In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group (−C(=O)−OH) [1] attached to an R-group. The general formula of a carboxylic acid is often written as R−COOH or R−CO2H, sometimes as R−C(O)OH 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. [2] For example, butyric acid (CH3CH2CH2CO2H) 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 (R−COO− or R−CO−2) 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.
| 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, body odour, milk, butter, cheese |
| 4 | Butyric acid | Butanoic acid | CH3(CH2)2COOH | Butter |
| 5 | Valeric acid | Pentanoic acid | CH3(CH2)3COOH | Valerian plant |
| 6 | Caproic acid | Hexanoic acid | CH3(CH2)4COOH | Goat fat |
| 7 | Enanthic acid | Heptanoic acid | CH3(CH2)5COOH | Fragrance |
| 8 | Caprylic acid | Octanoic acid | CH3(CH2)6COOH | Coconuts |
| 9 | Pelargonic acid | Nonanoic acid | CH3(CH2)7COOH | Pelargonium plant |
| 10 | Capric acid | Decanoic acid | CH3(CH2)8COOH | Coconut and Palm kernel oil |
| 11 | Undecylic acid | Undecanoic acid | CH3(CH2)9COOH | Anti-fungal agent |
| 12 | Lauric acid | Dodecanoic acid | CH3(CH2)10COOH | Coconut oil and hand wash soaps |
| 13 | Tridecylic acid | Tridecanoic acid | CH3(CH2)11COOH | Plant metabolite |
| 14 | Myristic acid | Tetradecanoic acid | CH3(CH2)12COOH | Nutmeg |
| 15 | Pentadecylic acid | Pentadecanoic acid | CH3(CH2)13COOH | Milk fat |
| 16 | Palmitic acid | Hexadecanoic acid | CH3(CH2)14COOH | Palm oil |
| 17 | Margaric acid | Heptadecanoic acid | CH3(CH2)15COOH | Pheromone in various animals |
| 18 | Stearic acid | Octadecanoic acid | CH3(CH2)16COOH | Chocolate, waxes, soaps, and oils |
| 19 | Nonadecylic acid | Nonadecanoic acid | CH3(CH2)17COOH | Fats, vegetable oils, pheromone |
| 20 | Arachidic acid | Icosanoic acid | CH3(CH2)18COOH | Peanut oil |
| Compound class | Members |
|---|---|
| unsaturated monocarboxylic acids | acrylic acid (2-propenoic acid) – CH2=CH−COOH, 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 |
| Divinylether fatty acids | 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 −C(=O)−) and hydrogen-bond donors (the hydroxyl −OH), 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. [3] 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 [H3O]+ cations and R−CO−2 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 [4] | pKa |
|---|---|
| Formic acid (HCO2H) | 3.75 |
| Chloroformic acid (ClCO2H) | 0.27 [5] |
| Acetic acid (CH3CO2H) | 4.76 |
| Glycine (NH2CH2CO2H) | 2.34 |
| Fluoroacetic acid (FCH2CO2H) | 2.586 |
| Difluoroacetic acid (F2CHCO2H) | 1.33 |
| Trifluoroacetic acid (CF3CO2H) | 0.23 |
| Chloroacetic acid (ClCH2CO2H) | 2.86 |
| Dichloroacetic acid (Cl2CHCO2H) | 1.29 |
| Trichloroacetic acid (CCl3CO2H) | 0.65 |
| Benzoic acid (C6H5−CO2H) | 4.2 |
| 2-Nitrobenzoic acid (ortho-C6H4(NO2)CO2H) | 2.16 |
| Oxalic acid (HO−C(=O)−C(=O)−OH) (first dissociation) | 1.27 |
| Hydrogen oxalate (HO−C(=O)−CO−2) (second dissociation of oxalic acid) | 4.14 |
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. The carbonyl carbon's partial positive charge is also weakened by the -1/2 negative charges on the 2 oxygen atoms.
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. [3] [6] 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.
Vilsmaier reagent (N,N-Dimethyl(chloromethylene)ammonium chloride [ClHC\dN+(CH3)2]Cl−) 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. [9]
The carboxyl radical, •COOH, only exists briefly. [10] The acid dissociation constant of •COOH has been measured using electron paramagnetic resonance spectroscopy. [11] The carboxyl group tends to dimerise to form oxalic acid.
In chemistry, an alcohol is a type of organic compound that carries at least one hydroxyl functional group bound to a saturated carbon atom. Alcohols range from the simple, like methanol and ethanol, to complex, like sugar alcohols and cholesterol. The presence of an OH group strongly modifies the properties of hydrocarbons, conferring hydrophilic (water-loving) properties. The OH group provides a site at which many reactions can occur.
In chemistry, an ester is a compound derived from an acid in which the hydrogen atom (H) of at least one acidic hydroxyl group of that acid is replaced by an organyl group. Analogues derived from oxygen replaced by other chalcogens belong to the ester category as well. According to some authors, organyl derivatives of acidic hydrogen of other acids are esters as well, but not according to the IUPAC.
In organic chemistry, a ketone is an organic compound with the structure R−C(=O)−R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group −C(=O)−. The simplest ketone is acetone, with the formula (CH3)2CO. Many ketones are of great importance in biology and in industry. Examples include many sugars (ketoses), many steroids, and the solvent acetone.
In organic chemistry, an aldehyde is an organic compound containing a functional group with the structure R−CH=O. The functional group itself can be referred to as an aldehyde but can also be classified as a formyl group. Aldehydes are a common motif in many chemicals important in technology and biology.
