carboxylic adj : relating to or containing the carboxyl group or carboxyl radical [syn: carboxyl]

English

Adjective

1. Of, or relating to the carboxyl functional group.

Derived terms

• carboxylic acid

Translations

Carboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the formula -C(=O)OH, usually written -COOH or -CO2H. Carboxylic acids are Brønsted-Lowry acids — they are proton donors. Salts and anions of carboxylic acids are called carboxylates.

The simplest series of carboxylic acids are the alkanoic acids, R-COOH, where R is a hydrogen or an alkyl group. Compounds may also have two or more carboxylic acid groups per molecule.

Physical properties

Carboxylic acids are polar, and form hydrogen bonds with each other. At high temperatures, in vapor phase, carboxylic acids usually exist as dimeric pairs. Lower carboxylic acids (1 to 4 carbons) are miscible with water, whereas higher carboxylic acids are very much less-soluble due to the increasing hydrophobic nature of the alkyl chain. They tend to be rather soluble in less-polar solvents such as ethers and alcohols.

Carboxylic acids are widespread in nature and are typically weak acids, meaning that they only partially dissociate into H+ cations and RCOO− anions in aqueous solution. For example, at room temperature, only 0.02 % of all acetic acid molecules are dissociated in water.

Since the carboxylic acids are weak acids, in water, both forms exist in an equilibrium:

RCOOH ↔ RCOO− + H+

The acidity of carboxylic acids can be explained by either the stability of the acid or the stability of the conjugate base using inductive effects or resonance effects.

Stability of the acid

Using inductive effects, the acidity of carboxylic acids can be rationalized by the two electronegative oxygen atoms distorting the electron clouds surrounding the O-H bond, weakening it. The weak O-H bond causes the acid molecule to be less stable, and causing the hydrogen atom to be labile, thus it dissociates easily to give the H+ ion. Since the acid is unstable, the equilibrium will lie on the right.

Additional electronegative atoms or groups, such as chlorine or hydroxyl, substituted on the R-group have a similar, though lesser effect. The presence of these groups increases the acidity through inductive effects. For example, trichloroacetic acid (three -Cl groups) is a stronger acid than lactic acid (one -OH group), which in turn is stronger than acetic acid (no electronegative constituent).

Stability of the conjugate base

The acidity of a carboxylic acid can also be explained by resonance effects. The result of the dissociation of a carboxylic acid is a resonance stabilized product in which the negative charge is shared (delocalized) between the two oxygen atoms. Each of the carbon-oxygen bonds has what is called a partial double-bond characteristic. Since the conjugate base is stabilized, the above equilibrium lies on the right.

Spectroscopy

Carboxylic acids are most readily identified as such by infrared spectrometry. They exhibit a sharp C=O stretch between 1680 and 1725 cm−1, and the characteristic O-H stretch of the carboxyl group appears as a broad peak in the 2500 to 3000 cm−1 region. .

• The addition of a carboxyl group to a compound is known as carboxylation; the removal of one is decarboxylation. Enzymes that catalyze these reactions are known as carboxylases (EC 6.4.1) and decarboxylases (EC 4.1.1).

Nomenclature and examples

The carboxylate anion R-COO– is usually named with the suffix -ate, so acetic acid, for example, becomes acetate ion. In IUPAC nomenclature, carboxylic acids have an -oic acid suffix (e.g., octadecanoic acid). In common nomenclature, the suffix is usually -ic acid (e.g., stearic acid).

Other carboxylic acids include:

• Short-chain unsaturated monocarboxylic acids
  • Acrylic acid (2-propanoic acid) – CH2=CHCOOH, used in polymer synthesis
• Fatty acids – medium to long-chain saturated and unsaturated monocarboxylic acids, with even number of carbons
  • Docosahexaenoic acid – nutritional supplement
  • Eicosapentaenoic acid – nutritional supplement

• Amino acids – the building blocks of proteins

• Keto acids – acids of biochemical significance that contain a ketone group
  • Pyruvic acid
  • Acetoacetic acid

• Aromatic carboxylic acids
  • Benzoic acid – C6H5COOH; sodium benzoate, the sodium salt of benzoic acid is used as a food preservative
  • Salicylic acid – found in many skin care products

• Dicarboxylic acids – containing two carboxyl groups
  • Aldaric acid – a family of sugar acids
  • Oxalic acid – found in many foods
  • Malonic acid
  • Malic acid – found in apples
  • Succinic acid – a component of the citric acid cycle
  • Glutaric acid
  • Adipic acid – the monomer used to produce nylon

• Tricarboxylic acids – containing three carboxyl groups
  • Citric acid – found in citrus fruits
  • Isocitric acid
  • Aconitic acid
  • Propane-1,2,3-tricarboxylic acid (tricarballylic acid, carballylic acid)
• Alpha hydroxy acids – containing a hydroxy group
  • Lactic acid (2-hydroxypropanoic acid) – found in sour milk

See also

• Acid anhydride
• Acid chloride
• Amide
• Ester

References

External links

• Carboxylic acids pH and titration - freeware for calculations, data analysis, simulation, and distribution diagram generation