3188
J. P h p . Chem. 1980, 84, 3188-3190
Homogeneous Unimolecular Gas-Phase Ellmlnatlon Kinetics of 2-Chloro-2-alkylpropanes. The Electronic Effect of Alkyl Substituents on the a-Carbon of Tertiary Chlorides Gabriel Chuchanl" and Ignaclo Martin Centro de Odmlca, Instltuto Venezolano de Investigaciones Cientificas, Apartado 1827, Caracas 10 10-A, Venezuela (Received: May 7, 1980; In Final Form: July 2, 1980)
The pyrolysis kinetics of 2-chloro-2-methylbutaneand 2-chloro-2,3-dimethylbutanehave been investigated, in a static system and seasoned vessel, over the pressure range of 50-280 torr and the temperature range of 260-320 "C. The reactions are homogeneous and unimolecular, follow a first-order law, and are invariable to the presence of a cyclohexene inhibitor. The temperature dependence of the rate coefficients is given by the log k1 (s-l) = (13.77 f 0.25) - (184.1 f 2.6) kJ-mol-' following Arrhenius equations: for 2-chloro-2-methylbutane, log k1 (9-I) = (13.33 f 0.18) - (175.3 f 1.9) kJ-mol-' (2.303RT)-'. (2.303R7')-1; for 2-chloro-2,3-dimethylbutane, The distribution of the olefin products from these reactions has been quantitatively determined and reported in details. The alkyl series ((CH3),C, (CH3)&H, CH3CH2,CH3, and H) in the tertiary halides, 2-chloro-2alkylpropanes, influence the rate of elimination by electronic effect. This is similar to those obtained with a- and P-alkyl-substituted ethyl chlorides. The plot of log k/ko vs. "*(R) gives a very good straight line with p* = -4.75, r = 0.994, and intercept = 0.048 at 300 "C. The previous and present results reveal that, if a reaction center at the transition state of an organic molecule is markedly polar, the +Iinductive electron release of alkyl substituents may affect gas-phase elimination processes.
Introduction The alkyl substituents were reported to exert a +I inductive electron-releasing effect in the gas-phase homogeneous unimolecular elimination of primary and secondary alkyl chlorides1i2. In both cases, the plot of log k / k o vs. CJ*((R)values3 yielded good straight lines, where p* = -1.81 for 0-alkyl-substituted ethyl chlorides,l and p* = -3.55 for a-alkyl-substituted ethyl chloridesS2These two results have additionally confirmed the heterolytic nature of these reactions4 and that the C-C1 bond polarization, in the sense of C"-Cl", is rate determining. The phenomenon of +I electron release of alkyls has not been observed in gas-phase elimination of esters. In these cases, the approximate linear relationship of log krelvs. E, values: for primary acetates with alkyl substituents on the carbon5 and for tertiary acetates with alkyl substituents on the a carbon,6 suggested that these eliminations are enhanced by steric acceleration. This means that esters are semiconcerted or semipolar in the transition state and less heterolytic than alkyl chlorides. The above-described facts thus imply that the +I electron release of alkyl substituents in organic molecules during gas-phase pyrolysis is only feasible and may be pronounced as long as the nature of the transition state at a reaction center is markedly polar. To shed additional light on such considerations, further evidence is therefore necessary. In this respect, as the alkyl substituents in tertiary acetates6affect the rate of elimination due to steric acceleration, the present work is a study, by analogy, of which factors influence the unimolecular decomposition of alkyl-branched tertiary chlorides. In reviewing the literature, the gas-phase pyrolysis of RC(CH3)2C17(R = CH3, CH3CH2,(CH3)&H, and (CH,),C) showed a gradual augmentation of the rate as the number of P-methyl in the R group increased from zero to three. The effect of Pmethyl substitution was described as second-order dependent on the C-C1 bond and not first order on the C r H bond. Unfortunately, this work did not report a detailed distribution of olefin formation at different temperatures, and just barely described the predominance of olefin products according to the Saytzeff rule. This means that 0022-3654/80/2084-3 188$01.OO/O
the effect of R toward the two methyl groups in RC(CH3)ZCl elimination cannot be assessed. Since the pyrolysis of tert-butyl chloride and 2-chloro-2,3,3-trimethylbutane yields a unique product of isobutene and 2,3,3-trimethyl-l-butene, respectively,7 we wanted to reinvestigate the gas-phase elimination kinetics of 2chloro-2-methylbutane and 2-chloro-2,3-dimethylbutane and, at the same time, to examine the olefin product distributions from these two reactions. Moreover, a linear correlation for R substituents in the gas-phase pyrolyses of RC(CH3)&1 is to be projected, if any.
