Competition between substitution ( S N 2), elimination ( E 2) and addition elimination ( AE ) reactions in the gas Phase

The competition between substitution ( S N 2), elimination ( E 2) and addition–elimination ( AE ) reactions in the gas phase has been studied by using OH − and NH 2 − as nucleophiles reacting with esters of acetylenedicarboxylic acid of general formula RO(O)CCCC(O)OR′. By selectively varying R and R′ it was possible to show that the E 2 reaction predominates over S N 2 and that the relative contributions of the three reactions with OH − follow the decreasing order AE > E 2 > S N 2, whereas for NH 2 − it is E 2 > AE > S N 2. The E 2 reaction is discussed in terms of substituent effects, relative acidity of the different β‐hydrogens and geometry of the transition state. For the AE reaction a leaving group order for this class of compounds was established.