Fluorocarbons, boiling point

Fluorocarbons have extraordinarily low boiling points when compared to hydrocarbons of the same molecular weight. 

The fluorocarbons have relatively low boiling-points, i.e. there is abnormally low attraction between the molecules, with little tendency to association, which makes their physical properties similar to those of the rare gases. 

To appreciate the significance of these two new compounds one must recall the striking properties of carbon compounds in which hydrogen is completely replaced by fluorine. The products are characterized by great chemical inertness, but have physical properties which are similar in many respects to those of the hydrocarbons. This is illustrated by the following summary (Table 3) of the boiling points of fluorocarbons and hydrocarbons of the two series C F2 +2 and... 

The properties of hydrofluorocarbons often fall between those of then corresponding fluorocarbons and hydrocarbons, but there can be significant differences The dielectric constants of C6H]3F and CF3(CF2)2(CH2)2CH3 and the surface tension and boiling point of C H F are notably higher than those of either C6F i4 or C6H14 These differences reflect the more polar character of the hydrofluoro carbons owmg to their net C-F and C-C bond dipoles... 

The boiling points of many halogen compounds are similar to hydrocarbons of the same molecular weight, but there are some conspicuous exceptions. lodomethane, for example, has about the same molecular weight as decane (MW 142), but the boiling point of iodomethane is 132° lower than that of decane. Likewise, fluorocarbons (e.g., tetrafluoromethane, CF4, MW 88, bp —129°) are far more volatile than hydrocarbons of similar weights (e.g., hexane, CBH14, MW 86, bp 69°). 

The fluorocarbons have extraordinarily low boiling points relative to the hydrocarbons of comparable molecular weight. As seen in Figure 14-3, their boiling points are nearly the same or even lower than those of the alkanes or cycloalkanes with the same number of carbons. Thus octafluorocyclobutane boils 17° lower than cyclobutane, despite an almost fourfold greater molecular weight ... 

Figure 14-3 Boiling points of straight-chain fluorocarbons (C F2 +2) and hydrocarbons (C H2n+2)...

Compared to the same size hydrocarbons, fluorocarbons have higher volatility and lower boiling points, unusual for halides. They are less reactive, more compressible, and more thermally stable than hydrocarbons. They also have low flammability and odor. 

Since the molecular weight of a fluorocarbon is considerably higher than that of the corresponding hydrocarbon, we might expect boiling points to be increased. 

FIG. 21.2. Boiling-points of hydrocarbons, fluorocarbons, and noble gases. 

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