Pentane interference

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Pentane interference or syn-pentane interaction is the steric hindrance that the two terminal methyl groups experience in one of the chemical conformations of n-pentane. The possible conformations are combinations of anti conformations and gauche conformations and are anti-anti, anti-gauche+, gauche+ - gauche+ and gauche+ - gauche of which the last one is especially energetically unfavorable. In macromolecules such as polyethylene pentane interference occurs between every fifth carbon atom. In cyclohexane derivatives, this type of interaction is called the 1,3-diaxial interaction. Pentane interference helps explain molecular geometries of many chemical compounds.

For instance in certain aldol adducts with 2,6-disubstituted aryl groups the molecular geometry has the vicinal hydrogen atoms in an antiperiplanar configuration both in a crystal lattice (X-ray diffraction) and in solution proton (NMR coupling constants) normally reserved for the most bulky groups i.d. both arenes [1]:

The other contributing factor explaining this conformation is reduction in allylic strain by minimizing the dihedral angle between the arene double bond and the methine proton.

References

  1. Effect of 2,6-Disubstituted Aryl Groups on Acyclic Conformation: Preference for an Antiperiplanar Orientation of the Geminal and Vicinal Hydrogens Paul R. Carlier, Yiqun Zhang, Carla Slebodnick, Michael M.-C. Lo, and Ian D. Williams J. Org. Chem.; 2006; 71(23) pp 8835 - 8841; (Article) doi: 10.1021/jo061495z

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