The kinetics of the hydrolysis of a series of imines derived...

The kinetics of the hydrolysis of a series of imines derived from benzophenone and primary amines reveals a normal dependence of mechanism on $\mathrm{pH}$ with rate-determining nucleophilic attack at high $\mathrm{pH}$ and rate-determining decomposition of the tetrahedral intermediate at low pH. The primary amines show a linear correlation between the rate of nucleophilic addition and the basicity of the amine. Several diamines, in particular 12-A, 12-B, and 12-C, all showed positive (more reactive) deviation from the correlation line for other primary amines. Why might these amines be more reactive than predicted on the basis of their basicity?

Key Concepts

Basicity and Nucleophilicity

Basicity refers to an amine's ability to donate electrons and accept protons, which is closely related to its nucleophilicity—the tendency to attack electrophilic centers. In many reaction series, a higher basicity generally correlates with a higher nucleophilic reactivity, leading to faster rates of reactions such as nucleophilic additions.

Reaction Mechanism and pH Dependence

Reaction mechanisms can vary with pH, where different steps become rate-determining under different conditions. For imine hydrolysis, nucleophilic attack is rate-determining at high pH, while the decomposition of the tetrahedral intermediate is rate-determining at low pH. This switch illustrates how changes in pH can alter the energy profile of the reaction pathway.

Neighboring Group Participation

Neighboring group participation involves the assistance of a functional group located near the reactive center in facilitating a reaction. In diamines, the presence of a second amine group can enhance the reactivity by providing intramolecular assistance, which can stabilize intermediates or transition states through hydrogen bonding or internal proton transfer.

Intramolecular Catalysis and Transition State Stabilization

Intramolecular catalysis occurs when a structure within the same molecule aids in lowering the activation energy needed for a reaction. In the case of specific diamines, the appropriate positioning of the second amine group can promote the formation of a favorable cyclic transition state or assist in proton shuttling, resulting in a reaction rate that is higher than what basicity alone would predict.

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