P . ~ N O Ri ~OF Y GEVERALMOTORSPROVING GROUND
Knocking Characteristics of Aromatic Hydrocarbons WHEELERG. LOVELL;JOHN 11. CAMPBELL,FRANKK. SIGNAIGO, AND T. -4.BOYD General RIotors Research Laboratories, Detroit, Mich. given in terins of the o c t a n e The reltztire fendencies to knock in a n engine publications n u m b e r s of t h e 20 per cent from this laboratory have hate been measured for j f t y - n i n e hydrocarbons blends. Further data in terms of described the knocking embracing the aromutic series. These measurecompression ratio have been recharacteristics of over one hunments were made not on the hydrocarbons alone, ported by Hofmann and others dred paraffin, olefin, and naph(7, 8) for b e n z e n e , t o l u e n e , but in admixture utith gasoline; the results have thene hydrocarbons of varied xylenes, ethylbenzene, styrene, been expressed, as in previous iuork, using the structure. This paper relates to phenylacetylene, cyclopentaa continuation of the previou3 antiknock effect of aniline as the standard of diene, and dicyclopentadiene. work and is concerned in parcomparison. The data given in the present ticular w i t h the b e h a v i o r of U p o n this basis there appear great differences paper include all but one of the aromatic hydrocarbons. hydrocarbons p r e v i 0 u s l y reamong tht: knocking properties of these comDuring the past decade, data ported upon and constitute in pounds. Certain relationships between molecular on the relative knocking chargeneral a confirmation of the acteristics of some of the more structure trnd the knocking properties of these published data as well as a concommon a r o in a t i c hydrocarcompounds are consistent with similar relationsiderable extention of these data. bons have gradually been acships preiiously observed among parafins, olejns, cumulating. I n 1921 Ricardo EVALUATION OF KNOCKING and naphthenes. (16) reported the relatively high PROPERTIES of bena n t i k n o c k -DroDerties As in previous work from these laboratories (11, 12, 13, 17) zene, toluene, and xylene with respect to certain naphthenes and paraffis. I n 1922 Midgley and Boyd (14) reported upon the tendencies of these hydrocarbons to knock have been combenzene, toluene, and xylene in solution with kerosene. They pared in a single-cylinder, variable-compression engine which found that the antiknock effect of these three hydrocarbons was equipped with an evaporative cooling system and fitted increased in the order of increasing molecular weight. I n with a bouncing-pin indicator for matching the fuels in re1929 Birch and Stansfield ( 2 ) reported data on benzene and spect to antiknock quality. This equipment and the general l,&cyclohexadiene. Since that time Howes and Sash (9) method of operating it have been described in previously pubhave published data on twelve aromatic hydrocarbons in 20 lished work (4). The relative knocking tendencies under these conditions per cent solution with gasoline and have expressed their results in terms of the concentration of tetraethyllead required have been evaluated in terms of aniline (a knock suppressor), in the reference gasoline to match the 20 per cent blends. and the unit of this evaluation has been called the “aniline More recently Garner and Evans (5)have reported upon some equivalent.” A positive aniline equivalent indicates that of the aromatics in 20 per cent solution with gasoline and the compound knocks less than the reference gasoline and r e p under different sets of engine conditions. Their data are resents the amount of aniline, expressed as the number of
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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. I n order to convey some conception of the magnitude of this scale of knock rating in units which may be more generally familiar than aniline equivalents, a compound having an aniline equivalent of 20 added in one-molar concentration to the reference gasoline having an octane number of 55 would produce a m i x t u r e h a v i n g an octane number of 65. This numerical ratio appears to be valid over the range of positive aniline equivalents used, but a distinction should be clearly recognized between aniline equivalents obtained a t onemolar concentration in gasoline and the behavior of compounds by themselves or not in admixture with gasoline. A t concentrations belou- about 40 per cent by volume the aniline equivalent computed from concentrations other than one mole per liter has been found to be substantially independent of concentration f o r a n u m b e r of MOLECULAR WEIGHT compounds investigated. But the aniline FIGURE1. RELATIVE KNOCKING CHARdCTERISTICS OF AROMATIC equivalent as computed a t concentrations HYDROCARBONS below 40 per cent by volume has been found to be not necessarily an index