T h e variation of boiling point with the number of carbon atoms has been determined for ten homologous series of mononuclear aromatic hydrocarbons. The equation for this relation is in the form: t = a log (n, 4.4) k. The variation of the difference in boiling point between an aliphatic compound and its phenyl derivatives with the number of carbon atoms has been determined for nine homologous series. The equation is in the Form: 6 = p q log n,. Comparisons have been made among phenylalkanes, phenylalkenes, and phenylalkynes; phenylalkanes and phenylcyclanes; 1-phenyl and Pphenyl compounds, and monophenyl and polyphenyl compounds.
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6 UNIVERSAL OIL PRODUCTS COMPANY, CHICAGO, ILL,
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points for nine series gave substantially a straight line when plotted against the logarithm of the number of carbon atoms in the aliphatic hydrocarbons, irrespective of branching. This relation may be expressed by an equation of the form:
HE boiling points of mononuclear aromatic hydrocarbons were recently collated and critically evaluated ( 3 ) . The relation between boiling point and structure will be treated here. Boiling points of a wide variety of hydrocarbons may be calculated by the use of equations and “boiling point numbers” developed by Kinney (5, 6, 7) or by similar relationships set forth by Klages (8). A different approach was used by Egloff, Sherman, and Dull (4) who determined the relation between boiling point and number of carbon atoms for thirty-one homologous series of aliphatic hydrocarbons. Iiinney’s and Klages’ methods are capable of application to a greater number of compounds; Egloff’s method generally yields more accurate results for those compounds which can be included in a homologous series. The latter method was used in this study. The data for boiling points of aliphatic hydrocarbons are well represented by an empirical equation (4)of the form: 1 = a log (nof 9)
2 = p i- q log 125
where 8 = boiling point difference ne = number of carbon atoms in aliphatic hydrocarbon The only marked deviation from this relation occurs when a tertiary carbon atom is present. An objection to this method is that unsymmetrical branchedchain hydrocarbons with the phenyl group a t one end would have the same calculated boiling point as the compound with the phenyl group a t the other end. For example,
c
C
CsH6-C-k-C-C and CSHS-C -C--&-C 1-Phenyl-2-methylbutane I-Phenyl-3-methylbutane are derived from the same aliphatic and would have the same calculated boiling point. Logically their boiling points ’R ould be expected t o differ. Few boiling points have been reported for compounds of this type, but available data indicate that the boiling points are not widely separated. The difference would probably increase with the length of the chain, but the only data available are on short-chain compounds. For these short branchedchain compounds the difference between the boiling point of the aromatic and that of the aliphatic is nearly the same as in thc case of the straight-chain compounds, and is usually within the limits of experimental error. Until more data are available, the inclusion of branched-chain phenyl-substituted hydrocarbons with the straight-chain compounds seem justified.
4- k
where t = temperature no = number of carbon atoms For thirty-one homologous series, including both saturated and unsaturated aliphatics, the constants a and b did not vary, while constant k differed from series to series. The values for a and b were: a = 745.42, 6 = 4.4. A similar study of boiling points of mononuclear aromatic hydrocarbons showed that an empirical equation of the same form could be used. The value of 4.4 for constant b fits the available aromatic data for all series except one. The value of constant a differs widely from one series to another, depending on the position of the phenyl group or groups, and on the position of the unsaturation in phenyl-substituted alkenes and alkynes. Constants a and IC were evaluated for ten homologous series by the method of least squares. The value of 4.4 was used for constant b in all series except one Later work may indicate that some other value should be assigned to this constant, but the paucity of data available a t present makes it impossible to determine whether any other value would fit the data better. Phenyl-substituted straight-chain alkanes, alkenes, alkynes, and phenyl-substituted cyclanes were the only types of compounds for which sufficient data were available to calculate an equation for a homologous seiies. I n an effort to include some branched-chain aromatics in the study, the differences between the boiling points of aliphatic compounds and the same compounds with phenyl substitutions were investigated. The boiling points of the aliphatic and alicyclic compounds used in this study were taken from Egloff (2). The boldface or “best” values were used whenever they were given. The differences in boiling
EQUATIONS AND GRAPHS
Table I lists the constants of equations for boiling point difference of a number of series. Figures 1, 2, and 3 show observed and calculated boiling points plotted against log (no 4.4); the boiling point differences are plotted against log nzswhere n, represents the number of carbon at,oms in the side chain. The graph of boiling points of toluene, diphenylmethane, triphenylmet’hane, and tetraphenylmethane plotted against the 4.4 showed a definite curvature. Hovever, logarithm of ne when the boiling points were plotted against the logarithm of n,, substantially a straight line resulted. The agreement, of the observed and calculated values is good in some series and poor in others. The deviation is usually greater in t,he higher members of a scries, probably because of the inaccuracy of the data for these compounds. The inaccuracy and scarcity of data for all except the first few members of a series are accounted for by the increased molecular weight and consequent high boiling points. This problem is not encountered %-it11the aliphatic hydrocarbons. I n many cases only one or two boiling
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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157
I
2 50 24 0
2x)
:E I80
I70
I60
I30 I50
low-I
I20
Ii O 0 I
6
7
8
Figure 1.
