May, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
purposes; and (3) an emetic to produce vomiting if enough of the methanol is taken to cause poisoning. It is believed that ample precautions will have been taken if these four means of warning are used-a poison label on the container, a color, an unpleasant taste, and an emetic, together with an educational program teaching that methanol never has been and cannot be used for beverage purposes without serious consequences. Summary
The information which the United States Bureau of Mines has obtained t o date indicates that there is no hazard to health from the reasonable use of methanol for antifreeze purposes. Many of the conditions of exposure to vapor and by contact with the skin, which were observed in the manufacture of methanol and for which no health hazard was found, are very comparable to the degree of exposure observed in usage as an antifreeze. Also, the exposure in both cases is below that which laboratory experiments with animals have indicated to be harmful either from inhalation of vapor or absorption through the skin.
555
The users, manufacturers, and distributors of methanol should not construe the finding that there is no apparent public-health hazard from the reasonable use of methanol as an antifreeze to mean that methanol is safe for all purposes and conditions of usage. Rational precautions must be observed for this compound, as is necessary in the safe manufacture, distribution, and use of many chemical commodities. When dealing with volatile combustible liquids, precautions should be taken, not only from a health consideration, but also from one of explosion and fire hazards. This precaution is emphasized because the investigations made by the Bureau of Mines have revealed instances of storage of both ethyl alcohol and methanol antifreeze in confined spaces and under conditions which constituted a fire and explosion hazard. Literature Cited (1) Sayers, R . R . , and Yant, W. P., Bur. Mines, I n f . Circ. 6415 (1930); IND. ENG.CIIEM., News Ed., 8, 4 (Dec. 10, 1930). 12) Sayers, R. R , Yant, W. P.. Waite, C. P . , and Patty, F. A,, U.S. Pub. Health Service, Pub. Health Repts. 45, 225 (1930).
Detonation Characteristics of Some Aliphatic Olefin Hydrocarbons' Wheeler G . Lovell, John M. Campbell, and T. A. Boyd GENERAL MOTORS RESEARCH LABORATORIES, DETROIT, MICH.
.
The relative tendencies to knock in an engine have been measured for twenty-five olefin hydrocarbons. These measurements were made, not on the hydrocarbon alone, but in admixture with gasoline, and the results have been expressed on a molecular basis using the antiknock effect of aniline as the standard of comparison. Upon this basis there appear very great differences among the knocking properties of these compounds, and even among isomers, including those in which the structural difference lies only in the position of the double bond. Definite relationships between molecular structure and tendency to knock have been found for these compounds.
With the straight-chain olefins it was observed (1) that in a homologous series the tendency to knock increased with increasing length of the saturated carbon chain, (2) that in an isomeric series the tendency to knock decreased progressively with centralization of the double bond, and (3) that the tendency to knock is roughly determined by the length of the longest saturated carbon chain. Branched-chain olefins show a somewhat similar behavior; the tendency to knock decreases upon the introduction of a double bond, and the knocking tendency apparently is related, not only to the position of the double bond, but also to the branched structure o the molecule.
I
The structure is to be considered not only as the arrangement of the carbon atoms, which have been found to be important in the case of the paraffins, but also as the position of the double bond in the molecule.
T HAS previously been found that the paraffin hydrocarbons exhibit a wide variation in their tendency to knock in an engine, even among different isomers (6). This present paper is concerned with the behavior of different aliphatic olefin hydrocarbons. It was pointed out long ago (7) that olefin hydrocarbons differ from the paraffins in their tendency to knock. Data have been published ( 1 , 2 , 8) on the knock characteristics of several isolated aliphatic olefins of definite structure-namely, 2-pentene, trimethylethylene, and diisobutylene-and as might be expected from analogy with the paraffin hydrocarbons, there appears considerable difference between the two isomeric pentenes upon which data are available. It is the purpose of this paper to present data upon twentyfive unsaturated aliphatic hydrocarbons of definite structures and to point out some of the rather simple relations between the knock characteristics of these hydrocarbons when burned in a gasoline engine, and their molecular structures. 1 Received March 17, 1931. Presented before the Division of Petroleum Chemistry at the 81st Meeting of the American Chemical Society, Indianapolis, Ind., March 30 to April 3, 1931.
