J. Phys. Chem. 1986,90, 431-433
k3 = (5.5 f 2.0)
X 10" cm3.mol-'.s-'
for the unimolecular decomposition of C6Hs0and the reaction of CH3 with C6H50,respectively. The relatively low values of the A factor and activation energy for the C 6 H 5 0decomposition reaction favor the mechanism that involves a bicyclic radical intermediate. Additionally, the modeling of C O yields in the very early stage of anisole decomposition at temperatures below 1200
431
K gave rise to the following approximate rate constant for the unimolecular decomposition process: (1.2 f 0.3) X 1OI6 exp(-33 10O/T) s-' kl The rate constant determined hereon for this important process is to be useful for the interpretation of the complex C6H.5 combustion chemistry. Registry No. C6HSOCH3,100-66-3.
The Elimination Kinetics of Methoxyaikyi Chlorides in the Gas Phase. Evidence for Neighboring Group Participation Gabriel Chuchani* and Ignacio Martin Centro de Quimica, Instituto Venezolano de Investigaciones Cient$cas, Apartado 1827, Caracas 101 0 - A , Venezuela (Received: July 22, 1985)
The rates of elimination of 3-methoxy-1-chloropropaneand 4-methoxy-1-chlorobutane have been determined in a seasoned, static reaction vessel over the temperature range of 410-490 OC and the pressure range of 56-181 torr. The reactions are homogeneous and unimolecular, follow a first-order rate law, and are invariant to the presence of a twofold or greater excess of the radical chain inhibitor toluene. The overall rate coefficients are given by the following Arrhenius equations: for 3-methoxy-l-chloropropane, log k , (8')= (12.92 f 0.48) - (226.0 f 6.8) kJ mol-' (2.303RT)-I; for 4-methoxy-l-chlorobutane, log k l (8)= (12.89 0.26) - (218.1 & 3.5) kJ mol-' (2.303RT)-'. The C H 3 0 group in 4-methoxy-1-chlorobutanehas been found to assist anchimerically the elimination reaction, where dehydrochlorination and tetrahydrofuran formation arise from an intimate ion pair type of mechanism. The partial rates for these parallel eliminations have been determined and reported. Participation of the CH30 in 3-methoxy-1-chloropropaneis barely detected. The present results give further evidence of intimate ion pair mechanism through neighboring group participation in the gas-phase elimination of certain types of organic molecules.
*
Introduction An intimate ion pair mechanism in the gas-phase pyrolysis of certain types of organic halides is thought to occur by way of neighboring group participation.' This phenomenon may arise whereby a substituent located in the same molecule assists in the stabilization of the reaction center in the transition state. In some cases, the participating atom of a substituent may form an irreversible or real bond whereby a leaving atom possibly migrates or rearranges through an intramolecular solvation or autosolvation in the intimate ion pair type of mechanism. In order to observe neighboring group participation in gas-phase reactions, according to our recent proposed generalizatiow2 it is necessary that (a) the transition state be very polar, (b) the participating atom be large and able to overlap, and (c) the participating atom be highly polarizable. The C H 3 0 substituent in the gas-phase elimination of CH30(CH2),0Ac (n = 2, 3, 4)3does not provide anchimeric assistance as does the (CH3)2Nsubstituent in the gas-phase elimination of (CH3)2N(CH2),0Ac(n = 2, 3,4)? These results suggest that the oxygen atom is not as polarizable as the nitrogen atom, and, moreover, the transition state of ester pyrolysis is not very polar. However, since the pyrolysis of alkyl halides in the gas phase are more heterolytic in nature than esters,' the present work is aimed at studying the influence and possible participation of the C H 3 0 (1) Chuchani, G.; Dominguez, R . M . Int. J . Chem. Kinet. 1983, 15, 795. ( 2 ) Chuchani, G.; Martin, I.; Martin, G.; Bigley, D. B. Int. J. Chem. Kinef. 1979, 11, 109. (3) Chuchani, G.; Rotinov, A. React. Kinet. Card Lett. 1978, 9, 359. (4) Chuchani, G.; Rotinov, A.; Dominguez, R. M.; Gonzilez, N. J. Org. Chem. 1984, 49, 4157. (5) Maccoll, A. Chem. Reu. 1969,69, 33.
