Carboxylic acid
From Freepedia
Carboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the formula -C(=O)-OH, usually written as "COOH". The salts and anions of carboxylic acids are called carboxylates generally.
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.
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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. The carboxylate anion R-COO- is usually named with the suffix "-ate", so acetic acid, for example, becomes acetate ion. 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 made 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
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 sulfonyl chloride (SO2Cl2) to form acyl chlorides. These are extremely reactive and useful to synthesise other organic compounds. Carboxylic acids can be reduced by LiAlH4 to form primary alcohols. 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.
Examples
Some carboxylic acids include:
- HCOOH formic acid (found in insect stings, formic refers to ants)
- all fatty acids, where R is an alkane in saturated acids and an alkene in unsaturated acids
- CH3COOH acetic or ethanoic acid (found in vinegar)
- CH3CH2COOH propionic acid
- C6H5COOH benzoic acid (sodium benzoate, the sodium salt of benzoic acid is used as a food preservative)
- butyric acid
- CH2=CHCOOH acrylic acid
- lactic acid, found in sour milk
- malic acid, found in apples
- oxalic acid
- malonic acid
- succinic acid
- glutaric acid
- adipic acid
- salicylic acid
- all amino acids