to the behavior centigram-moles per liter, which must be added to the refer- of the compound in greater conceikrations with gasoline, or ence fuel to produce a fuel that is equivalent in tendency to even in the pure state, and so extrapolation of values obknock to a one-molar solution of the compound in the refer- tained a t low concentrations to 100 per cent is not, as a ence gasoline-that is, to a solution containing one gram-mole general rule, recommended. The numerical values of the aniline equivalents reported in of the compound made up to a volume of 1000 cc. with the reference gasoline. A negative aniline equivalent indicates this investigation depend somewhat upon experimental conthat the compound knocks more than the reference gasoline ditions and, of course, upon the reference fuel. It is also recand represents the amount of aniline, expressed again as cen- ognized that the relative values of different aromatic hydrotigram-moles per liter which must be added to the molar solu- carbons may change with engine conditions, and some data tion of the compound in the reference gasoline to make it in this connection are presented. equivalent in tendency to knock to the reference gasoline. For example, the aniline equivalent of benzene (molecular weight, 78) is 10. This means that 78 grams of benzene made up to a liter with gasoline (about a 9 per cent solution of the hydrocarbon by volume) is equivalent in knock to a liter solution of aniline (molecular weight, 93) in gasoline containing 10/100 X 93 grams of aniline (about 0.9 per cent solution by volume). Similarly, the aniline equivalent of -3 for nheptylbenzene (molecular weight, 176) means that 176 grams of n-heptylbenzene, and 3/100 X 93 grams of aniline made up to a liter with gasoline knocks the same as the reference gasoline. This makes the solution contain about 20 per cent n-heptylbenzene and 0.3 per cent aniline by volume. The precision of the measurements obtained in this way has been found to be usually within one unit of aniline equivalent for compounds upon which two or more determinations FIGURE 2. EFFECT OF UNSATURATIOX IN THE have been made. Those compounds upon which check deSIDE CHAIN terminations have been made with different samples are so indicated in the tables. Most of these hydrocarbons have The use of a molecular basis for evaluating the knocking been available in limited quantities, so that only one determi- properties of the compounds is convenient because of the connation of the knocking tendency was possible, and, although sistent relationships between molecular structure and knock one determination represents the average of four or five sepa- rating which then become apparent.. It is the main purpose rate bouncing-pin readings, the precision is possibly less in of this paper to present and to discuss some of these relationsome cases than the figure given, owing to unavoidable varia- ships. tions in engine conditions during the period in which the measSOURCES OF HYDROCARBONS urements were being made. Most of the hydrocarbons used in t.his investigation were The aniline-equivalent method of expressing knock ratings provides a convenient and uniform basis for comparing com- synthesized in this laboratory. A few were obtained from the pounds representing a wide range in tendency to knock. This Eastman Kodak Company and elsewhere, and those com-
Ma?;, 1934
INDUSTRIAL I N D ENGINEERING CHEMISTRY
pounds which were suspected to be impure were purified by fractional distillation. I n general, methods of synthesis --ere chosen which were known to give products free from isomers and which would not yield any by-products that could not be removed by chemical means. The methylalkylbenzenes, with the exception of the dimethyl-, trimethyl-, and diethylbenzenes and p-cymene, were prepared by the alcohol-olefin-paraffin method n-hich has previously been described for the preparation of alkylcyclohexanes ( 1 7 ) . This preparation can be outlined as follows:
The three acetylene hydrocarbons were Prepared by the methods of Bourguel (S), and the trimethylphenylallene was prepared from bromobenzeneand mesityl oxide (10). The data are tabulated in Table I. The molecular weights and densities are also given in order to facilitate conversion of these values to other concentrations. Figure 1 is a graphical presentation of the alkyl benzenes in which aniline equivalents are plotted approximately against molecular weights. From this chart, relationships between m o l e c u l a r s t r u c t u r e and k n o c k i n g tendency m a y be more easily visualized than from the tabulation. Figure 2 shows the relationships between several saturated and unsaturated alkyl benzenes, and Figure 3 a comparison of some five- and six-carbon atom compounds in different states of hydrogenation.