Evaluation of Knocking Properties
As in previous work ( 6 ) , the tendencies of these hydrocarbons to knock have been compared in a single-cylinder, variable-compression engine which B-as equipped with an evaporative cooling system and fitted with a bouncing-pin indicator for matching the fuels in respect to antiknock quality. This equipment and the method of operating it have been described in previously published work (4). The relative knocking tendencies under these conditions have been evaluated in terms of aniline, a knock suppressor, and the unit of this evaluation has been called the aniline equivalent. A positive aniline equivalent indicates that the compound knocks less than the reference gasoline and represents the amount of aniline, expressed as the number of centigram-mols per liter, which must be added to the reference fuel to produce a fuel that is equivalent in tendency
INDUSTRIAL A N D ENGINEERING CHEMISTRY
556
to knock to a 1-molar solution of the compound in the reference g a s o l i n e t h a t is, to a solution containing 1 gram-mol of the compound made up to a volume of 1000 cc. with the reference gasoline. A negative aniline equivalent indicates that the compound knocks more than the reference gasoline, and represents the amount of aniline, expressed again as centigram-mols per liter, which must be added to the molar solution of the compound in the reference gasoline to make it equivalent in tendency to knock to the reference gasoline.
5
6
NUUEEP OF c4n8ON h
Figure 1-Detonation
7 S IN
8
MOLEWE
Characteristics of Straight-Chain Olefins
For example, the aniline equivalent of 3-heptene (mol. wt. 98) is 12. This means that 98 grams of the compound made up to a liter with gasoline (about a 14 per cent solution of the hydrocarbon by volume) is equivalent in knock to a liter solution of aniline (mol. wt. 93) in gasoline containing 12/1m x 93 grams of aniline (about 1.2 per cent solution by volume). Similarly, the aniline equivalent of -8 for 1-octene (mol. wt. 112) means that 112 grams of the octene and *//looX 93 grams of aniline made up to a liter with gasoline knocks the same as the reference gasoline. This makes the solution contain about 16 per cent octene and 0.9 per cent aniline by volume. The precision of the measurement obtained in this way has been found to be usually within * 1 unit of aniline equivalent for compounds upon which two or more determinations have been made. Those compounds upon which check determinations have been made with samples from different sources are so indicated in the tables. Unfortunately some of these hydrocarbons have been available in limited quantities, so that only one determination of knocking tendency was possible and, although one determination represents the average of four or five separate bouncing-pin readings, the precision is possibly less in some cases than the figure given, due to unavoidable variations in engine conditions during the period while the measurements were being made. I n order to convey some conception of the magnitude of this scale of knock rating, i t may be stated that, when tested alone or without admixture with gasoline, 1-octene, which has an aniline equivalent of -8, and 2,4-dimethyl-2-pentene,
Vol. 23, No. 5
which has an aniline equivalent of 18, begins to knock a t compression ratios of 3.4:l and 8.8:1, respectively, under certain specified conditions in the engine used in this work (6). These pure hydrocarbons are equivalent in knock to a mixture of 45 per cent 2,2,4-trimethylpentane in n-heptane and of 30 per cent benzene in the isooctane, respectively. I n other words, the respective octane numbers (3) are 4 5 and 130. The reference gasoline was equivalent to a mixture of 55 per cent of the isooctane and 45 per cent n-heptane; its octane number was 55. These data are indicative only of the range covered by the aniline equivalent scale; they do. not permit translation of this scale to another directly. The aniline-equivalent method of expressing knock ratings provides a convenient and uniform basis for comparing compounds representing a wide range in tendency to knock, This basis furnishes a measure of the partial knock effect of each compound in gasoline a t concentrations which are comparable with those in which such a compound might actually be present in commercial fuels. At concentrations below about 40 per cent by volume the computed aniline equivalent, either positive or negative, has been found to be substantially independent of concentration for a number of compounds investigated. This is, of course, due to the fact that there i s direct proportion between amounts of aniline and amounts of compound which are equivalent in respect to knock; and the aniline equivalent might be defined as the amount of aniline, per unit amount of compound, added to a given quantity of gasoline. But the aniline equivalent as computed a t concentrations below 40 per cent by volume has been found to be not necessarily an index to the behavior of the compound in greater concentrations with gasoline, or even in the pure state. Although the numerical values of the aniline equivalents reported in this investigation depend somewhat upon experimental conditions and, of course, upon the reference fuel, there is no evidence a t this time that the relative knocking tendency of one olefin hydrocarbon with respect to another when measured in this way is materially altered by reasonable changes in engine conditions. The use of a molecular basis for evaluating the knocking properties of the compounds is especially convenient because of the consistent relationships between molecular structure and knock rating which then become apparent. It is the main purpose of this paper to present and to discuss some of these relationships. Sources of Hydrocarbons
The twenty-five olefin hydrocarbons, data upon whose knocking behavior are presented here, have all been carefully synthesized by laboratory methods. Many of them have been prepared by P. L. Cramer, R. H. F. Manske, and F. K. Signaigo in the General Motors Research Laboratories. For a number of the olefins, and especially for the series of isomeric straight-chain octenes, the writers are indebted to Graham Edgar and the staff of the Ethyl Gasoline Corporation, and for the 2-hexene to G. M. Maverick and the Standard Oil Development Company. The sample of 2-methyl2-butene was donated by Thomas Midgley, Jr., and the diisobutylenes, by F. C. Whitmore. The preparation of the olefins in a state of absolute purity is not an easy matter. I n almost every case there is the possibility of the presence of a t least two isomers, the cis and trans modifications, and no attempt has been made to separate these isomers. In some cases, also, there is the possibility of isomerism in which the position of the double bond varies. For this reason only those olefins are included in which, judging from the method of preparation and the physical properties, the possibility of impurity from this.