Experimental Section The substrate 2-chloro-Zmethylbutane was acquired was from K + K Labs, but 2-chloro-2,3-dimethylbutane prepared by treating the corresponding alcohol with concentrated hydrochloric acid as describeds (bp 67-68 "C at 180 torr; lit. bp 67.6-70 "C at 180 torrg). These tertiary chlorides were distilled several times and the fraction with 99.4% purity (gas-liquid chromatography) was used. 2Methyl-l-butene and 2,3-dimethyl-l-butene were acquired from K + K labs, while 2-methyl-2-butene, 2,3-dimethyl-2-butene, and 3,3-dimethyl-l-butene were from Aldrich. The olefins were at least 99.0% pure and were used as standard references. A column of diisodecyl phthalate-5% Chromosorb G AW DMCS 60-80 mesh was used for the quantitative analysis of the alkyl chlorides, whereas a column of 12-ft bis(2-methoxyethyladipate)-5% Chromosorb G AW DMCS 80-100 mesh was used for the olefins. The purity of the halides and olefin products was verified with a mass spectrometer and by infrared and nuclear magnetic resonance spectroscopy. The leastsquares calculations were performed with a Digital PDP 1145 computer. The substrates were pyrolyzed in a static system with the vessel previously seasoned with allyl brornidel0~l1and the kinetic followed manometrically. The temperature was maintained within h0.2 "C with a calibrated platinumplatinum-13 % rhodium thermocouple. No temperature gradient was found at different point along the reaction vessel. The tertiary chlorides and butenes were injected 0 1980 American Chemical Society
The Journal of Physical Chemisfty, Vol. 84, No. 24, 1980 3189
Kinetics of 2-Chloro-2,-alkylpropanes
TABLE 11: Distribution (%) of Olefinsa
TABLE I: Stoichiometry of the Reactions 2-Chloro-2mmethylbutaneat 290.1 C 15.1 20.5 36.6 48.6 decomp % (press.) decomp 3'5 (chrom) 13.8 21.3 35.4 48.1
2-Chloro-2-methylbutane 50.3 51.9
temp,'C _. 270.1 290.1 310.2
S-Chloro-2,3-dimethylbutane at 299.9 C decomp % (press.) decomp % (chrom)
12.0 10.8
23.7 22.7
36.9 35.8
44.2 43.4
57.9 57.4
directly into the reaction vessel with a syringe through a silicon rubber sepLurn.l2
r3- r3
CH CHCl
CH,CH,C=CH,
4- C H 3 C H X C H 3 t HCI
(1)
21
la
lb
1 CH3 CH,
I
I
CH3 CH3
- ' 6 CH3CH-
%H,
,H3
t CH,C%CH,
CH3CH-rI CH3 2a
2
I
+
HCI
(2)
CH3
2b
of decomposition of 2-chloro-2-methylbutane (1) and of 2-chloro-2,3-dimethylbutane(2) indicates that for long reaction times the final pressure Pf should be twice the initial pressure Po. The average experimental results of Pf/Povalues at four different temperatures and ten halflives were 1.99 for 2-chloro-2-methylbutane and 1.95 for 2-chloro-2-dimethylbutane.Since the hydrogen chloride produced in the reactions rapidly recombined to the olefin products when condensed, the above stoichiometry could not be verified by comparing the percentage decomposition of the reaction with that determined by direct titration of HC1. However, additional agreement was possible by a comparison betwelen the percentages of decomposition from pressure measurements with those obtained from chromatographic analyses of the olefins after removing the HC1 by passage of the gaseous reaction mixture over soda lime (Table I). 