Fluorene, pKa

To examine the distribution of acidic C-H sites in PSOC 1197, derived in part from polynuclear hydroaromatic compounds, the pH 12 O-methyl coal was treated for 68 h at 0 °C, in separate experiments, with an excess of the conjugate bases of 9-phenylfluorene (pKa 18.5), fluorene (pKa 22), and triphenylmethane (pKa 31) as their lithium salts (BLi) THF was used as the solvent ... 

C-H sites in PSOC 1197 have a narrow range of acidities we conclude that the pKa values for these sites are comparable to that of fluorene (pKa 22). 

The acidity of 30 has been measured with reference to Streitwieser s acidity scale (87) a pKA value of 15.8 for 30 as compared to 22.8 for fluorene and 18.5 for 9-phenylfluorene demonstrates a significant increase in acidity (87). 

Attempts at correlation analysis for X—C—H acidity have been made for a long time. Bowden and colleagues192 examined the problem in 1970 for the pKa values of 9-X-fluorenes and related series. For a limited selection of substituents X in 9-X-fluorenes, a correlation with Taft s a values for X was found89. A more general correlation, albeit with considerable scatter, was found with AM, a parameter based on LCAO-MO calculations. Several dinitro-substituted diphenylmethanes fitted the line quite well and 2-nitrofluorene fitted passably 9-cyanofluorene also fitted quite well, but malononitrile deviated strongly. 

The data of Tables XII and XIII appear to demand an oxidation mechanism similar to that observed for fluorene itself and involving a carbanion intermediate. The greater acidity of fluorene and xanthene relative to diphenylmethane (approximately 10 pKa units) (28) apparently promotes ionization to yield a dianion which can react directly with oxygen or undergo a catalyzed reaction—e.g., by nitroaromatics or Fe111. 

Then, in 1973, Streitwieser reported that the polyarylmethane exchange rates measured in cyclohexylamine-cyclohexylamide correlate with equilibrium p/T values with a = 0.31.79 Apparently, when the proton is removed by cyclo-hexylamide, the polyarylmethanes have early transition states, just as the fluorenes did for proton removal by the weaker base methoxide.80 A short extrapolation of the Bronsted correlation led to a pKa for toluene of 40.9, about seven units higher than the value assigned in the MSAD scale. Furthermore, if we... 

Values up to pKa 33 (diphenylmethane) are those reported for equilibrium methods, and were measured either directly using the H acidity function or by comparing acidity with 9-phenyl-fluorene. Above pA 33, we assume a = 0.3 for toluene, cumene, and tripticene, and base other values on pKa = 43 for benzene and an assumed a of 0.9. The scale is based on the Langford and Burwell value of 18.5 for pKa of 9-phenylfluorene. 

Okamura and Katz have determined the pKa of 3H-pyrrolizine by measuring the rate of exchange of its protons with 5 M D20 in dimethyl-formamide containing 1 M triethylamine.115 The value of 29 seems surprisingly high compared with those of indene (18.2), cyclopentadiene (15), and fluorene (22.8). 

Explain why fluorene is a much stronger acid than most hydrocarbons. Its pKa is 23. 

Compare the piQ of cyclopentadiene with that of cycloheptatriene. Whilst the anion of the former has 6 7t electrons (which makes it isoelectronic with benzene), the anion of the latter has 8 ti electrons. Remember that on p. 176 we saw how 4n n electrons made a compound anti-aromatic The cycloheptatrienyl anion does have 4 7t electrons but it is not anti-aromatic because it isn t planar. However, it certainly isn t aromatic either and its pKa of around 36 is about the same as that of propene. This contrasts with the cyclopropenyl anion, which must be planar since any three points define a plane. Now the compound is anli-aromatic and this is reflected in the very high pKit (about 62). Other compounds may become aromatic on losing a proton. We looked at fluorene a few pages back now you will see that fluorene is acidic because its anion is aromatic (14 n electrons). 

The strength of the hydrogen bonds in the protonated amines can be increased by the use of larger and additional groups to provide further steric strain. For example, the difference in strain between 1,8-bis(dimethylamino)-naphthalene [33] and l,8-bis(diethylamino)naphthalene leads to a p a-value for the ethyl derivative enhanced by 0.5 units (Hibbert, 1974), and a similar difference is also observed in the fluorene series for [41] and [42]. Further strain can be introduced into the naphthalene series by the use of 2- and 7-methoxyl groups as substituents. This is illustrated by the p a-values of 1,8-bis(dimethylamino)naphthalene [pA"a = 12.1 (Hibbert, 1974)] and l,8-bis(di-methylamino)-2,7-dimethoxynaphthalene [p a = 16.1 (Hibbert and Simpson, 1987a)] and by the pK,-values of [35] [pKa = 7.49 (Hibbert and Hunte,... 

Tilorone hydrochloride, 2,7—bis [2-(diethylamine) ethoxy]-9H-fluorene-9-one dihydrochloride, is an orange solid which is highly water soluble at neutral and acidic pH. The compound is anhydrous and melts between 234-234.5 °C with decomposition. The molecular weight is 483.47. Tilorone-HC1 has an intense absorption band at 270 nm in water. The pKa of the amine functions are 8.64 and 9.27, respectively the compound is stable in acid or base. The synthesis of tilorone hydrochloride was outlined by Andrews et al.10), and Gaur and Wacker15). 

Streitwieser and co-workers reported extensive measurements of pKa values (designated as pKcsCHA values) of weak acids in cyclohexylamine (CHA) solution using the cesium salt of cyclohexylamine as the base. They also established a pKa scale involving Li and Cs " counterions in tetrahydrofuran (THF) solution. These pKg values are useful because THF is often used for synthetic reactions involving carbanions. The results suggested that lithium salts behave as solvent-separated ion pairs, while the cesium salts appeared to be contact ion pairs. The pKcs/xHP values of p-methylbiphenyl, fluorene, and 9-biphenylfluorene were found to be 38.73,22.90, and 17.72, respectively. ... 

Although fluorene and suberene can both act as carbon acids in Si state and have similar pKaS, they are acids of different families the former is a slow acid and the latter is a fast acid and this can be attributed to the antiaromatic character of suberenyl anion. There are a few other examples of carbon acids which are not slow acids. For example, hydrocyanic acid is virtually a normal acid in the ground state. Our studies [36b] show that the involvement of the CsN bond together with the CH (OH) bond in the reaction coordinate increases n and more than compensates (ca. lO O times) the increase in the effective force constant. The "increased-valence" structure -NsC-H-B leads to n =1.5 the value estimated from the experimental data is n =1.55 [36b]. 

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