CHEMISTRY OF ORGANIC FREE RADICALS IN THE VAPOR PHASE' II. Reactions of Alkoxy and Alkrlperoxy Radicals; Energetics of some Alkyl Radical Reactions HAROLD E. DE LA MARE and WILLIAM E. VAUGHAN Shell Development Company, Emeryville, California HYDROCARBON OXIDATION
A vast amount of work has been done in the field of oxidation, a field intimately associated with free radical chemistry; furthermore, many researchers are nowactively engaged in this field. The interaction of oxygen and hydrocarbons is, of course, of vital interest to petrochemical companies, and it is expected that the future will see more and more successful commercial applications of these oxidation reactions. It is again intended that the few leading references presented here will enable the interested student to probe the litewture more deeply. Basically the course of low-temperature2 hydrocarbon oxidations is the same in the liquid- and vaporphase reactions. There is ample evidence that hydroperoxides are formed as intermediates and that alkylperoxy and alkoxy radicals participate in the reaction mechanism (67). The over-all mechanism may be pictured simply as a free-radical chain reaction (equations (135) to (140)). The autocatalytic nature of the Initiation
R. +O*
fast
RO?. + R H - R 0 2 H + R . ( o r R 0 .
R.
+ RO*.
-
-
R . (or RO?H
RH
RO.
+ .OH)
ROY.
+RH
(135) (136)
A
ROH
+R.)
termination reactions leading to nonradieal products 4
products become those derived from R-0. and other secondary radical reactions. The utilization of experimental techniques employing a series of model aliphatic peroxides (68) enabled the formulation of a general mechanistic picture of the lolv temperature oxidation of hydrocarbons, supporting the mechanism previously outlined (equations (135) to (140)) and in particular elucidating the conversion of peroxy radicals to alkoxy radicals and hence to primary oxidation products. Figures 1 and 2 are
(137)
" Important
(138) (139) (140)
reaction is due to the production of the intermediate hydroperoxide which breaks down to initiate and propagate new chains as shown in (135) and (137). Under sufficiently high oxygen concentrations, termination proceeds principally by path (138) since reaction (136) is an exceedingly fast reaction and ROr radicals are the only species which build up any sensible concentration. By choice of low temperatures and conversions, it has been possible in many cases to isolate hydroperoxides in good yields. However, in many cases and particularly in the vapor phase reactions a t higher temperatures, extensive breakdown of the hydroperoxide occurs and the primary reaction For the first part of this paper, see J. CHEM.EDUC.34, 10 (1957). The numbers of equations and literature cited folloa eansecutively those assigned to the first paper. This is a rather arbitrary range which can be roughly defined as 25" to 300'C.
only st l o r oxygen
Figure 1.
eonoentration. Hydros.rbon Oxidation
L o w T.mper.tu.s
/.. 1..
R'CH20.(primary alkoxy radical)
b
R'.
+ CH?O
R'CH?OH R'CH?OR f R. or R'CHO RH
"\.. R'CHO
+ ROH
+
R'&HO.(seeondary
I
1
R'CHO
+ R'.
alkoxy
radical)
\
\
JRH 1"+ 1"".+ +
R'&HOH
R . R'&O
RH RICO
ROH
R'3C0.(tert,iary slkoxy radical)
A/
+ R'.
Rf9CO
Pigum 2.
\RE R'GOH
+ R.
Alkoxy Radicd Re.cti.3".
JOURNAL O F CHEMICAL EDUCATlON
presented as shown in the original data since they summarize clearly the details of low-temperature oxidation. The equations shown in Figures 1 and 2 are individually supported by experimental evidence obtained from controlled experiments with model peroxides. Further information relating to alkoxy radical stability mill be presented subsequently (see Peroxides). L41though aralkyl and olefinic hydrocarbons have not been specifically mentioned, they in general are oxidized in accordance ~ ~ thet mechanism h just outlined. Radical-double bond interactions do, however, play an additional role, but since most of the experimental work has been done in the liquid phase, these papers are beyond the scope of this review. The high temperature oxidation of isooctane and neohexane (69) provide some exceedingly interesting ohservations consistent with intermediate formation of hydroperoxides and the participation of alkyl, alkoxy and alkylperoxy radicals. The isolation of cyclic ethers, e.g., 2,2,4,4-tetra-methyltetrahydrofuran, is consistent with the partial reaction path shown in equations (141) to (143).
I
I
CH,
H
'04.
5
AH
\"
O-OH
Equation (143-a) depicts the formation of the hydroperoxide by intramolecular abstraction of hydrogen from the geometrically favorable tertiary position, and (143-b) indicates ether formation by an intramolecular alkyl radical displacement on the peroxide linkage. In the latter process the hydroxyl radical is displaced and can serve to propagate another chain. At the time this high-temperature oxidation of isooctane was done, it constituted the first evidence for free radical rearrangement in the vapor phase.a The isolation of 2,s-dimethylhexane from the oxidation mixture (after hydrogenation) suggests that the primary octyl radical rearranges to the isomeric tertiary radical (see eauation (144)). Additional evidence has already been presented from vapor phase brominstion studies.
