[Violagirl]

For two chair isopropylcyclohexane groups, I have a steric energy value of 10.8592 kcal/mol for the equatorial position and a value of 15.6631 for the isopropyl axial position. I next had to calculate the difference in steric energy for the conversion of the isopropyl group from the equatorial to the axial position and found a difference of 4.80 kcal/mol. I now next need to find Keq for the conversion of equatorial to axial isopropylcyclohexane. In searching online, it looked like the equation I need to use will be e^-(G/R*T). My G would be my steric energy value (would I use 4800 cal/mol or 4.80 kcal/mol?), my R value would be the constant 1.986 and T would be 298 K if calculated from room temperature. Am I right on this at all?


Also after determining Keq, I have to find the percentage of the axial chair conformation compared to equatorial isopropyl overall at room temperature but am not sure what I would do for this. Any help is greatly appreciated!

[Dan]

My G would be my steric energy value (would I use 4800 cal/mol or 4.80 kcal/mol?), my R value would be the constant 1.986 and T would be 298 K if calculated from room temperature. Am I right on this at all?

As I mentioned to you before the :delta: G/RT term should be unitless. If you are using R = 1.986 cal K^-1 mol^-1, then you should use cal mol^-1 for energy and not kcal mol^-1:

Considering units only for :delta: G/RT:

cal / (cal mol^-1 K^-1 mol^-1 K)

The terms that cancel are colour coded.

Also after determining Keq, I have to find the percentage of the axial chair conformation compared to equatorial isopropyl overall at room temperature but am not sure what I would do for this. Any help is greatly appreciated!

K = [products]/[reactants]

[Violagirl]

Thank you for your response! I got a value of 3329. Does that seem right??

[Dan]

As the process of equatorial to axial is going from a lower energy state to a higher energy state, the overall energy change, :delta: G = +4.8039 kcal/mol (avoid rounding until the end of the question). It is a thermodynamically disfavoured change.

As equatorial -> axial is disfavoured, the value of K should be less than 1.

So it doesn't look right.

Don't forget the minus sign in e^{-(:delta: G/RT)}