Relation of Properties to Molecular Structure for Petroleum

Relation of Properties to Molecular Structure for Petroleum Hydrocarbons CECIL E. BOORD

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: January 1, 1951 | doi: 10.1021/ba-1951-0005.ch030

The OhioStateUniversity, Columbus, Ohio

Two methods of approach are used in considering the relationship of properties to molecular structure for hydrocarbons: to observe the changes produced as one moves from point to point (1) along the subseries curve in the direction of increasing chain length, and (2) along a cross -sectionalline at any given carbon atom level. The actually measured values of the property under investigation or the magnitude of the deviation from corresponding point on the curve for the normal isomer may be observed. Chain length, chain branching, methyl groups, double bonds, and aromatic nuclei play a dominant role in determining the properties of a hydrocarbon. A careful analysis of the changes brought about by varying these elements of molecular structure leads to the conclusion that the properties,both physical and chemical, are a function of the electron distribution within the hydrocarbon molecule. Much has been accomplished in the synthesis, separation, purification, and careful determination of both physical properties and engine characteristics. Much still remains to be done, but the work is progressing. There still lie ahead a more careful correlation of chemical properties with structure,and a study of the relative rates of reactivity for the hydrogen atoms on the different parts of the hydrocarbon molecule and the effects which these differences impose upon the velocity of combustion.

Relation of Physical Properties and Chemical Constitution" was the title of a book published i n 1920 by Kauffmann (20). I t lists the freezing and boiling points of the normal paraffins and records the increments of rise with the addition of each methylene group. The same year Thomas Midgley (26) observed wide differences in the combustion of fuels i n internal combustion engines. The differences were found not only in different classes of hydrocarbons but also between isomeric hydrocarbons of the same class. The following year Ricardo (27) published the results of investigations on the highest useful compression ratio of a number of hydrocarbons and found a wide variation among different fuels i n this respect. Since that time the relations between physical properties of hydrocarbons and their molecular structure on the one hand, and the knocking character istics of the same hydrocarbons and their molecular structure on the other, have received more intensive study. Graham Edgar (12) i n 1927 proposed the use of mixtures of η-heptane and 2,2,4trimethylpentane, commonly known as isooctane, as standards for rating fuels for knock. Two years later the same investigator, together with Calingaert and Marker (13), made a 353

In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.


354

ADVANCES IN CHEMISTRY SERIES

NUMBER OF CARBON ATOMS comprehensive study of the synthesis and characterization of the isomeric heptanes. B y 1931 Shepard, Midgley, and Henne (31) had isolated, from a paraffinic gasoline, the normal paraffins from pentane to dodecane, inclusive, i n quantity and high p u r i t y ; and they had carefully determined the physical properties. The petroleum refiners were not idle. Around the turn of the century C . F . M a b e r y (26) had amply demonstrated the fundamental hydrocarbon nature of petroleum o i l . I n 1927 the American Petroleum Institute established at the National Bureau of Standards a research project for the purpose of separating and identifying the component parts of a typical mid-continent crude (29), This project, commonly known as A . P . I . - R . P . 6— first under the leadership of E . W . Washburn and, after his death, under the direction of F . D . Rossini—had by the end of 1948 separated and identified 91 hydrocarbons from the selected crude. The work i n the low boiling range has been completed and is continuing on the hydrocarbons of the high boiling range. The separation and identification of the components of a typical crude are not the sole accomplishments of D r . Rossini and his associates. The development and perfection of methods for the separation and purification of hydrocarbons and the establishment of criteria of purity have been of equal value CM). The automotive engineers were also busy. I n 1931 Lovell, Campbell, and B o y d (23) published data on an extended series of pure paraffin and olefin hydrocarbons in fairly dilute solution in gasoline. These data showed a number of definite relations between the molecular structure of the pure hydrocarbon and the tendency of the fuel to knock. The In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.

