Anti-periplanar

In organic chemistry, anti-periplanar, or antiperiplanar, describes the bond angle in a molecule. In this conformer, the dihedral angle of the bond and the bond is greater than +150° or less than −150° (Figures 1 and ). Anti-periplanar is often used in textbooks to mean strictly anti-coplanar, with an 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 and between +30° and −30° (see Figure 2). 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, σC–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 σC-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).