0022-3654/86/2090-0431$01.50/0
substituent in the gas-phase pyrolysis of CH30(CH2),C1, where n = 2, 3, and 4. Experimental Section
3-Methoxy-1-chloropropane. 3-Methoxy- 1-propanol (K & K, Labs) was added to PC13 in pyridine as described7 (bp 103 OC at 630 torr; lit. bp 110.4 O C at 756.6 torr'). The chloro ether was distilled to 97% purity as determined by GLC (FFAP 7%, Chromosorb G A W DMCS 80-100 mesh and Silicone DC 200/100, Chromosorb W AW DMCS 80-100 mesh). The pyrolysis intermediate, allylmethyl ether, was prepared by treating allyl bromide with C H 3 0 N a in CH,OH (bp 36 OC at 630 torr; lit. bp 46 "C at 760 torrs) and analyzed by using the FFAP column. Trimethylene oxide (Aldrich), propene (Matheson), and formaldehyde from 1,3,5-trioxane (Aldrich) were analyzed with a Porapak Q 80-100 mesh and Carbosieve B 60-80 mesh. 4-Methoxy-1-chlorobutane. Treatment of 4-methoxy-1-butanol (Sapon Lab.) with PC13 in pyridine yielded the corresponding chloro ether7 (bp 135-136 OC at 630 torr; lit. bp 142.5-142.8 "C at 760 torr7). This compound was distilled several times and a fraction of 98.7% purity (GLC, same FFAP column) was used. Tetrahydrofuran (Merck) was analyzed on the FFAP column, while CH$1 (Matheson) was analyzed on a Porapak Q 80-100 mesh column. 4-Methoxy-1-butene was prepared by pyrolyzing 4-methoxybutyl acetate as reported3 and 4-methoxy-2-butene was prepared from isomerization of 4-methoxy-1-butene with HCl in the gas phase. Both olefinic ethers were analyzed on the FFAP columns. ( 6 ) Kwart, H.; Sarner, S . F.; Slutsky, J. J . Am. Chem. SOC.1973, 95, 5234. (7) Palomaa, M. H.; Jansson, R. Berichte 1931, 64, 1606. ( 8 ) Bailey, W.; Nicholas, L. J . Org.Chem. 1956, 21, 648.
0 1986 American Chemical Society
432
Chuchani and Martin
The Journal of Physical Chemistry, Vol. 90, No. 3, 1986 60
TABLE I: Variation of Rate Coefficients with Initial Pressure 3-Methoxy-1-chloropropane at 460.1 O C Pa,torr 76 87 93 104 159 104k,,s-l 6.36 6.63 6.32 6.13 6.66
50
-
4-Methoxy-1-chlorobutaneat 430.0
Po,torr
40
104k1,SKI
u2 v)
81 4.75
99 4.88
126 4.84
O C
130 4.85
140 4.91
al L
The stoichiometry of eq 1 was confirmed by comparing, up to 50% reaction, the percentage decomposition of the substrate as determined by pressure measurements with that obtained by titrating the released HCl with 0.05 N NaOH solution (Figure 1). The stoichiometry for the pyrolytic elimination of 4-methoxy-1-chlorobutane, described in eq 3, requires that Pf = 2P0. The
a
30 0 .c
0
rr"
ae
20
IO
CH,OCH,CH,CH,CH,CI
-
+
CH,OCH,CH,CH=CH,
HCI
0 0
10
20
40
30
50
60
% H C I (titration)
Figure 1. Plot of percentage decomposition from pressure against the percentage decomposition by HCI titration for 3-methoxy-l-chloropropane at 460 O C .
The identities of substrates and products were also confirmed by mass spectrometric analyses. Kinetics. The chloro ethers were pyrolyzed in a static reaction ~ e s s e l seasoned ,~ with allyl bromide,' and in the presence of a twofold or greater excess of toluene inhibitor. The temperature was maintained within h0.2 OC with a calibrated platinumplatinum-I 3% rhodium thermocouple. The rate coefficient for 3-methoxy- 1-chloropropane was determined by titration of HC1 with a solution of 0.05 N NaOH. Rates for pyrolyses of 4methoxy- 1-chlorobutane were measured by GLC of unreacted starting material. The pyrolysis product 4-methoxy-1-butene was estimated on the basis of HCl titration, while tetrahydrofuran was estimated by chromatographic analysis of CH3Cl. No temperature gradient was found in the reaction vessel and the substrates were injected directly into the reaction vessel through a silicon rubber septum. Results and Discussion The gas-phase pyrolysis of 3-methoxy- 1-chloropropane as described in eq l requires that the final pressure, Pf,be three times CH,OCH,CH,CH,CI
-
+
CH,OCH,CH=CH,
HCI
l"i
(1)
CH2=CHCH3
+
CH,O
the initial pressure, Po (Pr = 3P0). This is necessary because decomallylmethyl ether (log A = 11.09; E, = 174.0 kJ poses extremely fast at any of the working temperatures used in the present work. The average experimental Pf/P0value measured after ten half-lives and at four different temperatures, each temperature in triplicate runs, is 3.16 f 0.05. The departure from theoretical stoichiometry is due to the formation of small amounts of ethane and other unidentified products. The yield of small quantities of methyl chloride and ethylene found in this elimination may be rationalized according to eq 2. That is, trimethylene oxide kH2-0
1
i CH,=CH,
+
CH,O
0.0%
(oxetane), which may be an intermediate, yields under the reaction condition ethylene and formaldehyde.