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BOORD—RELATION OF PROPERTIES TO MOLECULAR STRUCTURE

following year Garner and his associates (16), a group of British investigators, published similar data on blends of olefins and on naphthene and aromatic hydrocarbons. I n 1934 Lovell, Campbell, and B o y d (24) published extensive information on about 100 hydrocarbons i n the pure state. The petroleum refiner, by the mid-thirties, was faced with two problems: (a) W h a t hydrocarbon constituents did his gasolines and low boiling naphthas contain? and (b) What were the knocking characteristics of each of these constituents? It was i n this atmosphere that the American Petroleum Institute Hydrocarbon R e search Project, later to be known as A . P . I . - R . P . 45, was established i n 1938 (2). N o w after 13 years of operation, 298 pure hydrocarbons, as well as other products, have been tested, some under as many as 29 sets of engine conditions. Two hundred forty-six of these products were synthetically produced and/or purified i n the project's Hydrocarbon Laboratory at the Ohio State University. So acute was the need for accurate data on freezing point, boiling point, refractive index, density, and the spectrographic fingerprints of pure hydrocarbons that two new projects were set up. These included A . P . I . - R . P . 44, a project for the collection, evaluation, and distribution of physical, chemical, and thermodynamic data (1), and A . P . I . - R . P . 46, a project for the preparation and distribution of standard spectrometric samples (3). A . P . I . - R . P . 45 made substantial contribution to these new ventures.

2-M ETHYL ALKANES

FIGURE II MELTING POINTS N-ALKANES AND 2- METHYLALKANES

10 NUMBER OF CARBON ATOMS Just as the improvements in our knowledge of the physical properties and performance of the hydrocarbon constituents of gasoline enable us to make better gasolines, so would an extension of our knowledge of the properties and performance of lubricating oils enable us to make better lubricating oils. T o provide such data A . P . I . - R . P . 42 was established at Pennsylvania State College i n 1944 (4), first under the leadership of the late F . C . Whitmore and more recently under the direction of Robert Schiessler. These cooperative programs, with the generous support and contributions from the research laboratories of the petroleum and automotive industries, are filling i n a scientific background against which these industries can view their technical advances. Petroleum refiners can now evaluate their refining, cracking, and reforming processes scientifically upon the basis of the quality of the products these processes produce. A n d if they are sufficiently ingenious they can devise semisynthetic and total-synthesis methods for the production of specialty products. These refiners no longer operate by rule of thumb. In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.

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Table I.


Hydrocarbons

Series Deviation

Normal Value

Melting point deviation, ° C . Butanes -138.33 Pentanes -129.72 -95.32 Hezanes -90.60 Heptanes -56.80 Octanes -53.60 Nonanes

+8.61 +34.2 +4.72 +33.8 +3.2

Boiling point deviation, ° C . Butanes -0.50 Pentanes 36.07 Hexanes 68.74 Heptanes 98.43 125.67 Octanes 150.80 Nonanes

+36.57 +32.67 +29.69 +27.24 +25.13

(-)

Deviation of Physical Constants 3-Me

4-Me

-21.27 -30.17 -58.36 -27.68 -52.24 -26.8

-63.70 -53.9

-64.16 -59.6

-11.23 -8.22 -8.47 -8.38 -8.02 -7.66

-5.46 -6.48 -6.69 -6.62

-7.95 -8.32

-0.00405 -0.00341 -0.00274 -0.00250 -0.00239

+0.00166 +0.00101 +0.00057 +0.00007

+0.00047 +0.0006

-0.02160 -0.00657 -0.00622 -0.00503 -0.00470 -0.00430

+0.00496 +0.00432 +0.00322 +0.00300

+0.00203 +0.00220

2-Me

Refractive index deviation, n ° 2

Pentanes Hexanes Heptanes Octanes Nonanes

D

1.35748 1.37486 1.38764 1.39745 1.40549

+0.01738 +0.01278 +0.00981 +0.00804

Density deviations, d j ° Butanes Pentanes Hexanes Heptanes Octanes Nonanes

0.5788 0.62624 0.65937 0.68368 0.70260 0.71770

(-)