12 NO.OF C A R B O N A T O M S
9
IO
II
13
14
15
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3
0
Boiling Points and Boiling Point Differences for: I-Phenylalkane, and A I-Phenylcyclane Series
points for a compound are available, and critical evaluation cannot be carried out. Boiling points of many of the higher members of a series are reported a t reduced pressures, but these data are not consistent enough to permit their use in calculating an equation for relating boiling point l o structure. The poor agreement and brevity of most of the series make i t impossible to extrapolate the equations with any degree of confidence or to make comparisons between series on the basis of experimental values. However, comparison of the calculated values for the different series shows some trends. TRENDS IN CALCULATED VALUES
Change from a single to a double bond in the 1-position of both 1-phenyl and 1,l-diphenyl compounds causes a rise in boiling point (Figures l A , 2A, and 3 A ) . The increment decreases with increasing carbon content in the 1,l-diphenyl series. A 1-phenyl compound with a triple bond in the 1-position has a higher boiling point than the corresponding phenylalkene (Figure 2 A ) . The increment decreases with increasing carbon content. Replacement of a single bond by a double bond in the position farthest from the phenyl group in 1-phenyl compounds, on the other hand, lowers the boiling point; the increment increases with increasing carbon content (Figures 1A and 2-4). The differencein effect on the boiling point of position of double bond may be explained by comparison with the effect of unsaturation in the aliphatic hydrocarbons. Here the effect of a single unsaturation is to lower the boiling point, as is the case in the
4
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8 91011
NO OF CARBON ATOMSIN A L I P H A T I C
0 2-Phenylalkane,
aromatics when the double bond is not beta to the benzene ring. Alkadienes with conjugated double bonds, on the other hand, have higher boiling points than the corresponding alkanes (1). This is comparable to the aromatic compounds with a double bond in the position beta to the benzene ring:
c=c-c
0
Here the double bond in the side chain is conjugated with the double bonds in the benzene ring, and the boiling point is raised.
TABLE r. CONSTAXTS OF EQUATIONS Series I-Phenylalkane %€'hen> lalkane 1-Phenyl-1-alkene 1-Phenyl-(n 1)-alkene 1-Phenyl-1-alkyne 1 1-Diphenylalkane 1:n-Diphenylalkane 1,l-Diphenyl-1-alkene Phenylcsclane n-Phenylmethane
-
a
Boiling Point a k 712.38 679.54 537.36 699.81 527.78 406.87 617.38 498.20 776.30 576.2F
I n this equation n c was used instead of nc
+ 4.4.
B.P. Difference P
P
INDUSTRIAL AND ENGINEERING CHEMISTRY
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W . OF CARBON ATOMS Figure 2.