VOLUME
34,
NO. 2, FEBRUARY, 1957
CH, I
CH, I
CH, I
CHa I
In a rearrangement similar to that shown in equation (144), 2-methylbutenes are produced from rearrangement of the neopentyl radical. The latter radical (IV) arises from a @-cleavagereaction of one of two probable isooctyl radicals (see equations (145) and (146)).
It should be noted that most of the products in the high temperature oxidation of isooctane can be attributed to initial abstraction of the methyl hydrogen, i.e., formation of radicals I1 or 111. Ordinarily predominant attack mould have been predicated for the weaker tertiary and secondary hydrogens; however, these positions are shielded by the peripheral methyls in isooctane and, further, the high temperature conditions would be expected to reduce selectivity. For purposes of simplification, little has been said about the high temperature oxidation of hydrocarbons. Although the evidence clearly substantiates the presence of free radical chains, the nature of the intermediates involved is still somewhat in dispute. It has been proposed that the high temperature (70) (>40OoC.) oxidation of paraffins does npt proceed through intermediate peroxide formation. A series (71, 72, 73, 74, 76) of papers concerned ~ ~ i t h vapor phase homogeneous oxidation of hydrocarbons catalyzed by hydrogen bromide provide a significant finding in the development of the chemistry of hydrocarbon-oxygen interactions. It mas demonstrated that the oxidation of a large number of hydrocarbons could be effected efficiently a t relatively low operating temperatures in the presence of 2y0 to 4y0 hydrogen bromide. Thus, ethane was converted to acetic acid, other straight-chain hydrocarbons principally to k e tones, and branched-chain compounds to stable peroxides. Since the uxidation of a branched-chain hydrocarbon such as isobutane illustrates the principles basic to all of the oxidations, it will be discussed briefly. Tertiary hutyl hydroperoxide can be produced in yields as high as 75v0 (based on consumed oxygen) from a mixture of isobutane, oxygen and hydrogen bromide (10: 10: 1) under conditions (160°C.) where 87% of the oxygen is consumed. The principal reaction path is that shorn in equations (147) to (150).
HBr
+ O2
-
(HO?.?)
+ Br.
CH, C H d -I H
+Br
CHs CHs-(!-Os. I
(147)
CHI
+ HBr
+ HBr
CH-4
-
(148)
range 3 6 4 0 kcal. However, it is of interest to call attention to the paper of Lossing and Tickner (78) who combined the data of three different investigations which had utilized different techniques of measurement and operated in different temperature ranges. Table 1 shows the general agreement of this work; the composite value was obtained by the conventional plot of log k versus 1 / T from -430°K. to -625°K.
CHs
+ Br.
CHa-J-OzH I
TABLE 1 (150)
The facts are consistent with a free radical chain mechanism in which bromine atoms become the principal chain carrier (equation (148)) and hydrogen bromide the chief hydrogen donor for stabilization of peroxy radicals (equation (150)). I t should be emphasized that hydrogen bromide exerts a unique controlling effect on these hydrocarbon oxidations-no other material was found which exerted control of anywhere near comparable magnitude. The clean-cut character of the hydrogen bromide oxidations is attributed to (a) high specificity of attack by Br atoms (virtually exclusively a t the tertiary position in a branched chain hydrocarbon) and (b) the excellent hydrogen donating properties of HBr which facilitate the stabilization of the intermediate p e r o v radicals uia equation (150).
Thermal Decomposition of Di-tert-Butvl Peroxide
Vaughan el al. Sswarc et al. Lossing and Tickner Composite value
3 2 X lo'$ 4-i X 10"
39 1 36 3i 38
. ..
i X 10':
The decomposition of di-tert-amyl peroxide is principally a first order, homogeneous reaction (activation energy estimated as 3 7 4 1 kcal.) (76) but is complicated by the occurrence of higher order processes as well. The principal products of di-tert-amyl peroxide decomposition are acetone and butane (equations ( I N ) , 0551, (156)).
