S_N2 Reactivity of CH₃X Derivatives. A Valence Bond Approach

A method for conceptualizing reactivity trends in S_N2 reactions is formulated. The gas-phase S_N2 barrier arises from an avoided crossing of two electronic curves which contain the reactant-like and product-like Heitler-London VB forms N⁻: R· ·X and N· ·R:X⁻. The energy separation of the curves at each reaction end is the difference between the ionization energy of the nucleophile (I_N:) and the electron affinity of the substrate (A_rx). The reaction barrier is shown to be a fraction of this energy gap (I_N:. − A_RX), and the size of this fraction is shown to depend on the slopes of the intersecting curves. Thus, the height of the barrier is determined by the gap-slope interplay, and it can be represented by one general equation which applies to thermoneutral identity reactions as well as to exothermic nonidentity reactions. These reactivity factors are quantified for a variety of nucleophiles (N⁻⁰:) and CH₃X substrates and are used to discuss gas-phase S_N2 barriers for identity and nonidentity reactions. It is suggested that reactivity patterns fall into two categories: (a) electron-transfer-controlled patterns which are dominated by the size of energy gap 23 (these trends are found whenever N and X are varied down a column of the periodic table); and (b) slope-controlled patterns in which reactivity is determined by the strength of the (C∴X)⁻ and (C∴N)⁻ 3-electron bonds. Whenever these bonds are strong, reactivity is reduced. Increasing the number of possible leaving groups, e.g., as in CH₄, CH₂Cl₂, CCl₄, has a similar slope effect leading to low reactivity. The effect of ΔH is investigated. It is shown that whenever the gap factor and the slope factors operate in opposition, increasing the reaction exothermicity does not guarantee increasing reactivity. Thus, the dominant role of slope factors is suggested to be the root cause in the occasional breakdown of the Bell-Evans-Polanyi principle.