Cyclohexylamine, carbon acidity-carbanion

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

Allylic conjugation stabilizes carbanions and p/f values of 43 (in cyclohexy-lamine) and 47 8 (in THF-HMPA) were determined for propene. On the basis of exchange rates with cesium cyclohexylamide, cyclohexene and cycloheptene were found to have pK values of about 45 in cyclohexylamine. These data indicate that allylic positions have p/f 45. The hydrogens on the sp carbons in benzene and ethene are more acidic than the hydrogens in saturated hydrocarbons. A pK of 45 has been estimated for benzene on the basis of extrapolation from a series of halogenated... 

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