August i8, 1904J 



NATURE 



379 



propyl chloride, so that all its members contain one or more 

 C — CH, — C groups. 



In the case of the ethers, esters, and other compounds 

 which contain two alkyl radicals, a series is regarded as 

 homologous when one radical remains unaltered and the 

 other increases by stages of CH,. The variable radical only 

 is considered in dividing the series into the two groups ; 

 thus, although propionic acid contains a C — CH, — C group, 

 it remains unchanged in the propionic esters, the first group 

 of which consists of methyl, ethyl, and propyl propionate, 

 the second beginning with the last-named ester. 



Of the seventeen series of non-associating substances there 

 are only five for which the mean difference between the 

 calculated and observed values of A for the higher members 

 exceeds j°-5. 



1. The fM-xylene series. Here there is only one value, 

 which, I think, is doubtful. 



2. The olefines, H,C = CHR. Here two of the three 

 individual differences are less than i°-5 ; the temperatures 

 are all below o°, and are somewhat uncertain. 



3. The polymethylenes. The difference for penta- 

 methylene and he.\amethylene differs by less than i° from 

 the calculated value. The B.P. of heptamethylene appears 

 very doubtful. 



4. The amines. Differences somewhat erratic ; three 

 within i°.5 and two within o°-5. Octylamine and nonylamine 

 clearly incorrect and not included. 



5. The esters. Although Ramsay and Shields include 

 these substances as non-associating, there is, I think, reason 

 to suspect slight association. 



It will be seen that the differences are greater for 

 associating than for non-associating substances ; also that 

 they are greatest for the alcohols and least for the acids, 

 although the factor of association is very high for both 

 these series. In order to arrive at an explanation of these 

 facts the effect of replacing hydrogen by chlorine may 

 first be considered. 



The boiling-point of hydrogen chloride is not yet known 

 accurately, but it must be about —80°. Thus, by replacing 

 an atom of hydrogen in the hydrogen molecule by chlorine 

 the boiling-point is raised from 2o°-4 abs. to about 193° 

 abs., or about 173°. On replacing an atom of hydrogen 

 in methane by chlorine the rise of boiling-point is from 

 io8°-3 to 249°-3, or 141°. Ascending the series of paraffins 

 the rise of boiling-point due to the replacement of 

 hydrogen by chlorine diminishes rapidly at first, and then 

 more slowly, being only $^°-S in the case of octane. Thus 

 the influence of the chlorine atom becomes relatively 

 smaller as the formula weight of the alkyl group increases. 



Consider, now, the effect of replacing a hydrogen atom 

 by a hydroxyl group. In the formation of water from 

 hydrogen gas the boiling-point is raised no less than 

 352°-6, from 20°.4 abs. to 373° abs., or jp the ratio of 

 I : 18-3; in the case of methane the rise is 22i°-8, from 

 io8°.3 to 337°'7, or in the ratio of i : 312; with octane 

 the rise is 65°-4. from 3q8''-6 to 464° ; and with hexdecane 

 it is only 56°-5, from 560°-^ to 617°, the ratio being i : i-io. 



It will be seen that in the case of hydrogen the influence 

 of the hydroxyl is enormously greater, and in the case of 

 methane very much greater, than that of chlorine in rais- 

 ing the boiling-point, but that on ascending the series of 

 paraffins to octane the influence of the hydroxyl group 

 diminishes until it is little greater than that of the chlorine 

 atom, and it is quite probable that with hexdecane it would 

 be somewhat less. This is, no doubt, to be explained by 

 the fact that the molecules of water and of the lower 

 alcohols are highly associated in the liquid, but not in the 

 gaseous state, and therefore, in order to vaporise the 

 liquids, this molecular attraction must be overcome, and 

 the temperature must therefore be raised. The molecular 

 association diminishes, however, as the series of alcohols 

 is ascended, and is probably slight in the case of octyl 

 alcohol. If so, it would appear that the efl'ect of the 

 hydroxyl group — apart from association — in raising the 

 boiling-point is not very different from, and is probably 

 somewhat less than, that of the chlorine atom, and that the 

 difference between the boiling-points of the lower alcohols 

 and of the corresponding chlorides is entirely due to 

 molecular association in the liquid state. 



