597
LOW-TEMPERATURE OXYGENATOMADDITIONTO OLEFINS
Low-Temperature Oxygen Atom Addition to Olefins. 111. Transition State and the Reaction with cis- and trans-2-Butenes1
by Milton D. Scheer and Ralph Klein National Bureau of Standards, Washington, D . C .
20834
(Received August 5. 1 9 6 8 )
The reaction of ground-state oxygen atoms OpP), with either cis- or trans-2-butene at cryogenic temperatures produces cis- and trans-2, 3-epoxybutane12-butanone1 and isobutyraldehyde. Product ratios are different for the two 2-butenes. Two precursor states, one leading to the trans epoxide and 2-butanone1and the other to the cis epoxide and isobutyraldehyde, are implied by the constancy of the ratios trans-2,3-epoxybutane/a-butanone and cis-2 ,3-epoxybutane/isobutyraldehyde. A new transition state for the 0 atom-olefin reaction is proposed. A prediction based on the new transition state for the 0 atom reaction with 2-methyl2-butene was consistent with the experimental finding.
Results of observations on the reaction of 0 atoms with condensed olefins at low temperatures were given in the two previous papers of this s e r i e ~ . ~ JProducts of the low-temperature reaction with propene, butene-1 , and cis- and trans-2-butene were shown to be the same as those of the higher temperature gas phase results obtained by Cvetanovic4 exclusive of degradation products. Under the higher temperature conditions they arise from fragmentation of “hot” oxygen adducts and a t lower temperatures from ozonolysis when ozone is present. Cracking products can be eliminated under either condition of operation. In the present work, the reactions of 0 atoms with cis- and trans-2-butene are examined in considerable detail. Some of the previously developed concepts, particularly those relating to the transition state intermediate, require modification to be consistent with the results. Using a new model, prediction of the course of the reaction with 2-methyl2-butene was made and verified experimentally. The products of the 0 atom-2-butene reaction are trans- and cis-2, 3-epoxybutane, 2-butanone1 and isobutyraldehyde. The ratios are different for trans-2butene as compared to the cis-2-butene. There is also a temperature dependence and from its analysis the equilibrium constant for the transformation cis-2 ,3epoxybutane % trans-2 ,3-epoxybutane can be calculated.
Experimental Section The apparatus used to obtain these results has been described previo~sly.~JThe olefins were diluted with propane for the 77 and 90°K experiments and with 2-methylbutane and n-butane at 113°K. Boiling liquid nitrogen (77’K), oxygen (90”K), and melting isopentane (113°K) were used as the cryogenic baths for maintaining isothermal conditions in the vessel and at the reactant film. Gas chromatographic product analyses were obtJainedusing a 2 m long, 6 mm diameter
column, containing 2 ,4-dimethylsulfolane on chromosorb W. The reaction rates of films containing the 2-butenes were found to be zero order in olefin concentration. This is the same as the previously reported results3 for propene and 1-butene. Such observations suggest that the oxygen atoms easily penetrate hydrocarbon films. We are concerned here with the chemical reactions rather than the diffusion processes, however. Table I gives the fractional yields of the four products obtained with trans- and cis-2-butene at several temperatures. It is noteworthy that starting with trans2-butene, over 90% of the products from the 77°K reaction are trans-2,3-epoxybutane and 2-butanone. Even at 113°K they constitute about 85% of the products. It is of considerable interest that the ratio of trans-2 ,3-epoxybutene to 2-butanone is about 0.8 independent of whether the reactant olefin is cis- or trans-2-butene. The gas phase result of about 1.0 is not sufficiently different to cast doubt on the conclusion that trans4 ,S-epoxybutane and 2-butanone originate from the same precursor state. The data also imply that cis-2 ,3-epoxybutane and isobutyraldehyde have a common origin. Therefore, in the reaction of 0 with 2-butene, two precursor states are formed, one leading to trans-2 ,3-epoxybutane and 2-butanone, and the other to cis-2 ,S-epoxybutane and isobutyraldehyde. A pictorial representation embodying these criteria is shown in Figure l a and b for cis- and trans-2-butene reactants. Representation of the Precursor State. The representation of the precursor or transition state in the 0 atom reaction with olefins has previously been given as a triplet free radical with localized spins.5 For trans-2(1) Supported by the U. S. Public Health Service. ( 2 ) A. N. Hughes, M. D. Scheer, and R. Klein, J. Phys. Chem., 7 0 ,
798 (1966). (3) R. Klein a n d M. D. Scheer, (bid., 72, 616 (1968). (4)R. J. Cvetanovit, Advan. Photochem., 1, 115 (1963). (5) R.J. Cvetanovib, J . Chem. Phys., 2 5 , 376 (1966). Volume 79, Number 9 March 1969
598
MILTOND. SCHEERAND RALPHKLEIIS
Table I: Fractional Product Yields for the Reaction between O(*P) and cis- and trans-C4Hs-2 in the 77-300°K Temperature Range
a
Reactant
T,OK
trans-Z,3-Epoxybutane
cis-2,3-Epoxybutane
Isobutyraldehyde
2-Butanone
ITU~S-CeHs-2 trans-CaHs-2 ~TU~S-CaHg-2 trans-CaHs-2
77 90 113 300"
0.51 0.50 0.50 0.33
0.03 0.06 0.09 0.15
0.04 0.07 0.07 0.21
0.42 0.37 0.34 0.31
cis-CaHg-2 cis-CaHs-2 cis-CaEs-2 cis-CaHg-2
77 90 113 300"
0.29 0.30 0.35 0.26
0.26 0.24 0.23 0.25
0.20 0.21 0.15 0.23
0.25 0.25 0.27 0.26
Gas phase. These results are from R. J. Cvetanovik, Advan. Photochem., 1, 115 (1963).
