page_title: Anti-periplanar
section_title: Molecular orbitals
hierarchical_section_title: Anti-periplanar / Molecular orbitals
caption_attribution_description: English: Bonding and anti-bonding orbitals line up in the anti-periplanar geometry
context_page_description: Anti-periplanar is a term used in organic chemistry to describe the A–B–C–D bond angle in a molecule. In this conformer, the dihedral angle of the A–B bond and the C–D bond is greater than +150° or less than −150°. Anti-periplanar is often used in textbooks to mean strictly anti-coplanar, with an A-B C-D dihehedral angle of 180°. In a Newman projection, the molecule will be in a staggered arrangement with the anti-periplanar functional groups pointing up and down, 180° away from each other. Figure 5 shows 2-chloro-2,3-dimethylbutane in a sawhorse projection with chlorine and a hydrogen anti-periplanar to each other.
  Syn-periplanar is similar to anti-periplanar. In the syn-periplanar conformer, the A and D are on the same side of the plane of the bond, with the dihedral angle of A–B and C–D between +30° and −30°.
context_section_description: An important factor in the antiperiplanar conformer is the interaction between molecular orbitals. Anti-periplanar geometry will put a bonding orbital and an anti-bonding orbital approximately parallel to each other, or syn-periplanar. Figure 6 is another representation of 2-chloro-2,3-dimethylbutane (Figure 5), showing the C–H bonding orbital, σC–H, and the C–Cl anti-bonding orbital, σ*C–Cl, syn-periplanar. The parallel orbitals can overlap and become involved in hyperconjugation. If the bonding orbital is an electron donor and the anti-bonding orbital is an electron acceptor, then the bonding orbital will be able to donate electronegativity into the anti-bonding orbital. This filled-to-unfilled donor-acceptor interaction has an overall stabilizing effect on the molecule. However, donation from a bonding orbital into an anti-bonding orbital will also result in the weakening of both of those bonds. In Figure 6, 2-chloro-2,3-dimethylbutane is stabilized through hyperconjugation from electron donation from σC-H into σ*C-Cl, but both C–H and C–Cl bonds are weakened. A molecular orbital diagram shows that the mixing of σC–H and σ*C–Cl in 2-chloro-2,3-dimethylbutane lowers the energy of both the orbitals (Figure 7).
caption: Figure 6: The C–H bonding orbital is aligned with the anti-bonding orbital of C–Cl and can donate into the anti-bonding orbital through hyperconjugation.
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