Competition between nucleophilic substitution of halogen (SN Ar) versus substitution of hydrogen (SN ArH)-a mass spectrometry and computational study.

The mechanism of intramolecular gas-phase reactions of N-(2-X-5-nitrophenyl)-N-methylacetamide carbanions (X=H, F, Cl) has been studied using negative ion electrospray mass spectrometry ((-)ESI-MS) technique and modelled computationally. It was proven that all three anions form cyclic σ(H) adducts, which undergo elimination of water. In the case of X=F, formation of the σ(F) adduct, leading to SN Ar reaction, was a competing process. This is the first proof that also in the gas phase formation of σ(H) adduct proceeds faster than σ(X) adduct and only when X=F, rates of these two processes are comparable. The experimental results are in full agreement with quantum chemical calculations.