Early and late transition states
The structure of a transition state can often serve as the basis for predicting reaction rates.
In the transtion state of methane halogenation, the H-X bond is already partially constructed, while the methyl group also reflects a partial radical character at this point. In the respective halogens, the structures of the transition states display differences, which are caused by the different halogen electronegativities.
Due to its high electronegativity, methane is attacked by fluorine at an early stage of the reaction, namely when the C-H bond has still not been heavily altered. Therefore, the structure of the transition state resembles the starting products. This transition state is called an early transition state. It is located on the far left side of the reaction coordinate close to the starting products. According to the Hammond postulate, early transition states are characteristic of rapid exothermic reactions.
In contrast, iodine is known to have the lowest electronegativity of all halogens. In the transition state of a hypothetical methane iodination, the C-H bond has already been largely cleaved. Therefore, the structure of the transition state resembles the products. Such a transition state is known as a late transition state. It is found to the far right of the reaction coordinate close to the products. According to the Hammond postulate, late transition states are characteristic of slow endothermic reactions.