Editing Acetaldehyde (section)

==Reactions==
===Tautomerization to vinyl alcohol===
[[File:Ethanal Ethenol Tautomerie.svg|thumb|left|upright=1.3|Tautomeric equilibrium between acetaldehyde and vinyl alcohol.]]{{clear left}}

Like many other [[carbonyl compound]]s, acetaldehyde [[tautomer]]izes to give an [[enol]] ([[vinyl alcohol]]; IUPAC name: ethenol):

:{{chem2|CH3CH\dO  <->  CH2\dCHOH}} {{spaces}} {{spaces}} {{spaces}} {{spaces}} {{spaces}} {{spaces}} {{spaces}} {{spaces}} ∆''H''<sub>298,g</sub> = +42.7 kJ/mol

The [[equilibrium constant]] is 6{{e|−7}} at room temperature, thus that the relative amount of the enol form in a sample of acetaldehyde is very small.<ref>{{Cite journal | doi = 10.1021/ja00168a035| title = Keto-enol equilibrium constants of simple monofunctional aldehydes and ketones in aqueous solution| journal = Journal of the American Chemical Society| volume = 112| issue = 12| pages = 4862–8| year = 1990| last1 = Keeffe | first1 = J. R.| last2 = Kresge | first2 = A. J.| last3 = Schepp | first3 = N. P.}}</ref> At room temperature, acetaldehyde ({{chem2|CH3CH\dO}}) is more stable than [[vinyl alcohol]] ({{chem2|CH2\dCHOH}}) by 42.7 kJ/mol:<ref>Johnson, R.D. III "CCCBDB NIST Standard Reference Database". nist.gov</ref> Overall the keto-enol tautomerization occurs slowly but is catalyzed by acids.

Photo-induced keto-enol tautomerization is viable under [[Atmosphere|atmospheric]] or stratospheric conditions.  This photo-tautomerization is relevant to the Earth's atmosphere, because vinyl alcohol is thought to be a precursor to [[carboxylic acid]]s in the atmosphere.<ref>{{Cite journal | doi = 10.1038/nchem.1052| pmid = 21602858| title = Near-threshold H/D exchange in CD<sub>3</sub>CHO photodissociation| journal = Nature Chemistry| volume = 3| issue = 6| pages = 443–8| year = 2011| last1 = Heazlewood | first1 = B. R. | last2 = MacCarone | first2 = A. T. | last3 = Andrews | first3 = D. U. | last4 = Osborn | first4 = D. L. | last5 = Harding | first5 = L. B. | last6 = Klippenstein | first6 = S. J. | last7 = Jordan | first7 = M. J. T. | last8 = Kable | first8 = S. H. | bibcode = 2011NatCh...3..443H}}</ref><ref>{{Cite journal | doi = 10.1126/science.1220712| pmid = 22903524| title = Photo-Tautomerization of Acetaldehyde to [[Vinyl alcohol|Vinyl Alcohol]]: A Potential Route to Tropospheric Acids| journal = Science| volume = 337| issue = 6099| pages = 1203–6| year = 2012| last1 = Andrews | first1 = D. U.| last2 = Heazlewood | first2 = B. R.| last3 = MacCarone | first3 = A. T.| last4 = Conroy | first4 = T.| last5 = Payne | first5 = R. J.| last6 = Jordan | first6 = M. J. T.| last7 = Kable | first7 = S. H.| bibcode = 2012Sci...337.1203A| s2cid = 42079807}}</ref>

