A carboxylic acid is an organic compound that contains a carboxyl groupOH). The general
formula of a carboxylic acid is R-C(O)OH with R referring to the rest of the molecule. Carboxylic
acids occur widely, and include the amino acids and acetic acid.
Salts and esters of carboxylic acids are called carboxylates. When a carboxyl group is deprotonated,
its conjugate base forms a carboxylate anion. Carboxylate ions are resonance stabilized
and this increased stability makes carboxylic acids more acidic than alcohols. Carboxylic
acids can be seen as reduced or alkylated forms of the Lewis acid carbon dioxide; under
some circumstances they can be decarboxylated to yield carbon dioxide.
Example carboxylic acids and nomenclature Carboxylic acids are commonly named as indicated
in the table below. Although rarely used, IUPAC-recommended names also exist. For example,
butyric acid is, according to IUPAC guidelines, also known as butanoic acid.
To more easily understand much of the below discussion of reactions involving carboxylic
acids it can be helpful to notice that the carboxyl group itself is a "hydroxylated carbonyl
group" meaning that 2 of the Carbon atom's 4 bonds are to an Oxygen atom, the Carbon
atom's 3rd bond is to a second Oxygen atom, and the Carbon atom's 4th bond attaches to
R. [A Carbon atom double bonded to an Oxygen atom is a carbonyl group and 2 of the Carbon
atom's bonds remain available for bonding. A Hydrogen atom bonded to an Oxygen atom is
a hydroxyl group with the Oxygen atom's second bond available for bonding.]
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. In common nomenclature, the suffix is usually ic acid.
Carboxyl radical The radical ·COOH has only a separate fleeting
existence. The acid dissociation constant of ·COOH has been measured using electron
paramagnetic resonance spectroscopy. The carboxyl group tends to dimerise to form oxalic acid.
Physical properties Solubility
Carboxylic acids are polar. Because they are both hydrogen-bond acceptors and hydrogen-bond
donors, they also participate in hydrogen bonding. Together the hydroxyl and carbonyl
group forms the functional group carboxyl. Carboxylic acids usually exist as dimeric
pairs in nonpolar media due to their tendency to “self-associate.” Smaller carboxylic
acids are soluble in water, whereas higher carboxylic acids are less soluble due to the
increasing hydrophobic nature of the alkyl chain. These longer chain acids tend to be
rather soluble in less-polar solvents such as ethers and alcohols.
Boiling points Carboxylic acids tend to have higher boiling
points than water, not only because of their increased surface area, but because of their
tendency to form stabilised dimers. Carboxylic acids tend to evaporate or boil as these dimers.
For boiling to occur, either the dimer bonds must be broken or the entire dimer arrangement
must be vaporised, both of which increase the enthalpy of vaporization requirements
significantly. Acidity
Carboxylic acids are Brønsted-Lowry acids because they are proton donors. They are the
most common type of organic acid. Carboxylic acids are typically weak acids,
meaning that they only partially dissociate into H+ cations and RCOO– anions in neutral
aqueous solution. For example, at room temperature, in a 1-molar solution of acetic acid, only
0.4% of the acid molecules are dissociated. Electronegative substituents give stronger
acids. Deprotonation of carboxylic acids gives carboxylate
anions; these are resonance stabilized, because the negative charge is delocalized over the
two oxygen atoms, increasing the stability of the anion. Each of the carbon-oxygen bonds
in the carboxylate anion has a partial double-bond character.
Odor Carboxylic acids often have strong odors,
especially the volatile derivatives. Most common are acetic acid and butyric acid. On
the other hand, esters of carboxylic acids tend to have pleasant odors and many are used
in perfume. Characterization
Carboxylic acids are readily identified as such by infrared spectroscopy. They exhibit
a sharp band associated with vibration of the C-O vibration bond between 1680 and 1725 cm−1.
A characteristic νO-H band appears as a broad peak in the 2500 to 3000 cm−1 region. By
1H NMR spectrometry, the hydroxyl hydrogen appears in the 10–13 ppm region, although
it is often either broadened or not observed owing to exchange with traces of water.
Occurrence and applications Many carboxylic acids are produced industrially
on a large scale. They are also pervasive in nature. Esters of fatty acids are the main
components of lipids and polyamides of aminocarboxylic acids are the main components of proteins.
Carboxylic acids are used in the production of polymers, pharmaceuticals, solvents, and
food additives. Industrially important carboxylic acids include acetic acid, acrylic and methacrylic
acids, adipic acid, citric acid, ethylenediaminetetraacetic acid, fatty acids, maleic acid, propionic
acid, terephthalic acid. Synthesis
Industrial routes In general, industrial routes to carboxylic
acids differ from those used on smaller scale because they require specialized equipment.
Oxidation of aldehydes with air using cobalt and manganese catalysts. The required aldehydes
are readily obtained from alkenes by hydroformylation. Oxidation of hydrocarbons using air. For simple
alkanes, the method is nonselective but so inexpensive to be useful. Allylic and benzylic
compounds undergo more selective oxidations. Alkyl groups on a benzene ring oxidized to
the carboxylic acid, regardless of its chain length. Benzoic acid from toluene and terephthalic
acid from para-xylene, and phthalic acid from ortho-xylene are illustrative large-scale
conversions. Acrylic acid is generated from propene.
