INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1941
(5) Glasgow, J . Research Natl. Bur. Standards, 21,535 (1938). (6) Hicks-Bruun, Bruun, and Faulconer, J . A m . Chem. SOC.,61,3099 (1939). (7) Hufferd and Kranta, A. C. S,Meeting, Boston, 1939. (8) McArdle, E. H.,IND.ENQ.CHEM.,Anal. Ed., 11,450(1939). (9) McArdle, Moore, Terrell, Haines, and cooperators, Ibid., 11, 248 (1939). (10) Mair, Beveridpe, and Willingham, J. Research Natl. Bur. Standnrd* 21- , (iX.5 -. -_,-- - I1F)RR). - - - -, (11) Mair and Streiff, Ibid., 24,395 (1940). (12) Mikeska, IND.ENQ.CHEM.,28, 971 (1936). (13) Mikeska, Smith, and Lieber, J . Org. C h m . , 2,499 (1938). (14) Petrov, J . Gen. Chem. (U. 8.S . R.) 9,509(1939). (15) Philadelphia Paint and Varnish Production Club, Natl. Puint, Varnish Lacouer Assoc., Sci. sect., Circ. 546,273 (1937). (16) Ibid., 568 (1938). (17) Philadelphia Paint and Varnish Production Club, Oficial Digest Federation Paint & Varnish Production Clubs, 1939,115.
.
*
791
(18) Rossini, Proc. A m . Petroleum Inst., 111, 18,36-59 (1937). (19) Sweeney and Tilton, IND.ENQ.CREM.,26,693 (1934). (20) Tongberg, Fenske, and Nichols, Ibid., 29, 70 (1937). (21) Tongberg, Quiggle, and Fenske, Ibid., 28, 201 (1936). (22) Tongberg, Sweeney, and Fenske, Ibid., 30, 166 (1938). (23) U.8. Tariff Commission. Rept. 136,2nd series (1938). (24) Vluater, Waterman, and Van Western, J . Inst. Petroleum Tech., 21, 661 (1935). (25) Waterman. Ibid.. 25. 805 (1939). (26j Wetlaufer 'and Gregor, IND. ENG. CHEM.,Anal. Ed., 7, 290 (1935). (27) White and Glasgow, J . Research Natl. Bur. Standards, 22, 137 (1939). (28) White and Rose, Ibid., 21, 151, 167 (1938). (29) White et al., Ibdd., 22,316 (1939). PRESENTBD before the Division of Petroleum Chemistry at the 100th Meeting of the American Chemical Society, Detroit, Mich.
Calculation of the Boiling Points of Aromatic Hydrocarbons CORLISS R. ICINNEY University of Utah, Salt Lake City, Utah
T
HE method of calculating The boiling points of aromatic hydronucleus upon the boiling point boiling points from molecumay be by using the which has no precise counterpart in aliphatic structures. lar structure I'( has been boiling point equation and the boiling point applied to the aromatic hydroThe conjugation of double bonds characteristic Of the aromatic in open-chain derivatives raises carbons. The boiling point i s hydrocarbons. The method is useful in the boiling point; consequently, calculated from the boiling point number of the molecule (B. P. predicting the boiling points of new cornbenzene would be expected to pounds a s well a s checking the boiling have a higher boiling point also. N.) by means of the boiling point equation* The B**' N' is !he However, it is not possible to points of those appearing in the literature, summation of the boiling point estimate the amount. The certain Ones Of which are numbers (b. p. n.'s) of the variinB. P. N. adopted for benzene ous atomic and structural groupcorrect or have other structures. Also, of 20.0 allows only 0.8 for boiling point numbers may be used with the aromaticity of the ring. ings in the molecule. The limitations to determine the structure of This is much less than the value method, while empirical, approximates the complex forces of 2.4 found for the conjugated unknown hydrocarbons. which affect the boiling points triolefins. The b. p. n. of the of organic molecules and which phenyl group is obtained by cannot be evaluated quantitatively a t present. The relalowering the B. P. N. of 20.0 one unit for each hydrogen tive effects of varying structure upon the boiling point are atom displaced. brought out by this procedure, and consequently b. p. n.'s The following example demonstrates the method of calmay be used for structural studies in ways not possible culating the boiling point of a simple derivative of benzene. with other physical properties less sensitive to structural The boiling point may be calculated from the B. P. N. of the variations. molecule by means of the boiling point equation, 8
Boiling Points of Alkyl Benzenes The B. P. N. of a simple alkyl derivative of benzene is obtained by summing up the b. P. n.'s for the particular strutture, using the b. P. a ' s &en in 'l'able 1- Emphasis muat be laid on the requirement that the b. p. n.'s of all alkyl radicals attached to benzene be calculated by using the b. p. n.'s of 0.8 and 1.0 for each carbon and hydrogen atom, respectively, in the longest continuous chain beginning a t the point of attachment of the benzene ring. Any side chains not contained in the longest chain of each radical are assigned the b. p. n.'s of the branched alkyl groups. The boiling point of the benzene nucleus cannot be calculated from aliphatic b. p. n.'s alone because of the uncertainty of the effect of the
B. P. = 2 3 0 . 1 4 q B m - 543 or i t may be obtained from the previous calculations given in Table I1 of the work on aliphatic hydrocarbons (2). The calculation of the boiling point of sec-butylbenzene is as follows. Benzene less 1 hydrogen 3 crtrbon's in the main chain 6 hydro ens in the main chain 1 metha branched chain Calculated B. P. N. Calculated B. P . Observed B. P.
