Endergic reaction

If we consider reaction of mesylate ion with methyl bromide, we find that this is an endergic reaction thus the transition state lies along the reaction coordinate farther toward the products titan the reactants (Figure 5.11). The activated complex will therefore have a structure more resembling the products. There will be significant carbon-oxygen bond formation between the mesylate group and carbon and only a weak residual bond between carbon and bromine. The bromine... 

Figure 5.10 Reaction coordinate diagram for an endergic reaction in which the products are of higher energy than the reactants.

The Hammond postulate states that in endergic reactions, features which stabilize and thus lower the energy of a product lower the energy of the transition state leading to diat product. This is shown in Figure 5.12. If product 2 (P2) is lower in energy than product 1 (Pi), then transition state 2 ( 2) will be lower than transition state 1 ( 1). It will also be earlier. As a consequence, P2 will have a lower activation barrier and be formed faster than Pi. A simplified restatement of the Hammond postulate is that more stable products are formed faster. It must be remembered that this analysis is for endothermic reactions and assumes that the reactants have the same or similar energies. 

Figure 5.14 Relationship between product energy changes and transition states for endergic reactions.

For a reaction step in which the products are less stable titan the reactants (endergic), the transition state will lie farther along the reaction coordinate toward tile products and the activated complex will have a structure more similar to the products titan the reactants (Figure 5.10). 

---