+0.04744 +0.03313 +0.02431 +0.01892 +0.01510

Molecular structure i n organic chemistry is based upon the two fundamental principles of homology and isomerism. The hydrocarbons, the simplest of organic derivatives, are divided into two great groups, cyclic and noncyclic i n structure. The paraffins form the basis of the noncyclic group, inasmuch as the monoolefins, diolefins, and acetylenes may be regarded as derivatives of the parent hydrocarbon having the same arrangement of carbon atoms. I n like manner the cycloparaffins form the structural basis of the cyclic group. Cycloolefins, -diolefins, and -triolefins may be regarded as derivatives of the cycloparaffins having the same carbon skeleton. The aromatic series forms a particular case among the cyclohexatrienes, i n which the three double bonds are spaced symmetrically around a sixatom carbon ring. The term homologous series is used both generically and specifically. When one speaks of the paraffin series the term is being used generically. Such a concept does not express the whole truth. The normal paraffins form a simpler and more logical series, whereas the 2-methylalkanes, 3-methylalkanes, and 2,2-dimethylalkanes each form analogous series having a characteristic structural group. The paraffins are therefore to be regarded as a collection of many subseries, the collection growing more numerous and the structure more complex as the number of carbon atoms is increased. When the refractive indices of the paraffins are plotted against carbon content, a compact family of curves may be drawn, each representing a characteristic type of structure. Figure 1 shows the simpler paraffins plotted i n this manner. When one follows any one of these subseries curves from point to point, i n the direction of increasing carbon content, there is a n intensification of physical properties. T h e structural changes are those characteristic of homology and involve an increase i n both chain length and molecular weight. When one moves from point to point along the indicated cross section A-B i n Figure 1, the structural changes are those of isomerization. T h e isomerization m a y involve either a change i n the position of the side chain along the principal chain or an increase in the number of side chains. I n isomerization there is no change i n the carbon content or molecular weight, but a n increase i n the number of side ohains is accompanied b y a diminution i n the principal chain length. A n y study of the relation of physical constants or engine characteristics of a hydrocarbon to its molecular structure will, therefore, be made b y observing the changes produced either by moving from point along the subseries curve, increasing or decreasing the In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.


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of Normal, Methyl, and Dimethyl Alkanes


2,2-Me*

-•Isomer Deviation2,3-Met 2,4-Mea

2,5-Me

2

3,3-Μβ2

3,4-Mez

(-) -113.12 -4.41 -33.20 -64.38 (-)

-33.19 (-) (-) (-)

-28.63 (-) (-)

(-) -34.40 (-)

-43.86 - 69.20 (-)

(-) (-)

-26.57 -19.00 -19.22 -18.82 (-)

-33.19 (-) -10.06 (-)

-17.86 -16.24 (-)

-16.56 (-)

• 12.36 -13.70 (-)

-7.94 (-)

-0.00610 -0.00547 -0.00396 (-)

+0.00009 +0.00436 +0.00383 (-)

-0.00614 -0.00454 (-)

-0.00499

+0.00326 +0.00264 (-)

+0.00227 +0.01126 +0.00964 (-)

- 0.01088 -0.00224 (-)

... -0.00905 (-)

+0.00956 +0.00740 (-)

(-)

-0.01020 -0.00982 -0.00732

(-)

(-)

+0.00673 (-)

+0.01670 (-)

principal chain length, or from point to point along a cross-sectional line, A-B. Such a cross-sectional perspective may be had at any given carbon content and will become more and more complex as the number of carbon atoms is increased. The observations will be made i n the first case b y comparing the measured experi mental values, or the increments of change as one moves from point to point along the subseries curve. The observations will be made i n the case of isomerization either b y arranging the measured values i n an ascending or descending series or b y comparing the increments of deviation from the normal or parent hydrocarbon. Sometimes one and sometimes the other method of comparison seems to be the more revealing. Since data of highest accuracy are available for only a relatively small number of hydrocarbons, all of low molecular weight, one is not able to plot the subseries curves over an extended range. After the structural group characteristic of any subseries has been fully developed i t is usually true that the order of intersection along any cross-sectional line remains the same. The relative orders for boiling points, densities, and refractive indices are the same a l though the position of the group with respect to the normal or parent hydrocarbon may be changed. These cross-sectional relations are useful since they may be shown b y tabula tion as well as b y curves and are not invalidated by missing data. Tables I to I V show the melting points, boiling points, refractive indices, and densi ties of the simpler paraffins and monoolefins tabulated i n this manner. The comparisons are made on the basis of deviations at each cross-sectional level. Tables V to V I I show the same physical constants for the aromatic hydrocarbons and a group of cis-trans isomers. I n the latter cases the comparisons are made between the actually measured values.

