Dec. 20. 1964
COMMUNICATIONS TO THE EDITOR
0.001 v., which is in excellent agreement with the theoretical slope predicted by eq. 2 for the reversible two-electron dimer-monomer reaction 1 [ (2.3O3)XT/ 25 = 0.0293 v . a t 25O]. A wave analysis plot of Qz v s . Edrnewas rectilinear with a reciprocal slope of 0.044 v. This excludes any one-electron transfer process, whether reversible or irreversible, and is not consistent with Nernst control for a two-electron reduction. A Koutecky-type mathematical analysisa eliminated the possibility of a two-electron transfer "rate controlled" by slow steps other than diffusion. The remote alternative of spurious slope^,^^^^ due to extraneous effects (e.g., double layer), was ruled out in our results by considerations of internal consistency. Equation 2 predicts that the half-wave potential of hemin should shift with pH as shown in eq. 3. This (3) dependence has been previously reported in the literature.I1 From our data and the applicable expanded form of the Ilkovic equation,I2 the diffusion coefficient of the dimeric hemin ion A was evaluated as 1.72 X cm.2/sec. a t 25". It should be noted t h a t this assignment applies only to the species which controlled the current under the specific experimental conditions prevailing in this investigation, i e . , in a dilute aqueous solution (millimolar in total hemin) containing 0.1 M potassium hydroxide as the sole supporting electrolyte in the absence of any maximum suppressor. I t is well known that the degree of polymerization of hemin4 is highly dependent on environmental conditions. Studies are in progress for elucidating the polarographic properties of iron protoporphyrin aggregates in media of biological significance. Acknowledgment.-This investigation was supported by Public Health Service Grant No. 2 R 0 1 HE-02342 from the National Heart Institute and by the United States Atomic Energy Commission under Contract AT (30-1)-2133 with The Pennsylvania State Universi ty.
5691
ucts of such a reaction are hydrogen chloride and an olefin, and relative rates of elimination for a series of chlorides parallel the rates of solvolytic reactions for the same series. Rearrangements during the thermal decomposition of neopentyl chloride2 and bornyl chloride3 are also observed, and all of these facts suggest "quasi-hetrolytic"'a mechanisms analogous to those of solvolytic eliminations. Consequently, very polar transition statesla or ionic pair intermediates4 have been postulated for thermal dehydrohalogenations. The original analogy was based upon a comparison of primary, secondary, and tertiary halides whose relative rates of elimination a t 400" were in the approximate ratio 1 : 200 : 40,000, respectively.'",' We choose to compare a series of secondary chlorides which differ greatly in their solvolytic reactivity. Some relative rates of dehydrochlorination in the gas phase compared with solvolytic ( s N 1 or E l ) reactivities are presented in Table I . TABLE I RELATIVERATESOF THERMAL DEHYDROCHLORIXATION Reactant
400'
500'
Solvolysisa
Cyclohexyl chloride 1.00 1.00 1.00 2-Butyl chloride 0.85 1.12 1.51* Cyclopentyl chloride 1.20 0.93 14.1" ero-Norbornyl chloride 0.67 0.77 11i.d a-Phenylethyl chloride 7.76 6.92 12,5OOe &Butyl chloride 101 71 19,100f a Approximate values for 807, aqueous ethanol a t 8.5'. * The quoted value is for isopropyl chloride: A . Streitwieser, "Solvolytic Displacement Reactions," McGraw-Hill Book Co., New York, N . Y . , 1962, p. 96. J. D. Roberts, L. Urbanek, and R. Armstrong, J . A m . Chem. SOC.,71, 3049 (1949). J . D. Roberts, W. Bennett, and R . Armstrong, zbid., 72, 3329 (1950). e Extrapolated from 50": A. M . Ward, J . Chem. SOC., 445(192i). Extrapolated from 50": E. D. Hughes, zbid., 255 (1935).
