The reactions of O(3P) with the butanols

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10. H. H. GROTHEER andTH. JUST. Chem. Phys. Lett. 78,71 (1981). 11. D. G. KEIL, T. TANZAWA, E. G. SKOLNIK, R. B. KLEMM, and. J. V. MICHAEL. J. Chem.
The reactions of 0(3P)with the butanols JOHNM. ROSCOE Department of Chemistry, Acadia University, Wolfville, N.S., Canada BOP 1 x 0

Received April 26, 1983

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JOHNM. ROSCOE.Can. J. Chem. 61, 2716 (1983). ) the butanols were studied kinetically as a function of temperature and substrate concentration. The reactions of 0 ( 3 ~with The absolute rate constants for the gas phase reactions, in the units M-' s - ' , obey the following relations.

) other CH The results suggest that although the a-CH bond in these alcohols is the most reactive one, reaction of 0 ( 3 ~with bonds in the alcohols is also appreciable. The kinetic data for these and other alcohols are separated into contributions from the different types of CH bonds and the results are discussed in terms of linear free energy relations. JOHNM. ROSCOE.Can. J. Chem. 61, 2716 (1983). On a CtudiC la cinetique en fonction de la temperature et de la concentration de substrat, des reactions de O('P) avec les butanols. Les constantes absolues de vitesse des reactions en phase gazeuse a-yant pour unites M-' s-' obeissent aux relations suivantes:

0 0 0 0

+ butanol-1: + butanol-2: + methyl-2 propanol-1:

+ methyl-2 propanol-2:

*

In k = 24.12 0.16 - (1.78 r 0.19) x 103/T I n k = 22.22 ? 0.16 - (1.19 ? 0.19) X lo3/T In k = 22.02 r 0.13 - (1.10 0.15) X 1O3/T In k = 22.33 ? 0.20 - (2.19 ? 0.27) X 103/T

*

Ces resultats suggkrent qu'en depit du fait que la liaison CH-a de ces alcools soit la plus rtactive, 0 ( 3 ~reagit ) avec les autres liaisons de faqon appreciable. On a separt les donnees cinetiques de ces alcools et celles relatives ti d'autres alcools en des contributions provenant de diffirents types de liaison CH et on discute des resultats en fonction des relations lineaires d'tnergie libre. [Traduit par le journal]

Introduction The general features of the reactions of 0 ( 3 P )with alcohols have been thoroughly documented (1 -4). The primary reaction path is thought to be abstraction of the hydrogen atom a to the OH, although the reactivity of other CH bonds in the alcohols does not seem to have been evaluated directly. The overall reaction is generally discussed in terms of the following general mechanism under conditions of large excess of the alcohol. [l] 0

+ R-CHOH

[2] OH

I

+ R-

+ OH (R and R'

+ R-COH

I

CHOH + R-

I

COH

I

=

H or alkyl)

+ Hz0

OH OH [3] 2R-COH

I

+ R-C-C-R

I

+

R-CHOH I

[6] 0

+ R-COH

II

+ RC-R'

+ OH

The reactions of 0 ( 3 P ) with the butanols were studied for several reasons. 2-Methyl-2-propanol is the simplest alcohol lacking a-CH bonds. Its reactivity toward 0 ( 3 P ) would indicate whether corrections should be applied in the reactions of 0 ( 3 P ) with other alcohols to compensate for the reactivity of CH bonds that are not a - to OH. The other butanols include two primary alcohols, one of which is branched, as well as a secondary alcohol. Kinetic data for their reactions with 0 ( 3 P ) might be combined with other work in the literature to assess correlations of reactivity with structure. This might be useful in predicting the reactivity of other alcohols with 0(3P).

Experimental

R'

[4] 2R-COH I

0

II + RCR'

When 0 ( 3 ~ is) in excess, or when the alcohol is not in sufficient excess, reactions [2] to [4] become less important and one must include reactions such as (4, 5 )

'The experimental methods and apparatus used in this work have been described in detail elsewhere (4, 6). The reactions were studied in a conventional flow system at pressures from 0.7 to 1.2 Torr. Oxygen atoms were formed by titrating N(4S) atoms with NO in order to prevent interference from O2 (4, 5). N('S) was produced by dissociating purified N2 with a microwave discharge. The concentration ) in this way was kept small by passing only a small of 0 ( 3 ~produced part of the total nitrogen flow through the discharge, the remainder of the nitrogen mixing with the products of the discharge upstream from the NO inlet. This arrangement made it possible to achieve as much as 500-fold excess of alcohol, thereby avoiding interference from the

