with polarographic oxidation half-wave potentials. Other more elegant measurements of the ease of electron abstraction, such as ionization potential which for phenols is known to correlate with u+ values (34,are also available. For any specific substrate and condition of oxidative exposure there probably exists an optimum value range for this reactivity parameter. This optimum may possibly be located by a direct fundamental measurement of substrate reactivity; but it will always be empirically accessible by a single set of induction period measurements using a selected group of antioxidants of appropriately spaced reactivities. Given the apparently general correlation of phenolic antioxidant efficacy with the sum of electrophilic substituent constants, it is possible to predict antioxidant efficacy with fair confidence from chemical structure. This in turn suggests that for many systems massive inhibitor screening may profitably be replaced by the purposive tailoring of antioxidants for optimum performance under the desired conditions. Nomenclature
AH A. El/z
= = = k3,k5,etc. =
M
= =
n
RH R. ROO
= =
-
= =
S.C.E.
=
ti P
= =
U U+
~ 3 w6, , etc.
= =
phenolic antioxidant antioxidant-derived radical polarographic oxidation half-wave potential rate constants for indicated reactions gram moles per liter initiation efficiency factor hydrocarbon hydrocarbon radical organoperoxy radical saturated calomel electrode (reference potential) induction period Hammett reaction constant Hammett substituent constant electrophilic substituent constant rates of indicated reactions
Ac knowledgment
Polarographic potentials were measured by R. Bidwell and total oxygen by neutron activation analysis was determined by 0. U. Anders. We are indebted to H. G. Scholten and T. Alfrey, Jr., for valuable discussions.
Literature Cited
(1) Am. SOC. Testing Materials, “Standards,” ASTM Test D 525-55,Part 7, pp. 263ff, 1958. (2) Bickel, A. F., Kooyman, E. C., J . Chem. SOC.1953, p. 3211. (3) Bolland, J. L., ten Have, P., Discussions Faraday Sac. 2, 252 (1947). (4) Bolland, J. L.,ten Have, P., Trans. Faraday Sac. 43,201 (1947). (5) Boozer, C.E., Hammond, G. S., Hamilton, C. E., Sen, J. N., J . Am. Chem. Sac. 77, 3233 (1955). (6) Campbell, T.W., Coppinger, G. M., Zbid., 74, 1469 (1952). (7) Charton, M., J . Org. Chem. 28, 3121 (1963). (8) Conant, J. B., Pratt, M. F., Chem. Revs. 3, l(1926). (9) Davies, D.S.,Goldsmith, H. L., Gupta, A. K., Lester, G. R., J . Chem. Sac. 1956, p. 4926. (10) Fordham, J. W. L., Williams, H. L., Can. J . Res. 27B, 943 ( 1 949). (11) Fortuin, J. P., Waterman, H. I., Chem. Eng. Sci. 3, spec. suuul.. 60 (1954). (12)’Pudno,T.,Rde, T., Eyring,H., J . Phys. Chem. 63,1940(1959). (13) Gray, P., Pearson, M. J., J . Chem. SOG. 1964,p. 5725. (14) Hammond, G. S., Boozer, C. E., Hamilton, C. E.,. Sen,. J. N., J . Am. Chem. Sac. 77,-3238(1955). . (15)Hedenburg, J. F., Znd. Eng. Chem. Fundamentals 2, 265 (1963). (16) Hedenburg, J. F., Freiser, H., Anal. Chem. 25, 1355 (1953). (17) Helden, R.,van, Bickel, A. F., Kooyman, E. C., Rec. Trau. Chim. 80, 1257 (1961). (18) Hock, H., Siebert, M., Chem. Ber. 87, 546 (1954). (19) Hogg, J. S.,Lohmann, D. H., Russell, K. E., Can. J . Chem. 39, 1588 (1961). (20) Howard, J. A., Ingold, K. U., Zbid., 41, 1744 (1963). (21) Zbid., p. 2800. (22) Zbid., 43, 2724 (1965). (23) Ingold, K.U.,J. Phys. Chem. 64, 1636 (1960). (24) Ingold, K. U.,Zbid. 41, 2816(1963). (25) Lloyd, W. G., J . Polymer Sci. Al, 2551 (1963). (26) Lloyd, W. G.,Zimmerman, R. G., IND.END.CHEM.PROD. RES.DEVELOP. 4, 180 (1965). (27) Mayo, F. R., Miller, A. A., J . Am. Chem. Sac. 80, 2480 (1958). (28) Meier, K., Mebes, K., Farbeu. Lack 58,215 (1952). (29) Melville, H. W.,Richards, S., J . Chem. SOC.1954, p. 944. (30) Sethi, S. C., Aggarwal, J. S., Subba Rao, B. C., Indian J. Chem. 1,435(1963). (31) Stillson, G. H., Sawyer, D. W., cited in ( 75). (32) Stock, L. M., Brown, H. C., “Advances in Physical Organic Chemistry,” V. Gold, ed., pp. 35ff, Academic Press, New York, 1963. (33) Suatoni, J. C., Snyder, R. E., Clark, R. O., Anal. Chm. 33, 1894 (1961). (34) Tait, J. M. S., Shannon, T. W., Harrison, A. G., J. Am. Chem. Sod. 84, 4 (1962). RECEIVEDfor review April 21, 1966 ACCEPTED October 12, 1966
TELOMERIZATION OF ETHYLENE WITH TRIMETHYL BORATE W . T. H O U S E , S. D . S U M E R F O R D , l A . H . N E A L , A N D W . J . P O R T E R ’ Esso Research Laboratories, Baton Rouge, La.
