Inorganic chemistry,acarboxylic acidis anorganic acidthat contains acarboxyl group(−C(=O)−OH)[1]attached to anR-group.The general formula of a carboxylic acid is often written asR−COOHorR−CO2H,sometimes asR−C(O)OHwith R referring to anorganyl group(e.g.,alkyl,alkenyl,aryl), orhydrogen,or other groups. Carboxylic acids occur widely. Important examples include theamino acidsandfatty acids.Deprotonationof a carboxylic acid gives acarboxylateanion.
Examples and nomenclature
editCarboxylic acids are commonly identified by theirtrivial names.They often have the suffix-ic acid.IUPAC-recommended names also exist; in this system, carboxylic acids have an-oic acidsuffix.[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 theparent chaineven if there are othersubstituents,such as3-chloropropanoic acid.Alternately, it can be named as a "carboxy" or "carboxylic acid" substituent on another parent structure, such as2-carboxyfuran.
The carboxylate anion (R−COO−orR−CO−2) of a carboxylic acid is usually named with the suffix-ate,in keeping with the general pattern of-ic acidand-atefor aconjugate acidand its conjugate base, respectively. For example, the conjugate base ofacetic acidisacetate.
Carbonic acid,which occurs inbicarbonate buffer systemsin nature, is not generally classed as one of the carboxylic acids, despite that it has amoietythat 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 | Valerianplant |
| 6 | Caproic acid | Hexanoic acid | CH3(CH2)4COOH | Goatfat |
| 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 | Pelargoniumplant |
| 10 | Capric acid | Decanoic acid | CH3(CH2)8COOH | CoconutandPalm kernel oil |
| 11 | Undecylic acid | Undecanoic acid | CH3(CH2)9COOH | Anti-fungal agent |
| 12 | Lauric acid | Dodecanoic acid | CH3(CH2)10COOH | Coconut oiland 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 acidandeicosapentaenoic acid(nutritional supplements) |
| Amino acids | the building-blocks ofproteins |
| Keto acids | acids of biochemical significance that contain aketonegroup; examples:acetoacetic acidandpyruvic acid |
| Aromaticcarboxylic 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 acidthe monomer used to producenylonandaldaric acid– a family of sugar acids |
| Tricarboxylic acids | containing three carboxyl groups; examples:citric acid– found incitrus fruitsandisocitric acid |
| Alpha hydroxy acids | containing a hydroxy group in the first position; examples:glyceric acid,glycolic acidandlactic 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 |
Physical properties
editSolubility
editCarboxylic acids arepolar.Because they are both hydrogen-bond acceptors (thecarbonyl−C(=O)−) and hydrogen-bond donors (thehydroxyl−OH), they also participate inhydrogen 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 acidhas a low solubility in water (0.2 g/L), but its sodium salt is very soluble in water.
Boiling points
editCarboxylic acids tend to have higher boiling points than water, because of their greater surface areas and their tendency to form stabilized dimers throughhydrogen bonds.For boiling to occur, either the dimer bonds must be broken or the entire dimer arrangement must be vaporized, increasing theenthalpy of vaporizationrequirements significantly.
Carboxylic aciddimers
Acidity
editCarboxylic acids areBrønsted–Lowry acidsbecause they are proton (H+) donors. They are the most common type oforganic acid.
Carboxylic acids are typicallyweak acids,meaning that they only partiallydissociateinto[H3O]+cationsandR−CO−2anionsin neutralaqueoussolution. For example, at room temperature, in a 1-molarsolution ofacetic acid,only 0.001% of the acid are dissociated (i.e. 10−5moles out of 1 mol). Electron-withdrawing substituents, such as-CF3group,give stronger acids (the pKaof acetic acid is 4.76 whereas trifluoroacetic acid, with atrifluoromethyl substituent,has a pKaof 0.23). Electron-donating substituents give weaker acids (the pKaof formic acid is 3.75 whereas acetic acid, with amethyl substituent,has a pKaof 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 |
Deprotonationof carboxylic acids gives carboxylate anions; these areresonance 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 -1/2negative charges on the 2 oxygen atoms.