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FIG. 1. Dependence of pseudo first order rate constant on alcohol concentration 320 K; @, 0 + 2-methyl-1-propanol, 357 K; H, 0 + 2-butanol, 303 K. secondary reactions of 0 ( 3 ~ referred ) to in the Introduction. The concentration of O('P) was determined by measuring the intensity of the,emission from electronically excited NO2 produced by adding a small amount of NO in excess of that required to completely consume the N(4S).The rate constants obtained in this way were independent of the amount of excess NO used. The alcohols (Fisher, Certified) were purified by repeated fractional distillation until no impurities could be detected by gas chromatography. Their purity was estimated on this basis to be better than 99.999%. Gas chromatographic analysis of both starting materials and reaction products was done on an 8-foot column packed with Porapak Q, using a F and M model 700 gas chromatograph equipped with both a flame ionization detector and a thermal conductivity detector. The former detector was used for analysis of starting materials while the latter was used for analysis of reaction products because of the need to detect water. The reaction products were trapped at liquid nitrogen temperature, warmed to room temperature, and then removed from the flow system for injection into the gas chromatograph. Because of the large excess of alcohol used in these experiments, the alcohol could be used as an internal standard for the determination of other products. The method of data analysis differed somewhat from that used in earlier work (4, 6). This was necessary because of a complicating first ) was independent of both the alcohol order loss route for 0 ( 3 ~which and NO concentrations, was insensitive to temperature, and was only observed after the alcohol had been admitted to the reaction vessel. It seems probable that this loss route represents a heterogeneous process due to efficient adsorption of a reactant or product on the walls of the reactor. As indicated in Fig. 1, this behaviour is adequately represented by

At each temperature, first order rate constants (k,,,) were plotted as in Fig. 1 to yield a straight line whose slope was k, at that temperature. It is assumed that kR is the rate constant for the homogeneous reaction between 0 ( 3 ~and ) the alcohol and that k, refers to the extraneous first . the first order rate constants (k,,,) and order loss route for 0 ( 3 ~ )Both the Arrhenius parameters obtained from them were calculated by weighted least squares with weights y2/u2(y) where y = kR in the ) in the pseudo first order Arrhenius expression and y = 0 ( 3 ~signal analysis of O('P) decay. Data for both the rate constant determinations and the Arrhenius expressions were within two standard deviations of the regression line.

0, 0 + 2-methyl-2-propanol,

461 K;

0, 0 + 1-butanol,

Results and discussion' Gas chromatographic analysis of the reaction products indicated that the yield of water was equivalent to the consumption of 0 atoms. This suggests that the mechanism outlined in the Introduction adequately describes the reactions of the primary and secondary butanols with 0 ( 3 P ) under our experimental conditions. The reaction of 0 ( 3 P ) with 2-methyl-2-propanol would then be described by reactions analogous to [ l ] , [2], and [3] but involving P-CH bonds. This would be consistent with other work (1, 2, 4-6) and would mean that k , is the absolute rate constant for reaction [I]. No products other than water could be detected by gas chromatography. This could result if the organic radical formed in reactions [ l ] and [2] underwent recombination in preference to disproportionation. The glycols that would be formed would be expected to adhere to the glass tubing between the reaction vessel and the cold trap and would be lost to analysis. Moreover, the chromatographic column used gives poor results for high molecular weight glycols. Similar behaviour was experienced in our earlier work with the propanols (6) in which pinacol was detected, but could not be quantified and the glycol produced by the reaction of O('P) with 1-propanol could not be detected. This is also consistent with our experience that recombination products predominated over disproportionation products in the reactions of 0 ( 3 P ) with methanol and ethanol under similar experimental conditions (4). These results suggest that reaction [6] does not occur to any appreciable extent with the large a l ~ o h o l / O ( ~ Pratios ) used in this work. Pseudo first order loss of 0 (.3 P ) was observed for alcohol/atom concentration ratios ranging from about 50 to nearly 500 and for temperatures ranging from 300 K to 600 K. The temperature dependence of the homogeneous rate constants for the reactions of 0 ( 3 P ) with the butanols is summarized in Table 1 and Fig. 2. As anticipated, the rate ,

'Comp~eteset of raw data may be obtained, at a nominal charge, from the Depository of Unpublished Data, CISTI, National Research Council of Canada, Ottawa, Ont., Canada K I A 0S2.

