A METHOD OF ESTIMATING THE BOILING POINTS OF ORGANIC LIQUIDS
0
D. E. PEARSON Vanderbilt University, Nashville, Tennessee
TEE relationship of molecular structure to the boiling point of an organic liquid is discussed in this paper with the view of extending the enlightening contributions of McElvain (1) and Shiner and Fuson (8). The approach is simple enough and seemingly comprehensive enough to enable the interested reader to predict the boilmg point of most classes of organic liquids with an accuracy of +5'. If such is the case, the method must necessarily be consistent and cognizant of most of the important factors of molecular structure which influence boiling points. The method remotely resembles that of Kinney (3) to the extent that a boiling point estimation of an organic liquid is based on the boiling point of the parent hydrocarbon to which an increment factor is added for the particular functional group. A. The Parent Straight-Chain Hydrocarbons Table 1shows that the increment gradually decreases with increasing molecular weight, as to be expected for a proportional increment. Though it is not prictical to continue beyond CISor below C4, au estimation can be made if desired. For example, the boiling point of propane must differ from butane by a t least an increment of -35' and possibly another estimat,ed - 5O. The calculated boiling point of propane, therefore, is - 40°, and it is actually found to be - 42". The CI1 hydrocarbon must differ from the Cia hydrocarbon by TABLE 1 Boiling Points of Paraffin Hydmearbons (Straight Chain) Hydracarbn
Rounded Fiw@
Found
98.4 126 151 174 196 215
Ineremat
-25 25 25 20 20
20 235 235 Memory is aided by noting that C., G, and CII are O', 10Oe, and 200' (nearly).
CIS
an increment of 20' or less. The calculated boiling point of Clp is 255' and is actually 250'. At this point, an increment of 15" should be used and appears to be the limiting increment in further extensions. Obviously, considering both experimental determinations and the rough calculations, good correlation is not to be expected in or beyond this region of high molecular weight. The diierence in empirical formula between the para f f i and unsaturated hydrocarbon molecule is two hydrogen atoms (or a multiple thereof). Since the small hydrogen atom has little effect on the volume and the density of the individual molecule (two factors which affect the boiling point), the boiling points of straightchain olefins and acetylenes are found to be the same or only slightly less than those of the corresponding paraffin hydrocarbons.'
B. The Effect of Branching and Ring Formation. 1. For a single or double branching, subtract 10' from the boiling point of the parent hydrocarbon. This inore ment is smaller, the closer to the center of the chain the branched group is attached. 2. Two branches on a single 'arbon atom have a double effect-hence subtract 80'. This increment also becomes less as the double branch approaches the middle of the chain. 3. For a six-membered ring, whether aromatic or alicyclic, add 10' to the boiling point of the parent hydrocarbon. The above corrections are consistent with the conclusions that branching the chain decreases the effective volume of the individual molecule and that ring formation increases the effective volume. For example, pentane must have an elongated, "zig-zag" shape which together with its motion affords many more collision opportunities than the ball-shaped neopentane molecule, and, as a consequence, pentane (b. p. 35') h d s it more difficult to escape from the liquid than does neopentane (b. p. 10"). As another example, cyclohexane, like the doughnut, encloses space and again offers more collision opportunities than hexane. Thus, cyclohexane (b. p. 81') does not as readily escape from the liquid as does hexane (b. p. 70'). The explanation for the effect of cyclization is further strengthened by observation that the increment, +loo for six-membered rings, increases with enlargement of the ring system. This is Space does not permit the inclusion of tables of illustration.
It is left to the reader to check the general mlea given.
FEBRUARY, 1951
to be expected if more space is enclosed within the ring.% It will be noted in checking the above system of predict.. ing boiling points that a branched chain connected directly to a double-bond carbon does not lower the boiling point. C. Functional Group Equivalents (Non-associated Liquids) I t is implied in Table 2 that the substitution of another atom for a carbon atom in the orpanic molecule affects the boiling point to the extent that the atomic "density" and volume of that atom is related to the carbon atom, or, more properly, the methylene group. The above is true provided the bond angle attachment of the new group is approximately the same as the comparative carbon attachment.
