| Carboxylic Acid |
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Carboxylic acids are s — they are proton donors. Salt s and Anion s 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 s]] Carboxylic acids are Polar , and form Hydrogen Bond s 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.R.T. Morrison, R.N. Boyd. ''Organic Chemistry'', 6th Ed. (1992) ISBN 0-13-643669-2. Carboxylic acids are widespread in nature and are typically Weak Acid s, meaning they only partially Dissociate into H+ Cation s and RCOO− Anion s 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 Effect s or Resonance effects. Stability of the acid 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 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. 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 Methanol carbonylation with Carbon Monoxide , while long chain carboxylic acids are obtained by the hydrolysis of Triglyceride s 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 Carboxylic acids can be produced by Oxidation of Primary Alcohols and Aldehyde s with strong oxidants such as Jones Reagent , Potassium Permanganate , or Sodium Chlorite . They may also be produced by the oxidative cleavage of Olefin s 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 Nitrile s, Ester s, or Amide s, 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:
REACTIONS
:CH3COOH + NaHCO3 → CH3COONa + CO2 + H2O
: Reduction of a carboxylic acid to an alcohol.]]
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 ). Other carboxylic acids include:
SEE ALSO REFERENCES EXTERNAL LINKS
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