Influence of a β-OH substituent on SN2 reactions of fluoroethane: Intramolecular hydrogen bonding catalysis or inhibition? A theoretical study.

Computational investigations on the gas phase S N 2 reactions of X ions ( X - = OH - , F - , Cl - , Br - , and I - ) with fluoroethane and 2-fluoroethanol (β-hydroxyl ethyl fluoride) were performed by using MPW1K, M06 and CCSD(T) methods with a range of basis sets including 66-311++G ∗∗ , TZVP, aug-cc-pVDZ and SDD. To better understand the nature of the corresponding intermolecular interactions, we performed natural bond orbital (NBO) analysis and Bader’s quantum theory of atoms in molecules (QTAIM). In our calculations, we took into account two possible pathways for these reactions, i.e. S N 2 reactions of 2-fluoroethanol. In path( i ), β-hydroxyl group participates in hydrogen bonding (H-bonding) with fluoride ion as leaving group in transition state. However, in path( ii ), transition state includes H-bonding between hydroxyl group and nucleophile X − . Results indicate that in comparison with the absence of β-OH substituent i.e. reactions of fluoroethane, the activation barrier decreases in path( i ), but increases in path( ii ). The progress of each pathway depends on H-bonding character of nucleophile and leaving group. In addition, for X - = Cl - we compared β-OH group effect in the gas phase with its effect in dimethyl sulfoxide (DMSO). [ABSTRACT FROM AUTHOR]