Potential energy carbocation formation

There is another useiiil way of depicting the ideas embodied in the variable transition state theory of elimination reactions. This is to construct a three-dimensional potential energy diagram. Suppose that we consider the case of an ethyl halide. The two stepwise reaction paths both require the formation of high-energy intermediates. The El mechanism requires formation of a carbocation whereas the Elcb mechanism proceeds via a caibanion intermediate. 

With the potential energies shown on a common scale, we see that the transition state for formation of (CH3)3C is the highest energy point on the diagram. A reaction can proceed no faster than its slowest step, which is referred to as the rate-determining step. In the reaction of tert-hutyl alcohol with hydrogen chloride, formation of the carbocation by dissociation of the alkyloxonium ion is ratedetermining. 

Step 3 is bimolecular because two species, the carbocation and chloride ion, react together. Figure 4.9 is a potential energy diagram for this step, and Figure 4.10 shows the orbitals involved in C—Cl bond formation. 

Figure 6.3 compares potential energy diagrams for these two competing modes of addition. According to Hammond s postulate, the transition state for protonation of the double bond resembles the carbocation more than the alkene, and for formation of the more stable carbocation (tertiary) is less than that for formation of the less stable carbocation (primary). The major product is derived from the carbocation that is formed faster, and the energy difference between a primary and a tertiary carbocation is so great and their rates of formation so different that essentially all of the product is derived from the tertiary carbocation. 

Heats of formation of carbocations, which are the basis data for these scales have been derived from the heats of formation of the neutral precursors (e.g., radicals) and the adiabatic ionization energies [ = ionization potentials, Eq. (2)] [20]. 

The introduction of an arylthio (ArS) group at an ot-position of a heteroatom compound containing N or O causes a significant decrease in the oxidation potential, because the energy level of the S p-orbital is usually much higher than that of N and O p-orbital. In other words, the S p-orbital becomes the HOMO of the compound. One electron oxidation of such compounds leads to the formation of the radical cation, in which the C-S bond is cleaved selectively to generate a carbocation adjacent to the heteroatom Eq. 2. Therefore,... 

Each potential reaction pathway in Figure 3.76 involves a different carbocation-ic intermediate. Perhaps formation of a tertiary carbocation requires less energy than... 

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