Editing Phenol (section)

====Tautomerism====
[[File:Phenol tautomers.svg|class=skin-invert-image|left|thumb|upright=.75|Phenol-cyclohexadienone tautomerism]]
Phenol exhibits [[keto-enol tautomerism]] with its unstable keto tautomer cyclohexadienone, but the effect is nearly negligible. The equilibrium constant for enolisation is approximately 10<sup>−13</sup>, which means only one in every ten trillion molecules is in the keto form at any moment.<ref>{{cite journal |title=Ketonization equilibria of phenol in aqueous solution |first1=Marco |last1=Capponi |first2=Ivo G. |last2=Gut |first3=Bruno |last3=Hellrung |first4=Gaby |last4=Persy |first5=Jakob |last5=Wirz |journal=[[Canadian Journal of Chemistry|Can. J. Chem.]] |year=1999 |volume=77 |pages=605–613 |doi=10.1139/cjc-77-5-6-605 |issue=5–6}}</ref> The small amount of stabilisation gained by exchanging a C=C bond for a C=O bond is more than offset by the large destabilisation resulting from the loss of aromaticity. Phenol therefore exists essentially entirely in the enol form.<ref>{{Clayden|page=531}}</ref> 4,4' Substituted cyclohexadienone can undergo a [[dienone–phenol rearrangement]] in acid conditions and form stable 3,4‐disubstituted phenol.<ref>{{cite journal |last1=Arnold |first1=Richard T. |last2=Buckley |first2=Jay S. |title=The Dienone-Phenol Rearrangement. II. Rearrangement of 1-Keto-4-methyl-4-phenyl-1,4-dihydronaphthalene |journal=J. Am. Chem. Soc. |date=1 May 1949 |volume=71 |issue=5 |page=1781 |doi=10.1021/ja01173a071|bibcode=1949JAChS..71.1781A }}</ref>

For substituted phenols, several factors can favor the keto tautomer: (a) additional hydroxy groups (see [[resorcinol]]) (b) annulation as in the formation of [[naphthol]]s, and (c) deprotonation to give the phenolate.<ref>{{cite book|title=The Chemistry of Phenols|editor=Zvi Rappoport |series = PATAI'S Chemistry of Functional Groups | chapter=Tautomeric Equilibria and Rearrangements Involving Phenols |doi=10.1002/0470857277.ch11
| year=2003 |publisher=John Wiley & Sons| author=Sergei M. Lukyanov, Alla V. Koblik|pages=713–838 |isbn=0471497371 }}</ref>

Phenoxides are [[enolate]]s stabilised by [[aromaticity]]. Under normal circumstances, phenoxide is more reactive at the oxygen position, but the oxygen position is a "hard" nucleophile whereas the alpha-carbon positions tend to be "soft".<ref>{{cite journal |title=2,6,6-Trimethyl-2,4-Cyclohexadione |author1=David Y. Curtin |author2=Allan R. Stein |name-list-style=amp |journal=[[Organic Syntheses]] |year=1966 |volume=46 |pages=115 |url=http://www.orgsyn.org/orgsyn/prep.asp?prep=cv5p1092 |doi=10.15227/orgsyn.046.0115 |access-date=2010-03-31 |archive-url=https://web.archive.org/web/20110605132001/http://www.orgsyn.org/orgsyn/prep.asp?prep=cv5p1092 |archive-date=2011-06-05 |url-status=dead }}</ref>