MARGARET H. BACK^ AND R. J. CVETANOVIC DiBision of Applied Chemistry, National Research Council, Ottawa, Canada

Transcription

1 REACTIONS OF IODINE ATOMS WITH n-butenes 11. INTERCONVERSION OF BUTENE-1 AND BUTENE-2' MARGARET H. BACK^ AND R. J. CVETANOVIC DiBision of Applied Chemistry, National Research Council, Ottawa, Canada Received January 28, 1963 ABSTRACT 111 the vapor phase reaction of iodine atoms with n-butenes at 65' C appreciable double-bond migration takes place in addition to the readily occurring geometrical isomerization. Ko detectable quantities of iodine-containing hydrocarbons are observed. In contrast to this, appreciable amounts of 1,3-butadiene and the corresponding amounts of hydrogen iodide are formed although direct abstraction of a hydrogen atom from butene by an iodine atom is unlikely in view of the endothermicity of this reaction. Fortnation of 1,3-butadiene is counterbalanced by its hydrogenation back to the original butene and its isomers and a steady state conceritration is reached in the course of the reaction. As a consequence of this balancing of dehydrogenation and hydrogenation steps there is an induction period in the formation of butene-2 in the reaction of iodine atoms with butene-1 and a large part of butene-2 is found to be derived from the butadiene initially produced. To explain the occurrence of the endothermic steps, it is assumed that the initial iodine atoll1 - butene adduct (iodob~~tyl radical or x-complex) interacts with a second iodine atom. Reaction mechanism is tentatively discussed. ISTRODUCTION In the study of the interaction of iodine atoms with cis-butene-2 (1) (Part I) it mas found at large conversions, used in order to determine the final equilibrium ratio of transbutenelcis-butene, that small amounts of butene-1 and butadiene were formed as well. The following results report the investigation of this additional reaction of iodine atoins with n-butenes. EXPERIMENTAL The apparatus and methods were the same as those used in Part I. Butadiene was Phillips Research Grade and was analyzed by gas chromatography with the column described in-part I. Fully deuterated butadiene was qbtained from Merck, Sharpe and oh me,-montreal, and was shown by mass spectrometric analysis to consist of 66.7% 'o4d6 and 33.3% C4D6H. Hydrogen iodide was analyzed by titration with silver nitrate using eosin as indicator and also with-sodium hydroxide using phenolphthalein as indicator. Sinall amounts of oxygen were added by first mixing with a large amount of nitrogen. RESULTS The reaction of iodine atoms with butene-1 produced butadiene and cis- and transbutene-2 and the reaction with cis-butene-2 produced trans-butene-2, butadiene, and butene-1. Hydrogen iodide was analyzed from 11 experiments and found to be approxiinately twice the molar quantity of butadiene formed. The results are given in Table I. The variation in the yield of products with time of reaction is shown in Figs. 1 and 2. The forination of butadiene from cis-butene was about 15 times slower than the rate of cis-trans isomerization. The rate of production of butadiene was roughly about one and a half times as fast from butene-1 as from butene-2 (Fig. 3), while the overall conversion of butene-1 to butene-2 was about seven to eight tiines faster than the reverse reaction (Fig. 4). 'Issued as N.R.C. No National Research Council Postdoctoral Fellow Present address: Chenzistvy Department, Cninitlersity of Ottawa, Ottawa, Canada. Canadian Journal of Chemistry. Volume 41 (1963) 1406

2 BACK AND CVETANOVI~: REACTIOKS OF I ATOMS WITH n-butekes. I1 TABLE I Comparison of yields of HI and butadiene in the reaction of iodine atoms with butene-1 Expt. 2 Xbutadiene HI Method No. (pmoles) (pmoles) of titration TIME OF RRADiATON (hours) FIG. 1. Time dependence of the yields of products of the reaction of butene-1 with iodine atoms (temp. 65' C, \\ravelength > 5000 A, pressure of butene cm, pressure of 12 4 mm). FIG. 2. Time dependence of the yields of products of the reaction of rzs-butene-2 with iodine atoms (temp. 65" C, wavelength > 5000 A, pressure of czs-butene cnl, pressure of Ia 4 mm). The recovery of C4 hydrocarbons, or compounds volatile at -78' C, was never less than 9870 and the loss was not related to the extent of reaction. The same loss occurred in blank experiments with no irradiation in which no products were formed and was probably due to some absorption of butene in stopcock grease. It was therefore concluded that non-volatile products were not formed in the reaction to a significant extent. Addition of butadiene to butene-1 in approximately 1 :1 ratio increased the production of butene-2 by almost a factor of two. Results of experiments with added butadiene-d6 are shown in Table 11. Only about half of the product butene-2 was formed from the deuterated butadiene, although the deuterated butadiene was greatly in excess over that formed in the reaction. The composition of the butene-2 fraction from the reaction with butene-1 is shown as a function of reaction time in Fig. 5. Although the extrapolation is not precise it seems likely that cis- and trans-butene-2 are formed initially in approximately equal amounts, but by subsequent reaction with iodine atoms, gradually approach their equilibrium proportions.

