In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group attached to an R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl group (e.g., alkyl, alkenyl, aryl), or hydrogen, or other groups. Carboxylic acids occur widely. Important examples include the amino acids and fatty acids. Deprotonation of a carboxylic acid gives a carboxylate anion.
Carboxylic acids are commonly identified by their trivial names. They often have the suffix -ic acid. IUPAC-recommended names also exist; in this system, carboxylic acids have an -oic acid suffix.[1] For example, butyric acid is butanoic acid by IUPAC guidelines. For nomenclature of complex molecules containing a carboxylic acid, the carboxyl can be considered position one of the parent chain even if there are other substituents, such as 3-chloropropanoic acid. Alternately, it can be named as a "carboxy" or "carboxylic acid" substituent on another parent structure, such as 2-carboxyfuran.
The carboxylate anion (or) of a carboxylic acid is usually named with the suffix -ate, in keeping with the general pattern of -ic acid and -ate for a conjugate acid and its conjugate base, respectively. For example, the conjugate base of acetic acid is acetate.
Carbonic acid, which occurs in bicarbonate buffer systems in nature, is not generally classed as one of the carboxylic acids, despite that it has a moiety that looks like a COOH group.
| 0 | ? | OO | Air | ||
| 1 | Methanoic acid | HCOOH | Insect stings | ||
| 2 | Ethanoic acid | Vinegar | |||
| 3 | Propanoic acid | Preservative for stored grains, body odour, milk, butter, cheese | |||
| 4 | Butanoic acid | Butter | |||
| 5 | Pentanoic acid | Valerian plant | |||
| 6 | Hexanoic acid | Goat fat | |||
| 7 | Heptanoic acid | Fragrance | |||
| 8 | Octanoic acid | Coconuts | |||
| 9 | Nonanoic acid | Pelargonium plant | |||
| 10 | Decanoic acid | Coconut and Palm kernel oil | |||
| 11 | Undecanoic acid | Anti-fungal agent | |||
| 12 | Dodecanoic acid | Coconut oil and hand wash soaps | |||
| 13 | Tridecanoic acid | Plant metabolite | |||
| 14 | Tetradecanoic acid | Nutmeg | |||
| 15 | Pentadecanoic acid | Milk fat | |||
| 16 | Hexadecanoic acid | Palm oil | |||
| 17 | Heptadecanoic acid | Pheromone in various animals | |||
| 18 | Octadecanoic acid | Chocolate, waxes, soaps, and oils | |||
| 19 | Nonadecanoic acid | Fats, vegetable oils, pheromone | |||
| 20 | Icosanoic acid | Peanut oil |
| unsaturated monocarboxylic acids | acrylic acid (2-propenoic acid) –, 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; examples: acetoacetic acid and pyruvic acid | |
| Aromatic carboxylic acids | containing at least one aromatic ring; examples: 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; phenyl alkanoic acids – the class of compounds where a phenyl group is attached to a carboxylic acid | |
| 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; examples: citric acid – found in citrus fruits and isocitric acid | |
| Alpha hydroxy acids | containing a hydroxy group in the first position; examples: glyceric acid, glycolic acid and lactic acid (2-hydroxypropanoic acid) – found in sour milk, tartaric acid – found in wine | |
| Beta hydroxy acids | containing a hydroxy group in the second position | |
| Omega hydroxy acids | containing a hydroxy group beyond the first or second position | |
| containing a doubly unsaturated carbon chain attached via an ether bond to a fatty acid, found in some plants |
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 form the functional group carboxyl. Carboxylic acids usually exist as dimers in nonpolar media due to their tendency to "self-associate". Smaller carboxylic acids (1 to 5 carbons) are soluble in water, whereas bigger carboxylic acids have limited solubility due to the increasing hydrophobic nature of the alkyl chain. These longer chain acids tend to be soluble in less-polar solvents such as ethers and alcohols.[2] Aqueous sodium hydroxide and carboxylic acids, even hydrophobic ones, react to yield water-soluble sodium salts. For example, enanthic acid has a low solubility in water (0.2 g/L), but its sodium salt is very soluble in water.
