Activation energy and specific rate constant

Table 5-1. Comparison of the activation energies and specific rate constants for the bimolecular Diels-Alder cycloaddition reaction of cyclopentadiene giving endo-dicyclopentadiene in the gas phase and in solution at 20 °C [3, 33, 34],...

A typical example of such reactions is the exothermic Sn2 nucleophilic displacement reaction Cl -I- CH3—Br Cl—CH3 - - Br . Table 5-2 provides a comparison of Arrhenius activation energies and specific rate constants for this Finkelstein reaction in both the gas phase and solution. The new techniques described above cf. Sections 4.2.2 and 5.1) have made it possible to determine the rate constant of this ion-molecule reaction in the absence of any solvent molecules in the gas phase. The result is surprising on going from a protic solvent to a non-HBD solvent and then further to the gas phase, the ratio of the rate constants is approximately 1 10 10 The activation energy of this Sn2 reaction in water is about ten times larger than in the gas phase. The suppression of the Sn2 rate constant in aqueous solution by up to 15 orders of magnitude demonstrates the vital role of the solvent. 

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