Etbylation of Ethanol in the Gas PhaseIn the gas phase, alkyl halides RX lead either to alkylation' or protonation reactions. ' The reagent in these processes (RX", R+ or R-X+-R) can often be determined by ion cyclotron resonance but not so easily under chemical ionization conditions. In this work, CH, CD, I and CD, CH, I have been used to determine the mechanism of the ethylation of ethanol by CH, CH, I in the source of a mass spectrometer (VG ZAB 2F).In the ion source, equimolecular quantities of C, H, OH and of ethyl iodide lead to the formation of the symmetrical protonated ether CH, CH,-OH-CH, CH, . This ion has been selected with the magnetic field and characterized by the fragmentations observed in the 2nd field-free region (2nd FFR).In Table 1, the spontaneous fragmentations of metastable protonated diethyl ether 1 and its deuterated analogues la-lc have been compared to those of the corresponding ions 2 and 3 generated by ion-molecule reactions. The metastable ion 1 loses ethylene after H(B) migration leading to the CH,CH,OH, fragment ion (m/z 47). The results from la-lc show that the H(B) migration is selective; a strong primary isotopic effect (lc) and a less important secondary isotopic effect (lb) are observed: CH, CH,-OD--CH,CH,, la gives only CH, CH,OHD' + + m/z 48 + CH, CH,-OH--CD, CH, , l b leads to m/z 47 (43%), m/z 49 (56.5%), and + CH, CH,-OH-CH, CD,, lc leads to m/z 48 (29%) and m/z 50 (70%).The same results are obtained for the isomeric ions 2a, 2b and 2c generated by reaction of labelled ethanols with CH, CH, I (Table 1).However, the behaviour of 3a and 3b ions, formed by reaction between labelled alkyl iodides and ethanol, shows that two mechanisms are implied in the ethylation reaction. The first one is dominant and selective while, for the second one, hydrogens of the ethyl iodide exchange before alkylation. The non-specific mechanism corresponds, most probably, to the reaction between the ethyl cation and ethanol. Indeed, spontaneous and collision induced dissociations of [CD,CH,] + and [CH2CD3]+ metastable cations show that extensive H/D exchange occurs before the 2nd FFR.However, the collision-induced dissociations of [CH,CD,I]+' and [CD,CH,I]+' metastable ions indicate that in the 2nd FFR these ions still have their initial structures: H/D exchange is not observed. The same conclusion has been obtained by neutralization-reionization mass spectrometry., It is possible to conclude from these results that the reaction between [ICH,CH3]+' and C,H,OH corresponds to the dominant and selective mechanism.The existence of the adduct ion [CH,CH,I/CH,CH,OH]+ has been proved: this adduct ion has been selected with the magnetic field; in the 2nd FFR this ion leads to[CH3CH21]+' and CH,
