Cyclohexane Chair Conformation Stability: Which One Is Lower Energy?
How Do You Draw The Most Stable Conformation Of A Cyclohexane Chair? You’ve been given a structure and asked to draw the most stable conformation.
1. A-Values Are A Useful Measure Of Bulkiness
The energy difference in kcal/mol between axial and equatorial conformers on a cyclohexane ring was discussed in the previous post. The greater the A-value (bulk), the more favoured the equatorial conformer will be.
2. A-Values Are Additive
Calculating the torsional strain of a cyclohexane chair is as simple as adding up the A values of the axial groups in any chair conformation. Because A values are additive, adjacent carbons do not bump into one another.
3. Example: Determining The Most Stable Conformation Of cis- And trans– 1,2-Dimethylcyclohexane
We can use the A-values to determine which molecule is the most stable by drawing the two chair conformations of cis-1,2-dimethylcyclohexane and trans-1,.2-dimer and seeing which one has the highest energy.
4. To Determine Chair Conformation Stability, Add Up The A-Values For Each Axial Substituent. The Lower The Number The More Stable It Is
The di-equatorial conformer of trans-1,2,dimethylcyclohexane is the most stable of the two isomers of 1,2-dimethylcylohexane, and now that we’ve drawn all four possible isomers, we can rank them in order of stability.
5. Summary: Chair Conformation Stability
We’ll talk about 0cyclohexanoic rings in the next post, and see how we can use what we already know about them to learn more about them.
The 1,2-di-t-butyl cyclohexane is an exception to the “A values are additive” assumption, as the diaxial conformation is favored by 6.2 kcal/mol in the trans isomer! See the References section for more information.
(Advanced) References and Further Reading
In cyclohexane, A-values are thermodynamic preferences for a substituent to be in the axial or equatorial position; the total energy derived gives the difference in free energy between the all-axial and all-equatorial conformations.
How do you determine the most stable chair conformation?
Add up the A-Values for each Axial Substituent to determine chair conformation stability; the lower the number, the more stable it is.
Which chair conformation is most stable?
The chair form of cyclohexane is the most stable conformation; the C-C-C bonds are very close to 109.5sup>o/sup>, so it is almost free of angle strain, and it is also fully staggered, so it is free of torsional strain.
What is the most stable chair conformation for the following molecule?
Unlike the 2-D diagrams, cyclohexane exists in several 3-D conformations, the most stable of which is the chair conformation, which has the least ring and torsional strain. Other chair conformations include gauche, anti, eclipsed, and staggered.
What makes a chair conformation stable?
The chair conformation is more stable because there is no steric hindrance or steric repulsion between the hydrogen bonds; however, the equitorial form will be the most stable because the hydrogen atoms, or possibly other substituents, will not be touching each other.
Which of a )-( D is the most stable conformation?
As a result, the chair conformation (Option D) is the most stable cyclohexane conformation.
Is chair conformation more stable than boat?
The chair conformation of cyclohexane is more stable than the boat conformation because the C-H bonds are equally axial and equatorial in the chair conformation, i.e., six of the twelve C-H bonds are axial and six are equatorial, and each carbon has one axial and one equatorial C-H bond.
Why is a 1/3 cis disubstituted cyclohexane?
Because the methyl groups are not on adjacent carbons in the cyclohexane rings, gauche interactions are not possible. Cis and trans stereoisomers of 1,3-dimethylcyclohexane The other conformer has both methyl groups in equatorial positions, resulting in no 1,3-diaxial interaction.
Is a chair flip an enantiomer?
When a chair conformation “flips” from equatorial to axial and vice versa, there is no inversion of configuration; they are technically conformational isomers or the same molecule.
Are chair flips conformational isomers?
The Point of This Post Is To Describe How These Two Conformations Can Be Converted Into Each Other Through A Cyclohexane “Chair Flip” Same connectivity, different shape u2013 if ever there was a definition of ” conformational isomers,” this is it. The point of this post is to describe how these two conformations can be converted into each other through a series of bond rotations we call a “chair flip.”