An aldehyde or ketone is added to the Grignard reagent prepared from a bromotoluene. The addition product on hydrolysis yields a secondary or a tertiary alcohol which, on heating with iodine, is dehydrated to an olefin. This olefin is hydrogenated to the methylalkylbenzene with the use of a DISCUSSION OF RESULTS platinum oxide catalyst at 215room temperature and under T h e d a t a a s shown, a pressure of 2 to 3 atmosSECTIOV OF GEUERALMOTORS PROVIUG GROUXD especially in Figure 1, illusp h e r e s of h y d r o g e n using ethyl alcohol as the solvent, trate a number of interestunder which conditions the benzene ring is not hydrogenated. ing relationships betlyeen knocking tendency and nlolecular The 0-, m-, and p-bromotoluenes were prepared by the Eastman structure. Kodak Company from the corresponding toluidines by use of ADDITIOKOF STRAIGHT SIDECHAINSTO BENZENE.Up to the diazonium reaction. sec-Butylbenzene was prepared by this method from bromo- n-propylbenxene there is a progressire increase in aniline benzene and methyl ethyl ketone, and also cyclohexylbenzene equivalent with increasing length of tlle side chain. Further from bromobenzene and cyclohexanone. The olefins listed in Table I, with the exception of phenyl- increase in the length of the side chain results in progressive ethylene, were prepared by the first two steps in the above lowering of aniline equivalent as shown by the heavy line in synthesis. Figure 1. n-Heptylbenzene was a slight knock inducer in Cyclopentadiene was prepared by depolymerization of di- the reference gasoline. Thus it appears as if, after the alkyl cyclopentadiene. The two fulvene hydrocarbons were obtained from cyclopentadiene by condensing it n;ith acetone and aceto- side chain has reached a certain length, its lower antiknock character begins to weigh against the antiknock effect of the phenone (18). 1,3- and l,.l-cyclohexadiene were prepared by removal of benzene nucleus, and further increases in length of the alkyl hydrobromic acid from 1,2- and 1,4-dibromocyclohexane, re- side chain finally overcome the antiknock effect of the benzene. spectively ( I A , 6). l-Phenyl-1,3-butadiene was prepared from d D D I T I O N O F M E T H Y L GROUPS. SUcceSSive addition O f methyl iodide and cinnamic aldehyde as prescribed by Muskat methyl groups to benzene results in a progressive increase in (15).
TABLE I. ANILINE EQUIVALEKTS COXPOUND
MOL.
WT.
dit
ANILINE EQUIVALENT
Cvclonentadiene Dhyciopentadiene Dimethylfulvene hlethylphenylfulvene
66 132 106 168
34
65
61 36
S I X - C A R B O N ATOM FLING
78 80 80 92
0.878 0.828
0.842 0.866
10" 39 29 15"
Phenylacetylene Phenylethylene Ethylhenzene o-Xylene m-Xylene p-Xylene
102 104 106 106 106 106
0.930 0.903 0.868 0.879 0.865 0.861
5a 20
19 17" 23a 26a
3-Phenylpropyne 1-Phenylpropyne Indene p-Tolylethylene n-Propylbenzene Isopropylbenzene
116 116 116 118
0.939 0.939 1.006 0.896
8 11 210 30
Benzene 1,3-Cyclohexadiene 1,4-Cyclohexadiene Toluene
34
1,2-Methylethylbenzene 1 3-Methylethylbenzene 1:4-Methylethylbenaene Mesitylene Pseudocumene 2-o-Tolylpropene 2-m-Tolylpropene 2-p-Tolylpropene 1-p-Tolylpropene Tetrahydronaphthalene
132 132 132 132 132
0.892 0.899 0.898 0.900 0.975
25 37 37 29 19
WT.
