Ion–molecule reaction in the gas phase 3—nucleophilic substitution of 17ξ‐ R ‐5α,14β‐androstane‐14,17ξ‐diols under [NH 4 ] + chemical ionization conditions 1

Under Ammonia chemical Ionization conditions the source decompositions of [M + NH 4 ] + ions formed from epimeric tertiary steroid alchols 14 OH β , 17OH α or 17 OH β substituted at position 17 have been studied. They give rise to formation of [M + NH 4 H 2 O] + dentoed as [MH s H] + , [M s HH 2 O] + , [M s HNH 3 ] + and [M s HNH 3 H 2 O] + ions. Stereochemical effects are observed in the ratios [M s HH 2 O] + /[M s HNH 3 ] + . These effects are significant among metastable ions. In particular, only the [M s H] + ions produced from trans ‐diol isomers lose a water molecule. The favoured loss of water can be accounted for by an S N 2 mechanism in which the insertion of NH 3 gives [M s H] + with Walden inversion occurring during the ion‐molecule reaction between [M + NH 4 ] + + NH 3 . The S N 1 and S N i pathways have been rejected.