Competition between substitution and elimination reaction channels from fragmentation ratios of diquaternary ammonium bromide and formate ion pairs

• Gemini surfactants form ion pairs in the gas phase with bromide and formate.
• A collision initiates a reaction inside ion pair similar to as in the liquid.
• Formate pairs favor elimination while bromide substitution.
• Spacer size affects fragmentation ratio in a similar way regardless of kind.
• If there is labile hydrogen its transfer to anion is favored.

Fragmentation by collisionally induced dissociation in triple quadrupole mass spectrometer of pairs of six series of quaternary diamines (gemini surfactants) with two anions: formate and bromide has been studied. Amines included alkanediyl-α,ω-bis-(N,N-dimethyl-N-dodecyl ammonium bromide), alkanediyl-α,ω-bis-(N-hydroxyethyl-N-methyl-N-dodecyl ammonium bromide), oligo(oxa)ethyl-α,ω-bis-(N,N-dimethyl-N-dodecyl ammonium bromide) and three series of hexadecyl homologues. Range of atoms forming spacer varied from 2 to 23. It was confirmed that fragmentation occurs via either nucleophilic substitution or elimination reactions. All possible α-carbon atoms can be attacked in the substitution reaction, so can be all possible hydrogens of β-carbon in the elimination reaction. Additionally transferred to anion can be hydroxy group hydrogen. It leads together to eight different reaction paths. Depending on the spacer size and the kind of anion the ratio of fragmentation channels varies dramatically. For short spacers length ion pairs with formic anion dissociate through elimination, while bromide through substitution at spacer carbon. When spacer length is long enough to accommodate central anion between two quaternary nitrogens the substitution channel becomes the dominating one for both anions with exception when one of the substituents is hydroxyethyl. Then elimination is the only channel for ion pairs with formate anions, but for bromide anion division between substitution and elimination channels is not much affected. From the trends in channel ration and additional experiment with asymmetric geminis reveal it was concluded that the transition state is probably substantially different for long and short spacer geminis.

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