666
NOTES
7-Radiation-Induced Isomerization of Cyclohexanone to 5-Hexenal in the Liquid Phase"
by Ajit Singh and Gordon R. Freeman
-0
0
1
I
I
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada (Received J u l y 29, 10Sg)
When liquid cyclohexanone is irradiated with Corn y-rays, the compound isomerizes to 5-hexenal with a yield of 0.85 G unit.Ib The present communication includes evidence that the lowest triplet state of cyclohexanone is a precursor of or a reactant in the isonierization reaction.
0
2
5
I
I
I
I
4
4
6
8
1
0
4 6 8 €, X I 0
1
0
E, x IO
Experimental The techniques were similar to those described earlier.Ib Various solutions of benzene and of 2,3-dimethyllJ3-butadiene (DAIB) in cyclohexanone were irradiated e.v./e-, which is about to a dose of 5.92 =t0.12 X 1.9 X lozo e.v./ml., with CoB0 y-rays. The total dose was kept constant in units of e.v./e- so that the cyclohexanone in all solutions received the same dose. The dose rate was 5 X 10l8 e.v./ml. hr. and the temperature of the samples was 24 f 3". Two solutions of oxygen in cyclohexanone were irradiated to the above dose and analyzed to compare the effects of oxygen and DAIB on the product yields.
2 3 €, X I O 8
I
I
I
1
A
0
2
Figure 1. 5-Hexenal: e. = electron fraction of the solute; dose = 5.92 f 0.12 X 10-4 e.v./e-. Filled points refer to yield in pure cyclohexanone a t zero dose: A, benzene solutions; B and C, DMB solutions.
Results and Discussion The effects of benzene and DMB on the yield of 5-hexenal are shown in Figure 1. For the benzene solutions, the value of G(5-hexenal) increases with increasing benzene concentration and passes through a maxiniuin a t an electron fraction of benzene of about 0.6. For DAIB solutions, the yield of 5-hexenal drops very rapidly a t first and then decreases more slowly as the electron fraction of the solute, ea, is increased beyond about 2 X lops (Figures 1B and C). Oxygen also decreases the yield of 5-hexenal (Table I), and it is a iiiore efficient inhibitor than is DAIB. Blank analyses were made on all solutions and a very slow dark reaction between cyclohexanone and oxygen was noted. Benzene sensitizes the formation of 5-hexenal, whereas DAIB and oxygen inhibit it. Consideration of the energies of the lowest excited singlet and triplet states and the ionization potentials of the molecules involved (Table 11) indicates that these effects might be due to energy transfer processes that involve the The Joxrnal
of
Physical Chemistry
Table I : Oxygen Solutions (Dose
=
5.92 f 0.12 X lo-'
e.v./e-) Pressure of Oa in the sample,a mm.
0
71 412
Electron fraction of OP
0
2 5 x 10-5 14 6 X 10-6
G(C6 hydro-
G(5-Hexenal)
carbons)
0 56 f 0 03 0 22 f 0 02
0 29 f 0 02 0 21 f 0 02 0 20 f 0 02
0 15fO02
a Cyclohexanone, 2 ml.; oxygen, 10 ml. To calculate the electron fraction of dissolved oxygen, its solubility has been assumed to be 0.2 rnl./ml. of the ketone, on the basis of the values for the solubility of oxygen in some oxygenated organic compounds ("International Critical Tables").
(1) (a) This work received financial assistance from the National Research Council of Canada: (b) A. Singh and G . R. Freeman, Can. J . Chem., 42, 1869, 1877 (1964).
667
SOTES
lowest triplet state of cyclohexanone. The ground state of oxygen is a triplet, while those of the other molecules are singlets. Thus, for reasons of spin conservation, the excited singlet states of oxygen are t o be grouped with the excited triplet states of the other molecules.
