Carboxylic acid

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 stabilised dimers through hydrogen bonds. For boiling to occur, either the dimer bonds must be broken or the entire dimer arrangement must be vaporised, increasing the enthalpy of vaporization requirements significantly.

Carboxylic acid dimers

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 RCOO⁻ 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 pK_a of formic acid is 3.75 whereas trifluoroacetic acid, with a trifluoromethyl substituent, has a pK_a of 0.23). Electron-donating substituents give weaker acids (the pK_a of formic acid is 3.75 whereas acetic acid, with a methyl substituent, has a pK_a of 4.76)

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 (ν_C=O) between 1680 and 1725 cm⁻¹. A characteristic ν_O–H band appears as a broad peak in the 2500 to 3000 cm⁻¹ region.[3] 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.

In general, industrial routes to carboxylic acids differ from those used on a smaller scale because they require specialized equipment.

• Carbonylation of alcohols as illustrated by the Cativa process for the production of acetic acid. Formic acid is prepared by a different carbonylation pathway, also starting from methanol.
• 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, this method is inexpensive but not selective enough to be useful. Allylic and benzylic compounds undergo more selective oxidations. Alkyl groups on a benzene ring are oxidized to the carboxylic acid, regardless of its chain length. Benzoic acid from toluene, terephthalic acid from para-xylene, and phthalic acid from ortho-xylene are illustrative large-scale conversions. Acrylic acid is generated from propene.[5]
• Oxidation of ethene using silicotungstic acid catalyst.
• Base-catalyzed dehydrogenation of alcohols.
• Carbonylation coupled to the addition of water. This method is effective and versatile 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. Hydrocarboxylations involve the simultaneous addition of water and CO. Such reactions are sometimes called "Reppe chemistry."

HCCH + CO + H₂O → CH₂=CHCO₂H

• Hydrolysis of triglycerides obtained from plant or animal oils. These methods of synthesizing some long-chain carboxylic acids are related to soap making.
• Fermentation of ethanol. This method is used in the production of vinegar.
• The Kolbe–Schmitt reaction provides a route to salicylic acid, precursor to aspirin.

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 more suitable for laboratory conditions than the industrial use of air, which is "greener" because it yields less inorganic side products such as chromium or manganese oxides.
• Oxidative cleavage of olefins by ozonolysis, potassium permanganate, or potassium dichromate.
• Hydrolysis of nitriles, esters, or amides, usually with acid- or base-catalysis.
• Carbonation of a Grignard reagent and organolithium reagents:

RLi + CO₂ → RCO₂Li

RCO₂Li + HCl → RCO₂H + LiCl

• Halogenation followed by hydrolysis of methyl ketones in the haloform reaction
• Base-catalyzed cleavage of non-enolizable ketones, especially aryl ketones:[6]

RC(O)Ar + H₂O → RCO₂H + ArH

Many reactions produce 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.

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.

N,N-Dimethyl(chloromethylene)ammonium chloride (ClHC=N⁺(CH₃)₂Cl⁻) 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.[7]

• 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, a carboxylic acid on an aliphatic chain having a simple methylene bridge at the alpha position can have the chain shortened by one carbon. The inverse procedure is the Arndt–Eistert synthesis, where an acid is converted into acyl halide, which is then reacted with diazomethane to give one additional methylene in the aliphatic chain.
• Many acids undergo oxidative 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.
• In ketonic decarboxylation carboxylic acids are converted to ketones.
• Organolithium reagents (>2 equiv) react with carboxylic acids to give a dilithium 1,1-diolate, a stable tetrahedral intermediate which decomposes to give a ketone upon acidic workup.
• The Kolbe electrolysis is an electrolytic, decarboxylative dimerization reaction. It gets rid of the carboxyl groups of two acid molecules, and joins the remaining fragments together.