Odour
editCarboxylic acids often have strong sour odours.Estersof carboxylic acids tend to have fruity, pleasant odours, and many are used inperfume.
Characterization
editCarboxylic acids are readily identified as such byinfrared spectroscopy.They exhibit a sharp band associated with vibration of the C=O carbonyl bond (νC=O) between 1680 and 1725 cm−1.A characteristicνO–Hband appears as a broad peak in the 2500 to 3000 cm−1region.[3][6]By1HNMRspectrometry, thehydroxylhydrogen 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
editMany 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 ofaminocarboxylic acidsare the main components ofproteins.
Carboxylic acids are used in the production of polymers, pharmaceuticals, solvents, and food additives. Industrially important carboxylic acids includeacetic 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.
Synthesis
editIndustrial routes
editIn 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 theCativa processfor the production of acetic acid. Formic acid is prepared by a different carbonylation pathway, also starting from methanol.
- Oxidation ofaldehydeswith air using cobalt and manganese catalysts. The required aldehydes are readily obtained from alkenes byhydroformylation.
- 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 acidfromtoluene,terephthalic acidfrompara-xylene,andphthalic acidfromortho-xyleneare illustrative large-scale conversions.Acrylic acidis generated frompropene.[7]
- Oxidation of ethene usingsilicotungstic acidcatalyst.
- Base-catalyzed dehydrogenation of alcohols.
- Carbonylation coupled to the addition of water. This method is effective and versatile for alkenes that generate secondary and tertiarycarbocations,e.g.isobutylenetopivalic acid.In theKoch reaction,the addition of water and carbon monoxide toalkenesoralkynesis catalyzed by strong acids. Hydrocarboxylations involve the simultaneous addition of water andCO.Such reactions are sometimes called "Reppe chemistry."
- HC≡CH+ CO + H2O →CH2=CH−CO2H
- Hydrolysis oftriglyceridesobtained from plant or animal oils. These methods of synthesizing some long-chain carboxylic acids are related tosoap making.
- Fermentationof ethanol. This method is used in the production ofvinegar.
- TheKolbe–Schmitt reactionprovides a route tosalicylic acid,precursor toaspirin.
Laboratory methods
editPreparative methods for small scale reactions for research or for production of fine chemicals often employ expensive consumable reagents.
- Oxidation of primary alcoholsoraldehydeswith strongoxidantssuch aspotassium dichromate,Jones reagent,potassium permanganate,orsodium 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.[citation needed]
- Oxidative cleavage ofolefinsbyozonolysis,potassium permanganate,orpotassium dichromate.
- Hydrolysis ofnitriles,esters,oramides,usually with acid- or base-catalysis.
- Carbonation of aGrignard reagentandorganolithiumreagents:
- Halogenationfollowed by hydrolysis ofmethyl ketonesin thehaloform reaction
- Base-catalyzed cleavage of non-enolizable ketones, especiallyarylketones:[8]
- R−C(=O)−Ar+ H2O → R−CO2H + ArH
Less-common reactions
editMany reactions produce carboxylic acids but are used only in specific cases or are mainly of academic interest.
- Disproportionation of analdehydein theCannizzaro reaction
- Rearrangement of diketones in thebenzilic acid rearrangement
- Involving the generation of benzoic acids are thevon Richter reactionfrom nitrobenzenes and theKolbe–Schmitt reactionfromphenols.
Reactions
edit
Acid-base reactions
editCarboxylic acids react withbasesto form carboxylate salts, in which thehydrogenof thehydroxyl(–OH) group is replaced with a metalcation.For example,acetic acidfound invinegarreacts withsodium bicarbonate(baking soda) to formsodium acetate,carbon dioxide,and water:
- CH3COOH + NaHCO3→ CH3COO−Na++ CO2+ H2O
Conversion to esters, amides, anhydrides
editWidely practiced reactions convert carboxylic acids intoesters,amides,carboxylate salts,acid chlorides,andalcohols. Their conversion toestersis widely used, e.g. in the production ofpolyesters.Likewise, carboxylic acids are converted intoamides,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 ofamino acidsintopeptidesis a significant biochemical process that requiresATP.