Table 11: Energies (in e.v.) of the Lowest Excited States and Ionization Potentials Compound
Triplet
Cyclohexanone Benzene LIMB Oxygen
2.8” 3 58* -2.6’ 4 .35k
Singlet
4.27b 4.76bf” 5 . 4b 0 . 9gh 1 .64h
Ionization potential
9 . 14d 9 , 25d 4 . 7 0
1 2 ,2h
Estimated by comparison with the states of cyclopentanone.b,” “Ultra-Violet and Visible Spectroscopy; Chemical Applications,” Butterworth and Co., Ltd., London, 1961. S. R. LaPaglia and B. C. Roquitte, J . Phys. Chem., 66, 1739 K. Watanabe, T. Xakayama, and J. Mottl, J . Quant. (1962). Spectry. Radiative Transfer, 2, 369 (1962). e C. Reid, “Excited States in Chemistry and Biology,” Butterworth and Co., Ltd., G. S. Hammond, Yj. J. Turro, and P. A. LeerLondon, 1957. W. C. Price, R. makers, J . P h y s . Chem., 66, 1144 (1962). Bralsford, P. 5’. Harris, and R. G. Ridley, Speclrochim. Acta, 14, 45 (1959). G. Herzberg, “Molecular Spectra and Molecular Structure. I. Spectra of Diatomic Molecules,” 2nd Ed., D. Tan Nostrand Co., Toronid, 1950.
* C. N . R. Rao,
’
The quenching effects of oxygen2 and conjugated diolefins3 on triplet states have been discussed elsewhere. We suggest that the lowest triplet state of cyclohexanone is a precursor of 5-hexenal in the radiolytic system. The yield of 5-hexenal is reduced to about a third of the initial value (GI = 0.85) by about 2 X electron fraction of DAIB. The inhibition of the remaining third of the 5-hexenal requires a more than 100-fold larger concentration of DAIB (Figure 1B). Oxygen is an even inore efficient inhibitor than is DJIB because about 3 X lop5 electron fraction of oxygen reduces 5-hexenal to about one-fourth of the initial yield (Table I). JIuch larger concentrations of oxygen are required to inhibit the formation of the remaining one-fourth of the 5-hexenal (a sixfold increase in oxveeri concentration causes a relativelv small further decrease in the 5-hcxenal yield, see Table I). It thus appears that 5-hexetial has two precursors, one of which-(roughly . - - 70% of the total) is the IoIvest triplet, state of cyclohexarlone, The ot,her prectlrsor (roughly 30% of t’he total) has several possible identii n
ties, between which the present work does not distinguish. The present results are consistent with either a concerted or a diradical intermediate niechanism for the isomerization of cyclohexanone to 5-hexenal. It should be mentioned that all of the evidence that has been preseri ted in support of concerted mechanisms in the photoisonierization of cyclic ketones to open chain olefinic aldehydes4 can be equally well interpreted on the basis of mechanisms that involve diradical int ermediates . Kinetic Considerations. Crude values for diffusioncontrolled rate constants for reactions between cyclohexanone molecules and oxygen or D N B molecules can be calculated by a method reported earlier.5 The value for oxygen in liquid cyclohexanone at 25’ would be about 1 x 1010 l./mole sec. and that for D l I B in cyclohexanone would be about half this value. Approximately half of the precursors of 5-hexenal are quenched at a D l I B concentration of 2 X 14. The average time between encounters of any given cyclohexanone molecule and a D l l B niolecule would be 1X sec. a t this concentration. If the inhibition of 3-hexenal formation occurs by a diff usion-controlled reaction, the lifetinie of the precursor with respect sec. This assunies to the isomerization reaction is that D l I B deactivates the precursor at the first encounter. A similar calculation for the oxygen solutions also indicates that the 5-hexenal precursor has a lifetime of sec. It seciiis unlikely that a diradical would have a lifetiine as great as secSlfi so the oxygen and D l I B do not interact with diradicals io these systems. This conclusion has no bearing upon whet her or not diradicals are interniediates in the format ion of 5-hexenal. The conclusion is siniply that if diradicals are involved, oxygen and D l [B interfere with a precursor of the diradicals. This is consistent with the previous conclusion that it is the lowest triplet state of cyclohexanone that interacts with oxygen and DlIB. Othey Pj-oditcts qf C-C Bond Cleavage Reactions. Carbon inonoxide, Ca hydrocarboris, and ct hylerie are also products of the radiolysis of cyclohexanone.’ (2) It. 11. Hochstrnsser and 1 2 ~B. Porter, Quart. Rer. (London), 14, 146 (1960). (3) G . S. Hmrmond and 1’. A. Leermakers. .J. P h y s . Chcm., 6 6 , 1144, 1148 (1962). (4) (a) 11. Srinivnsaii, ,I. Am. C h m . SOC., 81, 2601 (1959): . (hi . ilJid,, 83, 4344, 4348 (19G1); ( c ) “Advances in l’hotocheinistry,” Vol. I , W . A. Noyes, J r . , G . S . Hanimond, and J. N. l’itts, J r . , I , X , Iiiterscience Publishers, Inr., New \*ark. N. \.., 1003, 1). 83. ( 5 ) G . It. Freeinmi. J Chrm. P h y s . . 39, 988 (1003) (6) T h e period for iiiterriai rotation i n the diratlic,al would he of the order of l o - ” sea., so i i i t r n n i o l w r i l n r recAonihination or tlisprol)ortionation would 1)rohablY O C C I I ~ ’in time 1iiut.h less than 1 0 - 7 sec.
