{{chembox | Verifiedfields = changed | Watchedfields = changed | verifiedrevid = 460108491 | Name = Crotonic acid | ImageFile = Crotonic acid.svg | ImageClass = skin-invert | ImageSize = 160px | ImageAlt = Skeletal formula of crotonic acid | ImageFile1 = Crotonic acid 3D ball.png | ImageClass1 = bg-transparent | ImageAlt1 = Ball-and-stick model of the crotonic acid molecule | PIN = (2''E'')-But-2-enoic acid | OtherNames = (''E'')-But-2-enoic acid<br />(''E'')-2-Butenoic acid<br />Crotonic acid<br />''trans''-2-Butenoic acid<br />β-Methylacrylic acid<br />3-Methylacrylic acid |Section1={{Chembox Identifiers | ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | ChemSpiderID = 552744 | ChEMBL_Ref = {{ebicite|correct|EBI}} | ChEMBL = 1213528 | PubChem = 637090 | InChI = 1/C4H6O2/c1-2-3-4(5)6/h2-3H,1H3,(H,5,6)/b3-2+ | InChIKey = LDHQCZJRKDOVOX-NSCUHMNNBH | SMILES1 = O=C(O)/C=C/C | StdInChI_Ref = {{stdinchicite|correct|chemspider}} | StdInChI = 1S/C4H6O2/c1-2-3-4(5)6/h2-3H,1H3,(H,5,6)/b3-2+ | StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} | StdInChIKey = LDHQCZJRKDOVOX-NSCUHMNNSA-N | CASNo_Ref = {{cascite|changed|??}} | CASNo = 107-93-7 | ChEBI_Ref = {{ebicite|correct|EBI}} | ChEBI = 41131

| UNII_Ref = {{fdacite|changed|FDA}} | UNII = YW5WZZ4O5Q

| DrugBank_Ref = {{drugbankcite|correct|drugbank}} | DrugBank = DB02074 | SMILES = C/C=C/C(O)=O }} |Section2={{Chembox Properties | C=4|H=6|O=2 | Density = 1.02 g/cm<sup>3</sup> | MeltingPtC = 70 to 73 | MeltingPt_notes = | BoilingPtC = 185 to 189 | BoilingPt_notes = | pKa = 4.69 <ref>{{cite book|last=Dawson|first=R. M. C.|display-authors=etal|title=Data for Biochemical Research|location=Oxford|publisher=Clarendon Press|date=1959}}</ref> }} |Section3={{Chembox Hazards | ExternalSDS = [http://siri.org/msds/mf/cards/file/0423.html SIRI.org] }} |Section4={{Chembox Related | OtherAnions = crotonate | OtherFunction_label = carboxylic acids | OtherFunction = propionic acid<br />acrylic acid<br />butyric acid<br />succinic acid<br />malic acid<br />tartaric acid<br />fumaric acid<br />pentanoic acid<br />tetrolic acid | OtherCompounds = butanol<br />butyraldehyde<br />crotonaldehyde<br />2-butanone<br />3-Butenoic acid }} }}

'''Crotonic acid''' ('''(2''E'')-but-2-enoic acid''') is a short-chain unsaturated carboxylic acid described by the formula CH<sub>3</sub>CH=CHCO<sub>2</sub>H. The name crotonic acid was given because it was erroneously thought to be a saponification product of croton oil.<ref>{{Cite EB1911|wstitle=Crotonic Acid|volume=7|page=511}}</ref> It crystallizes as colorless needles from hot water. With a ''cis''-alkene, Isocrotonic acid is an isomer of crotonic acid. Crotonic acid is soluble in water and many organic solvents. Its odor is similar to that of butyric acid.

