title: Ethyl anomaly in the nucleophilic substitution at a series of β-methylated alkyl RCH2Z and carbonyl RCOZ carbon centers for R = Me, Et, i-Pr, t-Bu, and Z = LG.
abstract: We have carried out DFT studies to explore the cause of anomalously fast reaction rates of ethyl group (R = Et) in the gas-phase S(N)2 reactions of RCH(2)Cl+Cl(-) and RCH(CN)Cl+Cl(-), and also for those in the cationic forms of RCH(2)(+) and RCH(CN)(+) with R = Me, Et, i-Pr, and t-Bu. The TS stabilization by hyperconjugative donor-acceptor vicinal charge transfers (CTs) from R to the major NBOs at the reaction center carbon in the S(N)2 TSs were estimated using natural bond orbital (NBO) analyses. In all cases the hyperconjugative CT stabilization increases in the order R = t-Bu < i-Pr < Me < Et in agreement with the experimental as well as theoretical rate orders, exhibiting an ethyl anomaly. We have also determined the reorganization energies and hyperconjugative CTs from R to the two major NBOs, C-O(-) and C-N(+), in the tetrahedral intermediate formed with five water molecules, T(5w), by transformation of sp(2) to sp(3) centers in the reactions of RC(═O)OC(6)H(5) with NH(3). The reorganization energy is the lowest and CT stabilization is the strongest with R = Et in line with the fastest experimental rate. We conclude that C-H is a better donor than C-C bond orbital and hyperconjugative vicinal σ chain extension leads to a stronger CT stabilization in the TS. The stronger CT stabilization for R = Et rather than Me is achieved by enhanced hyperconjugative CT to the reaction center in the TS as a result of narrower energy gap and greater overlap brought about by long-range orbital mixing as the C-H σ-chain is extended from n = 2 for Me to n = 3 for Et. We find that CT properties of the all-trans vicinal hyprconjugative C-H σ-chains are closely analogous to the corresponding conjugative polyene π-chains although skeletal patterns of bridge bonds are different and the stabilization energy gained by extension of the σ-chain is much weaker than that gained by the π-chain.
authors: Department of Chemistry, Dong-A University, Busan, Korea.
authors: Dae Dong
authors: DD
authors: Sung
authors: Ikchoon
authors: I
authors: Lee