Carboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the SMILES formula -C(=O)-OH, usually written as -COOH. In general, the 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

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

The two electronegative oxygen atoms tend to pull the electron located within the hydrogen atom away , and the remaining proton H⁺ can more easily leave. This is an explanation using what are called inductive effects. 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 oxygens has what is called a partial double bond characteristic.

The presence of electronegative groups (such as -OH or -Cl) next to the carboxylic group 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).

Carboxylic acids are most readily identified as such by infrared spectrometry, the characteristic O-H stretch of the carboxyl group appears as a broad peak in the 2500 to 3000 cm^-1 region.

In ¹H NMR spectrometry the hydroxyl hydrogen appears in the 10-13 ppm region.

Synthesis

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Carboxylic acids can be prepared in the laboratory by various methods some of which are as follows:

• Complete oxidation of primary alcohols and aldehydes. This can be done with the Jones reagent or Tollens' reagent.
• Alkene oxidatative cleavage by potassium permanganate or chromic acid.
• Alkylbenzene oxidation of potassium permanganate to benzoic acids.
• Acid or base hydrolysis of nitriles.
• Hydrolysis of amides or esters (saponification).
• Carbonylation (reaction with carbon dioxide) of Grignard reagents.
• Disproportionation of an aldehyde in the Cannizzaro reaction.
• Rearrangement of diketones in the benzylic acid rearrangement.
• halogenation followed by hydrolysis of methyl ketones in the haloform reaction
• Less-common reactions involving the generation of benzoic acids are the von Richter reaction from nitrobenzenes and the Kolbe-Schmitt reaction from phenols.

Reactions

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• Carboxylic acids react with bases to form carboxylate salts, in which the hydrogen of the hydroxyl (-OH) group is replaced with a metal ion. Thus, ethanoic acid (the IUPAC name for acetic acid) reacts with sodium bicarbonate (commercial baking soda) to form sodium ethanoate (sodium acetate), carbon dioxide, and water: CH₃COOH + NaHCO₃ → CH₃COONa + CO₂ + H₂O
• Carboxyl groups also react with amine groups to form peptide bonds and with alcohols to form esters in Fischer esterification or the Mitsunobu reaction.
• Carboxylic acids react with thionyl chloride (SOCl₂) to form acyl chlorides. These are extremely reactive and useful to synthesize other organic compounds.
• Carboxylic acids can be reduced by LiAlH₄ to form primary alcohols, although this reaction can be sluggish, as a first step is often formation of the lithium carboxylate salt. Another reducing agent for this reaction is borane.
  • The Arndt-Eistert synthesis inserts an α-methylene group into a carboxylic acid.
  • The Curtius rearrangement converts carboxylic acids to isocyanates.
  • Carboxylic acids are decarboxylated in the Hunsdiecker reaction and α-brominated in the Hell-Volhard-Zelinsky halogenation.
  • The Dakin-West reaction converts an amino acid to the corresponding amino ketone.

  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

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  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).

  Some representative carboxylic acids include:

  • Amino acids – the building blocks of proteins
  • Fatty acids – where R is an alkane in saturated acids or an alkene in unsaturated acids
    • Formic acid (methanoic acid) – HCOOH, found in insect stings (formic from the Latin word meaning ants)
    • Acetic acid (ethanoic acid) – CH₃COOH, the principal component of vinegar
    • Acrylic acid (ethenoic acid) – CH₂=CHCOOH, used in polymer synthesis
    • Propionic acid (propanoic acid) – CH₃CH₂COOH
    • Butyric acid (butanoic acid) – found in rancid butter
    • Lauric acid (dodecanoic acid) – found in coconut oil
    • Docosahexaenoic acid – nutritional supplement
    • Eicosapentaenoic acid – nutritional supplement
  • Keto acids – acids of biochemical significance that contain a ketone group
    • Pyruvic acid
    • Acetoacetic acid
    • Lactic acid (2-hydroxypropanoic acid) – found in sour milk
  • 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

  See also

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  • Acid anhydride
  • Ester

  External links

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

  Carboxylic acids | Functional groups | Acids | Organic chemistry

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