668
NOTES
Effects of Charge and Nickel Ion on Proton Chemical Shifts of Glycyl Peptides
by Raj Mathur and R. Bruce Martin
Th cyclohexanone molecule requires more e ergy to undergo reactions 3 and 4 than it does to undergo reaction 2 . It requires more energy to undergo reaction 2 t.han it does to undergo reaction 1. I n general, the higher the energy state of a species, the shorter is its lifetime, and the smaller is the probability that the species will enter into sensitization or quenching reactions. Thus, one would expect that the yield of Cs hydrocarbons would be less sensitive to the presence of additives than was the 5-hexenal yield. The yield of ethylene should be less sensitive than the C6 hydrocarbon yield. The sensitivity of the CO yield should be intermediate between those of the Cs and the ethylene yields. All these things were observed. (See Figure 2 and Table I. The effect of DMB on the Cs yield could not be measured due to analytical difficulty.)
2
c
' ' "-I
0
2
c
0
s 4 x
" 0 4
G 0 4
6 8 l o E, XI0
Figure 2. O t h e r products of C-C cleavage reactions: e, = electron fraction of solute; dose = 5.92 f 0.12 X e.v./e-. Filled points refer t o yield in p u r e cyclohexanone at zero dose: A, Cs hydrocarbons from benzene solutions; €3, CO from DMB solutions; C, CO from hensene solutions; D, C I H nfrom DiLlB sulritions; E, C?H4from benzene solutions.
Tho
.loirr
nal o/ Physical Chemistry
Cobb Chemical Laboratory, Univmsity of Virginia, Charlottemille, Virginia (Received August 6,1964)
Beginning with glycine and proceeding through tetraglycine, the series of glycyl peptides provides a set of compounds of known structure and increasing length upon which to study the effects of charge changes a t the termini on the proton magnetic resonance spectra of pairs of equivalent protons at varying distances from the protonation sites. Such a study on this comparatively simple series would seem a prerequisite to the understanding of the effects of protonating equilibria in more complicated systems. Furthermore, since there is an ammonium group a t one terminus and a carboxylic acid group a t the other, effects of ionization a t two different kinds of groups are simultaneously studied. I n this note the proton magnetic resonance spectra of cationic, dipolar ion, and anionic glycyl peptides are reported. When base is added to solutions containing divalent nickel ion and one equivalent of triglycine or tetraglycine, the color of the solution changes from blue to yellow. During the course of this color change, the nickel ion promotes ionization of amide hydrogens in a cooperative manner over a narrow pH range.l I n solution, a n equimolar mixture of triglycine and nickel chloride is blue and fully paramagnetic. Addition of one equivalent of base effects little change as only an ammonium hydrogen is removed, and nickel ion associates a t this nitrogen. Addition of a second equivalent of base yields a solution that is half as paramagnetic as the above and half as yellow as when three equivalents of base have been added, in which case the solution exhibits little Thus increasing yellow color directly parallels loss of paramagnetism and both changes are in accord with the cooperative nature of the transition deduced from titration data. After the addition of the second equivalent of base, half of the nickel complexes possess two ionized amide hydrogens and half retain both amide (1) R. B. Martin, M. Chamberlin, and J. T. E d s d l , J . Am. Chem. Soc.. 82, 495 (1960). (2) T. D. Coyle and R. B. Martin, unpublished experiments performed in Oxford, England, in 1961. The n.m.r. method of Evans3 was used to estimate the relative paramagnetic susceptibilities on a 30-Mc. machine with solutions containing 2% t-butyl alcohol. (3) D.
F. Evans. J . Chem. SOC.,2003 (1959).