butene this would be
The 0 atom, it is assumed, has added t o one of the two carbons of the double bond. It was postulated that in unsymmetrical double bonds, the addition is favored on the least-substituted carbon. This is, of course, not relevant to the 2-butenes. The triplet intermediate then traiisforms to ground state compounds by ring closure to form the epoxide, or by migration of one of the groups on the 0 bearing carbon to the carbon with the free electron. On the basis of the data previously at hand, the precursor state (I) provided an adequate rationale for the products observed in the 0 atomolefin reaction. A t 3OO0K, the differences between the cis- and trans-2-butene with 0 atom reactions were not striking. It did not appear to require any significant modifications of the intermediate state from I in the &-%-butene reaction. There n7as only weak maintenance of configuration as deduced from product analysis. The intermediate formed might also be shown in the cis form H O H
\.
c-c
I/
as opposed to I with the trans representation. Ring closure and migration must be postulated as competitive with the transformation I % I1 involving rotation about the 2 , 3 C-C bond since there is a difference in the cis-2 ,3-epoxybutane/trans-2 ,3-epoxybutane ratios for the two 2-butene isomers. The low-temperature results, indicating emphatically that trans-2,3-epoxybutane and 2-butanone have one precursor and cis-2,3-epoxybutane and isobutyraldehyde another, require a reformulation of the precursor states. The 2-butene molecule has a planar skeletal The Journal of Physical Chemistry
structure with orbitals perpendicular to the plane. It has been suggested that in the initial act, the approach of the 0 atom is in a direction perpendicular to the plane. The olefinic bond is transformed to a single bond type. h triplet radical with a completed C-0 bond and unpaired electrons localized as shown in I or I1 results. This niodel presents several difficulties. Transformation to final products requires a spin-forbidden transition. Compared to rotation about the central C-C pair, changes such as the processes of ring closure or H or CH, migration, with their change of multiplicity, would be expected to be considerably slower. On this basis identical products would be obtained regardless of whether the 2-butene reactant was cis or trans. That this is not the case for the 0 atom addition to the condensed olefins in the cryogenic temperature region is readily seen from Table I. The gas-phase results at 300°K do not show the striking difference in the products of the cis compared with trans; nevertheless, there is still a distinction. In all cases it is found that rotation about the C-C axis to give a configurational change does occur. The process of ring closure and group migration particularly in the low-temperature ranges are at least as efficient as the ro tation. Assuming I t o be the precursor to the tra?zs-2,3epoxybutane and I1 to cis-2 ,3-epoxybutane, it could not be inferred from the appropriate three-dimensional models why I should be the precursor for 2-butanone and I1 should be the precursor for isobutyraldehyde, as the experimental evidence demands. It would appear that the transition state (or precursor) in the 0 atom
(0)
(b)
Figure 1. Transition states in the reaction of O ( V ) with: (a) cis-2-butene; (b) tmns-2-butene.
