Theoretical Studies of Competitive Gas-Phase SN2 and E2 Reactions of NCCH2CH2Cl with OH- and SH

The gas-phase reactions of OH- and SH- with NCCH2CH2Cl have been studied with ab initio methods using the 6-31+G* basis sets. The electron correlation effect was accounted for at the MP2 level of theory. Both the SN2 and E2(anti) reactions are exothermic and have negative activation barriers when Cl- is the leaving group. Overall, E2 is preferred to SN2, and E2(anti) is favored over E2(gauche) for both OH- and SH-, indicating the preference of an antiperiplanar transition state in the gas phase. Since SH- is a weaker gas-phase nucleophile than OH-, the transition state occurs later on the reaction coordinate. The results of this work are consistent with those of gas-phase experimental studies which indicated that first-row nucleophiles undergo both SN2 and E2 reactions, whereas second-row nucleophiles have lower abilities to induce elimination reactions. Substitution of a CN group for a β-hydrogen of CH3CH2Cl to NCCH2CH2Cl is found to enhance both the SN2 and E2 reactivities and makes the β-H-elimination ...