Hanford and Joyce (5) first coined the term “telomin 1942, a considerable amount of research has been done in this field. An extensive review of the literature was made in 1958 by Fox and Field ( 4 ) . We report here the results of our studies on the telomerization of ethylene with trimethyl borate using di-tert-butyl peroxide as initiator. INCE
S erization”
Experimental
The reactor used was a 1-gallon stirred autoclave of Monel construction. A nominal charge of 2 liters of trimethyl 1 2
330
Present address, 2017 Hollydale Ave., Baton Rouge, La. Present address, Esso Chemical Co., Inc., New York, N. Y l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
borate (TMB) was used. The T M B was heated to reaction temperature, and its partial pressure was noted. Ethylene was pressured into the reactor to the desired ethylene partial pressure. A solution of di-tert-butyl peroxide in T M B was then pumped continuously into the reactor at a predetermined rate throughout a given run. At the end of a run, the total liquid product was withdrawn, and most of the unreacted T M B was removed by distillation. Methanol was then added to convert the higher borate esters to higher alcohols and trimethyl borate. The distillation was then continued to yield a bottoms product essentially free of methanol and TMB. The average molecular weight of the product was determined by freezing point depression, and the C, H, and 0 content was determined by direct analysis. The product was further characterized by infrared, gas chromatography, and chemical analysis for functional groups.
A systematic study of the effects of reaction variables on the telomerization of ethylene with trimethyl borate was carried out using di-ferf-butyl peroxide as an initiator. Long-chain alcohols were recovered from the resulting long-chain alkyl borates b y methanolysis. The yield of product per gram of initiator increased with increasing ethylene partial pressure, increasing temperature, or decreasing amount of initiator added. The molecular weight of the product increased with increasing ethylene partial pressure and decreasing tern perature. Hydrocarbons and ethers were also found in the product. A considerable amount of the alcohols recovered from the methanolysis was found to be secondary and the percentage of secondary alcohol varied inversely with the molecular weight of the product.
-
The desired long-chain alcohols are recovered by methanolysis of the product (Reaction 6). CHaO
\
+ CHIOH
B-O(CHz-CHz),-CH3 CH30
/
-L.
CHaO
\ B-OCH3
/
+ CH~-(CH~-CHZ),-OH
(6)
CH3O The above reactions represent the desired sequence and are an oversimplification of the actual case. The complicating side reactions are discussed below along with their effects on the product. In the following discussion, it is assumed that Reaction 3 represents the sum of all radical additions to ethylene and Reaction 4 represents all chain transfer reactions.