Melting Points It has long been known that the melting points of the normal paraffins do not fall on a smooth curve (10, 20). When the melting points are plotted against carbon content, alternately large and small increases i n the values are revealed. T w o smooth curves may be drawn, one connecting the melting points of the normal paraffins having even numbers of carbon atoms and the other connecting those having odd numbers. I n other words, the normal paraffins are separated into two series on the basis of their melting points, as shown i n Figure 2. Alternation is also characteristic of the melting points of the 2methylalkanes (Figure 2 and Table I ) , and the butylbenzenes (Table V ) . I t has fre quently been observed that the properties of the first member of an homologous series are In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.


358


anomalous. This is true for the melting points of the first members of both the odd and even series of n-paraffins. Closer examination of Figure 2 shows that the phenomenon of alternation is more a function of chain length than of carbon content, since i n the 2-methylalkane series the relative positions of the odd and even series are reversed from that characteristic of the normal series. The study of Tables I and I I has revealed the fact that isomerization of a normal paraffin t o a n isoparaffin produces certain specific and characteristic changes i n the physi cal constants as the methyl group is moved from point to point along the chain. I n a paper presented before the Refining Division of the American Petroleum Institute i n November 1942, attention was called b y this author to alternation i n the melting points of the n-octenes and π-octynes as the point of unsaturation is moved from position 1 to 2 to 3 to 4 (5). N o w that the cis-trans pairs have been separated and more highly purified, the alternation remains characteristic of both forms, although the data o n the cis forms are not fully verified. M E L T I N G POINTS O F TI-OCTENES A N D η - Ο ο τ τ Ν Ε β ,

cis 1 - Octene 2-Octene 3-Octene 4- Octene

-100.3 -126 -118.7

' C.

trans 102.4

1- Octyne 2- Octyne 3-Octyne 4- Oetyne

-87.4 -110.0 -93.7

-79.5 -62.0 -105.0 -109.9

Boiling Points T h e boiling points of the noncyclic hydrocarbons show the usual series relations. T h e increment of rise of the boiling point with increasing carbon content, for the lower members, is nearly constant, as illustrated b y the 2-methyl-, 3-methyl-, and 2,2-dimethylalkanes (columns 4, 5, and 7, respectively, of Table I ) . A s the carbon content is further increased, the increment becomes smaller, the subseries curves converging on the curve for the normal hydrocarbons. E v e r y isoparaffin boils at a lower temperature than does the corresponding normal paraffin of the same carbon content; this again shows the i n fluence of chain length. The increment of deviation from the normal isomer becomes less as the methyl group side chain is moved toward the middle of the principal chain, as is illustrated b y the 2-methyl-, 3-methyl-, and 4-methylheptanes or the 2,2-dimethyl- and 2,5-dimethyl- vs. the 3,4-dimethylhexanes (columns 4, 5, 6, and 7, 10, 12, respectively, Table I ) . Table IL Parent Hydro carbons

Normal Value

Deviation of Physical Constants of Normal Mono-olefins Series Deviation from Paraffin

2( = ) cis

- Isomer Deviation3 (= )

Melting point, ° C . Butane - 138.33 Pentane — 129.72 Hezane -95.32 Heptane -90.60 Octane -56.80

-47.02 -35.50 -44.58 -28.60 -45.6

+46.44 + 13.85 -1.32

+79.80 +24.98 +5.64

Boiling point, -0.50 Butane Pentane 36.07 Hezane 68.74 Heptane 98.43 Octane 125.67

+2:4

+l4!7

-5.76 -6.10 -5.28 -5.13 -4.40

+2.54 +7.13 +5.14 +5.2 +4.3

+5.38 +6.39 +4.44 +4.7 +3.7

Refractive index, n^>° Butane

e

e

6

(600/212° F.) 5.5 -1.55 3.95 -0.70 3.25 -0.25 3.0 -0.10 2.9 (600/350° F.) 5.3 -2.1 3.2 -0.2 3.0

+2.5 + 1.7 + 1.05 +0.65 +0.4

+0.95 +0.8 +0.5

+ 1.15 + 1.9 +0.65

+0.7

+8.7*

+0.6

····

* This datum constitutes, or is partly derived from, an extrapolated octane number based on critical compression ratios, measurement of which is not limited to the octane range as defined (0-100). . . . . 6 Calculated from the C F R Research octane number by the Army-Navy Aeronautical Board formula, P N - 2800/(127 - O N ) .