average experimental Pf/Povalue, each temperature in triplicate runs, is 2.36 f 0.10. The departure from the stoichiometry (eq 3) was found due to a small decomposition of the primary products 4-methoxy-I-butene and tetrahydrofuran. In this sense, 4methoxy- 1-butene decomposes, under similar experimental conditions of HCl and at least twofold of toluene, to I-butene and formaldehyde as described in eq 4. While pure tetrahydrofuran, CH,OCH,CH,CH=CH,
-
iro~/zo,ion
CH,OCH,CH=CHCH,
1
(4)
+
CH,CH,CH=CH,
CHg
7.7%
7H2-7H2 CH
&
CH2=CH, 7.2%
+
CH,=CHCH,
3.4%
+
CH,CH2CH3
+
0.4%
co (5) in the presence of HC1 and at least a twofold of toluene inhibitor, yields small amounts of propene, ethylene, and traces of propane (eq 5). The stoichiometry of eq 3 was verified by comparing, up to 63% decomposition, the chromatographic analyses of the unreacted amount of 4-methoxy- 1-chlorobutane with the chromatographic determination of CH$1 and the titrimetric determination of HC1. The yields of pyrolyses products within the range of rate determination are as follows: 3-Methoxy- l -chloropropane gave up to 50% decomposition, propene, formaldehyde, HCI and traces of ethylene. For 4-methoxy- 1-chlorobutane, the products are, up to 63% decomposition, mainly 4-methoxy- 1-butene, tetrahydrofuran, methyl chloride, and HCl, along with about 4.1% of the byproducts described in eq 4 and 5. The homogeneity of these pyrolytic eliminations was examined by using reaction vessels with different surface-to-volume ratios (Le., the packed vessel has a 6.0 times greater surface-to-volume ratio than does the unpacked vessel). When the packed and unpacked vessels were seasoned with the products of decomposition of allyl bromide, no differences in the rate coefficients were observed for either substrate. The kinetic determinations were carried out in the presence of toluene (the concentration of toluene is at least twice the initial pressure of the methoxyalkyl chlorides) to inhibit any possible radical chain processes of the substrate or the products. No induction period was observed. The rate coefficients are reproducible with a standard deviation not greater than h5% at a given temperature. The rate coefficients for pyrolytic eliminations of these chloro ethers have been found to be invariant of their initial pressure (Table I). The logarithmic plots are linear up to 50% decomposition for 3-methoxy-1-chloropropaneand 63% for 4-methoxy-1-chlorobutane. The rate coefficients with temperature in
The Journal of Physical Chemistry, Vol. 90, No. 3, 1986 433
Elimination Kinetics of Methoxyalkyl Chlorides TABLE II: Variation of Rate Coefficient8with Temperature 3-Methoxy-1-chloropropane temp, OC 104kl,s-I
430.0 440.0 450.0 460.1 1.36 2.23 4.29 6.42
TABLE I V Temperature Dependence in the Product Formation of 4-Methoxy-1-chlorobutane
470.1 480.1 490.0 10.61 17.24 29.72
temp, OC 104kA,s-l
4-Methoxy- 1-chlorobutane temp, OC 410.1 420.1 430.0 440.2 104kl,s-] 1.64 2.96 4.89 8.40
Product: 4-Methoxy-1-butene 410.1 420.1 430.0 0.98 1.74 2.95
440.2 5.13
Product: Tetrahydrofuran 410.1 420.1 430.0 0.68 1.20 2.00
440.2 3.60
temp, OC 104kA', s-l
TABLE 111: Arrhenius Parameters at 440 O C re1 rate compd 104k,,s-I per H E., kJ/mol log A, s-' 241.8 (14.2) 13.83 (10.20) CH3CH2CI" 1.34 1.0 244.7 (17.1) 14.06 (10.53) CH30CH2CH2Clb 1.36 1.5 226.0 (16.8) 12.92 (10.48) CH3OCH2CH2C2.29 2.6 H2Cl 218.1 (13.5) 12.89 (10.26) CH3OCH2CH2C8.13 9.0 H2CH2CI
the presence of toluene (Table 11) is given (80% confidence limit from a least-squares procedure) in the following Arrhenius equations: 3-methoxy- 1-chloropropane: log kl (s-I) = (12.92 f 0.48) - (226.0 f 6.8) kJ mol-I (2.303RT)-1 4-methoxy- 1-chlorobutane: (s-l)
=
(12.89 f 0.26) - (218.1 f 3.5) kJ mol-l (2.303RT)-' The influence of the methoxy substituent on the elimination rates of the chloro ethers relative to ethyl chloride can be seen in Table 111. The data in this table suggest that the CH30group is providing anchimeric assistance in the rate of elimination from 4-methoxy-1-chlorobutane. This interpretation is further supported by the formation of the ring closed product, tetrahydrofuran, in addition to the normal formation of the dehydrohalogenated product. We interpret the significantly higher relative rate for 4-methoxy- 1-chlorobutane as compared to ethyl chloride as being due to the participation of the neighboring C H 3 0 substituent through a favorable five-membered ring. Therefore, the mechanism may be explained in terms of an intimate ion pair, with intramolecular solvation or autosolvation of the chloride ion in two pathways (eq 6).