19.0 2.4 6.0 3.05 30.45 176.7' C. 173.5' C.
Certain polysubstituted benzene derivatives require the additional b. p. n.'s g v e n in Table I to obtain satisfactory
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
792
boiling points. o-Xylene, for example, has a higher boiling point than either the meta or para isomer and requires an additional b. p. n. to account for this difference, as do all similarly constituted methyl derivatives of benzene. A comparison of the B. P. Ne's of these compounds shows that the introduction of a methyl group ortho to another raises the B. P. N. by one unit for each methyl group so introduced. Thus, the B. P. N. of 1,2,3-trimethylbenzene should include an additional b. p. n. of 2.0 for the second and third methyl groups ortho t o the first. This makes a B. P. N. of 30.4 and a calculated boiling point of 175.3' C. The observed value is 176.1" C. I n hexamethylbenzene the first and sixth methyls are also ortho, and as might be expected, its boiling point requires the addition of 6.0 in calculating its B. P. N. and boiling point. The effect of the larger alkyl groups in the ortho positions becomes smaller as the size of the radical increases. In fact, only methyl and ethyl groups give an exaltation to the boiling point, while groups larger than ethyl or even several ethyl groups have a depressing effect upon the boiling point. The calculation of the boiling points of these more complex derivatives of benzene may be demonstrated from figures for 1methyl-3,5-diisopropylbenzene: Benzene less 3 hydrogens Methyl radical (0.8 3) 2 isopropyl radicals, Z(1.6 Structural b. D. n. Celoulated €3. P. K. Calculated B. P. Observed B. P.
+ + 4 -I-3.06)
17.0 3.8 17.3
.
-1.8 __
36.5 220.3' C . 217.0" C.
The structural b. p. n. is for a benzene derivative having three alkyl groups attached, two of which are larger than methyl (see section c under characteristic benzene structure, Table I). Since the two groups are propyl, the value - 1.6 is called for. If the methyl group had been a third propyl group, the value -3.2 would have been used, as in 1,3,5-triisopropylbenzene whose calculated boiling point is 242.5" C. and the observed, 235.0" C.: 17.0
+ 3(8.65) - 3.2 = 39.75
242.5' C.
If the groups requiring b. p. n.'s are not all identical, an average ia taken of those b. p. n.'s which would have been used if the groups requiring corrective b. p. n.'s were of the same kind. Thus, the boiling point of l-methyl-3-ethyl-6isobutylbenzene would be calculated using -1.3. The observed boiling point is 228.5" C.: 17.0
+ 3.8 + 6.6 + 11.45 - 1.3 = 37.55 = 227.7" C.
