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SN/E Competition - Role of the Temperature

The Role of the Reaction Temperature in the SN / E Competition

The number of particles in SN reactions does not increase during the reaction. A heteroalkyl compound and a nucleophile are converted into another heteroalkyl compound and a nucleofuge. Therefore, the translational entropy does not change as a result of the reaction.

On the other hand, the number of particles in eliminations increases during the reaction. The reaction of a heteroalkyl compound and a base yields an alkene, the acid conjugated to the base and the leaving group. Thus, two particles are converted into three particles.

Number of particles in eliminations and substitutions.

Consequently, in eliminations the translational entropy increases. Therefore, eliminations are more influenced by the reaction temperature than substitutions are, as the reaction entropy ΔS affects the thermodynamic driving force of the reaction, since it is a factor of the term - TΔS in the Gibbs free energy ΔG = ΔH - TΔS. Due to the increasing translational entropy, the reaction entropy of eliminations is usually positive. As a result, ΔG becomes smaller and smaller, or more and more negative, respectively, with the increase in reaction temperature (in - TΔS, T is the absolute temperature in kelvin and thus always positive). According to the Hammond postulate, the decrease of ΔG results in a reduction of the transition state's energy and, thus, in a reduction of the activation energy. The reaction rates of eliminations, therefore, increase more with the rising in reaction temperature than the rates of substitution reactions do.

An elevation of the reaction temperature therefore leads to a considerable preference for elimination in the SN1/E1, as well as in the SN2/E2 competition!
  • Click here for a detailed explanation of the Hammond postulate.
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