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Carboxylic-acid

Structure of a carboxylic acid

Carboxylic-acid-group-3D

The 3D structure of the carboxyl group

Carboxyl-3D-space-filling-labelled

A space-filling model of the carboxyl group

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. [1] Carboxylic acids are Bronsted 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 Edit

Carboxylic acid dimers

Carboxylic acid dimers

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, while 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 weak acids, meaning 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 either by the stability of the acid, or the stability of the conjugate base using inductive effects or resonance effects.

Stability of the acidEdit

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 baseEdit

Resonance stabilization of carboxylic acids

Resonance stabilization of carboxylic acids

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. The negative charge on the conjugate base of benzoic acid is delocalized over the two oxygen atoms, as well as the benzene ring. Thus benzoic acid is more acidic than acetic acid.

SpectroscopyEdit

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.

SourcesEdit

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 methanol carbonylation with carbon monoxide, while long chain carboxylic acids are obtained by the hydrolysis of 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 industrially.

Synthesis Edit

Carboxylic acids can be produced by oxidation of primary alcohols and aldehydes with strong oxidants such as Jones reagent, potassium permanganate, or sodium chlorite. They may also be produced by the oxidative cleavage of olefins by potassium permanganate or potassium dichromate. In particular, any alkyl group on a benzene ring will be fully oxidized to a carboxylic acid, regardless of its chain length. This is the basis for the industrial synthesis of benzoic acid from toluene.

Carboxylic acids can also be obtained by the hydrolysis of nitriles, esters, or amides, with the addition of acid or base. They can also be prepared from the action of a Grignard reagent on carbon dioxide, though this method is not used industrially.

Carboxylic acids may also form from the following reactions:

ReactionsEdit

Carboxylic acids react with bases to form carboxylate salts, in which the hydrogen of the hydroxyl (-OH) group is replaced with a metal cation. Thus, acetic acid found in vinegar reacts with sodium bicarbonate (baking soda) to form sodium acetate, carbon dioxide, and water:

CH3COOH + NaHCO3 → CH3COONa + CO2 + H2O

Carboxylic acids also react with alcohols and amines to give esters and amides. Like other alcohols and phenols, the hydroxyl group on carboxylic acids may be replaced with a chlorine atom using thionyl chloride to give acyl chlorides.

COOH reduced by LAH

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

Carboxylic acids may be reduced by lithium aluminum hydride or borane to give primary alcohols, though the former reaction is sluggish. Esters are more easily reduced, and it may be more feasible to esterify the acid before reduction.[2]

Like 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 Curtius rearrangement converts carboxylic acids to isocyanates.
  • 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. Enzymes that catalyze these reactions are known as carboxylases (EC 6.4.1) and decarboxylases (EC 4.1.1).

Nomenclature and examples Edit

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:

  • Short chain saturated monocarboxylic acids
  • Medium chain saturated monocarboxylic acids
  • Short chain unsaturated monocarboxylic acids
    • Acrylic acid (2-propenoic acid) – CH2=CHCOOH, used in polymer synthesis
  • 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

See alsoEdit

External links Edit

ReferencesEdit

  1. Compendium of Chemical Terminology, carboxylic acids, accessed 15 Jan 2007.
  2. 2.0 2.1 2.2 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.
  1. REDIRECT Template:Functional group
ar:حمض كربوكسيلي

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