==Production== Crotonic acid produced industrially by oxidation of crotonaldehyde:<ref name="Beyer">{{cite book|first1=Hans|last1=Beyer|first2=Wolfgang|last2=Walter|title=Organische Chemie|language=de|publisher=S. Hirzel Verlag|location=Stuttgart|date=1984|isbn=3-7776-0406-2}}</ref><ref name=Schulz/>{{rp|230}} :{{chem2|CH3CH\dCHCHO + }}{{1/2}}{{chem2| O2 -> CH3CH\dCHCO2H}}

A number of other methods exist, including the Knoevenagel condensation of acetaldehyde with malonic acid in pyridine:<ref name="Beyer"/>{{rp|229}}

:upright=2.5|Synthesis of crotonic acid by the Knoevenagel condensation of acetaldehyde and malonic acid

The alkaline hydrolysis of allyl cyanide followed by the intramolecular rearrangement of the double bond:<ref name="Tollens">{{cite journal|first1=A.|last1=Rinne|author2-link=Bernhard Tollens|first2=B.|last2=Tollens|title=Ueber das Allylcyanür oder Crotonitril|trans-title=On allyl cyanide or crotononitrile|journal=Justus Liebigs Annalen der Chemie|date=1871|volume=159|issue=1|pages=105–109|doi=10.1002/jlac.18711590110|url=https://zenodo.org/record/1427303}}</ref><ref name="Pomeranz">{{cite journal|first1=C.|last1=Pomeranz|title=Ueber Allylcyanid und Allylsenföl|trans-title=On allyl cyanide and allylic mustard oil|journal=Justus Liebigs Annalen der Chemie|date=1906|volume=351|issue=1–3|pages=354–362|doi=10.1002/jlac.19073510127|url=https://zenodo.org/record/1427569}}</ref>

:upright=1.5|Alkaline hydrolysis of allyl cyanide

Furthermore, it is formed during the distillation of 3-hydroxybutyric acid:<ref name="Beilstein">{{cite book|first1=F.|last1=Beilstein|title=Handbuch der organischen Chemie|language=de|edition=3rd|volume=1|publisher=Verlag Leopold Voss|date=1893|page=506|url=http://www26.us.archive.org/stream/handbuchderorgan00beil#page/506/mode/2up}}</ref>

:upright=1.5|Synthesis of crotonic acid from 3-hydroxybutyric acid

==Properties== Crotonic acid crystallizes in the monoclinic crystal system in the space group P21/a (space group 14, position 3) with the lattice parameters {{nowrap|''a'' {{=}} 971 pm}}, {{nowrap|''b'' {{=}} 690 pm}}, {{nowrap|''c'' {{=}} 775 pm}} and {{nowrap|''β'' {{=}} 104.0°}}. The unit cell contains four formula units.<ref name="Shimizu">{{cite journal|first1=S.|last1=Shimizu|first2=S.|last2=Kekka|first3=S.|last3=Kashino|first4=M.|last4=Haisa|title=Topochemical Studies. III. The Crystal and Molecular Structures of Crotonic Acid, CH<sub>3</sub>CH=CHCO<sub>2</sub>H, and Crotonamide, CH<sub>3</sub>CH=CHCONH<sub>2</sub>|journal=Bulletin of the Chemical Society of Japan|date=1974|volume=47|issue=7|pages=1627–1631|doi=10.1246/bcsj.47.1627|doi-access=free}}</ref>

==Reactions== Crotonic acid converts into butyric acid by hydrogenation or by reduction with zinc and sulfuric acid.<ref name="DOC">{{cite journal|last1=Heilbron|journal=Dictionary of Organic Compounds|title=Crotonic acid|volume=1|date=1953|page=615|url=https://archive.org/stream/dictionaryoforga011095mbp#page/n633/mode/2up}}</ref> :upright=1.5|Hydrogenation of crotonic acid

Upon treatment with chlorine or bromine, crotonic acid converts to 2,3-dihalobutyric acids:<ref name="DOC"/> :upright=1.5|Chlorination of butenoic acid

Crotonic acid adds hydrogen bromide to form 3-bromobutyric acid.<ref name="DOC"/><ref name="Loven">{{cite journal|first1=J. M.|last1=Lovén|first2=H.|last2=Johansson|title=Einige schwefelhaltige β-Substitutionsderivate der Buttersäure|trans-title=Some sulfur-containing β-substitution derivatives of butyric acid|journal=Berichte der deutschen chemischen Gesellschaft|date=1915|volume=48|issue=2|pages=1254–1262|doi=10.1002/cber.19150480205|url=https://zenodo.org/record/1426589}}</ref>

:upright=1.5|Reaction of crotonic acid with hydrogen bromide.