3 1408 CANADIAN JOURNAL OF CHEMISTRY. VOL. 41, 1963 TIME OF lrradlation(hourr1 TIME OF IRRADIATION (hours1 FIG. 3. Comparison of butadiene yields in the reactions of iodine atoms with cis-butene-2 and butene-l (temp. 65' C, wavelength > 5000 A; in both reactions iodine pressure 4 mm, olefin pressure 1.7 cni). FIG. 4. Comparison of the rates of double-bond isomerization in the reactions of iodine atoms with cis-butene-2 and butene-1 (temp. 65' C, wavelength > 5000 in both reactions iodine pressure 4 mm, olelin pressure 1.7 cm). TABLE I1 Isotopic conlposition of the product butene-2 formed in the reaction of iodine atoms with a 1:l mixture of C4H8-1 and 1,3-C4D6 Isotopic composition at various tinles of irradiation (70) Product 1 hr 2 hr 3 hr 1"' CaH s C4HiD C4H6D2 - FIG. 5. Time dependence of the isomeric compositio~l of butene-2 formed in the reaction of iodine atoms with butene-1 (temp. 65" C, wavelength > 5000 A, pressure of butene cm, pressure cf iodine 4 mm).

4 BACK AND CVETANOVI~: REACTIONS OF I ATOMS WITH n-butenes. I The effect of pressure of butene-1 on the rate of reaction (Fig. 6) was similar to that observed for the isomerization of butene-2, and, as in the latter reaction, the effect of FIG : 0.05 BUTADIENE TRANS-BUTENE BUTENE PRESSURE (cm) Dependence of the rates of product formatioil on pressure of butene-1 in the reaction of iodine atoms with butene-1 (temp. 65' C, wavelength > 5000 A, pressure of iodine 4 mm, irradiation time 1 hour). the butene as a third body for the recombination of iodine atoms presumably resulted in the tendency for the levelling off of the rate at high butene pressures. The rate of production of butadiene from butene-1 was a linear function of the light intensity over the range studied (Fig. 7). Since the rate of recornbination of iodine atoms PHOTOMETER READING (p ompr) PHOTOMETER READING (pomps) FIG. 7. Dependence of the reaction rates on light intensity in the reaction of iodine atoms with butene-1 (temp. 65" C, wavelength > 5000 A, pressure of butene-1 ca. 14 min, pressure of iodine 4 mm, 7, reaction ca. 2.5). FIG. 8. Light intensity dependence of the ratios of the products formed in the reaction of iodine atoms with butene-1 (temp. 65" C, wavelength > 5000 A, pressure of butene cm, pressure of iodine 4 mm). in the present system was proportional to (I)2, this suggests that the rate of formation of butadiene was also dependent on the square of the iodine atom concentration and mas probably a type of reaction similar to that suggested in Part I for iodine atom induced cis-trans isomerization of butene-2. The ratio of trafzs-butene-2/cis-butene-2 formed was practicallj7 independent of the light intensity for constant percentage conversion. The ratio of butadiene to trans-butene-2, however, increased with increasing light intensitj-. These ratios are shown in Fig. 8. Addition of nitrogen had no effect on the rate of reaction of butene-1, except for a

