4888 solution was heated t o 150° for 5 min [(Me&NilV] or 30 min
Acknowledgments. The authors wish to express their gratitude to the office of ~~~~l ~~~~~~~hand to the Army Research Office (Durham), and National Science Foundation Grant No. GP14372/22013.
( T M S N P ) and cooled and the absorption spectra were obtained t o yield A,. The rate constants were obtained from linear plots of -log ( A t - A,/Ao - A,) US. r. Plots of log k L’s.I / T gave the activation parameters.
Acidity of Hydrocarbons. XXXIX. H - Acidity Function for Methanolic Sodium Methoxide ( H ) Based on Ionization of a Hydrocarbon Acid‘ A. Streitwieser, Jr.,* C. J. Chang, and A. T. Young
Contribution from the Department of Chemistry, University of California, Berkeley, California 94720. Received April 7, 1971 Abstract: pK values of 1,3-diphenylindeneand fluoradene were determined to be 19.8 and 18.2, respectively, in methanol at 25”. An H- scale for methanolic sodium methoxide (Ha*)has been constructed from the ionization of 1,3-diphenylindene. This Hlr scale gives an excellent linear correlation with the logarithm of the pseudo-firstorder rate constants of base-catalyzed protodetritiation reaction of fluorene in NaOMe-MeOH.
T
he H- acidity functionZ measures the ability of a basic medium to ionize an acid, HA, to form the anion A-, and is defined as
H-
=
PKHA- log [HA]/[A-]
=
-log (aHifA-)/fHA (1)
For a methanol solution, pKH* is the ionization constant of HA in methanol and the activity coefficient, f, refers to methanol as standard state. Several authors have used HMfor H- in methanol, where the use of subscript, M, denotes the acidity function for methanol instead of water as standard state. The existing H M scales have been determined by using nitrogen and oxygen acids as indicators, whereas most kinetic investigations involve base-catalyzed breaking of C-H bonds. In fact, for acids of different structure the ratio f A - f H A cannot be assumed to be the same at a given solvent composition. Thus, H - ( H M ) scales depend on the type of indicator ~ s e d . ~With ,~ substituted anilines and diphenylamines as indicators, H M scales for methanolic sodium methoxide have been reported by Schaal and Lambert,s and More O’Ferrall and Ridd.6 The two scales agree satisfactorily when the same value of P K T X ~ is ~ Hused. Rochester’ used o-terr-butylphenol, di-o-tert-butylphenol, and pentamethylphenol to set up three H M scales which differ slightly from one another but differ substantially from the H M acidity function derived from amine indicators. (1) This research was supported in part by Grant No. 12855 of the National Institutes of Health, U. S. Public Health Service. (2) (a) L. P. Hammett and A . J. Deyrup, J . Amer. Chem. SOC.,54, 2721 (1932); (b) M. A. Paul and F. A. Long, Chem. Rea., 57, 1 (1957); (c) I