INDUSTRIAT, AND ENGINEERING CHEMISTRY
May, 1931
source is reduced to %: minimum. Perhaps the best indication of the purity of these compounds, and the structure as represented by the structural formulas given, is the consistency of the relations between their structure and their detonation characteristics, since a relation between these two sets of properties has previously been observed for the paraffin hydrocarbons. To present a complete picture of the aliphatic olefin hydrocarbons of 5 , 6,7, and 8 carbon atoms, or those boiling within the present gasoline range is a matter of considerable difficulty. This arises from the enormous number of isomers which are possible of existence. Thus the number of paraffin isomers of 5, 6, 7, and 8 carbon atoms are (excluding geometric isomers) 3, 5 , 9, and 18, respectively. The possible number of olefin hydrocarbons, however, is 5, 13, 27, and 68, respectively. Of the 68 possible octenes, only 8 have been previously reported in the literature as having been prepared in a state approaching purity. For these reasons the twenty-five hydrocarbons upon which data are presented have been selected so as to comprise, not only those whose probable purity was high, but also those which would exhibit clearly the general relations between their chemical structure and their behavior when burned in a gasoline engine.
557
hydrocarbons of low molecular weight. A general phenomenon similar to the regularity observed with the olefins is also marked by the line through 2-pentene, 2-hexene, and 2-octene, where again the lengthening of the saturated chain decreases the aniline equivalent or increases the knocking tendency. This relation also holds with the double bond in the 3 position, as in the case of the line drawn through 3-heptene and 3-octene. It is possibly significant that the lines as drawn are approximately parallel. It appears that the lengthening in this way of the longest saturated carbon chain of a straight-chain olefin produces a substantially constant change in the tendency of the olefin to knock. This relation is auite similar to the effect observed in oaraffin
Data on Straight-Chain Olefins
The data on the straight-chain olefins are shown in Table
I, which gives the structure of the compound, its aniline equivalent, and also the aniline equivalent of the corresponding paraffin. For convenience these data are shown graphically in Figure 1, where the aniline equivalent of each compound has been plotted against the number of carbon atoms in its molecule. The points representing the different compounds are designated by a conventional abbreviated structural formula of that hydrocarbon. In these designations, the hydrogen atoms are not represented; thus, the point C C = C C C C represents 2-hexene with 6 carbon atoms and an aniline equivalent of 12, and the point marked C = C C C CCC represents 1-heptene with 7 carbon atonis' and an aniline equivalent of 0. For comparison, the series of straight-chain paraffins is also represented in the figure.
+
b
4
I
''C,*+Cd'
+
Figure 2-Detonation
Table I-Straight-Chain
-,c
+
cg-
Characteristics of Branched-Chain Olefins
Olefins ANILIXE INCREASE EQUIVALENT I N AN~LITTE (2) Another relation between molecular structure and tendO F CORRE- EQUIVALEKT SPONDING DUETO ency to knock in an engine is illustrated by the vertical lines on ANILINE PARAF- UNSATURA- the chart. The vertical line through the octenes passes succes-
HYDROCARBON STRUCTURE EQUIVALENT 1-pentene C=C-C-C-(: 10 2-pentene C-C-C-C-C 16" 1-hexene C=C-C-C-C-C 8 2-hexene C-C=C-C-C-C 12 1-heptene C=C-C-C-C-C-C 05 3-heptene C-C-C-C-C-C-C 12" 1-octene C=C-C-C-C-C-C-C 8a 2-octene C-C=C-C-C-C-C-C 0 3-octene C-C-C-C-C-C-C-C 6 4-octene C-C-C-C=C-C-C-C 12" Two or more complete determinations.
