activated amino acid solutions for two or four hours prior to addition to the resin probe resulted in relatively small changes in the 0.5-2 range (DCC/amino acid), whereas substantial loss of acylating ability was observed a t high equivalents of DCC (e.g., 5/1) after four hours. The loss of activated amino acid a t high DCC levels during long preincubation times is of theoretical interest. However, operationally, the coupling reaction and acylating power of the solution appear to be independent of DCC to amino acid ratio during the initial phase of the reaction. Resin probe experiments, described in this report and previously, must be evaluated relative to the specified set of reaction conditions. If different conditions are employed (e.g., solvents, nucleophiles, concentrations, temperatures, etc.), it is prudent to evaluate the effects of the modification by carrying out new resin probe experiments rather than extrapolating from previous data. In addition, i t is desirable to maintain the relative coupling levels in the range of 40-80% in order to provide a system that is sensitive to change (i.e., increased coupling or inactivation). Often maximum sensitivity is obtained when the amount of activated amino acid present is equivalent to the free amine content of the resin. Dorman (7) has suggested an alternative to our resin probe method based on the quantitative isolation of solution components. Routine isolation of solution components can be difficult as the composition of the reaction mixture and/or the solvent is changed and different solvents are used. In contrast, resin probes provide ease of execution,
rapid separation and isolation of reactants and products, rapid quantitative analysis, and the general applicability for probing and optimizing reaction conditions. We feel that a comparison under a variety of experimental conditions will favor Resin Probe Analysis.
LITERATURE CITED (1) A. M. Tometsko, Biochem. Bbphys. Res. Commun.,50,886 (1973). (2) D. F. DeTar and L. Silverstein, J. Am. Chem. SOC.,88, 1020 (1966). (3) A. M. Tometsko, M. Schreiner. and J. Comstock, Anal. Biochem., 67, 182 (1975). (4) A. M. Tometsko and E. Vogelstein, Anal. Bbchem.. 84, 438 (1975). (5) J. Rebek and D. Feitler, J. Am. Chem. Soc., 95,4052 (1973). (6) J. Rebek and D. Feitler, J. Am. Chem. Soc., 96, 1606 (1974). (7) L. Dorman, Bbchem. Biophys. Res. Common.,60, 318 (1974). (8) J. C. Sheehan and G. P. Hess, J. Am. Chem. SOC.,77, 1067 (1955). (9) D. F. DeTar, R. Silverstein. and F. F. Rogers, Jr., J. Am. Chem. Soc.. 88, 1024 (1966). (10) L. Corley, D. H. Sachs, and C. B. Anfinsen, Biochem. Biophys. Res. Common.,47, 1353 (1972). (11) K. Esko, S. Karlsson, and J. Porath, Acta Chem. Scad., 22, 3342 (1968). (12) M. A. Tilak and C. S. Hollinder, TetrahedronLet?., 1297 (1968). (13) R. B. Merrifield, A. R. Mitchell, and J. E. Clarke. J. Org. Chem., 39, 660 /197A\ I . _ . .,.
(14) J. M. Stewart and J. D. Young, “Solid Phase Peptide Synthesis”, Freeman, San Francisco, Calif., 1969, p 24. (15) W. M. Monahan and C. Gilon, Biopolymers, 12, 2513 (1973).
RECEIVEDfor review May 5, 1975. Accepted July 21, 1975. This research was supported in part by the Monroe County Cancer and Leukemia Association, Inc, the Genesee Valley Heart Association, and by a Public Health Service Contract (Number N01-CP-45611) from the National Cancer Institute.
Determination of Gasoline Octane Numbers from Chemical Composition Mark E. Myers, Jr., Janis Stollsteimer, and Andrew M. Wims Analytical Chemistry Department, Research Laboratories, General Motors Corporation, Warren, Mich. 48090
The research and motor octane numbers (RON and MON, respectively) of a gasoline are measures of its quality of performance as a fuel. Octane number (rating) is affected by the isoparaffin, aromatic, lead (tetraethyl lead, TEL, and tetramethyl lead, TML), sulfur, and olefin contents of gasolines. The octane number is conventionally determined on a test engine by comparing the test gasoline with standard mixtures of 2,2,4-trimethylpentane (isooctane) and n-heptane (I). The octane number of a gasoline may range from that equivalent to isooctane (octane number of 100) to that of n-heptane (octane number of 0). This engine testing of fuels is somewhat expensive and time consuming. The purpose of this investigation was to develop a correlation between octane number and readily measurable characteristics of a gasoline (determined by conventional chemical instrumentation) using linear regression analysis. Such a method is of particular value when only a limited amount of gasoline is available. For engine testing, a 4000ml sample is normally used, but only 100 ml is required by the method to be described.
