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 (see Figure 4).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 bimolecular elimination reaction will occur in a molecule where the breaking carbon-hydrogen bond and the leaving group are anti-periplanar[4][5][6][7] (Figure 8).The term anti-periplanar was first coined by Klyne and Prelog in their work entitled "Description of steric relationships across single bonds", published in 1960.[13] In their article “Periplanar or Coplanar?” Kane and Hersh point out that many organic textbooks use anti-periplanar to mean completely anti-planar, or anti-coplanar, which is technically incorrect.
Figure 1: Functional groups A and D are anti-periplanar
Figure 2: Functional groups are considered periplanar if they have a dihedral angle less than −150° or greater than +150° or −30° to +30°. Adapted from a figure by Dreamtheater published on Wikimedia Commons.
[
3
]
Figure 3: Representation of a strictly anti-coplanar conformation. A, B, C, and D are in the same plane and the dihedral angle between A–B and C–D is 180°.
Figure 4: Newman projection showing A and D anti-periplanar.
Figure 5: Sawhorse projection of 2-chloro-2,3-dimethylbutane showing Cl and H anti-periplanar.
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.
Figure 7: The energy of both the C–H bonding orbital and the C–Cl anti-bonding orbital lower when they mix.
Figure 8: In an E
2
mechanism, the breaking C–H bond and the leaving group are often anti-periplanar. In the Figure B is a general base and X is a leaving group.
Figure 9: The C–H bonding orbital is mixing with the C–X anti-bonding orbital through hyperconjugation.
Figure 10: In an E
2
mechanism molecules generally prefer an anti-periplanar geometry because it aligns molecular orbitals and sets up the molecule to move electrons in a C–H bonding orbital into a π
C-C
bonding orbital.
Figure 11: Mechanism of a pinacol rearrangement. The C–C bonding orbital is aligned with the C–O anti-bonding orbital, which facilitates the methyl shift. H–A is a generic acid.
