Editing Carboxylic acid (section)

==Physical properties==

===Solubility===
Carboxylic acids are [[polarity (chemistry)|polar]]. Because they are both hydrogen-bond acceptors (the [[carbonyl]] {{chem2|\sC(\dO)\s}}) and hydrogen-bond donors (the [[hydroxyl]] {{chem2|\sOH}}), they also participate in [[hydrogen bond]]ing. 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.<ref name=M&B>{{cite book|last1=Morrison|first1=R.T.|last2=Boyd|first2=R.N.|date=1992|title=Organic Chemistry|publisher=Prentice Hall |edition=6th|isbn=0-13-643669-2}}</ref> 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.
:[[File:Solubility in different environments.jpg|class=skin-invert-image|500px]]

===Boiling points===
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 bond]]s. 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.{{cn|date=May 2025}}
:[[File:Carboxylic acid dimers.svg|class=skin-invert-image|thumb|Carboxylic acid [[Dimer (chemistry)|dimers]]|alt=|none]]

===Acidity===
Carboxylic acids are [[Brønsted–Lowry acid–base theory|Brønsted–Lowry acids]] because they are proton (H<sup>+</sup>) donors. They are the most common type of [[organic acid]].{{cn|date=May 2025}}

Carboxylic acids are typically [[weak acid]]s, meaning that they only partially [[Dissociation (chemistry)|dissociate]] into [[Hydronium|{{chem2|[H3O]+}}]] [[cation]]s and [[Carboxylate|{{chem2|R\sCO2−}}]] [[anion]]s in neutral [[Water (molecule)|aqueous]] solution. For example, at room temperature, in a 1-[[molarity|molar]] solution of [[acetic acid]], only 0.001% of the acid are dissociated (i.e. 10<sup>−5</sup> moles out of 1&nbsp;mol). Electron-withdrawing substituents such as [[Trifluoromethyl group|trifluoromethyl]] ({{chem2|\sCF3}}) give stronger acids (the p''K''<sub>a</sub> of acetic acid is 4.76 whereas trifluoroacetic acid, with a [[Trifluoromethyl group|trifluoromethyl substituent]], has a p''K''<sub>a</sub> of 0.23). Electron-donating substituents give weaker acids (the p''K''<sub>a</sub> of formic acid is 3.75 whereas acetic acid, with a [[Methyl group|methyl substituent]], has a p''K''<sub>a</sub> of 4.76){{cn|date=May 2025}}
{|class = "wikitable"
! Carboxylic acid<ref>{{cite book | editor-last= Haynes |editor-first=William M. | year = 2011 | title = CRC Handbook of Chemistry and Physics | edition = 92nd | publisher = [[CRC Press]] | isbn = 978-1439855119|pages=5–94 to 5–98|title-link=CRC Handbook of Chemistry and Physics}}</ref>
! [[Acid dissociation constant|p''K''<sub>a</sub>]]
|-
|[[Formic acid]] ({{chem2|HCO2H}}) || 3.75
|-
|[[Chloroformic acid]] ({{chem2|ClCO2H}}) || 0.27<ref name=metabolites>{{cite web | url = https://hmdb.ca/metabolites/HMDB0250109 | title = Chlorocarbonic acid | work =  Human Metabolome Database }}</ref>
|-
|[[Acetic acid]] ({{chem2|CH3CO2H}})|| 4.76
|-
|[[Glycine]] ({{chem2|NH2CH2CO2H}})||2.34
|-
|[[Fluoroacetic acid]] ({{chem2|FCH2CO2H}}) || 2.586
|-
|[[Difluoroacetic acid]] ({{chem2|F2CHCO2H}})|| 1.33
|-
|[[Trifluoroacetic acid]] ({{chem2|CF3CO2H}})|| 0.23
|-
|[[Chloroacetic acid]] ({{chem2|ClCH2CO2H}})|| 2.86
|-
|[[Dichloroacetic acid]] ({{chem2|Cl2CHCO2H}})|| 1.29
|-
|[[Trichloroacetic acid]] ({{chem2|CCl3CO2H}})|| 0.65
|-
|[[Benzoic acid]] ({{chem2|C6H5\sCO2H}})||4.2
|-
|[[2-Nitrobenzoic acid]] (''ortho''-{{chem2|C6H4(NO2)CO2H}})||2.16
|-
|[[Oxalic acid]] ({{chem2|HO\sC(\dO)\sC(\dO)\sOH}}) (first dissociation)
| 1.27
|-
|[[Oxalic acid#Acid-base properties|Hydrogen oxalate]] ({{chem2|HO\sC(\dO)\sCO2−}}) (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 −<sup>1</sup>/<sub>2</sub> negative charges on the 2 oxygen atoms.{{cn|date=May 2025}}

===Odour===
Carboxylic acids often have strong sour odours. [[Ester]]s of carboxylic acids tend to have fruity, pleasant odours, and many are used in [[perfume]]. {{cn|date=May 2025}}

=== Characterization ===
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''<sub>C=O</sub>) between 1680 and 1725&nbsp;cm<sup>−1</sup>. A characteristic ''ν''<sub>O–H</sub> band appears as a broad peak in the 2500 to 3000&nbsp;cm<sup>−1</sup> region.<ref name=M&B/><ref>{{cite journal |last1=Smith |first1=Brian |title=The C=O Bond, Part VIII: Review |url=https://www.spectroscopyonline.com/view/co-bond-part-viii-review |website=Spectroscopy |series=November 2018 |date=November 2018 |volume=33 |pages=24–29 |access-date=12 February 2024}}</ref> By <sup>1</sup>H [[Nuclear magnetic resonance|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.{{cn|date=May 2025}}