most stabe conformation

[holler01]

i thought the most stable conformation of an isomer was always higher priotity equatorial and lower prioity axial..but i did a question where trans 3- methyl cyclohexanol (one equat the other axial) is less stable than cis 3- methyl cyclohexanol (equatorial for both)

..so what is actually the most stabe? with both positions eqatorial or one equatorial otherr axial?

thanks!

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[Live4Life]

In general, if both substituents are able to be in the equatorial position, then that is the most energetically favored conformation. The more energetically favored conformation will always try to place all the bulky substituents in the equatorial position b/c 3,4-diaxial interactions place the molecule in a conformation that is energetically unfavored. Note, that having one substituent in the axial position and one in the equatorial position is better than having both in the axial position.

 

[MBHockey]

i thought the most stable conformation of an isomer was always higher priotity equatorial and lower prioity axial..but i did a question where trans 3- methyl cyclohexanol (one equat the other axial) is less stable than cis 3- methyl cyclohexanol (equatorial for both)

..so what is actually the most stabe? with both positions eqatorial or one equatorial otherr axial?

thanks!

That's the danger of trying to memorize hard and fast rules...conformations involving cis and trans configurations are situation-specific. Never use the word "always" with conformations and their associated stabilities!

But you pretty much answered your question. With both groups in the equitorial positions, there is much less van der waals strain between those groups and the hydrogens branching off the main cyclic carbon chain (and between the groups themselves) than with axial-axial or axial-equitorial positioning of the functional groups.

Do you have an organic chem model kit or modeling software? It's very easy to visualize this when making/viewing 3D models because you can easily judge the distances between groups and main-chain hydrogens or other groups. More distance between big groups = less strain energy = more stable (and thus, more common in nature).