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The stereoelectronic effect is the effect on molecular structures, physical properties and reactivities due to the molecules' electronic structures, in particular the interaction between atomic and/or molecular orbitals. Typical stereoelectronic effects with specific orbital overlaps generally lead to a specific molecular conformation or energy differentiation among various transition states that would lead to a particular reaction selectivity. The stereoelectronic effect, along with the steric effect, inductive effect, mesomeric effect, etc., is one of the key theories in illustrating unusual selectivity, reactivity and stability cases in the course of organic chemistry. Its application has widely spread in organic methodology and organic synthesis. This topic is now entering biochemistry and pharmaceutical chemistry. Stereoelectronic effect generally includes a donor–acceptor interaction. The donor is usually a higher bonding or nonbonding orbital and the acceptor is often a low-lying antibonding orbital as shown in the scheme below. As known from the orbital–orbital interaction requirement, if this stereoelectronic effect is to be favored, the donor–acceptor orbitals must have a low energy gap and they must retain antiperiplanar geometry to allow for perfect interacting direction. Stereoelectronic effect contains a large variety of subtopics, including anomeric effects and hyperconjugation. Orbital interactions in the stereoelectronic effect ==Trend of different orbitals== Take the simplest CH2X–CH3 system as an example; the donor orbital is σ(C–H) orbital and the acceptor is σ *(C–X). When moving from fluorine to chlorine, then to bromine, the electronegativity of the halogen and the energy level of the σ *(C–X) orbitals decreases. Consequently, the general trend of acceptors can be summarized as: π *(C=O)>σ *(C–Hal)>σ *(C–O)>σ *(C–N)>σ *(C–C), σ *(C–H). For donating orbitals, the nonbonding orbitals, or the lone pairs, are generally more effective than bonding orbitals due to the high energy levels. Also, different from acceptors, donor orbitals require less polarized bonds. Thus, the general trends for donor orbitals would be: n(N)>n(O)>σ(C–C), σ(C–H)>σ(C–N)>σ(C–O)>σ(C–S)>σ(C–Hal). The stereoelectronic effect in 1-halogen substituted ethane Stereoelectronic effect can be directional in specific cases. The radius of sulfur is much larger than the radius of carbon and oxygen. Thus the differences in C–S bond distances generate a much amplified difference in the two stereoelectronic effects in 1,3-dithiane (σ(C–H) → σ *(C–S)) than in 1,3-dioxane(σ(C–H) → σ *(C–O)).〔 The differences between C–C and C–S bonds shown below causes a significant difference in the distances between C–S and two C–H bonds. The shorter the difference is, the better the interaction and the stronger the stereoelectronic effect.〔 Directionality of stereoelectronic effects 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Stereoelectronic effect」の詳細全文を読む スポンサード リンク
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