With the acids there is association in the gaseous as well 

 as the liquid state, and since, according to the tables given 



NO t8i6, vol. 70] 



by Ramsay and Shields, the factor of association for a liquid 

 fatty acid at its boiling-point is rarely greater, and in 

 most cases is somewhat smaller, than for the correspond- 

 ing liquid alcohol, the molecular attraction to be overcome 

 on vaporisation must be considerably less for the acid 

 than for the corresponding alcohol, and the resulting rise 

 of boiling-point above the normal value must be less. An 

 explanation of the very low values of A for the alcohols and 

 the moderately low values for the acids is thus afforded. 



It would take up far too much time and space to give 

 full details of the boiling-points of all the compounds con- 

 sidered, with the observed and calculated values of A j but 

 it may, I think, be stated that the difference between the 

 boilipo--point of any non-associating organic compound 

 which contains at least one C — CH, — C group, and that of 

 its next higher homologue (at any rate up to temperatures 

 of about 300° C), may be calculated with an error rarely 

 exceeding I'-J, and generally under 1°, by means of the 

 formula A= '^'|^^ ,_ . The formula seems also to be 



applicable to any ester which contains at least five atoms 

 of carbon in the variable alkyl or acyl group (the mean 

 error for 40 values of A is -i-o''r)3'), and with smaller error 

 when the number of carbon atoms is still larger ; ' it is 

 probably also applicable to the higher fatty acids, cyanides, 

 ketones, and nitro-compounds. 



Comparison 0/ Molecular Volumes. 



The fundamental idea on which both Kopp and Schroder 

 based their methods of calculating the molecular volumes 

 of organic compounds from the atomic volumes of the 

 component elements was the constancy of the increase in 

 molecular volume for each addition of CH,. With regard 

 to this point the question was greatly discussed whether 

 the comparison should be made at the same temperature, 

 say 0° C, or at the boiling-points of the compounds under 

 the same pressure. Later, when Van der Waals brought 

 forward his conception of corresponding states, it was 

 thought probable that the comparison should be made at 

 corresponding or equal reduced temperatures ; that is to 

 say, at temperatures which bear the same ratio to the 

 critical temperatures. If the generalisations of Van der 

 Waals were strictly true, the boiling-points under 

 corresponding pressures would be corresponding tempera- 

 tures, but that is not usually the case. The comparison 

 may, therefore, be made either at equal reduced 

 temperatures or at the boiling-points under equal reduced 

 pressures ; or, lastly, it may be made at the critical points 

 themselves, and, thanks to the law of Cailletet and Mathias, 

 the critical volumes can be ascertained with a great degree 

 of accuracy. 



In order to find whether the difference in molecular volume 

 for each addition of CH^ is really constant it is best to 

 examine such perfectly normal substances as the paraffins, 

 and the data for four consecutive members of the series — ■ 

 »-pentane, u-hexane, n-heptane, and n-octane — are 

 fortunately available. 



In the table below the molecular volumes and the 



1 Thus the observed B.P. of w-hexyl formate is i53'"6, and the value of 

 -i c.ilculated from the formula is 22*8, giving i76°"4 a^ the B.P. of the next 

 higher homologue. This agrees very well with the observed B.P. of 

 «-heptyl formate, 176^ 7, but not with that of «-hexyI acetate, i69°'2. 

 Again, the observed B.P. of methyl caproate (hexoate) is i4g°'6, and the 

 calculated value of A is 2yo, giving 172 '6 as the B.P. of the next homo- 

 logue. The observed B.P. of methyl cLnanthylate (heptoate) is i72'^-i, but 

 that of ethyl caproate is only i66-'6. 



