Carboxylic acid



      Carboxylic acids are 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 group. Compounds may also have two or more carboxylic acid groups per molecule.

Physical properties

  Carboxylic acids are 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.[2]

Carboxylic acids are widespread in nature and are typically 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 resonance effects.

Stability of the acid

Using inductive effects, the acidity of carboxylic acids can be rationalized by the two 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, 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.[2]

In 1H NMR spectrometry, the hydroxyl hydrogen appears in the 10-13 ppm region, though it is often either broadened or not observed due to exchange with any traces of water.

Sources

Lower straight-chain aliphatic carboxylic acids, as well as those of even carbon number up to C18, are commercially available. For example, acetic acid is produced by triglycerides obtained from plant or animal oils.

Vinegar, a dilute solution of acetic acid, is biologically produced from the fermentation of ethanol. It is used in food and beverages, but is not used in industry.

Synthesis

  • Carboxylic acids can be produced by oxidation of primary alcohols and sodium chlorite.
  • They may also be produced by the oxidative cleavage of toluene.
  • Carboxylic acids can also be obtained by the hydrolysis of amides, with the addition of acid or base.
  • They can also be prepared from the action of a carbon dioxide, though this method is not used in industry.

Carboxylic acids may also form from the following reactions:

Reactions

CH3COOH + NaHCO3 → CH3COONa + CO2 + H2O
  • As with all carbonyl compounds, the protons on the α-carbon are labile due to keto-enol tautomerization. Thus the α-carbon is easily halogenated in the Hell-Volhard-Zelinsky halogenation.
  • The Arndt-Eistert synthesis inserts an α-methylene group into a carboxylic acid.
  • The isocyanates.
  • The amines.
  • Carboxylic acids are decarboxylated in the Hunsdiecker reaction.
  • The Dakin-West reaction converts an amino acid to the corresponding amino ketone.
  • In the Barbier-Wieland degradation (1912), the alpha-methylene group in an aliphatic carboxylic acid is removed in a sequence of reaction steps, effectively a chain-shortening [3] [4].
  • The addition of a carboxyl group to a compound is known as carboxylation; the removal of one is decarboxylation. 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 stearic acid).

Straight-Chained, Saturated Carboxylic Acids
Carbon atoms Common name IUPAC name Chemical formula Common location or use
1 Formic acid Methanoic acid HCOOH Insect stings
2 Acetic acid Ethanoic acid CH3COOH Vinegar
3 Propionic acid Propanoic acid CH3CH2COOH
4 Butyric acid Butanoic acid CH3(CH2)2COOH Rancid butter
5 Valeric acid Pentanoic acid CH3(CH2)3COOH
6 Caproic acid Hexanoic acid CH3(CH2)4COOH
7 Enanthic acid Heptanoic acid CH3(CH2)5COOH
8 Caprylic acid Octanoic acid CH3(CH2)6COOH
9 Pelargonic acid Nonanoic acid CH3(CH2)7COOH
10 Capric acid Decanoic acid CH3(CH2)8COOH
12 Lauric acid Dodecanoic acid CH3(CH2)10COOH Coconut oil
16 Palmitic acid Hexadecanoic acid CH3(CH2)14COOH
18 Stearic acid Octadecanoic acid CH3(CH2)16COOH

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
  • 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
  • Tricarboxylic acids – containing three carboxyl groups

See also

References

  1. ^ Compendium of Chemical Terminology, carboxylic acids, accessed 15 Jan 2007.
  2. ^ a b R.T. Morrison, R.N. Boyd. Organic Chemistry, 6th Ed. (1992) ISBN 0-13-643669-2.
  3. ^ Organic Syntheses, Coll. Vol. 3, p.234 (1955); Vol. 24, p.38 (1944) Link
  4. ^ Organic Syntheses, Coll. Vol. 3, p.237 (1955); Vol. 24, p.41 (1944) Link.
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