Initiator Efficiency The initiator efficiency is defined as the number of grams of product obtained per gram of initiator added. Examination of Reactions 1 to 6 indicates that the initiator efficiency should be determined by the ratio of the rate of Reaction 3 to the rate of Reaction 5. Initiator efficiency a
rate of Reaction 3 rate of Reaction 5
Although Reaction 4, chain transfer, stops one growing chain, it starts another and this reaction does not affect the initiator efficiency in grams per gram. As indicated in Figure 1, increasing the ethylene partial pressure increases the initiator efficiency. The effect here is 1 L3
3
/ CHiO CH30
\
/
i
1
1
1
181
16
CH3O CHiO
\
1
15
s
t
Z 14
100 150 200 250 300 350 400 C2H4 PARTIAL PRESSURE, PSI
Figure 1. Relation of CzH4 partial pressure to carbon number and efficiency 392' F., 4 g. di-terf-butyl peroxide per liter of trimethyl borate, 150 minutes VOL. 5
NO. 4
DECEMBER 1 9 6 6
331
to increase the rate of Reaction 3, which is dependent on ethylene concentration, without affecting the rate of Reaction 5. Increasing the temperature also increases the initiator efficiency, as iilustrated in Figure 2 . Reaction 5, a t least for combination of radicals, probably has little or no activation energy and its rate would not be greatly affected by temperature. Reaction 3, however, does have a small activation energy and its rate would be increased by increasing the temperature; thus, the ratio of the rate of Reaction 3 to that of Reaction 5 would be increased. Figure 3 shows that decreasing the amount of initiator increases the efficiency. A decrease in the amount of initiator effectively decreases the concentration of free radicals, decreasing the probability of a given radical’s reacting with another radical in termination without affecting the probability of its reacting with an ethylene molecule; thus, the rate of Reaction 5 would be decreased and the above ratio would be increased.
I
1
1
I 360
I 380
400
3 lOL
71
340
I
i
TEMPERATURE,
Figure 2. efficiency
Relation of
420
440
OF.
temperature
to
225 p.s.i. CzH4 partial pressure, 10 g. di-tert-butyl peroxide per liter of trimethyl borate, 60 minutes
Molecular Weight
The molecular weight of the product should be determined by the ratio of the rate of Reaction 3 to the rate of Reaction 4. Molecular weight a
rate of Reaction 3 rate of Reaction 4
‘
Increasing ethylene partial pressure would be expected to increase the rate of Reaction 3 without affecting Reaction 4, thereby increasing the above ratio and the molecular weight. Examination of Figure 1 shows that this is the case. Figure 4 shows that increasing the temperature decreases the molecular weight of the product. While the rates of both Reactions 3 and 4 would be expected to increase with increasing temperature, apparently Reaction 4 increases faster and the ratio is decreased. The larger effect of temperature on Reaction 4 is consistent with the higher energy of activation expected for this reaction (9).
Figure 3. efficiency
1
-c I9O
I n addition to reacting according to Reaction 2, the tertbutoxy radical can add ethylene (Reaction 7 ) or it can decompose (Reaction 8) CH 3
Relation of
initiator concentration to
3 9 5 ’ F., 2 3 0 p.s.i. CzH4 partial pressure, 2140 ml. of trimethyl borate, 150 minutes
3 U
Hydrocarbon and Ether Formation
-
8-
E9
I
I
I
1
1
I
I
I
t\
I
1
7
w 0
I 150
I
I
I
I
1 TEMP.,
Figure 4.
1
I
I
1
“F.
Relation of temperature to molecular weight
2 2 5 p.s.i. C2H4 partial pressure, 10 g. di-terf-butyl peroxide per liter of trimethyl borate, 60 minutes
CH~-C-O-(CHZ-CHZ),. I
(7)
CH3 into acetone and a methyl radical. 0
CH3
CHa-(2-0.
I
The methyl radical
I/
-+
CHa-C-CH3
+ CH3.
(8)
CH3 can abstract hydrogen as shown for the tert-butoxy radical in Reaction 2 or it can also add ethylene. T h e new growing chains can still abstract hydrogen from the trimethyl borate and thus form ethers and hydrocarbons. 332
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
Several of the telomer products were separated into three fractions by percolation through silica gel. The composition of the fractions obtained from a typical separation is shown in Table I. I n this separation 60 grams of telomer product is slurried with 25 grams of anhydrous silica gel and placed on a 4-foot X 13/4-inch chromatographic column containing anhydrous silica gel. The sample is first eluted with 3000 ml. of n-pentane, followed by 2500 ml. of benzene and then 2700 ml. of methanol. The products are recovered by vacuum stripping the fractions. T h e first fraction, which ranged from 10 to 30 weight yo of the product, consisted of hydrocarbons. The second fraction, which ranged from 10 to 20 weight yo of the product, consisted
CH30
\
300°F.
+-327". #
B-0-CH2-R
347'F.
0 392'F.
CHaO
/
+ Re
+
392-480".
1. 1 .