2-Chloro-2-methylbutane (1) yielded mainly 2-methyl-1-butene (la) and to a lesser amount 2-methyl-2-butene (lb), whereas 2-chloro-2,3-dimethylbutane (2) gave mostly 2,3-dimethyl-l-butene (2a),a small amount of 2,3-dimethyl-2-butene (2b), and traces of 3,3dimethyl-1-butene. The homogeneity of these eliminations was checked by using vessels with surface-to-volumeratio factors of 4.0 and 6.14 greater than that of the unpacked vessel. When the packed and unpacked vessels are seasoned with allyl bromide the reaction in both chlorides are homogeneous. However, in packed and unpacked clean Pyrex vessels a marked increase in the rates suggests some heterogeneous effects. The analyses of the decomposition products of both halides in an unpacked seasoned vessel are given in Table 11. The olefin ratios have been found to change slightly at the pyrolysis temperature. At the higher the pyrolysis temperature of the chlorides, as shown in Table 11, a very small increase of the more thermodynamically stable 2butene isomer is observed. Further examination of whether or not the formation of these olefins is invariable as the reaction progresses at a working temperature was performed. 'The daka of Table I11 confirm the results of Table 11. The pure olefins 2-methyl-1-butene and 2methyl-2-butene dal not isomerize when thermolyzed in a
2M2B
2MlB/2M2B
71.3 29.9 68.3 32.5 67.4 33.8 2-Chloro-2,3-dimethylbutane
2.4 2.1 2.0
2,3DMlB
2,3DM2B
2,3DM1 B/ 2,3DM2B
83.6 81.3 79.1
15.1 17.1 18.8
5.5 4.6 4.2
270.1 290.1 300.0
Results and Discussion The stoiclhiomet~y(eq 1and 2) based on the products CH
2M1B
a Vessel S/V = 1 and seasoned with allyl bromide. 2M1B = 2-methyl-1-butene; 2M2B = 2-methyl-2-butene; 2,3DMlB = 2,3-dimethyl-l-butene;2,3DM2B = 2,3dimethyl-2-buteine. The formation of 3,3-dimethyl-lbutene was not greater than 0.2% at the three working temperatures.
TABLE 111: Variation of Olefina Formation from Percentage Decomposition of the Halides at One Temperature product yield, % decomp, substrate % 2M1B 2M2B 2-chloro-2methylbutane at 310.1 "C
12
69.7
29.9
27 39 53
68.5 67.7 68.2
30.3 31.0 32.8
product yield, % 2,3DMlB 2,3DM2B 2-chloro-2,316 dimethylbutan,e at 290.3 a C 24 40 51
82.3
3,3DMlB
16.9
0.07
81.1 18.1 0.04 80.3 18.4 0.14 83.6 16.0 0.09 a 2M1B = 2-methyl-1-butene;2M2B = 2-methyl-2butene; 2,3DMlB = 2,3-dimethyl-l-butene; 2,3DM2B = 2,3-dimethyl-2-butene; 3,3DMlB = 3,3-dimethyl-l-butene. TABLE IV : Isomerization (%) of the Olefin Productsa in Seasoned Vessel of SlV = 1,in the Presence of HCl Gas,for 1h olefin 2-M1B 2-M2B
temp, O C 320.3 320.3
2M1B 99.3
2M2B 99.6
2,3DMlB
2,3DM2B
2,3DMlB 310.1 95.6 3.2 2,3DM2B 3113.1 1.1 98.8 3,3DMlB 310.1 See footnote to Table I11 for abbreviations.
3,3DMlB
99.4
TABLE V: Variation of Rate Coefficients with Temperature :Z-Chloro-2-methylbutane temp, " C 260.1 270.1 280.1 290.1 300.2 310.8 320.1 no.of 7 6 6 7 6 9 6 runs
104h,,s-' 0.55 1.15 2.43 4.74 9.79 19.05 37.89 2-Chloro-2,3-dimethylbutane temp, "C 270.2 280.1 290.0 299.9 306.1 309.9 no.of 6 9 12 10 10 8 runs 1 0 4 h , , s - ' 2.96 5.96 11.35 21.63 34.11 40.79
normal vessel with HC1 for 1 h at 320.3 OC (Table IV). However, under similar conditions but at 310.1 "C, pure
The Journal of Physical Chemistty, Vol. 84, No. 24, lQ80
3190
Chuchani and Martin
TABLE VI: Kinetic Parameters of RC(CH,),CI at 300.0 C 'CH,
R
104k,,s - '
(l-olefin),'~-~
E,, kJ/mol
log A,
s-l
log k , (1-olefin)
ref
H CH, CH,CH,
0.017 0.017 213.8 13.64 - 2.209 13' 4.13 2.75b 188.2 13.77 0.000 7c 10.96 184.1 13.82 7 9.76 6.54 184.1(f 2.6) 13.77(* 0.25) 0.383 this work (CH, )*CH 23.18 175.7 13.38 7 22.47 17.77 175.3(f 1.9) 13.33(*0.18) 0.811 this work (CH,),C 147.19 147.19 171.5 13.80 1.729 7 a kc^, is the rate coefficient of the two CH, substituents in RC(CH,),Cl pyrolyses. This k value corresponds to the elimination of two methyl group, thus allowing a determination of the effect of the third methyl substituent. Preferred value, see ref 14.
