acids are commonly present in soils as a result of root exudation and the microbially mediated decomposition of soil organic matter.
A batch experiment was conducted to examine the effects of six low molecular weight carboxylic acids (acetic, formic, malic, citric, oxalic, tartaric) on the mobilization of arsenic and trace metals from a range of contaminated soils. The results showed that the carboxylic acids behaved differently when reacting with soil-borne arsenic and trace metals. Oxalic acid and acetic acid had the strongest and weakest capacity to mobilize the investigated elements, respectively .
Fig. 2 Acid soil
Low molecular weight carboxylic acids (LMWCA) can alter minerals by complexing ions at the surface, weakening metal-oxygen bonds, and catalyzing dissolution reactions.
Fig. 3 Interfacial reactions between carboxylic acids and iron-bearing clay minerals
LMWCA may influence the mobility of iron in two ways. First, complexation of Fe(III), through formation of bidentate Fe(III)-LMWCA complexes (e.g. pyruvate, oxalate), may enhance the dissolution of Fe(III) (oxy)hydroxides or may prevent precipitation of Fe(III) (oxy)hydroxides. Second, monodentate LMWCA (e.g. acetate) can be used by iron-reducing bacteria as electron donors (e.g. Acidophilum spp.), with ferric iron as the electron acceptor .
Low molecular weight carboxylic acids and their derivatives are frequently used in beverage, food, and feed production. Acidic additives may act as buffers to regulate acidity, antioxidants, preservatives, flavor enhancers, and sequestrants. Beneficial effects on animal health and growth performance have been observed when using acidic substances as feed additives.
Fig. 4 Application in food and beverage
Alfa Chemistry provides various of low molecular weight carboxylic acids to our customers from grams to kilograms. We can also customize synthesis according to customers' requirements. Please feel free to contact us for more information.