VARIABLES AFFECTING OCTANE NUMBER Isoparaffins. It has been known for many years that a close correlation exists between the amount of branching in the structure of a paraffinic hydrocarbon and its octane
number. The role of branching is implied by the choice of the two pure hydrocarbons which define the extremes of the octane scale, and is further confirmed if one looks a t the experimental octane numbers of other pure hydrocarbons (2). A measure of the amount of branching is the “isoparaffin index”, which is the measured ratio of CH3:CHz in the paraffins. For 2,2,4-trimethylpentane, this ratio is 5.0 and for n-heptane it is 0.4. The CH3:CHz ratio for an unknown paraffin or for a mixture of paraffins can be determined experimentally by nuclear magnetic resonance (NMR) spectrometry. A typical NMR spectrum of a gasoline is shown in Figure 1. Also shown are the six principal spectral regions for gasolines and the types of protons they represent. The locations of these regions are indicated as parts per million (ppm) chemical shifts from the resonance of tetramethylsilane (TMS), a material added as an internal reference. The two regions of primary interest here are those due to methylene protons and methyl protons in paraffins. The integrals of these regions are referred to as E and F , respectively; F / 3 is proportional to the total number of methyl groups and E/2 is proportional to the total number of methylene groups. T h a t is, F / 3 = k (total number of CHs groups)
(1)
E / 2 = k (total number of CH2 groups)
(2)
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
2301
I f li
t
Table I. 1974 Automotive Gasolines Analyzed
A-/-----diik,: Spectrum '"Tegra'\
I 9
I-*
I
7
5
6 -Chem
Summer
sample No.
sample N o .
Regular Premium Sub regular Super premium Unleaded Low lead
1 to 15 15 to 29 29
36 to 49 49 to 62 76 77 62 to 76
30
31 to 34 34,35
k'-C+D+E+Fd
+B+
I 8
Winter Grade
3
4
2
t
0
caI Sh f t From TMS lupml
Figure 1. High resolution NMR spectrum of a premium grade gasoline
with the least precision. Thus, olefin concentration was ignored in the linear regression analysis. EXPERIMENTAL
Proton type
Chemical shift region
Ring aromatic Olefin cy-Methyl Methine (paraffins) Methylene (paraffins) Methyl (paraffins)
6.6 4.5 2 .O 1.5
to 8.0 ppm ( A )
to to to 1.0 to 0.6 to
6.0 ppm ( B ) 3 .O ppm (C) 2.0 ppm (D) 1.5 ppm ( E ) 1.0 ppm ( F )
Therefore, CH3:CHz = 2F:3E
(3)
One would expect the overall methyl to methylene ratio for the paraffins in gasolines to be between 5.0 and 0.4, which corresponds to the ratios for 2,2,4-trimethylpentane and n-heptane, respectively. The CH3:CHz ratio, measured by NMR, is thus taken as a principal entry in an equation to calculate octane number. This variable will be shown later to have high statistical significance in determining such an equation by linear regression analysis. Aromatics. The octane rating of a base stock gasoline is primarily due to its isoparaffin and aromatic content. By increasing the aromatic content, the blender can increase the octane quality to the required level. Aromatics are used because their individual octane ratings tend to be high ( 2 ) , typically 110 RON and 100 MON. Therefore a term for the aromatic content is the second important entry in the equation. Lead. Alkyl lead in the form of TEL and TML is added to gasolines to raise the octane numbers. They have been used since the early 1920's because only a few grams per gallon raises the octane rating by five to fifteen points. Hence, lead content is the third important entry in the equation. Sulfur. The average sulfur level of -0.02% by weight found in most gasolines significantly lowers the effectiveness of the alkyl lead additives ( 3 ) ,thus requiring the introduction of sulfur concentration as another variable. O t h e r Possible Variables. The density and the Reid vapor pressure were not considered to be important variables. The density varies at random with no obvious correlation to octane rating. The Reid vapor pressure usually is approximately constant for the winter and summer gasolines taken as groups. Another possible contributor to the octane number would be the olefins. The influence of the olefins on octane number is less certain; some olefins have high octane numbers (go's), while others have extremely low octane numbers (20's)-with a wide range between ( 2 ) . Of the three hydrocarbon types (paraffins, aromatics, and olefins), olefin concentration is usually relative low (-lo%), which is fortunate because the olefin group is determined 2302
Automotive Gasolines. For this study, 77 gasolines were analyzed that represented most of the various brands and grades available in the Detroit, Mich., area; they included 35 winter-grade gasolines (January 1974) and 42 summer-grade gasolines (July 1974). These samples are listed by grade in Table I with a reference number. Analytical Conditions. NMR. The volume percent aromatics present in each sample was obtained from the NMR integral spectra according to a method reported previously ( 4 ) . (This same information can also be obtained by the conventional FIA method (5)). Sample Preparation. The gasolines were diluted in deuterated chloroform to a concentration of about 30 volume %. Tetramethylsilane (TMS) was added as a chemical shift reference and for field locking; the concentration of TMS, about 3%,is not critical. Spectrometer Conditions. The determinations were performed on a Varian Model HA-100D NMR spectrometer. The absorption and integral spectra were obtained in separate scans from 0 to 10 parts per million (pprn) from TMS a t room temperature. The spectrometer operating conditions were the same as reported previously ( 4 ) . X - R a y Fluorescence and Atomic Absorption Spectrometry. X-Ray fluorescence was used to determine lead in concentrations 10.1 g/gal. Lead was determined in the remaining gasolines by atomic absorption spectrometry. X-Ray fluorescence was also used to determine the sulfur concentration. The results were obtained down to 0.004% (by weight), the lower limit of the method. Coulometric methods were used to check sulfur values
