PARAFFIN HYDROCARBONS Correlation of Physical Properties ALFRED W. FRANCIS
The boiling points, densities, and refractive indices of all possible octanes, nonanes, decanes, and undecanes have been calculated from the same properties of the next lower paraffins (precursors) from which they can be derived by substitution of a methyl group for a hydrogen atom by several defined modes. The agreement between the separate calculations of the same property, and between these and the observed values when available, is sufficiently good to justify the selections of means as properties of unknown isomers. In some cases these correlations suggest revisions in observed properties. Paraffin isomers with two branches on nonadjacent carbon atoms have almost identical boiling points. This is probably true also of isomers with two branches on adjacent carbon atoms.
Socony-Vacuum Oil Company, Inc., Paulsboro, N. J.
but with a judicious choice of the modes of change it holds fairly well. HE only change in structure here considered is the inTcrease in molecular size resulting from the substitution of a methyl group for a hydrogen atom (or in one case the intervention of a -CH*-group between two carbon atoms). The paraffin isomer so formed and its precursor are thus still corresponding homologs, though in a more general sense than in the earlier paper. Almost every isomer except the normal one in each group can be derived from two or three or even four precursors, so that a multiple check is possible on estimated values. Moreover, consistency in properties with those of higher paraffins of which an isomer is a precursor furnishes a further check. The several modes of formation selected are defined in Table I, and for clarity skeleton structures are given. Although somewhat arbitrary in some cases, these modes are logical; they are subdivided so that all changes in one class result in increments in physical properties which are reasonably consistent with one another. Further subdivision would increase this concordance slightly but would diminish the number of cases in each class. As more experimental data became available, the necessity for further subdivision may become apparent. The form of these modes indicates some factors in the effect of structure on physical properties which may apply to other organic compounds. The modes presented are sufficiently comprehehsive to cover all but 11 changes (out of 902) in the formation of heptanes, octanes, nonanes, decanes, and undecanes. The properties computed are boiling point, density, and refractive index, since they are generally reported experimentally and a means of comparison is thus afforded. Molal volumes and refractivities could be derived, but the relations between the directly observed properties are just as simple and theoretically sound. Other properties such as aniline point, octane number, and expansion coefficient can be estimated from the three presented by methods similar to those given previously (6). The increments in physical properties between nonanes and decanes, for example, are much less than those between hexanes and heptanes. For comparison of modes of formation in different groups, therefore, it is logical to use the ratio of any such increment to that of the corresponding increment for the normal isomer whose properties are known with adequate accuracy. Although such a method of adjustment would nearly compensate for decreasing increments among the higher members, it is not precise; and the accuracy can be increased by using as ratios of increments, not constants, but smooth functions of molecular size as recorded in Table I1 and Figure 1. Identical ratios are suitable for both density and index of refraction; but those for boiling point are quite different. The plots of the former are nearly straight lines sloping away
FORMER paper (6) presented empirical relations between different physical properties of the same paraffin hydrocarbon, and between the same physical properties of various paraffin hydrocarbons. Omitting (in most cases) the first member of each series of “corresponding” homologs, the boiling points, densities, and refractive indices showed good agreement with simple equations formulated for the series. The equations were used to suggest revisions in certain observations which are possibly inaccurate, and to estimate the properties of a few unknown isomers. The scope of these revisions and estimates was limited t o members of corresponding series (mode 1and parts of modes 2 and 9 in the tables of the present paper) ; this excludes most of the unknown isomers and many of those whose recorded properties are unreliable. The same limitation in scope applies to relations devised by other authors (1, 8, 6). I n fact, none of them attempts to evaluate more than twelve of the thirty-five nonanes or of the seventy-five decanes. Huggins (7) does give a function of molal refraction in terms of structure, but since this property involves density, it is useless in estimating any other property of an unknown isomer, unless the isomer belongs to one of a corresponding series. The present paper transcends this limitation, so that the properties of all paraffin isomers can be correlated; the recorded ones have been tested for some additional measure of reliability, and the unknown ones estimated. The hypothesis is that the same change in structure in a portion of a paraffin molecule produces substantially the same change (suitably expressed) in physical properties, regardless of the remainder of the molecule. This, of course, cannot be true precisely;
A
442
INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1943
443
OF HIQKER HOMOLOQS FROM NEXTLOWER ONES TABLE I. MODESOF FORMATION
(Italic C is the new carbon atom; R represents alkyl radicals, the same or different, which are constant during the change; carbon atoms marked with an asterisk may have additional branches) 1. Lengthening of n-propyl or longer R-C-C-C-C group 2. Lengthening of ethyl group atR tached to tertiary or quaternary I carbon atom R-C*-C-C-C 3. Substitution of ethyl for methyl group which is C a. One of two methyl groups attached to a tertiary carbon atom I R-C-C-C b . One of two or more methyl R groups attached to a quaternary I rarbon atom R-C-C-C
6. Attaching methyl group to secondary carbon atom in 3- or further position
tertiary tertiary
atoms are quaternary but neither is tertiary R C
I I R-C-C-C-R 1
I
or
C
R4-C-C R C
a. With no branch on adjacent carbon atom, and no quarternary carbon within two atoms
I
I
1
R-C-C-C R C
c. Ethyl group attached to qua-
ternary carbon atom
.
R-C-C-C
Attaching a second methyl group to a carbon atom in 2- position a. The 3-carbon atom is secondary, the 4-carbon atom is not quaternary
I
R
R 4 - C - CI C R C
I
R
l
l
R-C-C--C-R
I R
C
I
R-C-C-C--R
I c
