Carboxylic acid

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Image:Carboxylic-acid.png Image:Carboxylic-acid-group-3D.png Image:Carboxyl-3D-space-filling-labelled.png Carboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the 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.

Contents 1 Acidity, electron distribution and resonance 2 Synthesis 3 Reactions 4 Nomenclature and examples 5 See also

Acidity, electron distribution and resonance

Carboxylic acids are widespread in nature. Carboxylic acids are typically weak acids that 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 away from the hydrogen of the hydroxyl group, and the remaining proton H⁺ can more easily leave. The remaining negative charge is then distributed symmetrically among the two oxygen atoms, and the two carbon–oxygen bonds take on a partial double-bond character (i.e., they are delocalised).

This is a result of the resonance structure created by the carbonyl component of the carboxylic acid, without which the OH group does not as easily lose its H⁺ (see alcohol).

The presence of electronegative groups (such as -OH or -Cl) next to the carboxylic group increases the acidity. So, 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 helping group).

Synthesis

Carboxylic acids can be prepared in the laboratory by various methods:

• Complete oxidation of primary alcohols and aldehydes. This can be done with the Jones reagent or Tollens' reagent.
• Alkene oxidation by potassium permanganate.
• Alkylbenzene oxidation of potassium permanganate to benzoic acids.
• Acid or base hydrolysis of nitriles.
• Hydrolysis of amides and esters.
• Carbonylation (reaction with carbon dioxide) of Grignard reagents.
• Disproportionation of an aldehyde in the Cannizzaro reaction.
• Rearrangement of diketones in the benzilic acid rearrangement.
• 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 -OH group is replaced with a metal ion. Thus, ethanoic acid (the same as acetic acid) reacts with sodium bicarbonate (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.

Image:COOH reduced by LAH.png

• 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.

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 (e.g. stearic acid).

Some representative carboxylic acids include:

• Formic acid (methanoic acid) – HCOOH, found in insect stings (formic refers to ants)
• Acrylic acid (ethenoic acid) - CH₂=CHCOOH, used in polymer synthesis
• Lactic acid (2-hydroxypropanoic acid) - found in sour milk
• Amino acids – the building blocks of proteins
• Valproic acid (2-propylpentanoic acid) - pharmaceutical

• Fatty acids - where R is an alkane in saturated acids or an alkene in unsaturated acids
  • Acetic acid (ethanoic acid) – CH₃COOH, the principal component of vinegar
  • 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

• 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

• Diacids
  • 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

• Triacids
  • Citric acid - found in citrus fruits

See also

• Acid anhydride
• Esterar:حمض كربوكسيلي

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