1.41
\ CH~CHZ
0 2-
I
-0.6
-x
-1.0
\Hc - I 8-
-2 2 -26
I
I
I
I
I
where the pure olefins showed little or no isomerization when pyrolyzed in normal seasoned vessel and in the presence of HC1 gas. Therefore, it is possible, without fear of large errors, to establish a correlation between the effect of alkyl groups, R, and the relative rate of elimination of the tertiary alkyl chlorides, RC(CHJ2C1, in the gas phase. With the kinetic parameters listed in Table VI, an excellent linear relationship of log k/kovs. (I(~)* is obtained (Figure 1,p* = -4.75,r = 0.994,intercept = 0.048,at 300 "C). The present finding ratifies the polar effect of alkyl substituents in the gas-phase pyrolyses of primary and secondary alkyl chlorides.'I2 A very recent work15 has clearly demonstrated that any consideration concerning steric acceleration in branched alkyl chloride pyrolyses is unimportant, and that the paramount factor which determines the rate of decomposition is electronic in nature. According to previous and present results if the reaction center at the transition state of an organic molecule is sufficiently polar, the +Iinductive electron release of alkyl substituents may influence the gas-phase elimination reactions. The slopes of the lines for RC(CHJ2C1P * ( ~ = ) -4.75 at 300.0 "C, for RCHCICHs P * ( ~ )= -3.552 at 360 "C, and for RCH2CH2ClP * ( ~ )= -1.81' at 440 OC in the gas-phase elimination of these halides, obviously indicate, by extrapolation to one temperature (pT2/pTl = Tl/T2),that the positive nature at the a-carbon reaction center of the C-C1 bond in the transition state increases from primary to tertiary carbon atom.4
References and Notes Chuchani, G.; Herngndez A., J. A.; Avila, I. J. Phys. Chem. 1978, 82, 2767. Chuchani, G.; Martin, I.; Alonso, M. E. Inf. J. Chem. Kinet. 1977, 0, 819. Taft, Jr., R. W. "Steric Effect in Organic Chemistry"; Newman, M. S.,Ed.; Wlley: New York, 1956; Chapter 13. Maccoll, A. Chem, Rev. 1969, 69, 33 Chuchani, G.; Martin, I.; Avila, I . Int. J. Chem. Kinet. 1979, 1 1 , 561. Martin, I.; Chuchani, G.; Avila, I.; Rotinov, A.; Olmos, R. J. Phys. Chem. 1980, 8 4 , 9. Maccoll, A.; Wong, S . W. J. Chem. SOC.B 1968, 1492. ConDenhaver. J. E.: Whalev, A. M. "Organic - Synthesis Collect. Vol. I"; Wiley: New York, 1941; p 142. Brown, H. C.; Fletcher, R. S. J . Am. Chem. SOC.1949, 71, 1845. Maccoll, A. J. Chem. SOC. 1955, 965. Maccoll, A.; Thomas, P. J. J. Chem. SOC.1955, 979. Bridge, M. R.; Davies, D. H.; Maccoll, A.; Ross, R. A.; Banjoko, 0. J. Chem. SOC. B. 1968, 805. Tsang, W. J. Chem. Phys. 1964, 41, 2487. Benson, S. W.; O'Neal, H. E. Natl. Stand. Ref. Data Sef., AM/.Bur. Stand. 1970, 21. Chuchani, G.; Rotinov, A. React. Kinet. Catal. Lett. 1970, 12, 333.