===Addition and condensation reactions===
Acetaldehyde is a common electrophile in [[organic synthesis]].<ref>Sowin, T. J.; Melcher, L. M. (2004) "Acetaldehyde" in ''Encyclopedia of Reagents for Organic Synthesis'' (Ed: [[Leo Paquette|L. Paquette]]), J. Wiley & Sons, New York. {{doi|10.1002/047084289X}}</ref> In [[addition reaction]]s acetaldehyde is [[prochiral]]. It is used primarily as a source of the "{{chem2|CH3C+H(OH)}}" [[synthon]] in [[aldol reaction]]s and related [[condensation reaction]]s.<ref name="Behrens2004">{{OrgSynth | author = Behrens, C. | authorlink2 = Leo Paquette | author2 = Paquette, L. A. | title = ''N''-Benzyl-2,3-Azetidinedione (2,3-Azetidinedione, 1-(phenylmethyl)-)| collvol = 10 | collvolpages = 41 | year = 1998 | prep = V75P0106 | volume = 75 | pages = 106 | doi = 10.15227/orgsyn.075.0106}}.</ref> Grignard reagents and [[Organolithium reagent|organolithium]] compounds react with MeCHO to give [[hydroxyethyl]] derivatives.<ref name="Walter1955">{{OrgSynth | author = Walter, L. A. | title = 1-(α-Pyridyl)-2-Propanol (2-(β-Hydroxypropyl)pyridine) | collvol = 3 | collvolpages = 757 | year = 1943 | prep = cv3p0757 | volume = 23 | pages = 83 | doi = 10.15227/orgsyn.023.0083}}</ref> In one of the more spectacular addition reactions, [[formaldehyde]] in the presence of [[calcium hydroxide]] adds to MeCHO to give [[pentaerythritol]], {{chem2|C(CH2OH)4}} and [[formate]].<ref name="Schurink1941">{{OrgSynth | author = Schurink, H. B. J. | title = Pentaerythritol | collvol = 1 | collvolpages = 425 | year = 1925 | prep = CV1P0425 | volume = 4 | pages = 53 | doi = 10.15227/orgsyn.004.0053}}</ref>

In a [[Strecker amino acid synthesis|Strecker reaction]], acetaldehyde condenses with [[cyanide]] and [[ammonia]] to give, after [[hydrolysis]], the [[amino acid]] [[alanine]].<ref name="Kendall1941">{{OrgSynth | author1 = Kendall, E. C. | authorlink1 = Edward Calvin Kendall | author2 = McKenzie, B. F. | title = ''dl''-Alanine | collvol = 1 | collvolpages = 21 | year = 1929 | prep = CV1P0021 | volume = 9 | pages = 4 | doi = 10.15227/orgsyn.009.0004}}</ref> Acetaldehyde can condense with [[amine]]s to yield [[imine]]s; for example, with [[cyclohexylamine]] to give ''N''-[[ethylidene]]cyclohexylamine. These imines can be used to direct subsequent reactions like an aldol condensation.<ref>{{OrgSynth | authorlink1 = Georg Wittig | author1 = Wittig, G. | author2 = Hesse, A. | title = Directed Aldol Condensations: β-Phenylcinnamaldehyde (2-Propenal, 3,3-diphenyl-) | collvol = 6 | collvolpages = 901 | year = 1970 | prep = cv6p0901 |  volume = 50 | pages = 66 | doi = 10.15227/orgsyn.050.0066}}</ref>

It is also a building block in the synthesis of [[heterocyclic compounds]]. In one example, it converts, upon treatment with [[ammonia]], to [[5-Ethyl-2-methylpyridine|5-ethyl-2-methylpyridine]] ("aldehyde-collidine").<ref name="Frank1963">{{OrgSynth | author1 = Frank, R. L. | author2 = Pilgrim, F. J. | author3 = Riener, E. F. | title = 5-Ethyl-2-Methylpyridine (2-Picoline, 5-ethyl-) | year = 1950 | volume = 30 | pages = 41 | doi = 10.15227/orgsyn.030.0041}}</ref>