5 1410 CANADIAN JOURNAL OF CHEMISTRY. VOL slight decrease due to the competition of the nitrogen as third body for removal of iodine atoms. The ratios of the products, butadiene and cis- and trans-butene-2, remained the same. The effect of oxygen on the products at constant pressures of butene, iodine, and nitrogen and constant reaction time is shown in Fig. 9. The yield of butadiene was FIG. 9. Effect of additions of oxygen on the products formed in the reaction of iodine atoms with butene-1 (temp. 65' C, wavelength > 5000 A, pressure of butene cm, pressure of iodine 4 mm; HI was not determined). increased solnewhat with very small amounts of oxygen and then gradually decreased in a manner similar to the decrease in the rate of isomerization of butene-2 on addition of oxygen. The effect of oxygen on the production of butene-2 was more drastic. A ratio of oxygen/butene-1 of about 0.03 was sufficient to reduce the percentage formation of butene-2 from 1.5 to less than The effect of oxygen on the yield of butadiene as a function of time is shown in Fig. 10. A fixed quantity of air was added in each experiment TIME OF IRRADIATION imlnuter) FIG. 10. Effect of oxygen additions on the yields of butadiene at-various irradiation times in the reaction of iodine atoms with butene-1 (temp. 65' C, wavelength > 5000 A, pressure of butene cn~, pressure of iodine 4 trim). and the pressure of oxygen was sufficient to suppress completely the production of butene-2. The yield of butadiene was first below that found without oxygen, as was expected from Fig. 9. However, the final steady state yield of butadiene was about 50Yo greater than that observed with no added oxygen. In these experiments an estimate of the oxygen consumed was made by gas chromatographic analysis of the oxygen/nitrogen ratio before and after the irradiation. In each case the oxygen consumed was roughly equivalent to the yield of butene-2 in the absence of oxygen.

6 BACK ASD CVETANOVI~: REACTIONS OF I ATOMS WITH n-buteses. I DISCUSSION Butadiene was an unexpected product of the reaction of iodine atoms with butene-1 and butene-2 because hydrocarbons have not generally been found to lose hydrogen atoms b17 reaction with iodine atoms (2, 3). Excited molecules of iodine activated by 1849 a light (4) were found to attack C-H bonds in methane and ethane, but the energy content of such molecules was so large that the reaction was not endothermic. Recently (j), both n-propyl iodide and isopropyl iodide and hydrogen iodide have been found as products of the reaction of iodine atoms with propane at 49' C, although the quantum yield nras low, about 2X10-4. The reaction was thought to occur by the formation of a collision complex RHI with subsequent attack by another iodine atom. A similar type of mecha~lisrn is indicated by the present results and with olefins the second iodine atom reacts faster than with paraffins. The time dependence of the reaction products both from butene-1 and butene-2 and the production of deuterated butene-2 when deuterated butadiene was added to butene-1 are strong indications that butadiene was an intermediate product in the interconversion of butene-1 and butene-2 by reaction with iodine atoms. The butadiene was accompanied by hydrogen iodide and as this accumulated in the system a back reaction occurred to regenerate, finally, butene. In other words, the product butene was a secondary product and not formed by direct isomerization, through the formation of a short-lived C4HsI radical, for example. The rate of formation of butadiene and hydrogen iodide from butene-1 and butene-2 was roughly similar, but the rate of formation of butene-2 from butadiene was much faster than that of butene-1, as might be expected from the greater thermal stability of butene-2. Butadiene Formation It is rather unlikely that the initial step in the formation of butadiene is the abstraction of a hydrogen atom from butene by an iodine atom. This process is at least 10 kcal/mole endothermic and would be very slow at 65' C. Nevertheless the fornlation of butadiene from butene requires the removal of two hydrogen atoms by two iodine atoms to for111 two molecules of hydrogen iodide. This may occur by two slightly different mechanisms. (a) An iodine atom may abstract a hydragen atom from the radical or the 9-complex formed by addition of an iodine atom to butene (CdHEI): In either case the process would be exothermic and the product would be a molecule of iodobutene. Since the recovery of butene (as butene or butadiene) was virtually complete even at very long periods of irradiation the yield of this product could have been only very small so that fast subsequent reactions, keeping its steady state concentration low, would have to be assumed. A repetition of this sequence would yield butadiene: C4H7I I + CaH712 Dl CkH712 f 1 -+ CaHeI2 f HI [4 I C4H612 + C4H6 $ The formation of butadiene would thus occur in two steps, the molecule C4H7I being the intermediate. (b) The cis-trans isomerization of butene-2 was postulated in Part I to occur through the intermediate formation of 2,3-diiodobutane. The subsequent dissociation of this

