= Antiperiplanar conformation
An <antiperiplanar conformation> places two specified bonds in approximately the same plane with a <dihedral angle> of $180^\circ$. The designation concerns those particular bonds, not the planarity of the entire molecule.
In many <E2 elimination> reactions, the carbon-hydrogen bond being broken and the carbon-leaving-group bond align antiperiplanar. This arrangement supports the orbital overlap required to form the new <pi bond>. In a <cyclohexane> chair, the condition often requires a trans-diaxial pair, linking conformational geometry directly to which elimination pathway is accessible.
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