CAN. J . CHEM. VOL. 62. 1984

TABLE1. Summary of Arrhenius parameters for reactions of O('P) with alcohols -

Alcohol

Ax lo-.' (M-' S-I)

AE, (kJ mol-I)

A,x lo-' ( M ' s-I)

AE, (kJ mol-I)

Reference

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3 4

9 10 II 4 This work This work This work This work 6 6 "Several kinetic studies have given values which deviate markedly from the results tabulated here and from the predictions of correlations such as those of ref. 12. These values have been omitted from this table but may be found in refs. 13 to 16 inclusive. w h i l e the preexponential factors for these reactions are evidently too small and calculation of the components due to the a-CH bonds is not possible, these values are included because the activation energies correlate well with the other reactions.

.,

FIG.2. Arrhenius plots for the reaction of 0 atoms with the butanols. Absolute uncertainty in In k is 0.16. 0, 0 2-methylI-butanol; 0, 0 + 2-methyl- I-propanol; 0 2-propanol; e, 0 + 2-butanol.

+

+

constants for the reaction of 0 atoms with 2-methyl-2-propanol were much smaller, at a given temperature, than those for the corresponding reactions of the other alcohols. This is primarily due to the relatively large activation energy for the reaction with 2-methyl-2-propanol and confirms the earlier suggestion (1 -3, 6) that 0 ( 3 P ) reacts with alcohols primarily by abstracting the H atom a - to the OH group. While good Arrhenius behaviour was observed for all the reactions studied, it is evident that abstraction of hydrogens that are not a - to OH can be a significant reaction path for 0 ( 3 P ) . Moreover, the comparatively large activation energy for this path makes it increasingly important as the temperature increases. This situation may be described by

[8] k

=

k,

+ k'

= n,A, exp (-AE,/RT)

+ n'A'

exp (-AE1/RT)

where n, and n' are the number of CH bonds a- and not a - to OH, A, and A' are the corresponding Arrhenius preexponential

factors per CH bond, and AE, and AE' are the activation energies characteristic of abstraction of the two kinds of hydrogen atom. Equation [8] could be further refined by writing the last term on the right as a sum of contributions by primary, secondary, and tertiary CH bonds that are not a - to OH. This approach has been applied to the reactions of 0 ( 3 P ) with alkanes (7), but the smaller activation energies for the reactions of 0 ( 3 ~with ) the alcohols made this impractical. It has also been assumed that abstraction of the OH hydrogen is of negligible importance. While there is no direct proof that this is valid, it seems reasonable in view of the relatively small reactivity of CH bonds which are not a- to OH. It is also relevant ) alcohols has not that product analysis in reactions of 0 ( 3 ~with detected products which could be unambiguously attributed to radicals formed by abstracting the OH hydrogen from the alcohol (1-3, 6). If eq. [8] is to be applied to existing data, it is necessary to assume that all CH hydrogens not a - to OH have the same Arrhenius parameters and that these may be estimated from the data for the reaction of 0 ( 3 ~with ) 2-methyl-2-propanol. This is likely to be an oversimplification since, by analogy with the reactions of 0 ( 3 ~ with ) the alkanes, the activation energy should decrease in the order primary CH > secondary CH > tertiary CH (7). However, since 2-methyl-2-propanol contains only primary CH hydrogens and the contribution per CH is small in our temperature range, this approximation will at worst underestimate the contribution of hydrogens not a - to OH. The availability of kinetic data for a more extensive range of tertiary alcohols would clearly improve the accuracy of this analysis, particularly at the high temperatures typical of combustion where such contributions would be substantial. The Arrhenius parameters for abstraction of the a-hydrogen atoms, calculated from eq. [8], are given in Table 1. In making these calculations, the contribution of non-a CH bonds (n'A' exp (-AE1/RT) was calculated from the data for 2-methyl2-propanol and subtracted from the measured absolute rate constant for each set of experimental conditions. The resulting rate constants (k,) were then fitted to the Arrhenius equation by y-weighted least squares as described in the Experimental section. The resulting slope and intercept gave values of -AE, and rz,A, in the usual way. Also found in Table 1 are Arrhenius