The corrective b. p. n., -1.3, was obtained by averaging -0.6 (the value if both groups requiring a corrective b. p. n. had been ethyl) and -2.0 (the value if both groups had been butyl). If the methyl group had been larger-for example, propyl-it would have been proper to average in the value for the propyl group; but in that case b. p n.'s for three large groups would have been used, - 1.2, -3.2, and -4.0, respectively (see section c, Table I). In general the b. p. n.'s for the simple alkyl derivatives of benzene are applied t o the calculation of the B. P. N.'s of the polyphenyl derivatives. As would be expected, however, certain combinations of groups give rise to definite changes in the boiling point, and corrective b. p. n.'s should be used to account for these variations. One of the most striking of these structural effects is that produced by the attachment of two phenyl groups either directly together or by means of an unsubstituted carbon chain. The boiling points of these compounds are higher than expected and the effect is observed, as far as 1,5-diphenylpentane a t least (boiling points of longer
Vol. 33, No. 6
TABLEI. BOILINGPOIXTKUMBERSFOR AROMATICHYDROCARBOh-S
Carbon, in longest continuous chain Hydrogen, in longest continuoue chain Branched alkyl groups: Methyl Ethyl Propyl Isopropyl
3.05 5.5
7.0 6.0
0.8 1.0
9.7 12.0 14.2 -0.4
Butyl Pentyl Hexyl Groups ending in -C(CHa)a
Benzene, one unit less for each hydrogen replaced Characteristic benzene structures a. Vicinal methyl groups, for each methyl ortho to another (do not count the first methyl: for six adjacent count them all) b. Vicinal ethyl groups For an ethyl ortho to another (for additional ortho ethyl groups, see section c ) For an ethyl ortho to a methyl group c. Where three or more alkyl groups are attached to henzene and t,wo or more of these are ethyl groups or larger, subtract the b. p. n.'s given below. Where several different radicals are present, average the b. p, n.'s. Methyls are counted in totaling the number of groups, but no correctional b. p. n. is needed for them unless two or more are vicinal; in such cases apply the b. p. n.'s given in section a. Where two or more methyls are present and only one larger group, do not apply a correction for the larger group including ortho ethyl groups. The number of groups attached to benzene requiring corrective b. p. n.'s is as follows: Groups CiHs CsH7 C4Ho CaHii
Two -0.6 -1.6 -2.0 -2.4
Three -1.2 -3.2 -4.0
Four -1.8 -4.8
Five -2.4 -6.4
20.0
1.0
0.4 0.7
Six -3.6 -9.6
Derivatives of biphenyl (do not apply the b. p. n.'s given for benzene above) a. The biphenyl linkage 2.5 b . Methyl groups ortho to the other phenyl (b. p. n.'s of higher groups unknown) ; meta and para groups are treated normally as for benzene 2.5 c. Polyalkyl derivatives For the second methyl attached to the same phenyl group - 1.0 For the third methyl - 1.6 For the fourth methyl - 2.0 For the methyl-isopropyl combination - 4.6 Diphenyl derivatives of aliphatic chains (take the chain between the two phenyl groups as the normal chain, alkyl groups left over or attached to the normal chain have the usual b. p. n.'s of branched chains) a. Unsubstituted normal chain between the phenyls 1.8 b. Diphenylethane and 2,2-diphenylpropane 1.0 c . All other types 0.0 d . For alkyl groups attached to the phenyls of diphenylmethane use b. p. n.'s for biphenyl, part c Derivatives of terphenyl Meta 5.0 Para 9.0 1,1,2-Triphenylethane or ethene derivatives - 3.0 1,1,2,2-Tetraphenylethane or ethene derivatives - 6.0 Olefinic and acetylenic linkages have the usual b. p. n.'s depending upon substitution First phenyl attached to an olefinic carbon 1.0 Second phenyl attached t o an olefinic carbon 0.0 1.3 Phenyl attached to an acetylenic carbon Phenyl attached to alicyclic rings or to opposite end of a substituted chain from such a ring 1.5
chain derivatives are unknown), Other similarly constituted compounds have exalted boiling points, which appear to be characteristic of this structure. It is not clear why the carbon chain must be unsubstituted, but that it does may be shown by comparing the following derivatives. It should be noticed particularly that the methyl derivative has practically the same boiling point although it has a larger molecular weight.
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1941
The calculation of the boiling points of 1,3-diphenylpropane and 2-methyl-l,3-diphenylpropaneis as follows: Phenyl groups Carbons in main chain Hvdronens in main chain BiancGed chains jl.methyl) B. D. n. characteristic of position
1,3-Diphenylpropane 35.0 2.4 6.0
...
2-Methyl1,3-diphenylpropane 38.0 2.4 5.0 3.05
1.5
48.2 294.6 O C. 298.5* C.
Ca1e;lated B. P. N. Calculated B . P . Observed B. P.
48 I46 296.0' C. 297.5' C.
Three phenyl groups attached together cause a n even greater exaltation of the boiling point. Using the b. p. n. of 2.5 characteristic of the biphenyl structure for the third phenyl group to be attached gives a satisfactory calculated boiling point for m-terphenyl: 19
+ 18 + 19 + 2.5 + 2.5 = 61.0
=
363.0' C.
