Jan. 5, 1964
COMMUNICATIONS TO THE EDITOR
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racemizes to the extent of 1&1S70 during these periodss Control experiments showed that isolation and purification of active unsolvolyzed ester does not affect the rotation and, in fact, when racemization occurs (e.g., as in the absence of azide ion), k r a c can be determined TABLE 1 with good precision from the specific rotations of RATECONSTANTS FOR SOLVOLYSIS ( k e ) , OXYGENEQUILIBRATIONsamples of isolated ester. Thus i t is clear that racemi( k , , ) , AND RACEMIZATIOK (k,,,) ASSOCIATED WlTH THE SOLVOLYSIS zation would have been detected had it occurred. O F P-CHLOROBENZHYDRYL P-NITROBENZOATE I N 80% AQUEOUS This then shows that azide ion eliminates racemizaACETONEAT 99.6"" tion but not carboxyl oxygen equilibration. We feel [Ester], [Solute], k, this is compelling evidence that two intermediates are lO2M 102M Constant 102 h r . -1 involved; one which returns with preservation of con2.78 None k, 4 . 4 1 f 0.06' figuration and one which returns with racemization. I t ks 30.0 f 0 . 4 2.76 1 4 . 1 1 NaN, is the latter that is intercepted by azide ion. 2.43 Kone be, 6 . 5 f. 0.2' The observation that two intermediates are involved 2.80 1 4 . 1 1 NaN; ke, 5 . 9 =k 0 . 3 parallels the recent findings of Winstein and cokrw 2.76 f 0.02d 2.9 None w o r k e r ~ ~that . ~ so-called intimate (I) and solvent2.79 1 4 . 1 1 NaN3 krw 0.0 (11) ion pairs are involved in the ionization of separated krm 0.0 2.86 1 4 . 1 1 NaN, certain alkyl arenesulfonates in acetic acid. The a Solvent composition based on volumes of pure components at present results are nicely accommodated by the Win25" prior to mixing. * Taken from ref. 1. e I n this experiment stein scheme for ionization. According to this inter5 X Ad radioactive p-nitrobenzoic acid was added a t the outset and the second-order rate constant for exchange (5.4 f pretation internal return is completely stereospecific 0.1 X 1. mole-' h r . - l ) was also obtained using this reaction (retention) and external ion-pair return results in parmixture. Average value of two experiments in which different tial or complete racemization.1° Azide ion acts as a methods were used t o follow the reaction. scavenger for the solvent-separated ion pair I1 and In the presence of 0.14 M sodium azide the first-order thus eliminates racemization. solvolysis results in the formation of 0.63 equivalent of titratable acid. This shows that 37% of the ester is RX [R+X-] [R+i IX-] --+ products converted to p-chlorobenzhydryl azide (no acid proI I1 duced) and the rest to p-chlorobenzhydrol. The 6.8fold increase in rate of solvolysis (k,) is due in part to The stereochemistry of solvolysis was also investian azide-ion-promoted hydrolysis (with acyl-oxygen gated. Solvolysis of optically active carboxyl- l 8 0 cleavage6) which is superimposed on the carbonium ion ester in 80% acetone containing 0.14 M sodium azide reaction. In the presence of azide ion, 82% of the p gives p-chlorobenzhydrol with 85.2 f O.6yOretention chlorobenzhydrol (52y0 of the total product) is proof configuration and 81.9 f 1% acyl-oxygen cleavage. duced by acyl-oxygen cleavage ; solvolysis in the These values are averages for two samples isolated a t absence of azide ion involves exclusive alkyl-oxygen different times and the difference is considered to be cleavage.' Thus the ester undergoes the two simultanreal. This means that the carbonium ion portion of eous reactions a t about equal rates and the rate conthe solvolysis proceeds with excess retention of constant for solvolysis with alkyl-oxygen cleavage is ' 1 2 figuration. This parallels the observation that in the ks (3.4 times that in the absence of azide ion). absence of azide ion, where solvolysis involves alkylThe only complication resulting from the azide-ionoxygen cleavage exclusively, p-chlorobenzhydrol is promoted hydrolysis is that this side reaction consumes formed with 10% retention of configuration.' The phalf of the substrate and thus changes in the unsolvolychlorobenzhydryl azide derived from active ester is zed ester can only be observed for half as long as would also active. However, the relative configurations of otherwise be possible. Nonetheless, oxygen equilibrasubstrate and azide have not been established. tion (reaction 2 ) was followed to 27y0 completion (82% Acknowledgment.-This work was supported by a solvolysis). The value of k,, is the average of two grant from the National Science Foundation, Grant first-order constants calculated for 20 and 27y0 reacNo. G19244. tion. Under these conditions the second-order rate constant for exchange between unsolvolyzed ester and (8) If azide ion were n o t intercepting an intermediate, t h e recovered ester would be expected t o he substantially more t h a n 18% racemized he the sodium p-nitrobenzoate produced by the solvolysis cause of t h s positive salt effect o n t h e r a t e of ionization. is 0.54 X l o w 2 1. mole-' hr.-1.7 With this constant (9) S. Winstein, P. E. Klinedinst, J r . , and E Clippinger, J. A m . C k c m . it can be shown4 that