-
FIN
TION
1 1 -6 -6
- 14 - 14 -21 -21 -21 -21
9
18 14
18 14 26 13 21 27 33
The data plotted show a number of interesting relationships between the structures of these olefins and their tendencies t o knock. (1) , A first series of relationships is indicated by the dotted lines dlrected towards the lower right-hand corner of the chart. The line through the paraffins n-pentane, hexane, heptane, and octane shows a decrease in aniline equivalent with an increase in molecular weight. Similarly, the line drawn through 1-hexene, 1-heptene, and 1-octene, which is approximately parallel to the paraffin line, also shows the decrease in aniline equivalent accompanying the addition of CH2 groups to an olefin. It is to be observed that I-pentene does not lie on the straight line, and there are other indications that the lines for the n-paraffins and aolefins would cross if they could be extrapolated to the gaseous
sively through the isomeric 1-octene, 2-octene, 3-octene, and 4octene. In this isomeric series the aniline equivalent increases, and the tendency to knock decreases, as the double bond is centralized or is located closer to the center of the molecule. There is a similar relation in the case of the heptenes, hexenes, and pentenes. It is also interesting to see that each time the double bond is moved towards the center of the molecule by a distance of one carbon atom, in these isomeric series, it accompanies a substantially constant increase in aniline equivalent. (3) Let us also consider the relations as illustrated by the horizontal lines shown on the chart. The upper line passing approximately through 1-pentene, 2-hexene, 3-heptene, and 4octene represents a substantially constant aniline equivalent and also passes through compounds whose longest saturated carbon chain is 4 carbon atoms long. A similar line through 1-hexene and 3-octene indicates the 5-carbon chain and the line through 1heptene and 2-octene, a 6-carbon atom chain. Parallel lines may be drawn through 1-octene and 1-pentene, respectively. This series of horizontal lines has a substantially regular interval between them, indicating that, irrespective of molecular size, a decrease in the length of the longest saturated carbon chain produces an increase in the aniline equivalent. (4) The area on the chart occupied by the straight-chain olefins is possibly of significance. The line through these olefins defines a line of maximum knock and it appears that the aniline equivalents of these worst olefins decrease with increase in molecular size. A line through 2-pentene, 3-heptene, and 4-octene
INDUSTRIAL A N D ENGINEERING CHEMISTRY
558
defines a line of minimum knock, since these represent the most centralized structures possible for straight-chain olefins of their molecular size; and it appears that, as the molecular weight increases, these best of the straight-chain olefins have substantially the same aniline equivalents. The area occupied by the straight-chain olefins may also be compared with t h a t occupied by the paraffin hydrocarbons. To define the latter area, stars have been placed on the chart to represent the branched paraffins of highest aniline equivalent; a line through these points and the line through the normal or straight-chain paraffins defines the area occupied by the paraffins. The straight-chain olefins lie within this space. For a given molecular size and within the range covered by these data, any straight-chain olefin has a higher aniline equivalent than the lowest paraffin, but not so high as the highest paraffin. D a t a on Branched-Chain Olefins
The data on the branched olefins are given in Table 11, and for convenience are shown graphically in Figure 2. In this chart the aniline equivalents of the compounds have been plotted vertically and their structure indicated as before. On the chart also are the values for the corresponding saturated compounds, as indicated by the circles a t the lower ends of the dotted lines. The length of the dotted line, consequently, is a measure of the change in aniline equivalent brought about by the introduction of the double bond. Table 11-Branched-Chain
Olefins ARILINE INCREASE EQUIVALENT OF
IN
SPOND-
LENT
will be related both to the position of the double bond and to the arrangement of the carbon atoms. On account of the enormous number of possible isomers of 5 , 6, 7 , and 8 carbon atoms, which comprise 113 compounds, a complete survey of these hydrocarbons is out of the question a t present, and attention is confined to those upon which data are presented. It is clear from the chart that these hydrocarbons exhibit a wide variety of tendencies to knock, and show a number of interesting regularities: (1) All these hydrocarbons show greater aniline equivalents than the corresponding saturated paraffin compounds. The differences do not appear constant. By analogy with the straight-chain olefins, it might be expected that the greatest change would accompany the introduction of a double bond so as to break up a long chain of CHz groups, and there is some qualitative indication that this is so sometimes. (2) It is to be expected that an olefin of the paraffin of highest aniline equivalent would be a compound also of a high value. I n the case of the heptenes the most centralized structure is which has only one possible found in 2,2,3-trimethyl-3-butene, position for the double bond. This compound has an aniline equivalent greater than the corresponding paraffin, which is the highest heptane. An olefin of the highest octene, 2,2,3,3-tetramethylbutane, of most branched structure, does not exist. Approximations to the best octene would probably include 2,2,4trimethyl-3-pentene, which, however, is not so high as the tetramethylbutane. Absolute determination of whether the best aliphatic hydrocarbon of 8 carbon atoms is an olefin or paraffin would involve, of course, the preparation of an enormous number of hydrocarbons.