0
m 9
10
11
13
12
14
15
16
17
18
19
AVERAGE CARBON NUMBER
Figure 5. Relation of carbon number and temperature to production of 3-heptanol 150 to 600 p.s.i. C2H4 partial pressure, 1 .O to 4.2 g. di-ferf-butyl peroxide per liter of trimethyl borate, 150 minutes
Table I. Typical Silica Gel Separation
Product from run at 392" F., 403 p.s.i.g. ethylene, 2 liters of trimethyl borate, 3.59 grams of di-tert-butyl hydroperoxide added continuously over 150-minute period Fraction wt. % of Type of Product Eluted by Product n-Pentane 28 Hydrocarbon Benzene 'L 5 Ether 48 Alcohol Methanol 91yo recovery
.-
of ethers. The final fraction consisted of the desired alcohols. Average molecular weights were determined for each fraction by cryoscopic methods. The fractions were identified as to type by infrared spectra and by carbon, hydrogen, and oxygen determinations. These indicated that the second fraction contained one oxygen atom per molecule and infrared spectra, along with lack of chemical reactivity, indicated the material to be ethers. The alcohols were characterized by hydroxyl number and infrared spectra. In the distillation of the alcohol fraction, the molecular weights of the distillation fractions, estimated from boiling points, agreed very well with those obtained cryoscopically. All these products contained up to 35% unsaturation, indicating that a good portion of the termination reactions occurred by disproportionation. Secondary Alcohol Formation
Although the telomerization of ethylene with trimethyl borate has not been previously reported, several workers (7, 2, 3, 6) have reported the telomerization of ethylene with methanol. None of these workers reported the presence of secondary alcohols in the products; however, the products were not extensively characterized. Urry and coworkers (8) reported the telomerization of several olefins with various alcohols, and predicted that the initial reaction products from primary alcohols would react further a t high conversions to give secondary alcohols. We consistently found a relatively large amount of secondary alcohol in our products. T h e mass spectra of the silyl ethers of the alcohol product obtained by the technique described by Sharkey, Friedel, and Langer (7) indicated that the preponderance of these secondary alcohols were 3-alkanols. 3Heptanol was actually isolated and identified. The presence of small amounts of 5-alkanols was also indicated. T h e possible routes to Secondary alcohol are shown in Reactions 9 to 11:
In each case the new radical could add ethylene and lead to a secondary alcohol after methanolysis. Reaction 9 is not very likely, since the concentration of long chains is very small compared to the telogen and added reactivity of the substituted carbon is not sufficient to explain the large amounts of secondary alcohol formed. Calculations indicate that the reactivity of the hydrogen a t the substituted carbon atom would have to be about 45 times that of the unsubstituted even for conditions which led to relatively low (17%) secondary alcohol. In addition, this reaction would predict a random distribution of the position of the hydroxyl groups rather than a preponderance of 3-alkanols. Reaction 10 corresponds to an internal hydrogen abstraction, which apparently does occur to some extent to produce the 5-alkanols. If this were the main reaction leading to secondary alcohols, however, a preponderance of 5-alkanols rather than 3-alkanols would be expected. Reaction 11 is about the only possibility for explaining the preponderance of 3-alkanols and the amounts of secondary alcohols formed. This reaction would result in a distribution of carbon numbers for the second side chain similar to the distribution for the total product and the mass spectra of the silyl ethers indicate this to be true. In addition to reacting as depicted by Reaction 11, the radical could add another molecule of ethylene, the same as Reaction 3, and this would essentially stop the possibility of secondary alcohol formation. Thus, the per cent secondary alcohol in the total alcohol would be proportional to the ratio of rate of Reaction l l / r a t e of Reaction 3. Since Reaction 11 is somewhat analogous to Reaction 4, it might be expected to respond to the variables in the same way as Reaction 4. Thus, VOL. 5
NO. 4
DECEMBER 1 9 6 6
333
the per cent secondary alcohol could also be proportional to the ratio of rate of Reaction 4/rate of Reaction 3, the inverse of the ratio previously postulated as affecting molecular weight and it would vary inversely with the molecular weight. Such a correlation has been found and, in general, conditions which tend to give a low molecular weight tend to give high secondary alcohol content. Branched primary alcohols could also arise from the same type of reactions as 10 and 11 but would occur further out in the chain. There is also considerable evidence (gas chromatography plus infrared) for the presence of primary branched alcohol in these products.
Acknowledgment
The authors acknowledge the help of H. V. Drushel who interpreted the infrared spectra and of D. R. McAdams who interpreted the mass spectra.