LOW-TEMPERATURE OXYGENATOMADDITIONTO OLEFINS addition to an olefin as represented by I or I1 need be abandoned. The formulation of a structure which satisfactorily accounts for the low as well as the higher temperature observations follows. (1) The initial approach of the 0 atom to the olefinic bond is considered to be in the plane of the molecule rather than perpendicular to it. A reorganization of the electronic structure commences, but there appears to be no a priori reason why the uncoupled electrons need be localized. (2) An interaction between the 0 atom and any neighboring H atoms bonded to an olefinic carbon occurs. This is to be interpreted as not hydrogen bonding in the usual sense, but a loosely bound structure involving 0 and H. A pictorial representation embodying these criteria is shown in Figure l a and b for cis- and trans-&butene reactants. Two additional "rules" have to be postulated. (I) Interconversion between (a) and (b) must occur to a limited extent; that is rotation in the complex about C s C is allowed. (11) in the case of a migrating group, if there is a choice between H and CH3, the H rather than CHI migrates. This requirement arises from the fact that in the lomtemperature reaction with trans-2-butene, the principal products arc trans-2 ,3-epoxybutane and 2-butanone. It is tempting to generalize with the specification that the ease of migration is inversely related to the mass of the migrating group, but this awaits the development of observations on other molecules. The passage from the precursor state to final products, apart from interconversion between precursor states, requires electronic and spatial rearrangements. The four carbons and the two secondary C-H group hydrogens are coplanar in 2-butene. This is not the case for either 2-butanone or isobutyraldehyde. In 2,3epoxybutane, the COC ring is in a plane perpendicular to the original 2-butene skeletal moiety. The concept of two precursor states, one for the epoxide and the other for the aldehyde or ketone, is interesting since
+
599
correlation could be made with the geometry of the approach of the 0 atom. That perpendicular t o the %butene plane could be associated with epoxide formation, and parallel with aldehyde or ketone formation. Because the cis-2-butene results show that trans-:! 3epoxybutane is formed, together with its associated 2-butanone in a constant ratio for the temperature employed, regardless of starting isomer, the concept of one precursor for the epoxide and another for the carbonyl is not tenable. Consideration of the rotation about) the central C-C bond in the precursor state whereby (a) and (b) are interconverted (Figure 1) is of considerable interest insofar as reaction enthalpy release rate is concerned. Clearly equilibrium among the isomeric forms is not established, for the cis and trans epoxide are formed in comparable quantities from the cis-2-butene whereas the trans oxide is formed preponderantly from trans-2butene. Increase of initial temperature from 77 to 300°K changes the ratio of the cis to the trans isomers, as derived from either of the 2-butenes. The enthalpy release in the process 0 C4Hs 4C4H80 is about 4 X lo5J. Any effects of ambient temperature should be quite negligible if the isomer ratio were dictated by this heat. The conclusion is that the rotation about the C-C bond leading to isomerization occurs either before any appreciable enthalpy is released, or the heat transfer from the molecule to its matrix is rapid compared to the rotational process. The latter is considered unlikely. On the other hand, formation of the aldehyde or ketone requires breaking of a C-C or a C-H bond, an energetically costly process. The inference is strong that these processes occur during the period of the change from precursor to final products, while rotation is associated solely with the precursor state. The reaction between an oxygen atom and 2-butene is represented by eq 1 and 2. The mechanism below includes the provision of interconversion of intermediates. Starting with trans-Bbutene and the as-
+
4
L
c&
/CH3
C' -C
'K \o/ H' Volume 78,Number 8 March 1969
MILTOND. SCHEERAND RALPHKLEIN
600 sumption of a steady state for the intermediates, there is obtained trans epoxide cis epoxide
-_ k2b
)
(
+
k-lk-
(
)
=
!ql+*) IC-I
&&butene
I
I
I
1
I
\
(3)
t0.8
The cis-Zbutene as reactant yields cis epoxide trans epoxide
I
+ 1.0
“23)
h
tran8-2-bubne
I
. \
\
t0.6
(4) +0.4
-
k2b
The ratio of rates of isomerization of the precursor to ring closure, that is the formation of the epoxide, is readily calculated from (3) and (4). If the left side of (3) is designated as A and of (4) as B , then k l / h b , the ratio of the rate of isomerization of the trans intermediate t o the rate of formation of the trans-2 ,3-epoxybutane, is
+ +1
log kl kzb
t0.2
0.0
-0.2
-0.4
B(1+ D) C AB - 1
-0.6
where C and D are k2a/k2b and k3a/k3bl respectively. A , B , C, and D are experimentally determined quantities. The corresponding expression for k _ l / k a b is
+ +1
A ( 1 + C) D AB - 1
-0.8 103 T
Figure 3. Arrhenius plot for relative rates of rotation to epoxide formation from the cis complex.
Figures 2 and 3 give the Arrhenius plots for these rate constant ratios. The results are represented by IC-1
-= k3b
(10 f 1) exp[
]
-200 f 10
RT
(5)
1.1
1.0
\ 0.9
Uncertainties are standard errors of the reported values based on a least-squares treatment of the data. Both ratios approach 10 at high temperatures. An estimate can be made of the equilibrium trans-2,3-epoxybutane e cis-2 ,3-epoxybutane. This is klk3b/k2bk-l if it is assumed that k2b and k3b are approximately equal. Thus
\
\
\
0.8 log k3b
0.7
0.6
0.5
0.4 0
THIS WORK
0
CVETANOVIC
0.3 0.2
2
4
6
io3
8
IO
T
Figure 2. Arrhenius plot for relative rates of rotation to epoxide formation from the trans complex. The Journal of Physical Chemistry
12
14
with an indicated zero entropy change and an endothermic enthalpy of 1549 J/moP for the process transto cis-epoxide. The trans-2,3-epoxybutane is slightly more stable than its cis analog. The transition state model for the 0 atom reaction with olefins deduced here from a number of observations on the cis and trans-2-butene systems can, at least in one instance, be tested. For the reaction between 0 and 2-methyl-Z-butene, the new precursor state repre(13) Corresponds to 370 cal/mol.