ature, are all above 10, and those at 600 r.p.m. and 350° F . , while somewhat lower, are 6.9 or higher. The blending octane number of benzene is the lowest for any aromatic hydrocarbon thus far measured. W h e n measured b y either the Research or M o t o r method i t seems to form another case where the first member of an homologous series is anomalous. The blending octane members of the first five members of the monoalkylbenzenes as measured b y the M o t o r method show alternation. T h e same is true for the four monobutylbenzenes, as the b u t y l group is telescoped on the aromatic nucleus. This is also true of their critical compression ratios (Table X I ) . The engine characteristics of aromatic hydrocarbons seem strongly dependent upon the nature of the side chain. The alignment of the knock resistance of the polymethylbenzenes with structure corresponds quite closely with that found to be characteristic of the physical constants. T h e vicinal derivatives o-xylene, hemimellitine, and prehnitene


367


Characteristics of Isomeric Alkanes -Isomer Deviation2.2-ΜΘ2 2,3-Mej


3-Et

2,4-Me

2,5-Me

2

+65.0 53.8*

+23.8 +67.0 +92.8 +92.8°

+ 115.9° +91.1 +91.6°

+69.3 +66.0

+ 18 +67.4 +95.6 +91.0°

+68.3 +88.5 +95.5

+64 + 83

+38 +70 +89 +86

+77 +87 +90

+77 +84

+73 +64

+23 +75 +93 +91

+40 +90 +91

+78 +85

+ 10.7

+23.7 +50.2 + 57.8 +31.6

+54.9 +30.5*

+40.5 +25.7*

a

a

+0.6

a

a

+ 1.65 +2.70 +3.0 + 1.5

+5 +3 + 1

+ 1.3 + 2.1

+4 45

+83.1 +82.5°

+83.8 +83.5

2

+80.8 +90.8

+75.8"

3,4-Me

a

+86.6 +86,6°

a

+ 1.9 + 1.4

3,3-Met

+

+ 19.7°

+ 1.05

2

+96

+95

+83 102.4

+88 +81

+95

+37.4 +34.4°

+35.2*

+2.0 +1.75

+1.8

have the lowest blending octane numbers and critical compression ratios, each within its own group. The symmetrical derivatives—p-xylene, mesitylene, and durene—have the highest values, each within its own group. The unsymmetrical derivatives such as m xylene, pseudocumene, and isodurene have intermediate values i n each case. One cannot help being impressed b y the dominant character of the methyl group. It would seem that when the electron release of the methyl groups is balanced across the benzene nucleus the knock resistance is increased ; this indicates that the velocity of com bustion is slowed down. O n the other hand, when the electron releases of the methyl groups supplement each other, as i n the case of the vicinal derivatives, knock resistance is decreased; this indicates that the combustion velocity is increased. A n accumulation of methyl groups either upon the side chain, as i n ferJ-butylbenzene, or upon the nucleus, as in isodurene, seems to increase the knock resistance.

Discussion I n the preceding pages, an attempt was made to develop the relation of properties to molecular structure for hydrocarbons. T w o methods of approach were used. T h e first procedure is to follow the evolution of each typical subseries of homologs from point to In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.


368

Table IX. Hydrocarbon

Paraffin Value

Research octane number Pentane 61.7 Hexane 24.8 Heptane 0 Octane —20.3


Motor octane number Pentane 61.9 Hexane 26.0 Heptane 0 Octane -13.6

Engine Characteristics of Normal Olefins 2( = )

4(-) trans

90.9 76.4 28.7

(56.3)

77.1 63.4

Blending octane number (Motor) Butane 114 126 Pentane 67 109 Hexane 22 94 Heptane 0 46 Octane —15 ratio (600/212° F.) 5.5 7.1 3.95 5.55 3.25 4.35 3.0 2.9 3.25

Critical compression ratio (600/350° F.) Butane 5.3 5.3 Pentane 3.2 4.55 Hexane 3.0 3.6 Heptane ... 3.0 Octane ... 2.85

72.5

80.8

34.7

Blending octane number (Research) Butane 113 144 Pentane 63 119 Hexane 19 97 Heptane 0 68 Octane —19

Critical compression Butane Pentane Hexane Heptane Octane

3( = )

K = )

(56.5) 155 154 (75) 130 137

80.1 68.1

152 150 134

(68) 7.9 7.05

5.95 5.4 (3.25)

