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Early and late transition states

Fig.1
Fig.2

The structure of a transition state can often be used as a basis for the prediction of the reaction rate.

In the transition state of methane halogenation, the H-X bond has already partially been developed and the methyl group depicts a partial radical character. Differences in the structure of transition states of methane halogenations are caused by the different electronegativities of the halogens.

As a result of its high electronegativity, fluorine attacks the methane in a particularly early reaction stage when the C-H bond has still hardly been altered. Therefore, the transition state structure resembles the starting products. Such a transition state is called early transition state, which is closer to the starting products on the left of the reaction coordinate than to the products on the right. According to the Hammond postulate, early transition states are generally characteristic of rapid exothermic reactions.

In contrast to other halogens, iodine possesses the lowest electronegativity of all. In the transition state of a hypothetical methane iodination, the C-H bond is already largely cleaved. Therefore, the structure of this transition state resembles the products. Such a transition state is called late transition state, which is closer to the products on the right of the reaction coordinate than to the starting products on the left. According to the Hammond postulate, late transition states are generally characteristic of slow endothermic reactions.