2390
WARREN V. SHERMAN AXD SAULG. COHEN [CONTRIBUTIOS FROM THE
LT.s
LABORATORIES, S A T I C K , M A S S , AND THE BRASDEISUNIVERSITY, ~ ' A L T H A M , MASSj
/LRMY
Vol. 86 DEPARTMENT O F CHEMISTRY,
Hydrogen Transfer and Energy Transfer in the 7-Radiolysis of Benzophenone-2-Propanol. Effects of Naphthalene and of Phenyl Disulfide BY LVARRENV.SHERMAN' A N D SAULG. C O H E N ~ RECEIVED SOVEMBER 14. 1963 ?-Ray irradiation of 2-propanol leads to zero-order formation of acetone which is little affected by phenyl disulfide. ?-Ray irradiation of benzophenone in Zpropanol leads to benzpinacol, with rate zero order in benzophenone, and to an equimolar additional quantity of acetone. Saphthalene has essentially no effect in thi5 system, indicating t h a t triplet benzophenone may not be an intermediate in the -,-ray-induced reaction, in contrast t o the ultraviolet radiation-induced reaction. Phenyl disulfide inhibits both light and y-ra>--induced reductions of benzophenone. The radical ( C s H j ) ? C O H is an intermediate for consumption of benzophenone and formation of benzpinacol in both systems. Disulfide and mercaptan protect solvent and solute against radiation by repeated hydrogen transfer reactions (eq. 1 and 2).
Introduction Preliminary results have been reported, indicating that aromatic mercaptans and disulfides inhibit the 6oCo y-ray-induced conversion of benzophenone to benzpinacol in 2-propanol. The present work reports a more detailed study of the effects of phenyl disulfide and of naphthalene on the 6oCoy-ray-induced reaction, carried out to ascertain whether the first excited triplet of benzophenone and,,'or the benzophenone ketyl radical ((CoHj)zCOH,I ) and alcohol-derived radicals are important intermediates in this reaction. Measurements have been made of rates of formation of acetone and acetaldehyde, of disappearance of benzophenone, and of appearance of benzpinacol as functions of concentrations of benzophenone, phenyl disulfide, and naphthalene. Results and Discussion Radiolysis of %propanol led to acetone, G = 2.6, and a lower yield of acetaldehyde, G = 0.4, yields being directly proportional to dose and little if a t all affected by 2.56 X 31 phenyl disulfide. These two products of radiolysis of the solvent may be formed partly in radiation tracks and partly from fragments or other species which go into the solution and react too rapidly to be affected much by this solute present in low concentration. Irradiation of 0.1 and 0.5-1f solutions of benzophenone in 2-propanol led to disappearance of benzophenone and formation of acetone with rates zero order in benzophenone. The extents of these reactions appeared to depend only upon the quantity of radiation absorbed when followed to SOYo disappearance of benzophenone. The G-value for benzophenone disappearance was 0.4. In representative reactions, both under these conditions and in the presence of phenyl disulfide, benzpinacol was isolated and characterized, and found to account satisfactorily for the quantity of benzophenone which had reacted. The G-value for acetone formation was 5 . 3 . I t appears that approximately one molecule of acetone is formed per two molecules of benzophenone reduced to benzpinacol in addition to that which is normally formed from radiolysis of the solvent alone under these circumstances. This is similar to the relative yields in the p hotochemical reaction4 and confirms our preliminary L'. S A r m y Laboratories, h-atirk, 5lass I l e p a r t m m t of C h e m i s t r y , Brandeis U n i v e r s i t y , Waltham, >lass S G . Cohen, S. O r m a n , a n d D . A . Laufer. J . A m . C h e m . Sor , 84, 3905 ( 4 ) J . S . Pitts, Jr , R . I,. Letsinger, R . P. T a y l o r , J 5f Patterson, G R e c t e n w o l d . a n d R . B. IRIartin, rbid., 81, 1068 (1959)
r e ~ u l t s but , ~ indicates lower yield of acetone than reported previously.5 Acetaldehyde, formed with G = 0 . 3 from 2-propanol alone, showed increased G with increasing concentration of benzophenone, G = 1.0 and 1.3 in 0.1 and 0.5 M benzophenone, respectively. The reactions leading to this product appear relatively insensitive to the presence of either naphthalene or disulfide. I-Hydroxyethyl radical, CH3CHOH, a possible product of radiolysis of 2-propanol, may transfer hydrogen t o benzophenone, leading to increased yield of acetaldehyde and to the radical I, (C6Hj)zCOH, in a reaction analogous to t h a t proposed for the 2-hydroxy-2propyl radical. Naphthalene, 0.01 and 0.0511,had no effect which we could detect on the rate of reduction of benzophenone in the radiolysis of 0.1 31 benzophenone in 2propanol. This indicates that the y-ray-induced reaction does not involve the same metastable triplet state of benzophenone which is the chemically reactive intermediate in the light-induced process4j6and which is effectively quenched by naphthalene.7a,bz8 I t is not a t present clear whether other higher excited states of benzophenone, which may not be quenched by naphthalene, could themselves enter into chemical reactionsgasbleading to benzpinacol, or whether they would degrade very rapidly to the first excited triplet state.loa-c If the latter should be the case the insensitivity of the reaction to naphthalene would tend to rule out direct excitation of benzophenone by y-rays or energy transfer from excited solvent or solvent fragments as important mechanisms leading to disappearance of this solute. This is consistent with observations of liquid scintillation counters in which the energy efficiency of most scintillators in ?-irradiated organic solutions is less than 17c." Phenyl disulfide. in low concentrations, 11)-~~--10-2 M, has a marked effectiveness as a retarder and inhibitor ( 3 ) J G. B u r r a n d J D S t r o n g , J P h y s Chrtii , 63,87:1 (195W (6) G. S H a m m o n d and IT, 51 I l n o r e , .I A m C h i ~ m S n r , 81, f i W * (1955). (7) ( a ) G Porter and F. a'ilkinson, T y a i z s F a r a t f a y .So. Horner, H G Wippel. and G Klahre, ibid, 92, 24Y9 (1950); W. S . Wadsworth. J r , and W D Ernmons, . I , A m C h r n . Soc , 83, 1733 (1961)
Lohaus2 brieffy reported the synthesis of p-terphenyl (4) b y Diels-Alder addition of trans,trans- 1,4-diphenyl-
