13diaxial

13diaxial, more commonly written as 1,3-diaxial, is a term used in conformational organic chemistry to describe a specific type of steric interaction in cyclohexane rings. It refers to the repulsive interactions that occur between an axial substituent on one carbon and the axial substituents (usually hydrogens, but sometimes other groups) on the carbon three atoms away in the ring. In a chair conformation, these interactions arise when a substituent is oriented axially and the ring positions three bonds distant are also axial, creating a 1,3-relationship.

In practice, 1,3-diaxial interactions contribute to the destabilization of conformations in which bulky groups are axial. Because equatorial positions minimize these axial contacts, cyclohexane derivatives tend to favor chair conformations in which large substituents reside in the equatorial orientation. Ring flipping interconverts axial and equatorial positions, so the preferred conformation depends on the balance between diaxial strain and other substituent effects.

The magnitude of the diaxial penalty is often summarized by A-values, which quantify the energy difference between axial and equatorial positions for common substituents. For example, methyl groups have a relatively small A-value (about 1.7 kcal/mol), while larger groups such as ethyl, isopropyl, or tert-butyl raise the penalty considerably (roughly increasing with size). These values are approximate and context-dependent but provide a practical guide to predicting conformational preferences.

Overall, 1,3-diaxial interactions are a foundational concept for understanding the conformational behavior of substituted cyclohexanes and their reactivity, influencing which conformations are most populated under given conditions.