The observed boiling point is also 363" C. For p-terphenyl a much higher observed boiling point has been recorded-i. e., 383" C. This requires a b. p. n. of 9.0 which probably reflects the greater symmetry of p-terphenyl. The alkyl derivatives of the polyphenyl paraffins require, in certain instances, slightly different b. p. n.'s from those obtained for benzene. That is, methyl groups ortho to a phenyl group have a smaller effect upon the boiling point than those in the meta or para positions. When two or more alkyl groups are substituted on the same phenyl group of biphenyl or diphenylmethane, b. p. n.'s should be subtracted to account for the smaller observed boiling points. Without doubt other combinations of larger groups require additional b. p. n.'s but no examples appear in the literature. TABLE11. BOILING POINTDEVIATIONS
Alkyl derivatives
Phen"1 alicfclic hva(rocarbons Total
Av. Deviation from No. of Calcd., ' C. yo pf R . P.'s Hydrocarbons Irrespective Consider- within *lOo C. of sign ing sign of Calcd. Considered
149
4.75
-2.72
QO.0
67 70 20
5.36 5.Q4 8.14 5.17
t0.16 -2.12 -2.41 -0.2
91.0 84.3 70.0 100.0
9 __
4 75 -
4-2.8 -
88.9 -
323
5.64
-0.73
8
87.9
No additional b. p. n.'s are required for tri- and tetraphenylmethane whose boiling points may be considered to be normal. On the other hand, tri- and tetraphenylethane and -ethene derivatives have depressed boiling points. For these derivatives b. p. n.'s of 3.0 and 6.0 should be subtracted from the B. P. N.'s, respectively, to account for their structures. The boiling points of more highly phenylated ethanes, propanes, etc., are unknown and b. p. n.'s for these have not been obtained.
Boiling Points of Phenyl Olefins and Acetylenes An aryl group attached to an unsaturated carbon atom raises the boiling point of the compound beyond that to be expected. This is shown by comparing the boiling points of 1-phenyl-1-butene and 1-phenyl-2-butene, which are 189" and 176" C., respectively. The elevated boiling point is accounted for when calculating the boiling point by adding a b. p. n. of 1.0 for a phenyl group attached to an olefinic carbon. The b. p. n.'s of the variously substituted ethylenic linkage have been retained (B), and the only additional factor is the b. p. n. for the phenyl group attached to the unsaturated carbon. For a second phenyl attached to the same unsaturated carbon atom no additional b. p. n. is needed, but if
793
the second phenyl is attached to the other carbon atom of the double bond a second b. p. n. of 1.0 is added. If the chain connecting the two phenyl groups is not substituted further, the usual b. p. n. of 1.8 is included. When three or four phenyl groups are attached to the ethylenic linkage, 2.0 is added for the first two phenyl groups and 3.0 or 6.0 units are subtracted, respectively, for the concentration of phenyl groups on two adjacent carbon atoms. These general rules give satisfactory results with few exceptions, although the boiling points are often very high. The calculation of the boiling point of tetraphenylethylene follows: 4 phenyl groups 2 oarbon atoms I double bond of the type R&=CR$ 2 phenyle attached to double bond B. p. n. for 4 phenyls attached to adjacent
Calculated B. P. N. Calculated B. P. Observed B . P.
76 0 1.6 2.5 2.0 76.4 433.50 c. 425.0' C.
The calculation of the boiling points of the acetylenic derivatives follows the same general rules, but they are much simplified because only two substitutions are possible on the acetylenic linkage. If a phenyl group is attached to either carbon of the triple bond, the b. p. n. is 1.3 or 2.6 if both carbons are phenylated.
Cis-Trans Isomerism Among the aromatic derivatives of the olefins, no pairs of cis-trans isomers were found whose boiling points a t atmospheric pressure had been recorded. However, in vacuo boiling points of the stilbenes (6) and l-phenyl-l,3-butadienes (4) were determined. I n the first case the trans isomer (1) is the higher boiling, and in the second, the cis isomer. These contradictory data may be correlated with the 1,2-dihaloethylenes, on the one hand, in which the trans isomer is the higher boiling, and the 2-butenes as well as the esters of maleic acid and fumaric acid, on the other, in which the cis isomer is higher boiling. Apparently the cis and trans isomers are not so regular in their boiling points as in their melting points Additional data are needed to clarify these relations.
Boiling Points of Phenyl Naphthenes The combination of a phenyl group with a naphthenic ring affects the boiling point in much the same way as that produced by the combination of two phenyl groups. The b. p. n. for the direct attachment of a phenyl to an alicyclic ring or for the attachment of these two rings to the ends of an unsubstituted chain is 1.5, which is intermediate between the b. p. n. for the attachment of a phenyl to an ethylenic linkage and that for the attachment to another phenyl group.