Converting a carboxylic acid to an amide is possible, but not straightforward. Instead of acting as a nucleophile, an amine will react as a base in the presence of a carboxylic acid to give the ammoniumcarboxylatesalt. Heating the salt to above 100 °C will drive off water and lead to the formation of the amide. This method of synthesizing amides is industrially important, and has laboratory applications as well.[9]In the presence of a strong acid catalyst, carboxylic acids cancondenseto form acid anhydrides. The condensation produces water, however, which can hydrolyze the anhydride back to the starting carboxylic acids. Thus, the formation of the anhydride via condensation is an equilibrium process.
Under acid-catalyzed conditions, carboxylic acids will react with alcohols to formestersvia theFischer esterificationreaction, which is also an equilibrium process. Alternatively,diazomethanecan be used to convert an acid to an ester. While esterification reactions with diazomethane often give quantitative yields, diazomethane is only useful for forming methyl esters.[9]
Reduction
editLikeesters,most carboxylic acids can bereducedto alcohols byhydrogenation,or using hydride transferring agents such aslithium aluminium hydride.Strong alkyl transferring agents, such asorganolithiumcompounds but notGrignard reagents,will reduce carboxylic acids to ketones along with transfer of the alkyl group.
TheVilsmaier reagent(N,N-Dimethyl(chloromethylene)ammonium chloride;[ClHC=N+(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.[10]
Conversion to acyl halides
editThe hydroxyl group on carboxylic acids may be replaced with a chlorine atom usingthionyl chlorideto giveacyl chlorides.In nature, carboxylic acids are converted tothioesters.Thionyl chloridecan be used to convert carboxylic acids to their corresponding acyl chlorides. First, carboxylic acid1attacks thionyl chloride, and chloride ion leaves. The resultingoxonium ion2is activated towards nucleophilic attack and has a good leaving group, setting it apart from a normal carboxylic acid. In the next step,2is attacked by chloride ion to give tetrahedral intermediate3,a chlorosulfite. The tetrahedral intermediate collapses with the loss ofsulfur dioxideand chloride ion, giving protonated acyl chloride4.Chloride ion can remove the proton on the carbonyl group, giving the acyl chloride5with a loss ofHCl.
Phosphorus(III) chloride(PCl3) andphosphorus(V) chloride(PCl5) will also convert carboxylic acids to acid chlorides, by a similar mechanism. One equivalent of PCl3can react with three equivalents of acid, producing one equivalent of H3PO3,orphosphorus acid,in addition to the desired acid chloride. PCl5reacts with carboxylic acids in a 1:1 ratio, and producesphosphorus(V) oxychloride(POCl3) and hydrogen chloride (HCl) as byproducts.
Reactions with carbanion equivalents
editCarboxylic acids react with Grignard reagents and organolithiums to form ketones. The first equivalent of nucleophile acts as a base and deprotonates the acid. A second equivalent will attack the carbonyl group to create ageminalalkoxide dianion, which is protonated upon workup to give the hydrate of a ketone. Because most ketone hydrates are unstable relative to their corresponding ketones, the equilibrium between the two is shifted heavily in favor of the ketone. For example, the equilibrium constant for the formation ofacetonehydrate from acetone is only 0.002. The carboxylic group is the most acidic in organic compounds.[11]
Specialized reactions
edit- As with all carbonyl compounds, the protons on theα-carbonare labile due toketo–enol tautomerization.Thus, the α-carbon is easily halogenated in theHell–Volhard–Zelinsky halogenation.
- TheSchmidt reactionconverts carboxylic acids toamines.
- Carboxylic acids are decarboxylated in theHunsdiecker reaction.
