Relative reactivities of halogens toward methane

As an example, let us compare the reactivities of chlorine and bromine atoms toward methane that is, let us compare the rates, under the same conditions, of the two reactions  

Since temperature and concentration must be the same for the two reactions if we are to compare them under the same conditions, any difference in collision frequency would have to arise from differences in particle weight or size. A bro mine atom is heavier than a chlorine atom, and it is also larger as we have seen, the effects of these two properties tend to cancel out. In actuality, the collision frequencies differ by only a few per cent. It is generally true that for the same temperature and concentration, two closely related reactions differ but little in collision frequency. A difference in collision frequency therefore cannot be the cause of a large difference in reactivity. 

The nature of the probability factor is very poorly understood. Since our. two reactions are quite similar, however, we might expect them to have similar probability factors. Experiment has shown this to be true whether chlorine or bromine atoms are involved, about one in every eight collisions with methane has the proper orientation for reaction. In general, where closely related reactions are concerned, we may assume that a difference in probability factor is not likely to be the cause of a large difference in reactivity. 

We are left with a consideration of the energy factor. At a given temperature, the fraction of collisions that possess the amount of energy required for reaction depends upon how large that amount is, that is, depends upon the In our example act is 4 kcal for the chlorine reaction, 18 kcal for the bromine reaction. As we have seen, a difference of this size in the - act causes an enormous difference in the energy factor, and hence in the rate. At 275 , of every 10 million collisions, 250,000 are sufficiently energetic when chlorine atoms are involved, and only one when bromine atoms are involved. Because of the difference in act alone, then, chlorine atoms are 250,000 times as reactive as bromine atoms toward methane. 

As we encounter, again and again, differences in reactivity, we shall in general attribute them to differences in E t in many cases we shall be able to account for these differences in act on the basis of differences in molecular structure. It must be understood that we are justified in doing this only when the reactions being co/ i-pared are so closely related that differences in collision frequency and in probability factor are comparatively insignificant. 

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The halogenation reactions are quite similar and thus have similar entropy changes => the relative reactivities of the halogens toward methane can be compared on energies only. 

We shall want to know not only what these relative reactivities are, but also, whenever possible, how to account for them. To see what factors cause one reaction to be faster than another, we shall take up in more detail this matter of the different reactivities of the halogens toward methane. Before we can do this, however, we must understand a little more about the reaction itself. 

Before we leave this topic, one further point needs to be made. We have given explanations of the relative reactivities of the halogens toward methane that have been based on energy considerations alone. This has been possible only because the reactions are quite similar and thus have similar entropy changes. Had the reactions been of different types, this kind of analysis would not have been proper and might have given incorrect explanations. 

The reactivity of one substance toward another is measured by the rate at which the two substances react. A reagent that reacts very rapidly with a particular substance is said to be highly reactive toward that substance. One that reacts slowly or not at all under the same experimental conditions (e.g., concentration, pressure, and temperature) is said to have a low relative reactivity or to be unreactive. The reactions of the halogens (fluorine, chlorine, bromine, and iodine) with methane show a wide spread of relative reactivities. Fluorine is most reactive—so reactive, in fact, that without special precautions mixtures of fluorine and methane explode. Chlorine is the next most reactive. However, the chlorination of methane is easily controlled by the judicious control of heat, light, and concentration. Bromine is much less reactive toward methane than chlorine, and iodine is so unreactive that for all practical purposes we can say that no reaction takes place. 

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