Solubility
Carboxylic acids are polar. Because they are both hydrogen-bond acceptors (the carbonyl) and hydrogen-bond donors (the hydroxyl), 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 (1 to 5 carbons) are soluble with 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 enthalpy of vaporisation requirements significantly.
Acidity
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, only 0.02 % of all acetic acid molecules are dissociated. Electronegative substituents give stronger acids.
| Carboxylic Acids | pKa |
| Formic acid (HCO2H) | 3.77 |
| Acetic acid (CH3COOH) | 4.76 |
| Chloroacetic acid (CH2ClCO2H) | 2.86 |
| Dichloroacetic acid (CHCl2CO2H) | 1.29 |
| Trichloroacetic acid (CCl3CO2H) | 0.65 |
| Trifluoroacetic acid (CF3CO2H) | 0.5 |
| Oxalic acid (HO2CCO2H) | 1.27 |
| Benzoic acid (C6H5CO2H) | 4.2 |
Deprotonation of a carboxylic acid gives a carboxylate anion, which is resonance stabilized because the negative charge is shared (delocalized) between the two oxygen atoms increasing its stability. Each of the carbon-oxygen bonds in a carboxylate anion has partial double-bond character.
Odor
Carboxylic acids often have strong odors, especially the volatile derivatives. Most common are acetic acid (vinegar) and butyric acid (rancid butter). On the other hand, esters of carboxylic acids tend to have pleasant odors and many are used in perfumes.
Nomenclature
The simplest series of carboxylic acids are the alkanoic acids, RCOOH, where R is a hydrogen or an alkyl group. Compounds may also have two or more carboxylic acid groups per molecule. The mono- and dicarboxylic acids have trivial names.
Characterization
Carboxylic acids are most readily identified as such by infrared spectroscopy. They exhibit a sharp band associated with vibration of the C-O vibration bond (νC=O) 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 (component of vinegar, precursor to solvents and coatings), acrylic and methacrylic acids (precursors to polymers, adhesives), adipic acid (polymers), citric acid (beverages), ethylenediaminetetraacetic acid (chelating agent), fatty acids (coatings), maleic acid (polymers), propionic acid (food preservative), terephthalic acid (polymers).
Synthesis
Industrial routes
Industrial routes to carboxylic acids generally 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, generally 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 (-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 → CH3COO−Na+ + CO2 + H2O
Carboxylic acids also react with alcohols to give esters. This process is heavily used in the production of polyesters. Similarly 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 group in an aliphatic carboxylic acid is removed in a sequence of reaction steps, effectively 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 decarboxylation. Enzymes that catalyze these reactions are known as carboxylases (EC 6.4.1) and decarboxylases (EC 4.1.1).
- 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.
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).
Straight-Chained, Saturated Carboxylic Acids
Carbon atoms | Common name | IUPAC name | Chemical formula | Common location or use |
1 | Formic acid | Methanoic acid | HCOOH | Insect stings |
2 | Acetic acid | Ethanoic acid | CH3COOH | Vinegar |
3 | Propionic acid | Propanoic acid | CH3CH2COOH | Preservative for stored grains |
4 | Butyric acid | Butanoic acid | CH3(CH2)2COOH | Rancid butter |
5 | Valeric acid | Pentanoic acid | CH3(CH2)3COOH | Valerian |
6 | Caproic acid | Hexanoic acid | CH3(CH2)4COOH | Goat fat |
7 | Enanthic acid | Heptanoic acid | CH3(CH2)5COOH |
|
8 | Caprylic acid | Octanoic acid | CH3(CH2)6COOH | Coconuts and breast milk |
9 | Pelargonic acid | Nonanoic acid | CH3(CH2)7COOH | Pelargonium |
10 | Capric acid | Decanoic acid | CH3(CH2)8COOH |
|
11 | Lauric acid | Dodecanoic acid | CH3(CH2)10COOH | Coconut oil and hand wash soaps. |
12 | Myristic acid | Tetradecanoic acid | CH3(CH2)12COOH | Nutmeg |
13 | Palmitic acid | Hexadecanoic acid | CH3(CH2)14COOH | Palm oil |
14 | Stearic acid | Octadecanoic acid | CH3(CH2)16COOH | Chocolate, waxes, soaps, and oils |
205 | Arachidic acid | Icosanoic acid | CH3(CH2)18COOH | Peanut oil |
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Other carboxylic acids
| Compound class | Members |
| unsaturated monocarboxylic acids | acrylic acid (2-propenoic acid) – CH2=CHCOOH, used in polymer synthesis |
| Fatty acids | medium to long-chain saturated and unsaturated monocarboxylic acids, with even number of carbons examples docosahexaenoic acid and eicosapentaenoic acid (nutritional supplements) |
| Amino acids | the building blocks of proteins |
| Keto acids | acids of biochemical significance that contain a ketone group e.g. acetoacetic acid and pyruvic acid |
| Aromatic carboxylic acids | benzoic acid, the sodium salt of benzoic acid is used as a food preservative , salicylic acid – a beta hydroxy type found in many skin care products |
| Dicarboxylic acids | containing two carboxyl groups examples adipic acid the monomer used to produce nylon and aldaric acid – a family of sugar acids |
| Tricarboxylic acids | containing three carboxyl groups example citric acid – found in citrus fruits and isocitric acid |
| Alpha hydroxy acids | containing a hydroxy group example glyceric acid, glycolic acid and lactic acid (2-hydroxypropanoic acid) – found in sour milk tartaric acid - found in win |