Relative rates for the secondary chlorides were obtained by pyrolyzing a mixture of the required chloroalkanes in stirred flow reactor systems which have been described previously in detaiL5x6 The relative rate for t-butyl chloride is calculated from the Arrhenius (8) I,. Meites and Y. Israel, J. A m . Chem. SOL.,83, 4903 (1961). equation since the magnitude of the rate precluded (9) J. K u t a and J. Koryta, Summaries of Papers, Third International competitive experiments. Flow rates in these experiCongress of Polarography, Southampton, England. 1964, p. 48. ments were generally varied by a factor of ten and the (10) W. H. Reinmuth, L. B. Rogers, and L. E. I. Hummelstedt, J. A m . Chem. Soc., 81, 2947 (1959). usual precautions of surface inhibition were followed. IC (11) R . Brdicka and K. Wiesner, Collection Czech. Chem. C o m m u n . , 19, Four to eight runs were made for each pair and the 39 (1947). (12) I. Kolthoff and K. Izutzu, J. A m . Chem. Soc., 86, 1276 (1964). standard deviation of the resulting relative rate was (13) Based on part of a doctoral thesis by T. M . B. always less than 5%. In the pyrolysis of norbornyl DEPARTMENT O F CHEMISTRY JOSEPHJORDAN chloride, the corresponding olefin, bicyclo [2.2.1IhepTHEPENNSYLVANIA STATE THEODORE MARKBEDNARSKI'~ tene, was not isolated. As expected, the retro-DielsUNIVERSITY Alder reaction of the bicyclic olefin7 was so rapid t h a t UNIVERSITY PARK,PENKSYLVANIA 16802 only cyclopentadiene and ethylene could be identified, RECEIVEDOCTOBER 16, 1964 I n all of the other cases secondary processes did not occur to measurable extents. The Question of Heterolytic Mechanisms for 1958," B u t t e r w o r t h s Scientific Publications, L o n d o n , 19.59, p. 230; ( h ) Gas-Phase Pyrolyses of Alkyl Chlorides C. H. DePuy and R. W. K i n g , Chem. Rea.. 60, 431 (1960); (c) A. Maccoll, "Technique of Organic Chemistry," Vol. V I I I , Part I , Interscience PubSir: lishers, Inc., N e w York, N. Y . , 1961, Chapter X ; and (d) D. V. Banthorpe, "Elimination Reactions," Elsevier Publishing c o . , N e w York, A t elevated temperatures in the gas phase, in reN . Y., 1963, Chapter 7. (2) A. Maccoll and G. S. Swinbourne, Proc. Chem. Soc., 409 (1960). action vessels which have been seasoned by prolonged (3) A. Maccoll a n d R. Bicknell, Chem. I n d . (London), 1912 (1961) contact with reaction products, monochlorinated hy(4) C. K. Ingold, Proc. Chem. Soc., 279 (1957). drocarbons have usually been shown to decompose by (5) W. C. Herndon, M . B. Henley, a n d J . M. S u l l i v a n , J. P h y s . C h e m . , 67, 2842 (1963). homogeneous and unimolecular processes. The prod(1) For reviews on pyrolytic elimination reactions see (a) A. Maccoll, "Proceedings of t h e Kekule Symposium on Theoretical Organic Chemistry,
(6) W. C. Herndon a n d L. L. L o w r y , J . A m . Chrm. S o c . , 86, 1922 (1964). (7) W. C. Herndon, W . B. Cooper, Jr., and M . J. C h a m b e r s , J. P h y s Chem., 68, 2016 (1964).
5692
COMMUNICATIONS TO THE EDITOR
The differences in solvolytic reactivities would not be as large a t 400 or 500°, but one should find the same order of reactivity. One notes t h a t the reactivity of exo-norbornyl chloride is less than t h a t of cyclohexyl or cyclopentyl chloride, although an analogy to carbonium ion reactions in solution requires a greater reactivity. I n addition, the reactivity of a-phenylethyl chloride is comparable to t h a t of the other secondary chlorides. The analogy would predict a rate similar to t h a t of t-butyl chloride. Exner has recently suggested t h a t a parallelism of rates a t two different temperatures is sufficient to infer similarity or identity of mechanism in a reaction series.8 Inversion of reactivity due to inclusion of an isokinetic temperature in the experimental temperature rangeg is discounted. However, as one can see from Table I , isokinetic temperatures do intrude into the ordinary temperature range for thermal dehydrochlorination. The work reported herein shows clearly t h a t the "quasi-heterolytic" mechanismla for gas-phase thermal dehydrochlorination is not correct in detail, although the parallel reactivities to solvolytic reactions found previously now become more difficult to explain.