TABLE 3 Mjthylae Class
GTOUU
Epuiualent
weight alcohols, ketones, aldehydes, and amines will boil somewhat higher than predicted. However, as the molecular weight increases, the association factor levels off to the value given. Organic acids and amides also fall in this group. However, it is much easier to recall that formic acid boils a t 100" and the addition of one methylene group increases the boiling point by 20". I n terms of the exTABLE 2 planations previously used, this increment assumes that MethyGene association is considerable with the low molecular Class Group Equivalent weight acid (exactly a dimer for acetic acid) and diminChloride C1 2 4ishes with each additional methylene group a t such a Br 3 Bromide rate that the increment remains constant. Amides, on 4 I Iodide the other hand, are usually solids and are not con0 1Ether Ester anhydride, or acid sidered here, though they are very appreciably asso0 1 (for each oxygen) chlbride ciated. It is also questionable whether nitriles are S 3 Sulfide classified correctly. They would be expected to asso3 SH Meroaptan ciate through their active methylene groups, and, in The table is readily applied in one's mind as follows: fact, the predictions are quite good assigning a methylbutyl bromide has four carbons plus one bromine atom, ene equivalent of 3 to the nitrogen of the nitrile group. or the equivalent of three "methylenes"; four plus However, pivalonitrile (b. p. 10.5') has no active hydrothree methylenes is equivalent to the boiling point of gen and still boils a t the predicted temperature. heptane (100"). It is observed that the predicted boil- E. The Effect of Resonance If an olefin, ester, ether, phenol, or carbonyl group i s ing points of halogen-containing liquids check the experimental values closely with the exception of geminal conjugated with an aromatic ring (or attached to a benzyl halogen compounds. The striking deviations of this group) add +lo0 to the boiling point. This correction group parallel other unusual chemical and physical factor implies that the bond (or bonds) of the group are straightened and strengthened by the increased particproperties. Oxygen in an organic molecule is quite similar in den- ipation in resonance with the benzene ring. The sity and volume to a methylene group. Therefore, pro- straightening produces more collision possibilities. vided no association takes place, an oxygen atom is I t is undoubtedly too simple an explanation to attribute merely replaced by a methylene group in predicting boil- all factors involved in the +loo correction to resonance ing points, i. e., diethyl ether should boil a t the same alone, when the correction is applied to so many varied temperature as pentane, and butyl acetate a t the same structures. Certainly, there are varying degrees of temperature as octane. It will be noted that, follow- resonance within these structures which should require ing this procedure, the ethers boil 5 to 10' lower than a varying increment. However, the fact remains that the predicted values. This is attributed to the fact the correction factor works and that i t applies only to that the ether molecule is slightly folded a t the points of those compounds with the properly conjugated systems. attachment to the oxygen atom, thus reducing the ef- As an example, phenetole (b. p. 172') is predicted to boil a t 170"; calculation: Cs 01 i Z Ce E 150' fective volume or collision possibilities. SO0 (resonance correction) = It is rather surprising that mercaptans fall into the SO0 (ring structure) non-associated group. And, it should also be noted 170'. Other examples give better or almost as good that the very low molecular weight esters show some agreement with the exception of aromatic aldehydes. Their higher-than-predicted boiling points imply greater association. resonance (or perhaps association). Functional Group Equivalents (Associated L i M d s ) D. The considerable deviation of aromatic amines from The eouivalents in this ~ T O U D include an ~- - methvlene " - . association factor. The association factor, further- the predicted values can now be appreciated. Anilme more, is a minimum value, i . e., the lower molecular (b. p. 183") is predicted to boil a t 145'. The resonance must not only strengthen and straighten the bond 2 In this connection, the reported boiling point (4) of oycloundecane(b.p., 184'C.)isundoubtedly muchlower than itsactual between the aromatic ring and nitrogen but must also activate the hydrogens attached to nitrogen so that boiling point.
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JOURNAL OF CHEMICAL EDUCATION
62
association is appreciably increased over that of ordinani ali~haticamines. Sumrisinelv - " enoueh. - , tertiarv aromatic amines boil abnormally high also, which may indicate association through the para hydrogen atoms. In conclusion, it is interesting to speculate on the state of water at its boilinn noint in accordance with the principles of this paper, ~h~ monomer should be equivalent to one, or slightly more than one, methylene group. Since water boils at loo0, which is equivalent to seven methylene groups, it is possibly an aggregate of six to seven molecules a t 100" in the liquid state. Since water exists as a monomer in the gaseous state (61, a possible explanation for the large heat of vaporieation is that considerable energy is utilized to accom~~~~~~
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-
A
plish the dissociation of this large aggregate simultrtneouslv with its va~orization. LITERATURE CITED 1. MCELYAW, 5. M., "The Characterization of Organic Campounds," The MacMillan Co., New York, 1945. 2. SHRINER, R. L.,AND R. C. FUSON,"Systematic Identification of Organic Compounds," 3rd ed., John Wiley and Sons, New Yark, 1948. 3. KINNEY.C. R.. AND W. L. SPLIETHOFP. J. OW. Chem.. 14. 71 (1949); C. R. KINNEY,J . Am. ~h'hem.BOG., 60, 3032 (1938). 4. EGWFF, G., "Physical Constants of Hydrocarbons," Vol. 11, Reinhold Publishing Co., New York, 1940, p. 142. 5. DOESBY,N . E., "Properties of Ordinary water-substance," Reinhold Publishing Co., New York, 1940.
