The solvolysis of heavily branched tertiary alkyl halides in pure polar protic solvents heavily and — Haloalkanes and Haloarenes Chemistry Question
Question
The solvolysis of heavily branched tertiary alkyl halides in pure polar protic solvents heavily and notoriously favors unimolecular reaction pathways. Which of the following specific environmental variables serves to forcefully maximize the $E1$ elimination yield at the direct, calculated expense of the competing $S_N1$ substitution yield?
💡 Solution & Explanation
$E1$ and $S_N1$ reactions incredibly share the exact same rate-determining step (the formation of the carbocation) and only branch into different products in the second, fast step. Because an elimination reaction mathematically cleaves one reactant molecule into two (alkene + leaving group + protonated solvent), it strictly and dramatically increases the entropy of the system ($\Delta S > 0$). In stark contrast, substitution reactions generally maintain the same overall number of molecules. According to the Gibbs free energy equation ($\Delta G = \Delta H - T\Delta S$), a significantly higher temperature ($T$) massively amplifies the stabilizing thermodynamic effect of a positive entropy change. Therefore, elevating the reaction temperature forcefully drives the system toward the E1 elimination product over substitution.