Editing Acetaldehyde (section)

===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>