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Pentane interference
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. This is not to be confused with the 1,3-diaxial interactions of cyclohexane derivatives (gauche interactions shared between substituents and the ring). A clear example of the ''syn''-pentane interaction is apparent in the diaxial versus diequatorial heats of formation of ''cis'' 1,3-dialkyl cyclohexanes. Relative to the diequatorial conformer, the diaxial conformer is 2-3 kcal/mol higher in energy than the value that would be expected based on 1,3-diaxial interactions alone. Pentane interference helps explain molecular geometries in many chemical compounds, product ratios, and purported transition states. One specific type of ''syn''-pentane interaction is known as ''1,3 allylic strain'' or (A1,3 strain).
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:〔''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) 〕
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==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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