3394
C!:OBI~J.UIC.~~IONS TO
form'" in which the t-butyl groups can occupy equatorial-like positions. This was shown to be the case (see below), and consequently measurement of the equilibrium between cis- and trans-I is also a nieasuremexit of an equilibrium between the chair and boat forms. From the variation of the equilibrium constant with temperature, the enthalpy and entropy changes accompanying the isomerization were found by methods similar to those employed earlier." The entropy of the cyclohexane system may be presumed to be affected, perhaps seriously, by the presence of the t-butyl groups, but the value found for the difference in enthalpy between the boat and chair forins in this c;tse must be quite similar to that of any sitnple cyclohexane system. \I niixturc of the cis :mtl trams isoiiiers of I was prepared by reducing 2,1-cli-t-butylpliciiol using Ilancy nickel a t lSO", then chroinic acid oxidation o f the mixture of alcohols, and reductionI2 of the resulting ketones.13 Distillation of the mixture of hydrocarbons through a 4 f t . packed column gave the two isoiiiers: Cis-I,13 b.p. 223.3-223.5", X * ~ D 1.4531; id t r u ? ~ s - I , ' b.p. ~ 229.S-23O.O0, ?225D 1.4563. These isomers could be separated cleaiily by vapor phase chromatography using a coluinn of glyceryl p- tricyanoethyl ether on firebrick. At 118" the retention times were about 6 and 8 minutes for the cis and trans compounds, respectively. The analysis showed the trans isomer obtained was contaniinated with l . G % of the cis while the cis contained less than 0.5% trans. The cis structure was assigned to the isomer of lower enthalpy and physical constants.'4 Equilibrium between the cis and t r a m isoiiiers was established by heating sinal1 saniplcs miill palladium on carbon catalyst in scaled tubes. The resultiiig mixtures wcrc aiialyzcd by vapor phase Chromatography. Tlic results of the equilibratioii arc given in Table I.
TIIE
ELUPOR
VOl. 82
* l . O e.u. were calculated. This value of AI1 is in reasonable agreement with the predicted values (4.0,*4.7P and 5.17kcal./mole). The experimental value for the entropy clearly indicates that the trans isomer is not in the chair form. If it were, the entropy of the reaction would be no more than +1.4 e.u.16p16 (15) This value is calculated by recognizing t h a t the lranr isomer in the chair form is a d: mixture, while t h e c i s is mcso. It is a mnxinzum value, siuce t h e axial t-butyl group would have its rotational motions restricted much more t h a n would the corresponding equatorial group. (16) T h e possibility exists t h a t t h e t r a n s isomer is a mixture which contains a n appreciable amount of the chair form with an axial I-hutyl group. Even if this were the case, the value found for A H would still apply t o the boat-chair cquilibrium a s stated. IIEPARTMEST OF CHE?.IISTRY
SORMAN L. ALLINGER WAYNESTATE UNVERSITY LESLIEA. FREIBERC DETROIT 2 , MICHIG.4N RECEIVED J A N U A R Y 30. 1960
ON T H E MECHANISM OF THIAMINE ACTION. 2-ACETYLTHIAZOLIUM SALTS AS "ACTIVE ACETATE"
VI.'
Sir: Implicit in the mechanism of thiamine action which we have formulated2 is the requiremcnt that hcetylthiamine be able to acetylate phosphoric acid, and thus be yet another form of "active acetate." We wish to report evidence on the kinetic lability of 2-acetplthiazolium salts (I) as support for the idea of their thermodynamic instability as well.
CH3-.E'
s
'I