VOl. 86
above 190 mp. The presence of the -HC=CMegroup is indicated by the n . m . ~ -maxima .~ a t T 8.2
A. I
q C Q R
A
A
I1ROzC 3COzR
I11
ROzC
IV, R = H V, R = M e
VI, R = H VII, R = Sle
(intensity 3 ) and 4.5-5.0 (1) and confirmed by ozonolysis followed by hypobromite oxidation to the dicarboxylic acid IV (30%), m.p. 133'. This, with acetic anhydride, yielded the anhydride (m.p. 40-40.5') and with diazomethane, the ester (V) ; the infrared, ultraviolet, and n.m.r. spectra of these three compounds indicate the absence of olefinic groups, so that the acid IV is monocyclic. T h a t it is a glutaric acid Acknowledgment.-The support of the National is suggested by the maxima a t 1761 and 1805 ern.-' Science Foundation (Grant No. GP2-17) is greatly of the anhydride and confirmed by a reverse Michael appreciated. reaction.6 Thus, when the ester V was saponified ( 3 hr. refluxing with 10% potassium hydroxide in (8) 0 . Exner, Coiieclion Czech. Chem. Comm., 2 9 , I094 (1964). (9) J . E . Leffler, J . O i g . C h e m . , 20, 1202 ( l 9 5 , j ) . methanol) and the product re-esterified, gas chroma(IO) T o whom inquiries should be addressed a t Florida Atlantic Univertography indicated starting material (37.5%) and a new sity. isomer (VII, 5670). The same reaction was effected (11) National Defense Education Act Fellow. DEPARTMENT OF CHEMISTRY WILLIAMC. H E R N D O N ' ~ less cleanly by heating a 6y0solution of V in isooctane UNIVERSITY OF MISSISSIPPI J A C KM . SULLIVAN for 20 hr. a t 280". The corresponding new acid (VI) USIVERSITY,MISSISSIPPI MELVINB. HEN LEU'^ had m.p. 156-157', Amax 206 mp ( t 15,000), and, when treated with ozone followed by warm 30y0 hydrogen DEPARTMEXT OF CHEMICAL AND JERALD M . MAN ION^^ PHYSICAL SCIENCES peroxide, gave isopropylsuccinic acid (45%), identiFLORIDA ATLANTICUNIVERSITY fied by infrared spectrum and mixture melting point 33432 BOCARATON,FLORIDA with a synthetic' specimen. RECEIVED JULY 6, 1964 The photoproduct I11 is thermally stable, having a half-life a t 300° of 40 min. ; five pyrolysis products, probably isomers of the starting material, are formed. When heated at 240' for 5 hr. with 5% platinized Synthesis of a Bicyclo[Z.l.l]hexene charcoal, m-cymene and p-cymene were obtained Sir: (8 and 5%, respectively, from a 96% sample); these The recent photoisomerization of 1,j-hexadiene to were isolated by gas chromatography and identified bicyclo [2.1.1]hexane' suggests a new general approach by their infrared spectra. Buchi and Goldmans to the synthesis of this interesting ring system.2 This have described the acid-catalyzed aromatization of communication records a cognate preparation of the another bicyclo[2.l.l]hexane. first unsaturated member (111) of the series. The mass spectrum of I11 further confirms the presence of an isopropyl group, having its base peak a t Ultraviolet irradiation of wphellandrene (I) was reported to give a mixture of trienes (11), which dismie = 93 and the complementary ion peak in comappeared on prolonged i r r a d i a t i ~ n . ~In the present parable abundance (58%) a t mle = 43.9 I t is thus work a 3yo ethereal solution of this terpene (Roy0)was surprising that the n.m.r. spectra of neither IV nor irradiated with light above 250 mp until i t showed no V show splitting of the isopropyl peak a t 7 8.9-9.1, absorption in this region. The crude product showed while those of the anhydride of IV and the hydroonly a weak maximum a t 1950 cm.-' due to allenic carbon I11 both show this peak as only partially rephotoproducts4 and yielded one principal photosolved doublets ( J = 3 and 1.6 c.P.s., respectively). product (35%) on distillation. ( 5 ) Kun in CDCln on a Varian A-60. T h e author is deeply indebted t o This compound (111) has b.p. 55" (18 mm.), is Drs. L. Stautzenberger a n d K . M. Geudin. Celanese C o r p . . Clal-kwood' Texas, for t h e n . m . r . a n d mass spectral d a t a given in this war-k optically inactive, and shows no ultraviolet maximum (I) K . Siinivasan, 3 P h y s . Chenr , 67, 1367 ( 1 0 6 3 ) : J . A m . C ~ P SI O~L.. , 8 6 , 819 (1963)
( 2 ) For a review see J. M e m w a l d , Record C h e m P Y O W ,2 2 , 39 (1961). ( 3 ) I < . J d e K o c k , S . G hlinaard. a n d E Havinga, R e c . ira7'. chrm.. 7 9 , 922 (1960) (4) k: J CI-owley, Pror. Chrin Soc., 1 7 (1064).
(6) C / . L Crombie, J . Crossley, a n d D . A. Mitchard. J . Chem Soc., 49.57 (1963). ( 7 ) P . A. S. S m i t h a n d J . P. Horowitz, J . A m . Chrm. S O L , 71, 3418 (1949). ( 8 ) G . Buchi a n d I. h l . Goldman, ibid., 79, 4741 (19.57). (9) R . I . Reed in " M a s s Spectrometry of Organic Ions," F. W. McLafferty. E d . , Academic Press, I n c . , 1963, p p . 656 el s e q .