NO, OFCARBQN ATOMS IN ALS PHATlC
Phenylcyclanes have higher boiling point3 than the correspondIng phenylalltanes; t,he increment iric CY with the sizc of the ;ing (Figure 1A). h shift of t,lie phenyl group Erom tlic 1- l o thc 2-position on an alkane lowers the boiling point. Thc increment increases with increasing chain length (Figure I d ) . Data on compounds with the phenyl group in positions other thmi 1- niicl 2- are too fev- to permit comparison. Equations for boiling point incrcmerits between series were worked out by subtracting the calciilated boiling points for one series from the corresponding c:ilculnted values of another serics. The increments thus obtained were correlated with the number of carbon atoms by the method of least squares: I-phenylalliane -+ 1-phcnyl-1-alkcnc 207.68 - 173.01 log ( n c 4.4) At 1,I-diphenylalltaiie --+ 1,l-diphenyl-1-alkene At = 98.20 log (n, 4.4) - 119.385 1-phenylalkane +I-phenyl-1-alkyne At = 231.87 - 185.03 log (n,f 4.4) 1-phenyl-1-alkene --+ 1-phenyl-1-alkyne At = 23.85 - 11.58 log (n,f 4.4) I-phenylalkane --+ I-phenyl-(n - 1)-alkene At = 10.54 - 12.69 log (no 4.4) 1-phenylalltane --+1-phenylcyclane At = 63.65 log ( n c f 4.4) - 82.60 I-phenylalliane --+ 2-phenylalltane At = 29.52 log ( T Z ~ 4-4.4) - 24.21
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8 I-Phenyl-I-alkene,
-- ---
Boiling Points and Boiling Point Differences for:
A I-Phenl-?-alkyne, and
Vol. 38, NO. 2
1-Phenylalkane Series
l-Phenyl-I(n
- 1)-alkene,
The effect of introducing phenyl groups into hydrocarbons is, of course, to raise the boiling point because of the increase in molecular R-eight. The increase in molecular weight resulting from addit,ion of a phenyl group precludes the possibility of comparing a monophenyl compound with the corresponding polyphenyl compound on the basis of the total number of carbon atoms. When the boiling point increments are plotted against thc logarithm of 4.4 plus the number of carbon atoms in the alkane or alkene to xhich the phenyl gIoups are attached, a straight line results (Figures l A , 2A, and 3 A ) . Equations for boiling point increments folloir. In these equations ne refers to the nuinber of carbon atonis in the aliphatic chain: I-phenylalkane --+ 1,l-diphenylallianc At = 324.23 - 237.15 log (ne 4.4) 1-phenylalkane + 1,n-diphenylalkanc At 278.71 - 157.79 log (n, 4.4) 1-phenyl-1-alkene +1,l-diphenyl-1-nlkenc At = 204.48 - 108.32 log (ns 4.4)
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The effect of adding a phenyl group t o an aromatic compound cannot readily be compared with the effect of adding a phenyl to an aliphatic compound because the increment in the latter case is linear with respect to t,he logarithm of no,but not linear with respect to nc 4.4. Qualitatively, the effect on the boiling point' of adding the first phenyl group is much greater than the effcct of further addition of phenyl groups. The data for polysubstituted benzenes are too inconsistent to be used for any quantitative relatione. Only a few general con-
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
159
430
390
230 NO.OF CARBON ATOMS
Figure
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I 2
3
I
I
I
I
4
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6
1
W . O F C A R B O N ATOMS I N ALIPHATIC
3. Boiling Points and Boiling Point Differences for: 0 l,l-Diphenylalkane, A Ill-Diphenyl-I-alkene Series
i,n-DiphenyIalkane, and
clusions can be drawn. For compounds of eight and nine carbon atoms, the boiling point is higher for hydrocarbons with more than one substitution than for compounds of the same molecular weight with only one substitution. For example, the xylenes have higher boiling points than ethylbenzene has. Above Cg the boiling points are, in general, lower in disubstituted compounds than in their monosubstituted isomers. Trisubstituted compounds are, almost without exception, higher boiling than disubstituted compounds of the same molecular weight. A further increase in the number of substituents usually continues to raise the boiling point. I n the disubstituted compounds the effect on the boiling point of the relative position of the substituents is not consistent. In almost every case the metacsubstituted compound has a lower boiling point than either the ortho or para derivative, but with di-n-propylbenzene and methylisopropenylbenzene the para compound is lower boiling than the meta. The ortho and para compounds are variable. I n some instances (especially the lowermolecular-weight hydrocarbons) the ortho compound has a higher boiling point than the para; in other instances this is reversed. LITERATURE CITED
(1) Boord, C. E., in “Scienoe of Petroleum”, Vol. 11, pp. 1349-56, London, Oxford Univ. Press, 1938. (2) Eeloff,G., “Physical Constants of Hydrocarbons”, A.C.S. Monograph 78,Vol. I (1939),Vol. I1 (1940). (3) Ibid., Vol. 111,in press. (4) Egloff, G., Sherman, J , and Dull, R. B., J . Phys. Chem., 44, 730 (1940). ( 5 ) Kinney, C. R., IND. ENG.CEEM.,32,559 (1940). (6) Ibid., 33, 791 (1941). (7) Kinney, C. R., J . Am. Chem. Soc., 60,3032 (1938) (8) Klages, F., Ber., 76, 788 (1943). PRESENTED on the program of the Division of Petroleum Chemistry of the 1945 hfeeting-in-Print, AMERICAN C m n r I c A r . SOCIETY,
Catalytic Polymerization Unit of Universal
Oil Products Company
