Everything about Carboxylic Acids totally explained
Carboxylic acids are
organic acids characterized by the presence of a
carboxyl group, which has the
formula -C(=O)OH, usually written -COOH or -CO
2H. Carboxylic acids are
Brønsted-Lowry acids — they're 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
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, whereas 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.
Carboxylic acids are widespread in nature and are typically
weak acids, meaning that 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 by either the stability of the acid or the stability of the
conjugate base using
inductive effects 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. .
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 (for example, octadecanoic acid). In common nomenclature, the suffix is usually -ic acid (for example, stearic acid).
Other carboxylic acids include:
Short-chain unsaturated monocarboxylic acids
- Acrylic acid (2-propanoic acid) – CH2=CHCOOH, used in polymer synthesis
Fatty acids – medium to long-chain saturated and unsaturated monocarboxylic acids, with even number of carbons
Amino acids – the building blocks of proteins
Keto acids – acids of biochemical significance that contain a ketone group
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
Tricarboxylic acids – containing three carboxyl groups
Alpha hydroxy acids – containing a hydroxy group
- Lactic acid (2-hydroxypropanoic acid) – found in sour milk
Further Information
Get more info on 'Carboxylic Acids'.
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