2.4 de Forcrand and Villard – Career Gas Hydrate Researchers
After submitting a thesis entitled Recherches sur les hydrates Sulfhydrés to the Faculté des Sciences de Paris in 1882 [57], Robert Hippolyte de Forcrand (Figure 2.3) spent the next 43 years contributing prolifically to gas hydrate research. Nominally, de Forcrand worked in the organic chemistry laboratory of le Collège de France in Paris under the direction of Marcellin Berthelot, although most of his work was performed in Lyon under A. Loir.
Some 30 years previous, Loir [58] had formed a solid compound from chloroform, hydrogen sulfide, and water at room temperature. Similar compounds were formed when methyl chloride, 1,1‐dichloroethane, ethyl chloride, methyl bromide, or methyl iodide were used instead of chloroform, or hydrogen selenide was used instead of hydrogen sulfide. Since Loir had only measured the relative proportions of chloroform and hydrogen sulfide, the composition of these compound hydrates remained essentially unknown. Since de Forcrand found that the pressure–temperature stability conditions for the compound hydrates (double hydrates, in today's parlance) were more convenient to work with than those of the single component hydrates, he chose the former for analysis.
The term hydrate sulfhydré was coined by de Forcrand to distinguish the compound hydrate of volatile organic liquids with hydrogen sulfide from the simple gas hydrate of hydrogen sulfide itself. Since there is no simple English equivalent of this term, it is best rendered as the awkward “double hydrate with hydrogen sulfide.” de Forcrand pointed out [59] that the compound hydrates appeared to be similar to the double hydrate of phosphine and carbon disulfide, newly formed under pressure by Cailletet and Bordet [42].
Using the production of crystals when hydrogen sulfide was bubbled through an organic liquid layer underlying a layer of water at a temperature near 0 °C as the criterion of hydrate formation, de Forcrand identified double hydrates of the 33 organic compounds, mainly halogenated hydrocarbons, listed in Table 2.1. No doubt, these organic compounds, many of them synthesized by de Forcrand himself, varied greatly in purity and the boiling points (b.p.) given are mid‐points of the observed ranges of the boiling point. Similarly, a number of organic liquids were found to form “hydrates selenhydrés,” double hydrates with hydrogen selenide gas substituted for hydrogen sulfide, such as Loir had found [58] for chloroform. Hydrogen selenide was also found [59] by de Forcrand to form a hydrate by itself, although apparently not soon enough for inclusion in his thesis.
Double hydrates with hydrogen sulfide were formed only by the organic halides which boiled below about 110 °C, a result now known to be due to a rough correlation between boiling point and molecular size (i.e. organic halides with boiling points greater than 110 °C appear to be too large to fit into the clathrate hydrate cages).4 The presence of hydrogen sulfide is not required for some of the compounds given in Table 2.1, but the stability of these hydrates is much less in the absence, than in the presence of H2S.
Three years before, in his first publication [60] on the subject of gas hydrates, de Forcrand had reported the formation of methyl iodide hydrate by the method said by Berthelot [33], to give a snow of CS2 hydrate: bubbling moist air through the volatile liquid at a rate fast enough to cause substantial cooling. The new methyl iodide hydrate melted at approximately −4 °C and was found to have the composition CH3I·H2O. “Analogous hydrates” were said [59] to be formed by chloroform, ethyl bromide, and ethyl iodide. These results were not mentioned in his thesis, and by then, de Forcrand probably had doubts of their authenticity. In retrospect, this method of preparation of hydrates of volatile liquids is very likely to produce hydrates that contain varying amounts of air; thus, they were double hydrates in their own right. In turn, this would give considerable variability to the decomposition temperatures of the double hydrates and thus the difficulties in obtaining reproducible results. Since oxygen and nitrogen hydrates were not reported until 1960, these gases were assumed to be inert with respect to hydrate formation by the hydrate researchers of the day.
Table 2.1 Molecules found by de Forcrand to form double hydrates with H2S [1, 26].
| Molecule | Boiling point (°C) | Molecule | Boiling point (°C) |
|---|---|---|---|
| CH3Cl | −23 | C2HCl3 | 75 |
| CH2Cl2II | 40 | C2H5BrIII | 38 |
| CHCl3III | 61 | CH3CHBr2III | 115 |
| CCl4III | 78 | C2H3BrIII | 20 |
| CH3Br | 5 | CH2CBr2 | 91 |
| CH2Br2 | 80 | C2H5IIII | 71 |
| CH3III | 41 | C2H3I | 56 |
| CBrCl3II | 104 | n‐C3H7Cl | 46 |
| CCl3NO2III | 110 | n‐C3H7BrII | 71 |
| C2H5ClII | 10 | i‐C3H7Br | 60 |
| CH3CHCl2 | 64 | CH2CHCH2Cl | 46 |
| CH3CCl3III | 75 | CH2CHCH2Br | 70 |
| CH2ClCCl3II,H | 102 | i‐C4H9ClH | 67 |
| CH2ClCH2ClIII | 83 | i‐C4H9BrH | 90 |
| C2H3Cl | 18 | CH3NO2 | 101 |
| CH2CCl2 | 40 |
C2H5NO2
|