===Polymeric forms===
{{multiple image|caption_align=left
| align = left
| direction = horizontal
| total_width  = 320
| image1 = Paraldehyde structure.svg
| image2 = Metaldehyde.svg
| footer = Cyclic [[oligomer]]s of acetaldehyde ({{chem2|CH3CHO)_{n}|}}: [[paraldehyde]] (''n'' = 3, left) and [[metaldehyde]] (''n'' = 4, right)
}}
Three molecules of acetaldehyde condense to form "[[paraldehyde]]", a cyclic trimer containing C-O single bonds. Similarly condensation of four molecules of acetaldehyde give the cyclic molecule [[metaldehyde]].  Paraldehyde can be produced in good yields, using a sulfuric acid catalyst.  Metaldehyde is only obtained in a few percent yield and with cooling, often using HBr rather than {{chem2|H2SO4}} as the catalyst.  At {{convert|−40|C|F}} in the presence of acid catalysts, polyacetaldehyde is produced.<ref name=Ull/> There are two [[stereomer]]s of paraldehyde and four of metaldehyde.

The German chemist Valentin Hermann Weidenbusch (1821–1893) synthesized paraldehyde in 1848 by treating acetaldehyde with acid (either sulfuric or nitric acid) and cooling to {{convert|0|C|F}}. He found it quite remarkable that when paraldehyde was ''heated'' with a trace of the same acid, the reaction went the other way, recreating acetaldehyde.<ref>{{cite journal |first=H. |last=Weidenbusch |trans-title=On some products of the reaction of alkalies and acids with acetaldehyde |title=Ueber einige Producte der Einwirkung von Alkalien und Säuren auf den Aldehyd |journal=Annalen der Chemie |volume=66 |pages=152–165; see pp. 155–8 |date=1848 |url=https://babel.hathitrust.org/cgi/pt?id=chi.47401598;view=1up;seq=584}}</ref>

Although [[vinyl alcohol]] is a polymeric form of acetaldehyde ({{slink||Tautomerization to vinyl alcohol}}), [[polyvinyl alcohol]] cannot be produced from acetaldehyde.

===Acetal derivatives===
[[File:Acetal formation 2.png|thumb|450px|left|Conversion of acetaldehyde to [[1,1-diethoxyethane]], R<sup>1</sup>={{chem2|CH3}} R<sup>2</sup>={{chem2|CH3CH2}}]]

Acetaldehyde forms a stable [[acetal]] upon reaction with [[ethanol]] under conditions that favor dehydration. The product, {{chem2|CH3CH(OCH2CH3)2}}, is formally named [[1,1-diethoxyethane]] but is commonly referred to as "acetal".<ref name="Adkins1941">{{OrgSynth | author1 = Adkins, H. | authorlink1 = Homer Burton Adkins | author2 = Nissen, B. H. | title = Acetal | collvol = 1 | collvolpages = 1 | year = 1923 | prep = CV1P0001 | volume = 3 | pages = 1 | doi = 10.15227/orgsyn.003.0001}}</ref> This can cause confusion as "acetal" is more commonly used to describe compounds with the [[functional group]]s RCH(OR')<sub>2</sub> or RR'C(OR<nowiki>''</nowiki>)<sub>2</sub> rather than referring to this specific compound — in fact, 1,1-diethoxyethane is also described as the diethyl acetal of acetaldehyde.

===Precursor to vinylphosphonic acid===
Acetaldehyde is a precursor to [[vinylphosphonic acid]], which is used to make adhesives and [[ion conductivity|ion conductive]] membranes.  The synthesis sequence begins with a reaction with [[phosphorus trichloride]]:<ref name= Lavinia>{{cite journal | last1 = Lavinia | first1 = M. | last2 = Gheorghe | first2 = I. | year = 2010 | title = Poly(vinylphosphonic acid) and its derivatives | doi = 10.1016/j.progpolymsci.2010.04.001 | journal = Progress in Polymer Science | volume = 35 | issue = 8| pages = 1078–92 }}</ref>
# {{chem2|PCl3 + CH3CHO -> CH3CH(O−)PCl3+}}
# {{chem2|CH3CH(O−)PCl3+ + 2 CH3CO2H -> CH3CH(Cl)PO(OH)2 + 2 CH3COCl}}
# {{chem2|CH3CH(Cl)PO(OH)2 -> CH2\dCHPO(OH)2 + HCl}}