Slice-Selective NMR for Quantitative Analysis of Liquid–Liquid Biphasic Systems: A Tool for Organic Chemists

Biphasic systems are ubiquitous across modern chemistry,
yet direct
and quantitative analysis of solute distribution across phases remains
experimentally nontrivial. While many chemists rely on qualitative
intuition to reason about partitioning, confirming these assumptions
typically requires time-consuming sample manipulation. We demonstrate
that slice-selective NMR spectroscopya straightforward technique
available on modern NMR instrumentscan be used to directly
measure spatially resolved concentrations. This allows users to quantify
how species partition between immiscible phases without disrupting
the equilibrium, offering a powerful and broadly accessible tool for
reaction monitoring, mechanistic study, and process troubleshooting.
We demonstrate the application of this technique to measure logP values,
monitor phase-transfer catalysis, study biphasic reaction kinetics,
and quantify the influence of counterion identity and ionic strength
on phase distribution. In each example, slice-selective NMR enables
an understanding of underlying chemical behavior that would otherwise
be difficult to capture. We aim to highlight this technique not as
a niche spectroscopic curiosity but as a general-purpose tool that
can be readily adopted by synthetic and process chemists. This work
serves as both a tutorial introduction and a practical demonstration
of how spatially selective NMR can demystify the behavior of biphasic
systems and bring quantitative rigor to problems often approached
qualitatively.