# Revealing the Mechanism of Alcohol Side Product Formation in Crown Ether-Mediated Nucleophilic Fluorination Using Acetonitrile as Solvent

## TL;DR

This study explains how alcohol side products form during a fluorination reaction and suggests ways to prevent them.

## Contribution

The paper reveals a new mechanism involving the HF2– ion in alcohol side product formation during fluorination.

## Key findings

- Deprotonation of water is driven by the formation of the stable HF2– ion.
- The KOH(18-crown-6) complex is more reactive than the KF(18-crown-6) complex in the SN2 process.
- The KHF2(18-crown-6) complex can inhibit alcohol side product formation.

## Abstract

Nucleophilic fluorination of primary alkyl halides using
KF salt
and catalyzed and mediated by crown ether and bulky alcohols is an
established method for monofluorination of organic compounds. However,
in the presence of a small concentration of water molecules in the
organic solvent, alcohol side products are formed. This is an intriguing
finding because water molecules are unreactive toward the SN2 reaction. Further, the formation of a hydroxide ion via deprotonation
of water by the fluoride ion faces the problem of very different pK
a values in acetonitrile solution, which were
calculated to be 19.5 for HF and 41.9 for H2O. This work
explores the mechanism behind this side reaction via theoretical calculations
and experiments. We found that the deprotonation of H2O
is driven by the formation of the stable HF2
– ion, leading to the small concentration of the KOH­(18-crown-6) complex.
This species exhibits higher reactivity compared to the KF­(18-crown-6)
complex in the SN2 process, which offsets its lower concentration
and results in a competitive side reaction. Thus, the present study
elucidates the mechanism involved in the alcohol side product formation
and indicates that the presence of the KHF2(18-crown-6)
complex can inhibit this side reaction. Furthermore, this work indicates
that complex reaction systems require an analysis beyond the comparative
barriers in the free energy profile, and multiple equilibria must
be accounted for.