The competition between substitution (sN2), elimination (E2) and additiorrelimination (AE) reactions in the gas phase has been studied by using OH-and NH,as nucleophiles reacting with esters of acetylenedicarboxylic acid of general formula RO(O)CCZCC(O)OR'. By selectively varying R and R' it was possible to show that the E2 reaction predominates over SN2 and that the relative contributions of the three reactions with OH-follow the decreasing order AE > E2 > SN2, whereas for NH,-it is E2 > A E > SN2. The E2 reaction is discussed in terms of substituent effects, relative acidity of the different phydrogens and geometry of the transition state. For the AE reaction a leaving group order for this class of compounds was established.
The mechanism of water loss from the 1-beptanol molecular ion was studied by using a set of deuterium 1heptanols labelled at each position of the chain. The carbon-bound hydrogen atom lost in the water comes mainly from C(4) (69%) but also from C(2) and C(3) (10 and 13%, respectively), and from C(5) and C(6) (5 and 4%, respectively). No measurable contribution by an H atom from C(7) and C(l) was found. The structure of the resulting [ C,HI41+' ion was studied by using the MIKE and CADWIKE techniques applied to unlabelled, and 'H and "C labelled compounds and by ionization energy and appearance energy measurements.The results clearly indicate that at the onset of the reaction, the [ C,HI4]+' ion formed has the propylcyclobutane structure. At 70 eV electron energy, the [ C,H,,]+' ion formed is indistinguishable from molecular ions of heptenes.
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