ANILIKE EQUIVALENT
n-Butylbenzene Isobutylbenaene sec-Butylbenzene tert-Butylbenzene 1,2-Methyl-n-propylbenzene 1,3-1Methyl-n-propylbenzene 1 4-Methyl-n-propylbenzene l:2-Methylisopropylbenzene 1 3-Methylisopropylbenaene 1:4-Methylisopropylbenzene
134 134 134 134 134 134
0.880 0.864 0.861 0.885 0.857
19 25 25 20 26 27
1 3-Diethylbenzene 1:4-Diethylbensene 1-Phenyl-1,a-butadiene n-Amylbenzene tert-Amylbenzene
134 134 140 148 148
0.860 0.865 0.928 0.862 0.866
30 34 33 17 23''
12-Methyl-n-butylbenzene 1 3-Methyl-n-butylbenzene
1:4-Methyl-n-butylbenzene Trimethylphenylallene
148 148 148 158
0.874 0.869 0.860 0.922
21
[ethylisoamylbenzene ~,3-.Methylisoamylbenzene 4isoamylbenzene
162 162 162 162 162 162
0.874 0.861 0.857 0.869 0.858 0.858
14 14 18 16 20 26
158 160 168 176
0.986 0.948 1.006 0.867
26 -7 26 -3
SIX-CARBON ATON RING
0.803 0.978 0.885 1.021
MOL.
d;: (cont'd) 134 0.862 134 0.855 134 0.860 134 0.867
CONPOUND
F I V E - C A R B O N ATOM RING
Phenylcyclohexene Phenylcyclohexane Diphenylmethane la-Heptylbenzene a T w o or more complete deterlninations.
0.865
20Q 20 21a 25a
17
25 36
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RELATION TO OTHERTYPES OF HYDROCARBONS Compared with data previously reported in these investigations (11, 12, 13, 17), the aromatic hydrocarbons as a class are higher in antiknock quality than either the paraffins, the olefins, or the naphthenes, although there are individual compounds in the latter groups which are closely comparable to the aromatics. Comparing individual aromatic compounds with the corresponding naphthenes, it is clear that hydrogenation of the benzene nucleus materially lowers the aniline equivalent. This decrease in aniline equivalent with hydroP O S I T I OISOMN genation of the alkyl benzenes increases with molecular weight ERISM. The effect so that there is a much greater difference between mesitylene of position isomer- and the corresponding naphthene, for example, than between ism is illustrated by benzene and cyclohexane. the xylenes. Here I n Figure 3 a comparison is made between the five- and the p-xylene is higher six-carbon ring compounds in various degrees of hydrogenathan m-xylene, and tion and some of the corresponding six-carbon open-chain m-xylene is higher compounds. This figure shows the comparatively powerful than o-xylene. This antiknock effect that is sometimes exerted by a conjugated r e l a t i o n s h i p be- double bond as it is found in cyclopentadiene, 1,3-cyclohexat w e e n the ortho, diene and 2,4-hexadiene. This antiknock effect seems to be m e t a , a n d p a r a characteristic for certain types of conjugation and was most p o s i t i o n s h o Id 3 strikingly shown by dimethylfulvene, which has an antiknock in general for the value0.6 of that of aniline. This was the highest aniline higher alkyl methyl- equivalent observed among 180 hydrocarbons, with the excepI cccccc benzenes, as is in- tion of dicyclopentadiene which may be considered a polymer d i c a t e d b y t h e of cyclopentadiene. FIGURE3. COMP4RISON OF FIVEspread between tlle ~ N D SIX-CARBOUATOM RING dotted lines in FigSUSCEPTIBILITY TO CHANGES IN ENGINE CONDITIONS COWPOUNDS ure 1; it a p p e a r 5 The aniline equivalents which have been reported thus far also that the effect of position isomerisni on aniline equivalent may be very were obtained under conditions of engine operation equivalent to those specified in the method of knock rating now genergreat. BR.~NCHINGSIDECHAIXS. Branching of the side chains ally known as the Research method (19). Aniline equivacontaining more than three carbon atoms resulted in an ap- lents determined by the Motor method (1, 19) now in general preciable increase in aniline equivalent. It may be that the use sometimes vary appreciably from those made by the Reeffect of branching is greater when the alkyl group contains a search method of knock rating. The essential difference begreater number of carbon atoms; and such an effect would be tween the Research and the Motor methods is that the latter consistent with the conception previously expressed, that the procedure is carried out a t a higher speed and a t a higher mixantiknock effect of an alkyl benzene appears to be a balance ture temperature. The aniline equivalents of a few typical between the effect of the ring and the effect of the side chain. hydrocarbons representing aromatics, naphthenes, olefins. The differences between aniline equivalents of the isomeric