ANILINE CORRE- EQUIVA-
HYDROCARBON Cs: 2-methyl-%butene
STRUCTURE C C-LC-C
C-C-2-C-C 2,3-dimethyl-2pentene 2,4-dimethyl-2pentene 2,2-dimethyl-4pentene
LENT
FIN
TION
23
9
14
14
8
6
21
12
9
200
8
10
c c c--&-Lc-c c c c-c=c-c-c I t
Another variation in structure with the aliphatic olefins is the case of the introduction of two double bonds into the molecule, forming diolefins. Attention is confined to the four diolefins, data upon which are shown in Table 111.
27
I
8
19
c c
c - L A C I
c
ANILINE INCREASE EQUIVAIN OF LENT ANILINE CORRE- EQUIVARPOND-
C
c-&-c-C=C
Table 111-Diolefins
ANILINB
C
2,2,3-trimethyl-3butene
Data on Aliphatic Diolefins
C
Cs: 3-methyl-2-pentene c7:
ANILINE ING DUETO EQUIVA-PARAF-UNSATURA-
Vol. 23, No. 5
23"
19
HYDROCARBON 2,3-dimethyl-1,3butadiene 1,5-hexadiene 2,4-hexadiene
4
I
LENT
ING DUETO PARAP- UNSATURAPIN
TION
c c
C=&-L=C
38
C=C-C-C-C=C C-C-C-C=C-C
6 29
C
19 -6 -6
I9 12 36
!. c
3-ethyl-1,3pentadiene
C
EQUIVASTRUCTURE LENT
I c=c-c-c-c
24
4
20
C
3-ethyl-2-pentene
I
C-C-C-C-C
20
4
16
8
0
8
9
3
6
32
16
16
30
16
14
C
2-methyl-5-hexene
I
C-C-C-C-C-C C
3-methyl-5-hexene
Ce: 2,2,4-trimethyl-4pentene
C-C-&-C-C=C
c I
C-C-C-&=C
I
C
c 2,2,4-trimethyl-3pentene a
c
C-C!2-C=C-C
I
c I
C T w o or more complete determinations on samples from separate sources.
These compounds show a behavior, with respect to their tendency to knock in an engine, in good agreement with what might be expected from the known behavior of the olefins already considered. Thus, the two hexadienes have aniline equivalents greater than n-hexane, and the 2,Phexadiene, with more centralized double bonds, has a much greater aniline equivalent than 1,5-hexadiene1 whose double bonds are much less centralized and leave a greater length of unbroken and unbranched carbon chain. The 3-ethyl-2, 4pentadiene, and the 2,3-dimethyl-1 ,a-butadiene in which there are present the conjugated double-bond systems as well as side chains, exhibit a somewhat similar behavior. Literature Cited
These olefins with branched chains are of special interest as they represent the structural condition of a double bond combined with a varied branched structure. The position doubtless affects the detonation characteristics, as was found to be the case for the straight-chain olefins; and it has previously been found that the arrangement of a branched structure is of great influence on the aniline equivalent. It is to be expected that the hocking tendencies of these compounds
(1) Birch and Stansfield, Nature, 123, 490 (1929). (2) Birch and Stansfield, I b i d . , 128, 639 (1929). (3) Boyd, 11th Annual Meeting Proceedings, American Petroleum Institute. 11, No.74, 38 (Dec. 31, 1930). (4) Campbell, Lovell, and Boyd, I N D . E N G . CHEM, 20, 1046 (1928). (6) Campbell, Lovell, and Boyd, J . SOC.Aafomotiue Eng., 26, 163 (1930). (6) Lovell, Campbell, and Boyd, I N D . END.CHEM.,2 & 2 6 (1931). (7) Midgley, J. Soc. Automotive Eng., 7, 489 (1920). (8) Nash and Howes, Nolure, 123, 276 (1929).