INFLUENCE OF MICELLES ON TITRATIONS OF SOAPSOLUTIONS
60 1
sentation would be
hyde, and 3-methyl-2-butanone. The ratio of dimethylpropionaldehyde to 3-methyl-2-butanone was found to be approximately 4 to 1. The results decisively verify the prediction based on the transition-state model proposed here.
It would be predicted, therefore, that the principal
Summary
carbonyl product would be dimethylpropionaldehyde. The localized triplet state model
The reaction of ground-state oxygen atoms with cisor trans-2-butene at low temperatures yields cis- and trans-2 ,3-epoxybutane, 2-butanone1 and isobutyraldehyde. These occur in different proportions depending on whether the cis- or the trans-2-butene is the reactant. The ratio of trans-2,3-epoxybutane to 2-butanone is constant as is that of cis-2 ,3-epoxybutane to isobutyraldehyde. A new model of the precursor or transition state for the O(3P)-olefin reaction is proposed. The model correctly predicted that dimethylpropionaldehyde would be the main carbonyl product in the 0 atom addition to 2-methyl-2-butene.
CHs
\.
0
I
/'-'iEHa
CHa
(111)
would lead to the prediction that 3-methyl-Bbutanone would be the major product, The reaction between 0 ("p) and 2-methyl-2-butene was effected at 90°K with propane as a diluent in a ratio of 10 to 1. The products were 2,3-epoxy-2-methylbutane,dimethylpropionalde-
The Influence of Micelles on Titrations of Aqueous Sodium and Potassium Soap Solutions18 by Myron E. Feinsteinlb and Henri L. Rosano Department of Chemistry, The City College of the City University of il.'ew York, New York, N e w York (Received August 8 , 2 9 6 8 )
10031
Sodium and potassium caprate, laurate, and myristate solutions below and above the critical micelle concentration were titrated with HC1. The cationic activity (Hf, Sa+, K+) was monitored during the titrations using a pH glass electrode and a cationic glass electrode. The curves of pH us. HCl added can be theoretically calculated for these systems from equilibrium considerations. The simultaneous monitoring of Hf and Kf activity during the course of a titration of a micellar laurate solution leads to the conclusion that Hf ions compete with Kf ions at the negatively charged micelle-solution interface. The effect of charged interfaces on the ionic distribution is discussed in terms of the apparent surface concentrations of counterions.
Introduction In a previous publication12 it has been shown that when a micellar potassium laurate solution is titrated with HC1 the titration curve looks like a titration curve of a diacid. The interpretation of this result was based on the marked influence of molecular aggregate surfaces (i.e., soap micelles) on the bulk pH. In another p~blication,~ using cationic glass electrodes, it was established that the potassium ion activity is lowered in micellar soap solutions. The interpretation advanced was that the charged micelle-solution interface depletes the bulk concentration of potassium ion. In the present study, it was decided to expand the previously advanced interpretation by simultaneously monitoring the activity of the hydrogen and potassium ions while titrating potassium soap solutions. Lucassen has investigated the hydrolysis and precipi-
tates in potassium carboxylate solution^.^ We have used a similar theoretical approach to interpret our titration curves. Although this equilibrium treatment quantitatively describes the systems studied it does not provide an adequate qualitative physical description.
Experimental Section The soap solutions were prepared by neutralizing a weighed amount of the melted fatty acid with a known (1) (a) Submitted in partial fulflllment of the thesis requirement for the Ph.D. of M.E. F.; presented a t the Symposium on Colloidal Electrolytes, 154th S'ational Meeting of the American Chemical Society, Chicago, Ill., Sept 1967; (b) Unilever Research Laboratory, Port Sunlight, Chesire, England. ( 2 ) (a) H. L. Rosano, K. Breindel, J. H. Schulman. and A. J. E y d t , J. ColZoid Interface Sci., 2 2 , 58 (1966); (b) H. L. Rosano and M. E. Feinstein, Rev. Franc. Corps Gras, 13, 661 (1966). (3) M. E . Feinstein and H. L. Rosano, J. Colloid Interface Sci., 24, 7 3 (1967). (4)J. Lucassen, J. Phys. Chem., 7 0 , IS24 (1966).
Volume 75, Number S March 1989