r1--7---butylbenzenes is considerable, and a relatively small amount of branching of the compounds containing the amyl groups appears to have a great effect on antiknock quality. The general effect of branching appears consistent with the previously observed behavior of the paraffin and naphthene hydrocarbons. DISTRIBUTIONOF CARBONATOMS AMOXG SIDE CHAINS. Among compounds of given molecular weight, the aniline equivalents vary with the distribution of the carbon atoms in the side chains. It appears as if, in general, the more symmetrical and centralized the molecule, the higher the aniline 5 IO I5 equivalent, although this cannot be estimated quantitatively. Ib.CXhSE IN OCTAhL NUMBER/GM. MOL /LITER Thus, among the compounds having a molecular weight of 134 the aniline eqaiyalents varied between 20 and 34. In OF ANILINE EQUIV~FIGURE4. COMPARISON terms of octane numbers this means that the octane numbers LENTS WITH OTHER PUBLISHED DATA of the blends, in the concentrations a t which the aniline equivalents were determined, which was 1 gram mole per liter, or and paraffins, as determined by the two methods of test, are about 15 per cent by volume for these compounds having a compared in Table 11. These data indicate that the aromatic molecular weight of 134, varied between 65 and 72; that iq, hydrocarbons as a class are more susceptible to changes in the increase in octane number accompanying these additions engine conditions than the other classes of hydrocarbons. This is in agreement with the conclusions reached by Garner varied between 10 and 17. and Evans (6) who have made similar comparisons in an Ethyl UNSATURATION IN SIDE CHAINS. Figure 2 illustrates the effect of unsaturation in the side chain. For these few com- Gasoline Corporation Series 30 engine a t jacket temperatures pounds a double bond in the side chain adds slightly to the of 212' and 300' F. (100' and 148.9' C.), respectively. The aniline equivalent, and a triple bond in the side chain causes data in Table I1 also present some evidence which indicates a decided reduction in aniline equivalent with respect to the that the relative antiknock properties of two hydrocarbons, even of the same class, may vary somewhat with widely varycorresponding saturated alkyl benzene. aniline equivalent. Thus toluene is higher in antiknock quality (molecule for molecule) than benzene, the xylenes are higher than toluene, and mesitylene is higher than the xylenes. The effect of the addition of methyl groups to an alkyl benzene, in general, is not so simple. Thus the addition of methyl to n-propylbenzene may result in a slight increase in the aniline equivalent, in a decrease, or in substantially no change, depending upon the position of the added methyl group. A somewhat similar situation also prevails for the introduction of a methyl group to butyl- and amylbenzenes.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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479
From these octane numbers the increase in octane number per gram mole per liter has been calculated and plotted against the corresponding aniline equivalent in Figure 4. Similarly the data of Garner and Evans ( 5 ) obtained in 20 volume per cent solution have been converted to a molecular basis and plotted in Figure 4. For the majority of the compounds, agreement is well within the limits of experimental error. Lack of good agreement among these different groups of data in the relative antiknock effects of these hydrocarbons TABLE 11. COMPARISON OF AVILINE EQUIVALENTS DETERMINEDis shown by the following compounds: isopropylbenzene, nUNDER DIFFERENT TESTCONDITIONS butylbenzene, sec-butylbenzene, tert-amylbenzene, mesitylene, and p-cymene. After this comparison was made, the aniline --ANILINE EQCIVALENTResearch Motor equivalents of freshly distilled samples of these hydrocarbons, method method Difference COMPOUND as procured from the Eastman Kodak Company, were rede.4ROYATICR termined ; the results checked previous determinations. 10 6 4 Benzene
ing engine conditions. Thus 1,3-diethylbenzene was approximately equivalent to mesitylene by the Research method, but by the Motor method the diethylbenzene was definitely higher in antiknock value than mesitylene. I n connection with this discussion of aniline equivalents as determined by the two methods of test, the octane number calibration for aniline in the reference gasoline is not changed between one method and the other.