Hydroboration–oxidation reaction is a two-step hydration reaction that converts an alkene into an alcohol. The process results in the syn addition of a hydrogen and a hydroxyl group where the double bond had been. Hydroboration–oxidation is an anti-Markovnikov reaction, with the hydroxyl group attaching to the less-substituted carbon. The reaction thus provides a more stereospecific and complementary regiochemical alternative to other hydration reactions such as acid-catalyzed addition and the oxymercuration–reduction process. The reaction was first reported by Herbert C. Brown in the late 1950s and it was recognized in his receiving the Nobel Prize in Chemistry in 1979.
In chemical nomenclature, the IUPAC nomenclature of organic chemistry is a method of naming organic chemical compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC). It is published in the Nomenclature of Organic Chemistry. Ideally, every possible organic compound should have a name from which an unambiguous structural formula can be created. There is also an IUPAC nomenclature of inorganic chemistry.
Fischer esterification or Fischer–Speier esterification is a special type of esterification by refluxing a carboxylic acid and an alcohol in the presence of an acid catalyst. The reaction was first described by Emil Fischer and Arthur Speier in 1895. Most carboxylic acids are suitable for the reaction, but the alcohol should generally be primary or secondary. Tertiary alcohols are prone to elimination. Contrary to common misconception found in organic chemistry textbooks, phenols can also be esterified to give good to near quantitative yield of products. Commonly used catalysts for a Fischer esterification include sulfuric acid, p-toluenesulfonic acid, and Lewis acids such as scandium(III) triflate. For more valuable or sensitive substrates other, milder procedures such as Steglich esterification are used. The reaction is often carried out without a solvent or in a non-polar solvent to facilitate the Dean-Stark method. Typical reaction times vary from 1–10 hours at temperatures of 60-110 °C.
Diethyl malonate, also known as DEM, is the diethyl ester of malonic acid. It occurs naturally in grapes and strawberries as a colourless liquid with an apple-like odour, and is used in perfumes. It is also used to synthesize other compounds such as barbiturates, artificial flavourings, vitamin B1, and vitamin B6.
In organic chemistry, an acyl chloride is an organic compound with the functional group −C(=O)Cl. Their formula is usually written R−COCl, where R is a side chain. They are reactive derivatives of carboxylic acids. A specific example of an acyl chloride is acetyl chloride, CH3COCl. Acyl chlorides are the most important subset of acyl halides.
Tollens' reagent is a chemical reagent used to distinguish between aldehydes and ketones along with some alpha-hydroxy ketones which can tautomerize into aldehydes. The reagent consists of a solution of silver nitrate, ammonium hydroxide and some sodium hydroxide. It was named after its discoverer, the German chemist Bernhard Tollens. A positive test with Tollens' reagent is indicated by the precipitation of elemental silver, often producing a characteristic "silver mirror" on the inner surface of the reaction vessel.
The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro, is a chemical reaction which involves the base-induced disproportionation of two molecules of a non-enolizable aldehyde to give a primary alcohol and a carboxylic acid.
In organic chemistry, a carboxylate is the conjugate base of a carboxylic acid, RCOO−. It is an ion with negative charge.
Nucleophilic acyl substitution (SNAcyl) describes a class of substitution reactions involving nucleophiles and acyl compounds. In this type of reaction, a nucleophile – such as an alcohol, amine, or enolate – displaces the leaving group of an acyl derivative – such as an acid halide, anhydride, or ester. The resulting product is a carbonyl-containing compound in which the nucleophile has taken the place of the leaving group present in the original acyl derivative. Because acyl derivatives react with a wide variety of nucleophiles, and because the product can depend on the particular type of acyl derivative and nucleophile involved, nucleophilic acyl substitution reactions can be used to synthesize a variety of different products.
The Dakin oxidation (or Dakin reaction) is an organic redox reaction in which an ortho- or para-hydroxylated phenyl aldehyde (2-hydroxybenzaldehyde or 4-hydroxybenzaldehyde) or ketone reacts with hydrogen peroxide (H2O2) in base to form a benzenediol and a carboxylate. Overall, the carbonyl group is oxidised, whereas the H2O2 is reduced.
Organozinc chemistry is the study of the physical properties, synthesis, and reactions of organozinc compounds, which are organometallic compounds that contain carbon (C) to zinc (Zn) chemical bonds.
Methanesulfonic anhydride (Ms2O) is the acid anhydride of methanesulfonic acid. Like methanesulfonyl chloride (MsCl), it may be used to generate mesylates (methanesulfonyl esters).
Alcohol oxidation is a collection of oxidation reactions in organic chemistry that convert alcohols to aldehydes, ketones, carboxylic acids, and esters where the carbon carries a higher oxidation state. The reaction mainly applies to primary and secondary alcohols. Secondary alcohols form ketones, while primary alcohols form aldehydes or carboxylic acids.
In organic chemistry, carbonyl reduction is the conversion of any carbonyl group, usually to an alcohol. It is a common transformation that is practiced in many ways. Ketones, aldehydes, carboxylic acids, esters, amides, and acid halides - some of the most pervasive functional groups, -comprise carbonyl compounds. Carboxylic acids, esters, and acid halides can be reduced to either aldehydes or a step further to primary alcohols, depending on the strength of the reducing agent. Aldehydes and ketones can be reduced respectively to primary and secondary alcohols. In deoxygenation, the alcohol group can be further reduced and removed altogether by replacement with H.
An insertion reaction is a chemical reaction where one chemical entity interposes itself into an existing bond of typically a second chemical entity e.g.:
Alpha-substitution reactions occur at the position next to the carbonyl group, the α-position, and involve the substitution of an α hydrogen atom by an electrophile, E, through either an enol or enolate ion intermediate.
