. Can. J. Chem. 54, 3949 (1976). The hydrogenolysis of a series of alkylcesiums in cyclohexylamine solvent has been studied spectrophoton~etrically. The cesium salt of p-phenyltoluene reacts with H2 (1 atm, room temperature), but the cesium salts of di-2,4-xylylmethane and of diphenylrnethane are unreactive. The results show that the acidity of Hz is intermediate between those ofp-phenyltoluene and of di-2,4-xylylniethane in this reaction system. Possible effects of ion association phenomena on the equilibrium processes are considered.ERWIN BUNCEL ei BALACHANDRAN C. MENON. Can. J. Chem. 54, 3949 (1976). On a etudie spectrophotometriquement I'hydrogCnolyse d'une serie d'alkylcesiuni dans la cyclohexylan~ine cornme solvant. Le sel de cesium du p-phenyltolukne reagit avec H2 (1 atm, B temperature de la piece) mais ies sels de cesium du dixylyl-2,4 methane et du diphenylmethane ne reagissent pas. Les resultats montrent que. dans ce systkme de reactions, l'acidite de Hz est intermediaire entre les acidites du p-phCnyltolukne et du dixylyl-2.4 methane. 0 1 1 considkre les effets possibles du phenomkne d'association des ions sur les processus d'equilibration.[Traduit par le journal]The pK, of molecular hydrogen, or deuterium: is of fundamental importance to the interpretation of the effect of structural changes on acidity (1-4). Literature estimates of this pKa vary between 29 and 38 ( 5 , 6), although no experimentally determined value has been given.In relation to our studies of isotopic exchange of molecular hydrogen (deuterium) in strongly basic systems (7-9), it was of interest to have direct knowledge of the acidity of H2 (El2), in order to be able to relate the kinetic and th, errnodynamic acidity parameters.Our approach to the determination of the acidity of hydrogen is based on quantitative measurements of the equilibrium between H2 and an organometallic indicator:
It is clear from the reaction depicted in [I] that ifId2 is a stronger acid than RH the equilibrium will lie to the right hand side, and vice versa. Thus, by studying a series of hydrocarbons of increasing p&, a break in reactivity should be observed. When the pK, of the hydrocarbon R H is within 1-2 pK units of that of H2, the reaction [ l ] will proceed to equilibrium and the equilibri-'Carbanion mechanisms. Part V. The previous parts in the series are considered to be those in ref. 26. These publications concern the carbanions derived from i,3,5-trinitrobenzene (26a), 2,4,6-trinitrotoluene (26b), 1,3-dinitrobenzene (26c), and dimethyl sulfoxide (26d).
urn constant (Krel) will be related to the Ka's of H Z and of R H via [2] and [3]:The method for pKa determination of H2 outlined above is analogous in principle to the method originally put forward (10) for pKa determination of carbon acids, via [4]:When the anions R1-and W2-have characteristic spectral absorptions in the ultraviolet or visible region, the position of the equilibrium in [4] is conveniently measured spectrophotometrically. In the present system, though one of the anionic species (H-...