T o1ue n e +Xylene m-Xylene
15 17 23 26 19 31 30 21 33
Ei3iiYZzene Mesitylene 1 3-Diethylbeneene I;>--1-
8
11 13 13 11
16 24 16 30
7 6 10 13 8 15 6 5 3
ACKNOWLEDG~VENT The authors wish to thank P. L. Crarner of this laboratory for helpful suggestions in connection with the preparation of many of t,he hydrocarbons listed in this work. LITERATURE CITED
NAPHTHENEl
Cyclopentene Cyclo entane CycloEexene Cyclohexane Methylcyclohexane 1 2-Dimethylcyclohexane 1'2-Methylethylcyclohexa~ie 1:2-Methyl-n-propyIcyclohexaue 1,2-Methyl-n-biitylryclohexane
16 14 10
14
7 5 6 0 -5
6 5 2 0 -6 - 13
16
13 15 27
- 16
12
(1)
9
23 31
1
-3
3 8 4
COMPARISON WITH
-6
-6
0
16
-13 13
-1 3
- 14
dpecifica-
I
PAHhFFIN8
n-Hexane n-Heptane 2,2,4-Trimethylpentane
(I). 7 4 j ,
tion D 357-33 T (1933) (1A) Baeyer, Ber., 25, 1840 (1892).
OLEFINS
2-Pentene 2-Methyl-%-butene Dikobutylene
Sou. T e s t h q Materials, P r o c e e d i n g s , 33
OTHER DATA
In Figure 4 comparison is made between the aniline equivalents presented here and some related (lata previously published by other investigators. The data of Howes and Nash (9) originally expressed in centimeters of tetraethyllead per gallon were converted to equivalent octane number by Garner and Evans (.5).
(2) Birch a n d Stansfield, ,Vuture, 123, 490 (1929) (3) Bourguel, Ann. chim., [ l o ] 3, 379 (19%). (4) Campbell, Lovell, a n d Boyd, IND. EXG.CHEX.,20, 1045 (1928) (5) G a r n e r a n d Evans, J . Inst. Petroleum Z'rch.. 18, 751 (1932). (6) Harries, Be,..,45, 811 (1912). (7) H o f m a n n , Lang, Berlin, anti Schniiclt, Hrr,rLnsto~-('her,r.,13, 161 (1932). (8) Ibid., 14, 103 (1933). (9) Howes a n d Nash, J. Sac. Chetn. I d . , 49, 16 (1930) (10) Klages. Ber., 37, 2305 (1904). (11) Lovell, Campbell, a n d B o y d , TND. Erti. CHmr.. 23, ? t i (1931). (12) Ibid., 23, 555 (1931). (13) Ibid., 25, 1107 (1933). (14) Midgley a n d B o y d , Ibid., 14, 589 (1922). (15) M u s k a t a n d H e r r m a n , J . Am. Chem. SOC., 53, '5s (1W1). (16) Ricardo, Automobile Engineer, 11, 92 (1921). (17) Signaigo a n d Cramer, J . Ani. C'hern. SOC.,55, 3326 (1933), (18) Thiele, Ber., 33, 671 (1900). (19) Veal, Best, Campbell. and Holarlay. ,$, :I. E. . T o j A r r w / , 32, 105 (1933). RECEIVED January 24. 1934
STEEPGRADEAT GENERAL MOTORSPROVING GROUND