Carboxylic acids tend to have higher boiling points than water, because of their greater surface areas and their tendency to form stabilized dimers through hydrogen bonds. For boiling to occur, either the dimer bonds must be broken or the entire dimer arrangement must be vaporized, increasing the enthalpy of vaporization requirements significantly.
Carboxylic acids are Brønsted–Lowry acids because they are proton (H+) donors. They are the most common type of organic acid.
Carboxylic acids are typically weak acids, meaning that they only partially dissociate into cations and anions in neutral aqueous solution. For example, at room temperature, in a 1-molar solution of acetic acid, only 0.001% of the acid are dissociated (i.e. 10−5 moles out of 1 mol). Electron-withdrawing substituents, such as -CF3 group, give stronger acids (the pKa of acetic acid is 4.76 whereas trifluoroacetic acid, with a trifluoromethyl substituent, has a pKa of 0.23). Electron-donating substituents give weaker acids (the pKa of formic acid is 3.75 whereas acetic acid, with a methyl substituent, has a pKa of 4.76)
| Carboxylic acid[3] | pKa |
|---|---|
| Formic acid | 3.75 |
| Chloroformic acid | 0.27[4] |
| Acetic acid | 4.76 |
| Glycine | 2.34 |
| Fluoroacetic acid | 2.586 |
| Difluoroacetic acid | 1.33 |
| Trifluoroacetic acid | 0.23 |
| Chloroacetic acid | 2.86 |
| Dichloroacetic acid | 1.29 |
| Trichloroacetic acid | 0.65 |
| Benzoic acid | 4.2 |
| 2-Nitrobenzoic acid (ortho-) | 2.16 |
| Oxalic acid (first dissociation) | 1.27 |
| Hydrogen oxalate (second dissociation of oxalic acid) | 4.14 |
Carboxylic acids often have strong sour odours. Esters of carboxylic acids tend to have fruity, pleasant odours, and many are used in perfume.
Carboxylic acids are readily identified as such by infrared spectroscopy. They exhibit a sharp band associated with vibration of the C=O carbonyl 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.[2] [5] 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.
Many carboxylic acids are produced industrially on a large scale. They are also frequently found 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 (a flavor and preservative in food and beverages), ethylenediaminetetraacetic acid (chelating agent), fatty acids (coatings), maleic acid (polymers), propionic acid (food preservative), terephthalic acid (polymers). Important carboxylate salts are soaps.
In general, industrial routes to carboxylic acids differ from those used on a smaller scale because they require specialized equipment.
Preparative methods for small scale reactions for research or for production of fine chemicals often employ expensive consumable reagents.
Many reactions produce carboxylic acids but are used only in specific cases or are mainly of academic interest.
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. For example, acetic acid found in vinegar reacts with sodium bicarbonate (baking soda) to form sodium acetate, carbon dioxide, and water:
Carboxylic acids also react with alcohols to give esters. This process is widely used, e.g. 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 significant 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.
Like esters, most carboxylic acids can be reduced to alcohols by hydrogenation, or using hydride transferring agents such as lithium aluminium hydride. Strong alkyl transferring agents, such as organolithium compounds but not Grignard reagents, will reduce carboxylic acids to ketones along with transfer of the alkyl group.
The Vilsmaier reagent (N,N-Dimethyl(chloromethylene)ammonium chloride;) is a highly chemoselective agent for carboxylic acid reduction. It selectively activates the carboxylic acid to give the carboxymethyleneammonium salt, which can be reduced by a mild reductant like lithium tris(t-butoxy)aluminum hydride to afford an aldehyde in a one pot procedure. This procedure is known to tolerate reactive carbonyl functionalities such as ketone as well as moderately reactive ester, olefin, nitrile, and halide moieties.
The carboxyl radical, •COOH, only exists briefly.[8] The acid dissociation constant of •COOH has been measured using electron paramagnetic resonance spectroscopy.[9] The carboxyl group tends to dimerise to form oxalic acid.