text: Take the simplest CHX–CH 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).
  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.