5.95 5.4 4.5

101

7.0 k.25

(3.9)

99

120 85'

8.7 7.05 5.4

74.3

137 95'

128 134 129

73.3

.25

5.2 3.6

point i n the direction of increasing carbon content or chain length. The subseries curve takes up a fixed position with respect to the normal series curve, which is determined by its characteristic grouping. The second procedure is to examine i n detail the order of intersection of such a bundle of curves as one moves from point to point along a crosssectional line at any given carbon atom level. The first procedure is the one usually followed by those investigators who attempt to find a mathematical formula for calculating or predicting physical properties (14). T h e second procedure has the advantage that once the general pattern has been established, it is not too greatly disturbed by missing data. I n either case, comparisons may be made between actually measured values or between the increments of deviation from point to point. B o t h methods of comparison have their value. Some investigators use the second procedure i n making predictions and some of them use a combination of both procedures (15). N o method of prediction is quite so satisfying as having at hand experimentally determined values, measured on products of high purity. A s the experimentally determined data become increasingly accurate and as the data increase in volume, the predictions can be and are made with greater and greater accuracy. One of the most fundamental elements of molecular structure is chain length. I t serves to fix the hydrocarbon's position on its own subseries curve and thus becomes a factor i n determining its physical constants. I t also is a major factor i n determining the hydrocarbon's rate of combustion and hence its octane number and critical compression ratio. The methyl group and the double bond are two other structural elements which play a dominant role i n determining the properties of a hydrocarbon. A s a methyl group is moved from carbon to carbon atom along a hydrocarbon chain, i t alters the physical properties of the hydrocarbon. H o w does it do this? I t seems obvious that the change in the electronic pattern of the molecule, as the methyl group is moved from point to point, effects these changes. I t also interrupts the effective chain length and determines the In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.



369

principal point of oxidative attack on the hydrocarbon. The combustion velocity is altered and hence the octane number and critical compression ratio are changed. The double bond functions i n a very analogous manner. I t too interrupts effective chain length and determines the principal point of oxidative attack. I n a manner quite analogous to the methyl group, through changes i n the combustion velocity, the double bond also alters the octane number and critical compression ratio. The implications of these deductions seem to lead to the conclusion that such structural factors as chain length, methyl groups, and double bonds influence not only the physical properties but the chemical properties of the hydrocarbon as well. Hydrocarbons are limited i n their chemical reactivity. The paraffins are compounds having small affinity. The hydrocarbon molecules are armor-plated with hydrogen. Since structure plays so vital a role i n the rate of combustion, there must be a fundamental difference in the relative reactivity of the hydrogen atoms. Experimental evidence that such is the case is accumulating. There are reasons to believe that methods are at hand b y which the Table X. Hydrocarbon Structure

Olefin Type

Engine Characteristics of Branched Olefins

Research Octane No.

Motor Octane No.

Blending Octane No. (Research)

Blending Octane No. (Motor)

Critical Compression Ratio (600/212° F.)

92.3

90.3

99.0

104

5.65

5.1

129

125

8.3

5.45

133

7.7

5.45


2-Methylbutane derivatives C

c—è—c—c c

—

i

-

(volatile)

c = c

C

I 0=0—0—0

III

+0.3

81.9

IV

97.3

87.4

73.4

73.5

I

95.7

80.9

II cis

99.3

84.3

III

95.1

iv

c

c—b=c—c

6.45

2- Methylpentane derivatives C 79

4.3

3.65

108

6.8

5.5

130

128

7.7

5.5

78.9

126

114

6.75

5.15

97.8

83.0

159

148

7.05

5.25

74.5

74.3

86

81

4.2

3.7

I

96.0

81.2

113

114

7.0

5.4

III

98.3

79.4

143

129

7.0

5.4

125

113

130

118

C — C — 0 — C

83

C C—i—C—C=C C C—i—C=C—C C C—C—0=0—C

II trans

C C=è—C—C— C C

c—i=c—c—c

3- Methylpentane derivatives C 0—C—i—C—C

C—C—i—C=C C C—C—è—C—C C C—d—C=C—C

I V cis

5.C

C C—C—C=C—C

IV trans

97.2

81.0

7.15


5.0


370

Table XI. Research Octane No.

Hydrocarbon

Motor Octane No.

