A value

A value

An "A" value is a measure of the conformational preference of an equatorial compared to an axial substituent in a monosubstituted cyclohexane. [GoldBookRef |title=A value |file=A00012 |year=1994] It is defined as the Gibbs free energy difference between the equatorial and the axial conformers. A positive "A" value indicates preference towards the equatorial position. The magnitude of the values ranges from nearly zero for very small substituents such as deuterium, to about 5 kcal/mol for very bulky substituents such as the tert-butyl group.

The "A" values are also known as Winstein–Holness "A" values after Saul Winstein and N. J. Holness, who proposed them in 1955. [cite journal |title=Neighboring Carbon and Hydrogen. XIX. t-Butylcyclohexyl Derivatives. Quantitative Conformational Analysis |author=S. Winstein, N. J. Holness |journal=J. Am. Chem. Soc. |year=1955 |volume=77 |issue=21 |pages=5562-5578]

The following table lists some common "A" values in kcal/mol. [cite book | author=Eric Anslyn, Dennis A. Dougherty | year=2006 | title=Modern Physical Organic Chemistry|isbn=9781891389313| pages=104 |publisher=University Science Books ]

:

ee also

* Cyclohexane conformation

References


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