Keto-enol tautomeric constants and ionisation constants have been meas
ured for the keto and enol tautomers of 9-formyl-, 9-acetyl- and 9-ben
zoyl-fluorene in aqueous solution at 25 degrees C. Values of pK(E) (K-
E = [enol]/[ketone] and pK(E) = -log K-E) are - 1.22, 2.28.and 1.91, r
espectively, and the corresponding pK(a)s for enolate anion formation
are 6.19, 9.94 and 9.44 for the keto tautomers and 7.41, 7.66 and 7.53
for the enols. The measurements demonstrate the effectiveness of the
fluorenyl group in increasing enol stability and ketone-acidity. For 9
-formylfluorene, for which the enol is the stable tautomer, K-E is inc
reased by a factor of more than 10(7) and the acidity of the keto taut
omer by more than 10(9) relative to acetaldehyde (pK(E) = 6.17, pK, =
16.73). For 9-acetyl- and 9-benzoyl-fluorenes tautomeric :constants: w
ere determined kinetically by combining rate constants for ketonisatio
n measured spectrophotometrically following quenching of their enolate
anions in. carboxylic acid buffers with rate constants for enolisatio
n measured by halogen trapping under the same conditions. For 9-formyl
fluorene rate constants for enolisation were measured by generating it
s unstable aldehyde tautomer from an ethanethiol hemithioaetal by reac
tion with iodine. Combining these rate constants with rate constants f
or ketonisation from trapping the aldehyde with bisulfite ion gave the
tautomeric constant. In aqueous solution the aldehyde tautomer of 9-f
ormylfluorene is appreciably hydrated and an equilibrium constant K-h
= [hydrate]/[aldehyde] = 5.6 was derived from measurements of the (slo
wer) equilibration of enol and hydrate following enolisation in acetic
acid buffers. In aqueous Solution therefore the enol (71%) and hydrat
e (24%) are the principal species. log k-pH profiles for enolisation,
ketonisation and hydration reactions are reported. Intrinsic reactivit
ies of the three enolate anions towards protonation by H3O+ and carbox
ylic acids are compared within an extended Bronsted plot of log k vers
us Delta pK with measurements by Kresge for the corresponding enolate
anions derived from fluorene-9-carboxylic acid and its methyl, methylt
hio and methylthione esters. Surprisingly, not only are the thio and t
hione esters less acidic than the oxygen ester, but intrinsically less
reactive.