Blending Octane No. (Research)

Blending Octane No. (Motor)


Critical Compression Ratio (600/350° F . )

Benzene Toluene Ethylbenzene n-Propylbenzene n-Butylbenzene

+5.8 +0.8 + 1.5

+2.7 +0.3 97.9 98.7 95.3

99 124 124 127 114

91 112 107 129 117

15.' 13.5 11.9 10.2

ii-Propylbenzene Isopropylbenzene

+ 1.5 +2.1

98.7 99.3

127 132

129 124

11.9 14.5

11.35 8.2 8.7 6.9 8.7 8.9

95.3

114 122 116 138

117 118 117 127

10.3 13.5 12.5 13.2

6.9 8.6 8.25 9.8

n-Butylbenzene Isobutylbenzene sec-Butylbenzene ierl-Butylbenzene


Engine Characteristics of Monoalkyl benzenes

> +3.0

+0.8

relative reactivity pattern of the hydrogen atoms of a hydrocarbon may be determined. If this can be done with accuracy, i t should not only give us an insight into the cause of the variation of combustion velocity with structure but also form a foundation for an attack upon the stability of the carbon skeleton of the hydrocarbon. Table XII. Hydrocarbon

_

Engine Characteristics of Methylbenzenes

Blending octane No. (Research) CeHe CeH CH CeH (CH )2 (1,2) CeH (CH )3 (1,2,3) CeHî(CH )4 (1,2,3,4) 6

3

4

3

8

3

• Position Isomers Unsymmetrical

Vicinal

120 118

(1,3) (1.2,4) (1,2, 3, 5)

103 105

(1,3) (1.2,4) (1,2, 3, 5)

3

Blending octane No. (Motor) CeHe CeH CH CeH (CH ) (1,2) CeH (CH ) (1.2,3) CeH (CH ) (1,2,3,4) 6

3

4

3

2

3

3

3

2

3

4

Critical compression ratio (600/212° F.) CeHe CeH»CH CeH (CH ) (1,2) CeH (CH ) (1, 2, 3) CeH (CH ) (1.2,3,4) 3

4

3

2

3

3

3

2

3

4

Critical compression ratio (600/350° F.) CeH. CeHeCHs CeH4(CH ) (1,2) CeHa(CH ) (1,2,3) CeH (CH ) (1,2,3,4) 2

3

2

3

3

3

4

13.0 12.0

7.1 7.9

(1.3) (1.2,4) (1,2,3,5)

99 124

91 112

15

11.35 (1,3) (1.2,4) (1,2, 3, 5)

Symmetrical

145 148 154

(1.4) (1.3. 5) (1.2,4, 5)

146 171

124 124 128

1.4) (1.3. 5) (1.2,4, 5)

127 137

15.5 12.8 12.7

(1,4) (1,3, 5) (1,2, 4, 5)

15.7 14.0

11.5 8.7 8.7

(1,4) (1.3, 5) (1,2,4. 5)

11.5 10.6

The structure of cyclic hydrocarbons is very complex. The number of isomers i n creases rapidly with carbon content. Although the data at hand are extensive, many more are needed to allow a fundamental analysis of the relation between properties and structure. The aromatic hydrocarbons form a unique case of cyclic structure. T h e benzene ring, like the methyl group and double bond, exerts a powerful influence upon the properties of any hydrocarbon of which i t is a part. A l l aromatic hydrocarbons have high boiling points, high densities, and high refractive indices. They also have high octane numbers and critical compression ratios. The hydrogen atoms of the benzene ring are not readily subject to free radical stripping (8), Variations i n the engine characteristics are, therefore, chiefly due to difference i n the structure of the side chains. One of the most striking illustrations of the effect of structure upon properties for aromatic hydrocarbons is to be found i n the distribution of polymethyl groups about the benzene ring. The effect is observable i n both the physical properties and i n the engine characteristics. This effect of symmetry vs. dissymmetry, together with the increased reactivity of the hydrogen atoms on the alpha carbon atoms of side chains, seems to lend added support to the belief that these differences are effected through changes in the electron pattern of the molecule.