The reaction with alkaline potassium permanganate solution affords 2,3-dihydroxybutyric acid.<ref name="DOC"/> :upright=1.5|Reaction of crotonic acid with alkaline potassium permanganate solution.

Upon heating with acetic anhydride, crotonic acid converts to the acid anhydride:<ref name="Clover">{{cite journal|first1=A. M.|last1=Clover|first2=G. F.|last2=Richmond|title=The Hydrolysis of Organic Peroxides and Peracids|journal=American Chemical Journal|date=1903|volume=29|issue=3|pages=179–203|url=https://archive.org/stream/americanchemical291903balt#page/178/mode/2up}}</ref>

Esterification of crotonic acid using sulfuric acid as a catalyst provides the corresponding '''crotonate''' esters:

:upright=1.5|Preparation of crotonate esters.

Crotonic acid reacts with hypochlorous acid to 2-chloro-3-hydroxybutyric acid. This can either be reduced with sodium amalgam to butyric acid, can form with sulfuric acid 2-chlorobutenoic acid, react with hydrogen chloride to 2,3-dichlorobutenoic acid or with potassium ethoxide to 3-methyloxirane-2-carboxylic acid.<ref name="Beilstein2">{{cite book|first1=F.|last1=Beilstein|title=Handbuch der organischen Chemie|edition=3rd|volume=1|language=de|publisher=Verlag Leopold Voss|date=1893|page=562|url=http://www26.us.archive.org/stream/handbuchderorgan00beil#page/562/mode/2up}}</ref>

:upright=1.5|Reaction of crotonic acid into 2-chloro-3-hydroxybutanoic acid and subsequent reactions

Crotonic acid reacts with ammonia at the alpha position in the presence of mercury(II) acetate. This reaction provides {{sc|dl}}-threonine.<ref>{{OrgSynth|last1=Carter|first1=H. E.|last2=West|first2=H. D.|title=''dl''-Threonine|collvol=3|collvolpages=813|year=1955|prep=cv3p0813}}</ref>

==Use== Crotonic acid is mainly used as a comonomer with vinyl acetate.<ref name=":0" /> The resulting copolymers are used in paints and adhesives.<ref name=Schulz/>

Crotonyl chloride reacts with ''N''-ethyl-2-methylaniline (''N''-ethyl-''o''-toluidine) to provide crotamiton, which is used as an agent against scabies.<ref>{{cite book|first1=A.|last1=Kleemann|first2=J.|last2=Engel|title=Pharmazeutische Wirkstoffe: Synthesen, Patente, Anwendungen|edition=2nd rev. and updated|volume=5|publisher=Georg Thieme Verlag|location=Stuttgart & New York|isbn=3-13-558402-X|page=251}}</ref>

:none|550px|Crotamiton synthesis

==Safety== Its {{LD50}} is 1&nbsp;g/kg (oral, rats).<ref name=Schulz>{{Ullmann|first1=R. P.|last1=Schulz|first2=J.|last2=Blumenstein|first3=C.|last3=Kohlpaintner|title=Crotonaldehyde and Crotonic Acid|year=2005|doi=10.1002/14356007.a08_083}}</ref> It irritates eyes, skin, and respiratory system.<ref name=":0">{{RömppOnline|ID=RD-02-03102|Name=Butensäuren|Datum=January 7, 2020}}</ref>

== See also == * Crotyl * Crotyl alcohol * Isocrotonic acid * Methacrylic acid

== References == {{reflist}}

Category:Alkenoic acids Category:Crotyl compounds