In some solvents, such as K^+.-OR-DMSO, it can be shown that the internal return equilibrium cha

step 1 For A, let's write the chemical equation for the self -dissociation reaction. DMSO, which is dim

In some solvents, such as K^+.-OR-DMSO, it can be shown that...

In some solvents, such as $\mathrm{K}^{+.-}$OR-DMSO, it can be shown that the internal return equilibrium characterized by $k_{1} / k_{-1}$ is fast relative to the dissociation process characterized by $k_{2}$. In this process, the base returns the proton to the carbanion faster than the proton donor exchanges with other molecules from solution. If internal return is important under a given set of conditions, how might that affect the correlation between observed kinetic and thermodynamic acidity? How can the occurrence of internal return be detected experimentally?

Key Concepts

Kinetic Versus Thermodynamic Acidity

This concept distinguishes between the rates at which proton transfer reactions occur (kinetic acidity) and the equilibrium position of the protonated/deprotonated species (thermodynamic acidity). When internal return is operative, the fast, reversible reprotonation can make the observed rate (kinetic data) appear inconsistent with the underlying equilibrium constant (thermodynamic data), since the base quickly returns the proton before a full equilibration with the bulk solvent takes place.

Internal Return Mechanism

The internal return mechanism refers to a reversible proton transfer event that occurs within a reactive complex, where the base reprotonates the carbanion rapidly before the dissociative step in which the proton donor exchanges with solvent molecules. This fast intramolecular reprotonation can lead to an apparent decoupling of the observed kinetic rates from the thermodynamic acid strength, complicating the interpretation of kinetic measurements.

Proton Transfer Dynamics

Proton transfer dynamics involve the detailed steps that occur during the movement of a proton, including the formation of a contact ion pair, reversible protonation (internal return), and the slower, solvent-mediated exchange process. Understanding these dynamics is crucial for interpreting how different steps contribute to the overall reaction rate and how the kinetics might not directly reflect the acid–base equilibrium.

Experimental Detection of Fast Internal Return

Experimentally, fast internal return can be detected by employing kinetic techniques such as competition experiments or using isotopic labeling to measure kinetic isotope effects. These methods are sensitive to rapid, reversible proton transfers and can reveal discrepancies between the observed kinetic rates and the expected thermodynamic acidities. Time-resolved spectroscopic techniques that capture rapid proton movements also assist in identifying the presence of this fast internal reprotonation process.

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