A value

A-values are numerical values used in the determination of the most stable orientation of atoms in a molecule (conformational analysis), as well as a general representation of steric bulk.

Due to the larger number of possible conformations of ethyl cyclohexane, the A value is reduced from what would be predicted based purely on enthalpic terms.

Due to these favorable entropic conditions, the steric relevance of an ethyl group is similar to that of a methyl substituent.

One of the original experiments performed by Winston and Holness was measuring the rate of oxidation in trans and cis substituted rings using a chromium catalyst.

The large tert-butyl group used locks the conformation of each molecule, placing it equatorial (cis compound shown).

The proposition was that the large hydroxyl group in the axial position was disfavored and formed the carbonyl more readily to relieve this strain.

Using the A-values of the hydroxyl and isopropyl subunit, the energetic value of a favorable intramolecular hydrogen bond can be calculated.

The longer bond allows for less interactions with neighboring substituents, which effectively makes the trimethylsilyl group less sterically hindering, thus, lowering its A-value.

The A-value for a methyl group is 1.74 as derived from the chemical equilibrium above. This means it costs 1.74 kcal/mol (7.3 kJ/mol) of energy to have a methyl group in the axial position compared to the equatorial position.
A methyl substituent has a significantly smaller A-value than a tert -butyl substituent; therefore the most stable conformation has the tert -butyl in the equatorial position.
Possible axial conformations of ethyl cyclohexane.
Possible equatorial conformations of ethyl cyclohexane.
Possible chair conformations of cis -4- tert -butyl-cyclohexan-1-ol
Chromium oxidation of cis -4- tert -butyl-cyclohexan-1-ol
Possible chair conformations and the favorable hydrogen bond available in the conformation where both hydroxyl substituents are equatorial
Equilibrium representation of a chair flip of a carboxylic acid; the axial position is preferred due to favorable electronic factors, despite a steric bias favoring the equatorial position.