(6)
+
CH30CH2CH,CH=CH2
HCI
CH2-CH2
I
CH
I
+
CHGI
The small amount of methyl chloride obtained in the pyrolysis of 3-methoxy- 1-chloropropane implies that in this case the CH30 substituent participates only weakly. This is in agreement with expectations since methoxy group participation requires formation of a strained four-membered conformation. There a r e few ex~
~
~~
product 4-methoxy-1butene tetrahydrofuran
104k,, per H to s-I CH,CH,CI" 5.13 5.7 3.63
'See Table 111. equivalent to one H.
'See ref 9. bSee ref 2.
log k l
TABLE V Partial Rates and Arrhenius Parameters for the Elimination Products of 4-Methoxy-1-chlorobutane at 440 OC re1 rate
~
~~
(9) Evans, P. J.; Ichimura, T.; Tschiukow-Roux, E. Inr. J. Chem. Kinet. 1977, 9, 819.
8.1b
E., kJ/mol 221.6 (13.5)
log A, s-l 12.94 (0.26)
222.1 ( f 8 . 6 ) 12.83 (10.64)
Formation of tetrahydrofuran is statistically
amples of solvolysis in the literature where neighboring group participation occurs via a four-membered ring.I0 The effect of the C H 3 0group in the dehydrochlorination process can be assessed if it is projected on a recently reported Taft correlation of the log k,, of substituted ethyl chlorides, i.e. ZCH2CH2Clvs. u* values." We find that our data for 3-methoxy-1-chloropropane(i.e., Z = CH30CH2,u* = 0.64)12is within experimental error of this Taft correlation. That the electron withdrawing substituents CH30CH2 and CH30 both enhance the pyrolytic elimination rate can be rationalized as reported previously." These groups acidify the hydrogen adjacent to Z and thus assist the leaving chlorine atom. This effect accounts for the observed rate enhancement with respect to ethyl chloride. The partial rate of product formation from the pyrolysis of Cmethoxy- 1-chlorobutane up to 63% decomposition was estimated by titration of HCl for the formation of CH30CH2CH2CH=CH2 and by the quantitative chromatographic analysis of CH$l for the yield of tetrahydrofuran. The temperature dependence of the rate coefficients for the formation of these products (Table IV) gives the Arrhenius parameters shown in Table V. The summation of the rates in Table IV is within experimental error of the overall rate coefficient for 4-methoxy-1-chlorobutane obtained as described above (Table 11). The higher relative rate for each elimination product with respect to the value of ethyl chloride (compare Table I11 and V) implies the anchimeric assistance of the C H 3 0 substituent in the stabilization of the transition state as shown in eq 6. Thus, we conclude that the present experimental results supports Maccoll's view of polar transition state5 and provides additional evidence for an intimate ion pair mechanism in the gas-phase pyrolytic elimination of some organic halides.
Acknowledgment. We are grateful to the Consejo Nacional de Investigaciones Cientificas y Tecnoldgicas (CONICIT) for their support through Project No. 51.78.31, S1-1072, and to Matilde Gdmez for mass spectrometry analyses. R@try NO. CH30CH2CH2CH2CI,36215-07-3; C H 3 O C H 2 C H 4 H2, 627-40-7; CH30CH2CH2CH2CH2CI,179 13-18-7; 3-methoxy- 1propanol, 1589-49-7; allyl bromide, 106-95-6; 4-methoxy-l-butanol, 11 1-32-0. (10) Capon, B.; McManus, S.P. "Neighboring Group Participation", Vol. Plenum: New York, 1976. (11) Chuchani, G.;Martin, I.; Rotinov, A.; Hernlndez, A., J. A,; Reikonnen, N. J . Phys. Chem. 1984,88, 1563. (12) Hansch, C.; Leo, A. J. "Substituent Constants for Correlation Analysis in Chemistry and Biology"; Wiley: New York, 1979. 1;