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ROSCOE

parameters for other alcohols as reported in the literature and the corresponding values of the parameters for abstraction of the a-CH hydrogens calculated as above. As with the lower alcohols, the preexponential factors are some 10 to 100 times smaller than those obtained for the reactions of O('P) with the alkanes (3, 4, 7). However, no clear relation between the preexponential factor and molecular structure is evident other than a purely statistical dependence on the number of a-CH bonds. Our earlier results for the reactions of the propanols with O('P) (6) give preexponential factors which are much too small in comparison with the results for the other alcohols. This problem was referred to in our earlier work although no explanation could be offered. The activation energy for abstraction of the a-hydrogen by 0('P) also shows little variation among the alcohols. The value for ethanol seems smaller than for the other primary alcohols, but this may be accidental as a result of the limited temperature range used in that work and the relatively large uncertainty reported for the activation energy. The activation energies for the reactions of 0('P) with 2-propanol and 2-butanol are similar and are smaller than the values obtained with the primary alcohols, as might be expected due to the somewhat weakened a-CH bond in the secondarv alcohols. It is useful to compare the activation energies for the reactions of O('P) with alcohols to those for the corresponding reactions of alkanes. In so doing, one must allow for the differences in reactivity of primary, secondary, and tertiary CH bonds referred to earlier. Since primary alcohols except methanol have secondary CH bonds, primary alcohols should be compared with alkane secondary CH bonds and secondary alcohols should be compared with alkane tertiary CH bonds. With this in mind, the ratio of activation energies for a-hydrogen abstraction from primary and secondary alcohols is about the same as the ratio of activation energies for abstraction of secondary and tertiary hydrogens in the alkanes. However, the activation energies for the reactions of 0('P) with the alcohols are some two to four times smaller than those for the alkane reactions as a result of the inductive effect of the alcohol oxygen atom. This makes differences in activation energy for different kinds of CH bonds in the alcohols more difficult to measure than in the alkane reactions. It is also of interest that the activation energy for the reaction of 0('P) with 2methyl-2-propanol is comparable to that for abstraction of a secondary CH hydrogen from an alkane. Since 2-methyl-2propanol contains only primary CH bonds this implies that the inductive effect of OH is significant more than one carbon atom into the aliphatic chain. Useful qualitative information may often be obtained by correlating relative rate constants with functional group parameters through a linear free energy relation. An appropriate correlation of this type is the Taft equation (8) which estimates the sensitivity of a reaction to inductive donation of electron density at the reactive site. A comparison of some abstraction reactions of O('P) and of OH radicals is provided in Table 2 and in Fig. 3. The Taft polar reaction coefficient, p*, is negative for all the reactions tested, indicating that 0('P) is an electrophile as expected (12). The steric sensitivity parameter, 6, is negligibly small for all the reactions, suggesting that structural variation within a given class of compound does not significantly alter the steric constraints on oxygen atom attack at the reactive site. This would not preclude the existence of some constant steric feature such as the postulated preferential collision of the elec-

TABLE2. Summary of data for Taft correlations" k29trb

Reaction

( M - I s- I)

&:

E,

Reference

0.49 0.49 0.49 0.49 0.00 0.00 -0.10 -0.1 15 -0.115 -0.19

1.24 1.24 1.24 1.24 0.00 0.00 -0.07 -0.36 -0.36 -0.47

4 9 10 II 1 4 12 12

Standard deviation 0.49 0.00 -0.10 -0.1 15

This work This work =

0.17

=

17 17 17 12 0.06

1.24 0.00 -0.07 -0.36

Standard deviation 0.49 0.00 -0.30

1.24 0.00 -1.54

12 12 12 Standard deviation = 0

0.49 0.00 -0.30

1.24 0.00 -1.54

Standard deviation

-0.19

=

-0.47

Standard deviation

12 12 12 0

=

12 0.08

=

12 0.09

0

I1

OH + HCH 0

II

OH+CH,CH

-0.19

-0.47

Standard deviation

"In all cases the reference compound is the one for which a* = E , = 0.00 by definition. bRateconstants for the reactions of 0 i 3 P ) with the alcohols were calculated for abstraction of hydrogen from a single a-C-H bond. 'Calculated in ref. 12 from linear free eneray -. correlations with other reactions.

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VOL. 62.

1984

linear free energy correlation of Gaffney and Levine (12) i n which the slope (1.364) of their correlation of log k(o,lp,, with log k(oH, for hydrogen atom abstraction can b e identified a s p*coc~p,,/p*coH, in the Taft analysis.

Acknowledgment

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Financial support for this work was provided by the Natural Sciences and Engineering Research Council of Canada.

.,

FIG. 3. Taft plots for 0, 0 + alkanes containing only primary C-H bonds; e, OH + alkanes containing only primary C-H bonds; 0, 0 + aldehydes; OH + aldehydes; A,0 + primary alcohols; A , OH + primary aIcohols. trophyllic 0 ( 3 P ) with the OH group of a n alcohol (3), which is unreactive and could lead t o a n unusually small Arrhenius preexponential factor for all the reactions in a n homologous series of alcohols. T h e values o f p* become less negative a s the strength o f the reactive C H bond decreases. This is in agreement with the theoretical result (18) that the absolute value of the selectivity parameter (p* in the Taft equation) should become smaller a s the general reactivity increases. T h e sensitivity t o polar effects also appears somewhat greater for reactions of 0 ( 3 P ) than for the corresponding reactions of OH. This is consistent with the

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