Accuracy of Calculated Boiling Points The boiling points of a total of 323 aromatic hydrocarbons were obtained from the literature and compared with the calculated values. The results are arranged in Table 11. Usually boiling points of organic compounds are given over a range of temperature. To avoid this complication the mean temperature was taken as the observed boiling point. This probably does not always give the best value but has been followed for the sake of uniformity. The boiling points were taken from all of the usual sources, and no boiling point was discarded for any reason. As would be expected because of the uncertainty of the effect of cis-trans isomerism, the poorest agreement was observed in the olefin series. For the other types the results were more satisfactory. The average deviation of about 5" C. was undoubtedly much larger than necessary because it appears likely that a number of the observed boiling points appearing in the literature have been incorrectly determined. Such deviations are made apparent by a comparison with the calculated values. This has been done in
INDUSTRIAL AND ENGINEERING CHEMISTRY
794
TABLE111. AROMATIC HYDROCARBONS WHOSEOBSERVEDBOILINGPOINTS DEVIATEFROM THE CALCULATED BY MORRTHAN 110'' C. B P.N.
Hydrocarbon
(Calcd ) 35.3 40.2 42.25 36.05
sene
1-(1-Methylethyl) -2- (1,l-dimethylethyl) benzene 1,4-Di( 1-methylpropyl) benzene 1 4-Dibutylbensene
l:Methyl-2-(3-methylbutyl)-4-( 1methylethyl) benzene
B. P. B. P. Dexiation, (Calcd.), O C. (Obsvd.). a C. C. 212.9 245.4 258.6 217.3
1-propene 1-Phenyl-1-methylene-2-methylpropane 1-(4-Methylphenyl)-l-methylene-2methylpropane 3-Phenyl-3-ethyl-1-propene 1-Phenyl-1 2-dimethyl-1-butene 1-Phenyl-2iethyl-1-butene 1-Phenyl-1-pro ylidenebutane 6-Phenyl-2-met%yl-2-pentene 1-Phenyl-2 4-dimethyl-1 3-pentadiene l-Phenyl-lknethylene-5-'methyl-4hoxene Polyphenyl olefins Diphenyl(2-methylpropylidene) methane 1 Z-Diphenylethene(tmns) 1:2-Diphenyl-l-rnethyl-2-ethylethene 1 3-Diphenylpropene 1:3-Diphenyl-2-ethylpr?pene 1,4-Diphenyl-l,3-butadlene (trafls-
-
trans)
Phenyl acetylenes (none) Phenyl alicyclic hydrocarbon 1-Phenyl-3-etbyloyolopentane
-15.9 -40.4 -17.6 -12.3
32.1
187.7
170.0
-17.7
223.6 249.9 269,5
210.0 237.0 225,O
-13.6 -12.9 -44.5
41.7
256.1
245,O
-10.1
39.4 38.0
240.1 230.7
228,O 245.5
-12.1 +14.8
254.8 293.4 246.7 284.0
242.0 271.5 232.5 242.6
-12.8 -21.9 -14.2 -41.5
265.1
277.0 287.5 288.0 316.5 311.1j 383.0
+11.9
274.4 303.2 296.0 296.0 421.5 186.2 204.7
175.0 191.3
-11.2 -13.4
40.15
245.0
226.6
-18.5
30.8
178.4
191.6
+13.1
33.6 32.4 37.1 37.0 39.2 36.25 40.5
199.6 190.7 224.6 223.9 238.8 219.3 247.4
211.0 173,O 205,O 205,O 228.0 196.6 236.0
+11.4 -17.7 -19.6 -18.9 -10.8 -22.8 -11.4
42.55
260.5
246.0
-14.5
47.3 52.95 49.1 52.2
50.85
309.6 289.2 321.2 299.7 317.1
298.5 306.5 298.0 276.0 306.5
-11.1 +17.3 -23.2 -23.7 -10.6
53.6
324.7
35O,O(oa.)
+25 3
40.5
247.4
270.0
f22.6
41.65 48.0 40.4 1:3:5-Trimethyl-2:4,6-triethylbenzene 46.4 Polyphenyl paraffins 43.8 3-Metbylbi henyl 44.8 2,3'-Dimet&lbiphenyl 3.5.3'.5'-Tetramethvlbi~henvl 49.7 . , ~ .~ , ~ 1-Phenvl-I1 2,5 dim&hclphe