PEROXIDE DECOMPOSITIONS
Elucidation of the chemistry and energetics of alkyl hydroperoxide and dialkyl peroxide decompositions has received considerable attention in the last decade (76, 77, 78, 79, 80, 81, 8g, 86). The findings have done much to stimulate free radical research both in the liquid and vapor phase. Dialkyl Peroxides. The decompositions of di-tertbutyl and di-tert-amyl peroxide in the vapor phase have been studied kinetically. The vapor phase decomposition of di-tert-butyl peroxide (140'-160°C.) is a homogeneous, unimolecnlar non-chain process proceeding with an activation energy of 39.1 kcal. (76). The rate determining step is scission of the 0 - 4 bond, and the activation energy has been identified with the bond dissociation energy of the peroxide linkage. The chemistry of the decomposition is shown in equations (151) to (153) ( 7 8 The production of tert-butoxy and methyl radicals is well substantiated, and di-tert-butyl peroxide bas become a widely used radical source. CHa C&-A-0-0-A-CH,
CHa I
-
Although the cleavage of the tert-amyloxy radical is principally a t the weaker carbon-carbon bond (equation (155)) i t is not exclusively so and a small amount of methyl radical is produced. Thus, as expected, methyl ethyl ketone, propane (Me. Et. +) and ethane (2Me.-) are produced in IOU- yield by decomposition of di-tert-amyl peroxide in the vapor phase (76,81). Rebbert and Laidler4 have investigated the decomposition of diethyl peroxide in the vapor phase (200'250°C.) utilizing the toluene carrier method. The main products observed r e r e ethane and formaldehyde, together with some methane and bihenzyl. The main products were explained by the reaction paths depicted in equations (157) to (159); methane and bibenzgl arise as a result of methyl radical attack on toluene at
+
CH,
1
2CH-C-0.
(151)
I
the relatively low temperatures involved. First order rate constants, based on an assumed stoichiometry. were given by the expression, k = 2 X loLae-31.70"RT. The activation energy of 31.7 kcal./mole is identified again with the 0 4 bond dissociation energy, a value All of the literature values for ESct in the thermal decomposition of di-tert-butyl peroxide fall in the
' Ref. (5s)in Part I : REBBERT,R. E., AND K. J. LAIDLER,J. Chem. Phys., 20, 574 (1952). JOURNAL OF CHEMICAL EDUCATION
substantiallv lower than that found with di-tert-butvl peroxide. -4lkory Radical Stability. The relative stabilities of a series of alkoxy radicals (85) have been determined by decomposing a series of dialkyl peroxides in cyclohexene as a hvdrogen " - donor (tem~eratureof 195°C.). Basically, the competitive paths which an alkoxy ra&cal can take under the conditions imposed are (a) hydrogen abstraction, (b) carbon-carbon bond rupture with concomitant carbonyl formation, and (c) carbon-hydrogen bond rupture with aldehyde formation. The relative stability of an alkoxy radical was defined by the authors as the ratio of the number of moles of alcohol produced (process a) t o the sum of the alcohol plus the moles of product produced by carbon-carbon bond rupture (process b); paths (a) and (b) constitute the principal modes of decomposition. A good example of this research on alkoxy radical stability is the decomposition of isopropyl tert-butyl peroxide. The fate of the isopropoxy and tertbutoxy radicals is shown in equations (160) to (164) and (165) to (168), respectively.
accord with the bond dissociation energies of the carbon-carbon bonds which are ruptured (process b). The data clearly show that the tert-butoxy radical is an excellent methyl radical source a t 195' due to the facility with which the carbon-carbon bond rupture occurs, e.g., in the cyclohexene environment only 9 moles of tert-butyl alcohol were produced compared to 95 moles of acetone. Diacyl Peroxides. The kinetics of decomposition of several diacyl peroxides have been studied in the gas ~ h a s ebv A. Rembaum and M. Szwarc (84, . . . 86) usingflow technique with toluene (or benzene) as the carrier gas a t temperatures in the range 100'-250°C. The peroxides studied were found to decompose by first-order kinetics, and it was concluded that the scission of the 0-0 bond was the ratedetermining step in the decompositions. The expressions for the rate constants of diacetyl, dipropionyl and dibutyryl peroxides are listed in Table 2 for comparison. Within experimental error it was found that the activation energy for each of the decompositions was ca. 30 kcal./mole. The activation energy is identified with the bond dissociation energy of the 0 4 bond, and the conclusion, therefore, is that structural changes in the alkyl radical have essentially no effect on the bond dissociation energy of the acyl-type peroxide linkage. The basic chemistry of the diacyl peroxide decompositions may be represented by the mechanism shown in $quat,ions(170) to (172).
a
0
- L rate deternuning
( 8 - )
2R-8-0,
R-4-0. R.
The observed stabilities of the alkosy radicals xere found t o follow the order (decreasing stability)
This order of stability mas shown to he qualitatively in
0
--
CO,
+ R.
+ R.
R-R
(170) (171) (172Y
In accordance with this scheme, ethane, butane, and hexane are the principal hydrocarbon products produced respectively from diacetyl, dipropionyl, and dibutyryl peroxides. Furthermore, the observed ratio of carbon dioxide to decomposed peroxide is roughly two in all cases. TertButyl ~ydroperoxide. The vapor phase decomposition of tertbutyl hydroperoxide has been studied (86) a t 195', where it was shown to be relatively stable (