Literature Cited (1) Banes, F. W., Fitz erald, W.P. Nelson, J . F. (to Standard Oil Development C o j , U. S. Patent 2,655,525 (Oct. 13, 1953). ( 2 ) Banes, F. W., Fitzgerald, W. P., Nelson, J. F., Gilliland, E. R., (to Esso Research and Engineering Co.), Ibid., 2,668,181 (Feb. 2, 1954). (3) Erchak, M., Jr. (to Allied Chemical & Dye Co.), Ibid., 2,713,071 (July 12, 1955); 2,717,910 (Sept. 13, 1955). (4) Fox, R. B., Field, D. E., U. S. Naval Research Lab., Rept. 5190 (Nov. 19, 1958). ( 5 ) Hanford, LV. E., Joyce, R. M., Jr. (to E. I. du Pont de Nemours & Co.),U. S. Patent 2,440,800(May 4, 1948). (6) Hanford, \V. E., Roland, J. R.(to E. I. du Pont de Nemours & Co.), Ibid., 2,402,137(June 18, 1946). (7) Sharkey, A. G., Jr., Friedel, R. A., Langer, S. H., Anal. Chem. 29.771 (19571. (8) Urry, h'.H:, Stacey, F. W., Huyser, E. S., Juveland, 0. O., J . Am. Chem. Soc. 76, 450 (1954). (9) Walling, C., "Free Radicals in Solution," p. 152, Wiley, New
York, 1957.
RECEIVED for review February 1, 1966 ACCEPTED August 11, 1966
VAPOR PHASE ISOMERIZATION OF RB'0 R NE STELVIO PAPETTI, CLAYTON OBENLAND, AND THEODORE L. H E Y I N G
Olin Research Center, New Haven, Conn. An improved process for the conversion of o-carborane to its meto and para isomers involves heating carborane vapors, admixed with an inert carrier gas, in a heated tube a t atmospheric pressure. This accomplishes the isomerization rapidly in a continuous fashion, eliminating the need for the heavy pressure equipment required in previous methods, Optimum conversion of 0 - to m-carborane occurred a t a tube temperature of 600" C., with a residence time of 0.30 to 0.48 minute and mass flows of 0.73 to 2.98 grams per Mixtures of meta and para isomers were minute; the meta isomer was recovered in yields up to 98%. formed a t 700" C., the yield of p a r a isomer being about 22% under optimum conditions. HE field of boron chemistry, especially the reactions of Torganoboron derivatives, has undergone a rapid expansion during the past decade. I n the past few years, a new family of unique organoboron compounds has been discovered and investigated. These are evolved from the pyrolysis of diborane to decaborane(l4) (ZO), followed by subsequent reaction of the decaborane with alkynes such as acetylene itself in basic solvents. The organoboron compounds from this type of reaction are known as carboranes and their preparation and chemical properties have been discussed in a number of papers (7,3-7, 73-75,27,22, 24). Recent work has shown that boron-containing polymers, based upon monomeric carborane derivatives, possess outstanding advantages in the formulation of heat-resistant materials (2, 70-72, 76, 78, 79). This has led to further development and evaluation of these materials, with a corresponding requirement for increased amounts of the parent carboranes, especially m-carborane. The first member of the B10C2H12 family of carboranes to be discovered and prepared in quantity was the ortho isomer, 1,2-dicarbadodecaborane(12) (4, 74). Somewhat later, the meta isomer, 1,7-dicarbadodecaborane(I2), was prepared by thermal rearrangement of the ortho isomer (8, 9, 25) in a sealed system. Extensions of this technique resulted in the formation of the para isomer, 1,12-dicarbadodecaborane(12) (77). The structures assigned to the three isomers are shown in Figure 1 (77, 23, 26).
334
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
ORTHO Figure 1 .
META
PARA
Icosahedral carborane configurations
Boron atoms shown as white circles, with two carbon atoms in black; hydrogen atoms omitted far clarity
The conversion of the ortho to the meta isomer has become of more importance as uses for the meta isomer have increased. The original method for this conversion involved the batch rearrangement of o-carborane, in an autoclave at a temperature of 470" to 480" C., with conversion tjmes ranging from 2 to 48 hours. Batch sizes were necessarily small (approximately 110 to 125 grams) and an extended cooling time was required before the container was safe to open. Yields were high (>90y0)and conversion was complete, but the method was slow and cumbersome. Pressures on the order of 200 to 300 p.s.i.g. were usually developed in the chamber during the heating. At 600' to 615" C., a mixture of meta and para isomers resulted. These processes, therefore, were hardly