Base-catalyzed dehydrogenation of alcohols. Carbonylation is versatile method when coupled
to the addition of water. This method is effective for alkenes that generate secondary and tertiary
carbocations, e.g. isobutylene to pivalic acid. In the Koch reaction, the addition of
water and carbon monoxide to alkenes is catalyzed by strong acids. Acetic acid and formic acid
are produced by the carbonylation of methanol, conducted with iodide and alkoxide promoters,
respectively, and often with high pressures of carbon monoxide, usually involving additional
hydrolytic steps. Hydrocarboxylations involve the simultaneous addition of water and CO.
Such reactions are sometimes called "Reppe chemistry":
HCCH + CO + H2O → CH2=CHCO2H Some long-chain carboxylic acids are obtained
by the hydrolysis of triglycerides obtained from plant or animal oils; these methods are
related to soap making. fermentation of ethanol is used in the production
of vinegar. Laboratory methods
Preparative methods for small scale reactions for research or for production of fine chemicals
often employ expensive consumable reagents. oxidation of primary alcohols or aldehydes
with strong oxidants such as potassium dichromate, Jones reagent, potassium permanganate, or
sodium chlorite. The method is amenable to laboratory conditions compared to the industrial
use of air, which is "greener," since it yields less inorganic side products such as chromium
or manganese oxides. Oxidative cleavage of olefins by ozonolysis,
potassium permanganate, or potassium dichromate. Carboxylic acids can also be obtained by the
hydrolysis of nitriles, esters, or amides, in general with acid- or base-catalysis.
Carbonation of a Grignard and organolithium reagents:
RLi + CO2 → RCO2Li RCO2Li + HCl → RCO2H + LiCl
Halogenation followed by hydrolysis of methyl ketones in the haloform reaction
The Kolbe-Schmitt reaction provides a route to salicylic acid, precursor to aspirin.
Less-common reactions Many reactions afford carboxylic acids but
are used only in specific cases or are mainly of academic interest:
Disproportionation of an aldehyde in the Cannizzaro reaction
Rearrangement of diketones in the benzilic acid rearrangement involving the generation
of benzoic acids are the von Richter reaction from nitrobenzenes and the Kolbe-Schmitt reaction
from phenols. Reactions
The most widely practiced reactions convert carboxylic acids into esters, amides, carboxylate
salts, acid chlorides, and alcohols. Carboxylic acids react with bases to form carboxylate
salts, in which the hydrogen of the hydroxyl group is replaced with a metal cation. Thus,
acetic acid found in vinegar reacts with sodium bicarbonate to form sodium acetate, carbon
dioxide, and water: CH3COOH + NaHCO3 → CH3COO−Na+ + CO2 +
H2O Carboxylic acids also react with alcohols
to give esters. This process is heavily used in the production of polyesters. Likewise,
carboxylic acids are converted into amides, but this conversion typically does not occur
by direct reaction of the carboxylic acid and the amine. Instead esters are typical
precursors to amides. The conversion of amino acids into peptides is a major biochemical
process that requires ATP. The hydroxyl group on carboxylic acids may
be replaced with a chlorine atom using thionyl chloride to give acyl chlorides. In nature,
carboxylic acids are converted to thioesters. Carboxylic acid can be reduced to the alcohol
by hydrogenation or using stoichiometric hydride reducing agents such as lithium aluminium
hydride. N,N-dimethylchloromethylenammonium chloride
is a highly chemoselective agent for carboxylic acid reduction. It selectively activate the
carboxylic acid and is known to tolerate active functionalities such as ketone as well as
the moderate ester, olefin, nitrile, and halide moeties.
Specialized reactions As with 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 Schmidt reaction converts carboxylic acids
to amines. 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, the alpha-methylene
bridge in an aliphatic carboxylic acid is removed in a sequence of reaction steps, in
effect a chain-shortening. The inverse procedure is the Arndt-Eistert synthesis, where an acid
is converted into acyl halide and reacts with diazomethane to give the highest homolog.
Many acids undergo oxidative decarboxylation. Enzymes that catalyze these reactions are
known as carboxylases and decarboxylases. Carboxylic acids are reduced to aldehydes
via the ester and DIBAL, via the acid chloride in the Rosenmund reduction and via the thioester
in the Fukuyama reduction. In ketonic decarboxylation carboxylic acids
are converted to ketones. The Kolbe electrolysis is an electrolytic,
decarboxylative dimerization reaction. In other words, it gets rid of the carboxyl groups
of two acid molecules, and joins the remaining fragments together.
See also Acid anhydride
Acid chloride Amide
Ester List of carboxylic acids
Pseudoacid References
External links Carboxylic acids pH and titration – freeware
for calculations, data analysis, simulation, and distribution diagram generation