Competition Experiments and Product Analyses

In Section 8.8 we examined several tools for deciphering reaction mechanisms, one of which was competition experiments, and another was examining the products. In a reaction that produces two or more products, a single reactive intermediate may be involved, with the two products arising from different reaction paths emanating from that intermediate. Chemists check for this possibility by generating the same intermediate from two different reactants. If we see the same product ratio from the two different reactants, a common intermediate is likely involved. 

Because eliminations are in competition with substitutions (see Sections 10.13.3 and 11.5.16), these two paths can be used to check for a common intermediate in the two different 

Gas Phase Sn2 Reactions—A Stark Difference in Mechanism from Solution 

As shown in the drawing, the barrier to the Sn2 reaction in the gas phase can be comparable to, or even below, the energy of the separated nucleophile and electrophile. For example, the transition state enthalpy for reaction of chloride with methylbromide in the gas phase is actually 2.5 kcal / mol lower than the separated reactants (AE = 2.5). The reason that the rate constants can be very small, even though the enthalpy barrier is low relative to the reactants, is entropy control. The change in entropy going from the loose complex to the transition state is very large and negative. 

The differences in the mechanisms in the gas and solution phases necessarily arise from differences in solvation. In the gas phase the nucleophile and electrophile solvate one another prior to reaction, and achieving the correct orientation between the nucleophile and electrophile for backside attack leads to a substantial entropy barrier. The rate constants therefore have little correlation with the enthalpy of the reaction. In solution, however, the electrophile and nucleophile are solvated prior to the reaction, the transition state is solvated during the reaction, and the product is solvated after the reaction. This yields rate constants that correlate with the heat of the reaction. 

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