952
Vol, 66
KOTES
were formed from piperylene, suggesting direct hydrogenation by hydrogen. I n addition, some hydrogenation of cyclopentene to cyclopentane mas observed in the presence of hydrogen. I n the case of piperylene, hydrogen transfer reactions also appear to occur, as evidenced by the formation of CS olefins from the piperylene in the complete absence of hydrogen, Le., in the presence of helium as a carrier gas. The piperylene also was found to cyclize to cyclopentene in the presence of hydrogen. Some double bond migration to other pentadienes also was observed.
EQUILIBRILX COKST-IYT OF A HYDROGEN BOXDING SYSTEbI: PHEXOL-PYRIDINE BY ASISH KUMAR CI-IANDR~ AND SHANTIMOY BASERJEE Depariment of Chemistry, Uniuersitzl College of Science and Technologv, Calcutta 9, India Received November 6,1961
Although the existence of intermolecular hydrogen bonds in solution can be recoqnized from the observation of anomalous absorption spectra of n-electron systems, few values have been reported for hydrogen bond energies of various proton Discussion donors interacting with proton acceptors. In a Double bond migration and skeletal isomeriza- previous communication, Chandra and Basul tion of olefins over alumina have been interpreted reported the free energy change of hydrogenin terms of carbonium ion mechanisms.6 I n a bonded complexes of some ?r-electron systems with similar manner, the hydrogen transfer and cycliza- different aliphatic alcohols. In their paper, equition reactions of piperylene observed in this work librium constants were measured from the n d n * can be explained by carbonium ion mechanisms blue-shift and the optical density measurements at the peak, while Nagakura and Baba2 measured the equilibrium constants and the hydrogen H+ + + bond energies of phenol with ether and dioxane c=c-c=c-c~c=c-c-c-c,c=c-c-c-c from the F T * red-shift of the aromatic molecule and the temperature dependence of the epectra. I n all the previous cases, the physico-chemical basis underlying the method of calculating the + c=c-c-c-c equilibrium constants was the Benesi and Hildebrand equation or its modifications. These methods have a common failing, namely, they cannot be c=c-:-c applied to the cases where both the donor and the acceptor have comparable absorption in the wave length region of interest. It is the purpose of the c=c-c-c present note to develop a simple expression which I O + H + will treat the spectral data specifically in the region C where both of the components, donor and acceptor, and the complex have appreciable absorpwhere H : represents a hydride ion donated by tion and permit a direct evaluation of equilibrium piperylene or another hydrocarbon molecule in the constants with minimum computation. system. I n the case of cyclization of piperylene, a P+ Method of Calculation.-Let the equilibrium be primary carbonium ion is an intermediate. Since a represented by primary carbonium ion is quite unstable comC A + B I A B IC= pared to secondary and tertiary ions, the extent of ([AI - CX[Bl - C) (') cyclization is limited. One would expect hexadiene, for example, to cyclize more readily to a five- where [A] and [B] represent the initial concentramembered ring structure (methylcyclopentene) tions of the two components, and C, the equilibrium since the intermediate would be a secondary carbo- concentration of the hydrogen bonded complex. The optical density per cm. of path (O.D.) of a nium ion. solution containing the molecular species (1) is That alumina possesses hydrogenation activity given by is evident from the observed hydrogenation of O.D. = €8+ E,( [A] - C) CZ( [BJ - C) (2) some of the olefins in the presence of hydrogen. The hydrogenation activity of alumina has nrhere E, €1, and €2 are the molar extinction cobeen noted The effect of hydrogen on efficients of AB, A, and B, respectively. the ring splitting reaction also has been observed in Suppose [B] = m [ A ] (3) a previous study on methylcyclopentane18in which it was suggested that hydrogen might enhance the whence acidity of the alumina and hence increase the cataO D . = (e, - E1 - E*)C 4- (E1 rnEd[AI (4) lytic activity for these reactions. However, it is Let us write also conceivable that the role of hydrogen may be D = O.D. - ( € 1 + mezl[A] = iC (5) one of keeping the surface free of carbonaceous where residues which lower the activity of the alumina.8 B = ( E , - €1 - €2)
o+ it
+
+
(5) (6) (7) (8)
W. 0. Haag and H. Pines, J . A m . Chem. Sac., 82, 2488 (1960). V. C. F.Holm end R. 'A'. Blue, Ind. Eng. Chem., 43, 501 (19.51). 8. W. U eller and S. G. Hmdin, J . Phgs. Chem., 60, 1501 (1956). J. €I. Sinfelt and J. C. Rohrer, ibzd., 65, 2272 (1961).
( 1 ) A. I
