Low Molecular Weight Acids

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Low Molecular Weight Acids List


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.

Structure of carboxylic acidsFig. 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.

Properties of Carboxylic Acids

  • Physical properties

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.

  • Chemical properties

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

Application of 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.

  • In soils

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 [1].

Acid soilFig. 2 Acid soil

  • In minerals

Low molecular weight carboxylic acids (LMWCA) can alter minerals by complexing ions at the surface, weakening metal-oxygen bonds, and catalyzing dissolution reactions.

Interfacial reactions between carboxylic acids and iron-bearing clay mineralsFig. 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 [2].

  • In beverage, food, and feed production

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.

Application in food and beverageFig. 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.


  1. Obinna Elijah Nworie, Junhao Qin and Chuxia Lin. Differential Effects of Low-Molecular-Weight Organic Acids on the Mobilization of Soil-Borne Arsenic and Trace Metals. Toxics. 2017, 5, 18.
  2. Bernhard Dold, David W Blowes, etc. Low Molecular Weight Carboxylic Acids in Oxidizing Porphyry Copper Tailings. Environ. Sci. Technol. 2005, 39, 2515-2521.

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