371


Conclusions The conclusions which one draws i n consideration of the above facts are that both the physical and chemical properties of hydrocarbon molecules are largely a function of the electron distribution. The structural elements which play a prominent role i n this distribution are chain length, chain branching, methyl groups, double bonds, and benzene nuclei. M u c h has been accomplished i n the synthesis, separation, purification, a n d careful determination of both physical constants and engine characteristics. M u c h still remains to be done but the work is progressing. There still remains to be accomplished a more careï'ul correlation of the chemical properties with structure; a study of the relative rates of reactivity for the hydrogen atoms on different parts of the hydrocarbon structure ; and the effects which these differences impose upon the velocity of combustion.

Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33)

American Petroleum Institute-R.P. 44, Chem. Eng. News, 25, 904 (1947). American Petroleum Institute-R.P. 45, Ibid., 28, 2578 (1950). American Petroleum Institute-R.P. 46, Ibid., 24, 2020 (1946). American Petroleum Institute-R.P. 42, Proc. Am. Petroleum Inst. III (1946). Boord, C. E., paper presented before Div. of Refining, 23rd meeting, Am. Petroleum Inst., Chicago, November 1942. Boord, C. E., Science of Petroleum[II],1353(1938). Boord, C. E., Symposium on Combustion Chemistry, Cleveland meeting, Am. Chem. Soc., April 1951. Cramer, P. L . , J. Am. Chem.Soc.,56, 1234 (1934); 60, 1406 (1938). Cullis, C. F., and Hinshelwood, C. N., Faraday Soc. Discussions, 2, 117 (1947). Deansley, R. M . , and Carleton, L . T., J. Phys. Chem., 45, 1104 (1941). Derfer, J . M . , private communication. Edgar, Graham, IND. ENG. CHEM., 19, 145 (1927). Edgar, Graham, Calingaert, George, and Marker, R. E., J. Am. Chem.Soc.,51, 1483 (1929); Edgar, Graham, and Calingaert, George, J. Am. Chem. Soc., 51, 1540 (1929). Egloff, Gustav, Sherman, J . , and Dull, R. B., J. Phys. Chem., 6, 730 (1940); Corbin, N . , Alexander, M . , and Egloff, G.,IND.ENG.CHEM.,38,156 (1946); Francis, A. W., Ibid., 33, 554 (1941). Francis, A . W., Ibid., 35, 442 (1943); Calingaert, George, and Hladky, J. W., J. Am. Chem. Soc., 58, 153 (1936). Garner, F . H . , et al., J. Inst. PetroleumTechnol.,18, 751 (1932). Greenlee, K. W., and Fernelius, W. C., J. Am. Chem.Soc.,64, 2505 (1942). Greenlee, K . W., and Shepherd, J . W., A . P . I . - R . P . 45, Monthly Rept., p. 3, January 1950. Hall, H. J., and Mikos, I. Α., Anal. Chem., 21, 422 (1949). Kauffmann, H. J., "Relation of Physical Properties to Chemical Constitution," pp. 135, 151, Stuttgart, Ferdinand Enke, 1920. Kistiakowsky, G. B., et al., J. Am. Chem.Soc.,57, 876 (1935). Kurtz, S. S., and Ward, A. W., J. FranklinInst.,222, 563 (1936); 224, 583, 697 (1937). Lovell, W . G., Campbell, J . M . , and Boyd, Τ. Α., IND. ENG. CHEM., 23, 26-9 (1931); 23, 558 (1931). Ibid., 26, 1105 (1934). Mabery, C. F., Am. Chem. J., 25, 253 et seq. (1900). Midgley, Thomas, Jr., J. Soc. Automotive Engrs., 7, 489 (1920). Ricardo, H . R., Automobile Engrs., 11, 51, 92 (1921). Rossini, F . D., etal.,J. Research Natl. Bur. Standards, 26, 591 (1941); 35, 355 (1945). Rossini, F. D., etal.,Petroleum Refiner, 21, 377 (1942). Schiessler, R. W., etal.,Proc. Am. Petroleum Inst., 27, III (1946). Shepard, A . W., Midgley, Thomas, Jr., and Henne, A. L . , J. Am. Chem.Soc.,53, 1948 (1931). Sherrill, N . L . , and Matlock, E . S., Ibid., 59, 2134 (1937). Smittenberg, J . , "Refractivity Intercept and Refractivity Quotient of Series of Homologous Hydrocarbons," 3rd World Petroleum Congress preprint (1951).

R E C E I V E D M a y 31, 1951.


Relation of Properties to Molecular Structure for Petroleum

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