Strain (chemistry) - Kinds of Strain - Van Der Waals Strain - 1,3-diaxial Strain

1,3-diaxial Strain

1,3-diaxial strain is another form of strain similar to syn-pentane. In this case, the strain occurs due to steric interactions between a substituent of a cyclohexane ring ('α') and gauche interactions between the alpha substituent and both methylene carbons two bonds away from the substituent in question (hence, 1,3-diaxial interactions). When the substituent is axial, it is brought near to an axial gamma hydrogen. The amount of strain is largely dependent on the size of the substituent and can be relieved by forming into the major chair conformation placing the substituent in an equatorial position. The difference in energy between conformations is called the A value and is well known for many different substituents. The A value is a thermodynamic parameter and was originally measured along with other methods using the Gibbs free energy equation and, for example, the Meerwein-Pondorff-Verley Reduction/Oppenheimer Oxidation equilibrium for the measurement of axial versus equatorial values of cyclohexanone/cyclohexanol (0.7kcal/mol).

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