In organic chemistry, anti-periplanar, or antiperiplanar, describes 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°[1] (Figures 1 and 2). Anti-periplanar is often used in textbooks to mean strictly anti-coplanar,[2] with an A−B C−D dihedral angle of 180° (Figure 3). 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). Figure 5 shows 2-chloro-2,3-dimethylbutane in a sawhorse projection with chlorine and a hydrogen anti-periplanar to each other.
Syn-periplanar or synperiplanar 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° (see Figure 2).
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Molecular orbitals
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).
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Examples of anti-periplanar geometry in mechanisms
E2 mechanism
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). This geometry is preferred because it aligns σC-H and σ*C-X orbitals.[8][9] Figure 9 shows the σC-H orbital and the σ*C-X orbital parallel to each other, allowing the σC-H orbital to donate into the σ*C-X anti-bonding orbital through hyperconjugation. This serves to weaken C-H and C-X bond, both of which are broken in an E2 reaction. It also sets up the molecule to more easily move its σC-H electrons into a πC-C orbital (Figure 10).
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Pinacol rearrangement
In the pinacol rearrangement, a methyl group is found anti-periplanar to an activated alcohol functional group.[10][11] This places the σC–C orbital of the methyl group parallel with the σ*C–O orbital of the activated alcohol. Before the activated alcohol leaves as H2O the methyl bonding orbital donates into the C–O antibonding orbital, weakening both bonds. This hyperconjugation facilitates the 1,2-methyl shift that occurs to remove water. See Figure 11 for the mechanism.
History, etymology, and misuse
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.[12] ‘Anti’ refers to the two functional groups lying on opposite sides of the plane of the bond. ‘Peri’ comes from the Greek word for ‘near’ and so periplanar means “approximately planar”.[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.[14]