CARBOXYLIC ACID

A space-filling model of the carboxyl group

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

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

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

• 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.

Lithium aluminium hydride reduction of a carboxylic acid to an alcohol.

• 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

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

• Acid anhydride
• Ester

External links

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