7 1412 CANADI.4N JOURNAL OF CHEMISTRY. VOL. 41, 1963 unstable con~pound, usually to give butene and iodine, could be assumed to give, a small fraction of the time, butadiene and two molecules of hydrogen iodide in one step: C4H812 + C4Hs + 2HI. An analogous process is direct formation of 2HI and butadiene in reaction [2]. The probability of each of these mechanisms will be considered later. Butene Formation A detailed mechanism of the reverse formation of butene from butadiene would be difficult to propose at present. Several broadly similar alternatives are possible. The process probably begins by addition of an iodine atom to form a C4H61 radical (or a n-complex) followed by reaction with hydrogen iodide. This could give one of the iodobutene isomers and a further similar series of steps could then yield C4H812. Since significant amounts of non-volatile products were not forined it may be concluded that the diiodobutane molecule, C4H812, dissociates rapidly to butene and iodine and the I,&-diiodobutane (a stable compound) is not formed. This would require that the initial addition of iodine atoms occurs at one of the two internal carbon atoms and, furthermore, that the addition and abstraction occur in such a way that both butene-1 and butene-2 may be formed. The following reaction scheme, for example, would meet these requirements: If the radical formed in [9] is stable and reacts with hydrogen iodide before decomposing the result would be the formation of 2,3-diodobutane, which would then decompose to give butene-2 and iodine. However, the formation of butene-1 could not occur without hydrogen-atom migration. Intermediate -formation of resonance-stabilized methallyl radicals, as in reactions [lo] and [Ill, would be more likely to produce butene-1 and butene-2 in ratios consistent with their thermal stability by abstracting hydrogen from HI (reaction [12]). Inasmuch as methallyl radicals are indeed formed as intermediates, it is not necessary to postulate the unlikely non-terminal addition of iodine atoms in reaction [7]. The probability of mechanism (a) and (b) may now be discussed in more detail. The fact that the production of butadiene is not, or is very slightly, inhibited by oxygen may be adequately explained by mechanism (b) whereby butadiene is formed without the intermediate formation of a radical. However, the probability of simultaneous formation of two molecules of HI in reaction [6] may be quite low. hlechanism (a) accounts more easily for the results of the experiments with added butadiene-ds which showed that about half the product butene was not formed from butadiene. If the molecule C4HTI appears in both the breakdown to butadiene and the buildup to butene it follows that the C4H71 molecule may be converted back to C4He without necessarily going all the way to C4H6. This would still allow an induction period for the formation of the product butene-2 since the reaction [12] depends on a sufficient concentration of hydrogen iodide in the system. Thus the product butene may not all be formed by the buildup from butadiene. [GI

8 BACK AND CVETANOVIC: REACTIONS OF I ATOMS IVITH n-butenes. I The effect of oxygen on the series of reactions [I] to [5] is difficult to predict. The production of butadiene may not be inhibited if oxygen reacts with the C4HgI radical in the same way as the iodine atom reacts, to abstract a hydrogen atom. This may even increase the production of butadiene and in fact a small increase was observed. This effect is shown both in Fig. 8 and Fig. 9, where the increase occurs at a different percentage reaction for different amounts of added oxygen. With larger amounts of oxygen the additional effect of an efficient third body for recombination of iodine atoms may lower the overall rate of reaction as discussed in Part I. The abstraction of hydrogen by oxygen may also inhibit the re-formation of butene-1 and butene-2 since the hydrogen atom thus abstracted would very likely be made unavailable for the reactions involving conversion of butadiene to butene. Inhibition by oxygen in this sequence could, however, also occur after reaction [ll]. The linear dependence of the rate of production of butadiene on light intensity is the result of either mechanism (a) or (b). In each case the rate of butadiene formation is proportional to (I)2 and since (I)2 is proportio~lal to I,, the rate of formation of butadiene is thus proportional to I,. It should be noted that in contrast to the cis-trans isomerization of butene-2 no evidence for an additional rate proportional to the first power of the iodine atom concentration was observed. The authors are grateful to the Analytical Section of this division for hydrogen iodide determinations by NaOH titration and to Dr. A. W. Tickner for mass spectrometric analyses. REFERENCES 1. M. H. BACK and R. J. CVETBNOVI~. Can. J. Chetn. This issue. 2. J. F. HORNIG,_G. LEVY, and J. E. WILLARD. J. Chem. Phys. 20, 1556 (1952). 3. J. ZIMMERMAN and R. M. NOYES. J. Chetn. Phys. 18, 658 (1958). 4. T. A. GOVER and J. E. WILLARD. J. Am. Chem. Soc. 82, 3816 (1960). 5. S. V. FILSFTH and J. E. WILLARD. J. Am. Chem. Soc. 84, 3806 (1962).