- TheDakin–West reactionconverts an amino acid to the corresponding amino ketone.
- In theBarbier–Wieland degradation,a carboxylic acid on an aliphatic chain having a simplemethylene bridgeat the Alpha position can have the chain shortened by one carbon. The inverse procedure is theArndt–Eistert synthesis,where an acid is converted into acyl halide, which is then reacted withdiazomethaneto give one additional methylene in the aliphatic chain.
- Many acids undergooxidative decarboxylation.Enzymesthat catalyze these reactions are known ascarboxylases(EC6.4.1) anddecarboxylases(EC 4.1.1).
- Carboxylic acids are reduced toaldehydesvia theesterandDIBAL,via the acid chloride in theRosenmund reductionand via the thioester in theFukuyama reduction.
- Inketonic decarboxylationcarboxylic acids are converted to ketones.
- Organolithium reagents (>2 equiv) react with carboxylic acids to give a dilithium 1,1-diolate, a stabletetrahedral intermediatewhich decomposes to give a ketone upon acidic workup.
- TheKolbe electrolysisis an electrolytic, decarboxylative dimerization reaction. It gets rid of the carboxyl groups of two acid molecules, and joins the remaining fragments together.
Carboxyl radical
editThe carboxylradical,•COOH, only exists briefly.[12]Theacid dissociation constantof •COOH has been measured usingelectron paramagnetic resonancespectroscopy.[13]The carboxyl group tends to dimerise to formoxalic acid.
See also
editReferences
edit- ^IUPAC,Compendium of Chemical Terminology,2nd ed. (the "Gold Book" ) (1997). Online corrected version: (2006–) "carboxylic acids".doi:10.1351/goldbook.C00852
- ^Recommendations 1979.Organic Chemistry IUPAC Nomenclature. Rules C-4 Carboxylic Acids and Their Derivatives.
- ^abMorrison, R.T.; Boyd, R.N. (1992).Organic Chemistry(6th ed.). Prentice Hall.ISBN0-13-643669-2.
- ^Haynes, William M., ed. (2011).CRC Handbook of Chemistry and Physics(92nd ed.).CRC Press.pp. 5–94 to 5–98.ISBN978-1439855119.
- ^"Chlorocarbonic acid".Human Metabolome Database.
- ^Smith, Brian."The C=O Bond, Part VIII: Review".Spectroscopy.Retrieved12 February2024.
- ^Riemenschneider, Wilhelm (2002). "Carboxylic Acids, Aliphatic".Ullmann's Encyclopedia of Industrial Chemistry.Weinheim: Wiley-VCH.doi:10.1002/14356007.a05_235.ISBN3527306730..
- ^Perry C. Reeves (1977). "Carboxylation of Aromatic Compounds: Ferrocenecarboxylic Acid".Org. Synth.56:28.doi:10.15227/orgsyn.056.0028.
- ^abWade 2010, pp. 964–965.
- ^Fujisawa, Tamotsu; Sato, Toshio."Reduction of carboxylic acids to aldehydes: 6-Ooxdecanal".Organic Syntheses.66:121.doi:10.15227/orgsyn.066.0121;Collected Volumes,vol. 8, p. 498.
- ^Wade 2010, p. 838.
- ^Milligan, D. E.; Jacox, M. E. (1971). "Infrared Spectrum and Structure of Intermediates in Reaction of OH with CO".Journal of Chemical Physics.54(3): 927–942.Bibcode:1971JChPh..54..927M.doi:10.1063/1.1675022.
- ^The value is pKa= −0.2 ± 0.1.Jeevarajan, A. S.; Carmichael, I.; Fessenden, R. W. (1990). "ESR Measurement of the pKaof Carboxyl Radical and Ab Initio Calculation of the Carbon-13 Hyperfine Constant ".Journal of Physical Chemistry.94(4): 1372–1376.doi:10.1021/j100367a033.
External links
edit
- Carboxylic acids pH and titration– freeware for calculations, data analysis, simulation, and distribution diagram generation
- PHC.