Enthalpy differences of starting alkenes and transition states in bromination

The same ions can also be produced by electrophilic attack on alkenes by species that should generate positive halogen  

The kinetics of brominations are often complex, with at least three terms making contributions under given conditions  

In methanol, pseudo-second-order kinetics are observed when a high concentration of Br is present. The dominant contribution to the rate comes from the third term of the general rate expression under these conditions. The occurrence of third-order terms suggests the possibility of a mechanism like that proposed for addition of 

de la Mare and R. Bolton, Electrophilic Additions to Unsaturated Systems, Elsevier, New York, 1966, Chap. 7. 

As in the case of hydrogen halide additions, this attack should lead to anti addition. Alternatively, one can account for the observed third-order kinetics and retain the bromonium ion as a key intermediate by postulating that the bromonium ion is in equilibrium with such a complex  

---

Fig. 6.2. Enthalpy differences of starting alkenes and transition states in bromination.

---