[luvmath03]

I was helping another student and now I have a question. 

1 methyl cyclohexane  chair conformations first when methyl is axial then when equitorial....shows 7 kJ/mol energy change....

now same molecule but take carbons 3,5  make them oxygens.   axial vs equitorial methyl is 3 kJ/mol difference in energy. 

Explain this smaller change..

I thought electrons  around O were going to push electrons floating on H's of methyl group away more than just H electrons...of carbons but that was same answer she gave and it was wrong. 

Can you help?

[contra]

Perhaps the electrons on the oxygen are attracted to the hydrogens on the methyl group?

Rather than being opposed to the bonding electrons.

[BluePill]

On top of my head, I'm thinking of 1,3-diaxial interactions. The methylcyclohexane (axial methyl) has 1,3-diaxial interaction. Without the -CH₂- on both 3,5 position, there is no 1,3-diaxial interactions. That's the simplest idea I have.

[fledarmus]

BluePill is right - the lone pair electrons on oxygen are sterically smaller than the hydrogen bonded to a carbon would be. 1,3-diaxial interactions are much less. I couldn't find a table that quoted an A-value (http://en.wikipedia.org/wiki/A_value) for a lone pair, but I seem to remember it being around -1 to -1.7 kcal/mol.

The conformation of the ring also changes slightly for 1,3-dioxane - the chair conformation of cyclohexane is slightly distorted. The O-C-O section of the ring is more bent and the C-C-C section of the ring is flatter in 1,3-dioxane than cyclohexane, meaning that the axial 5-Me substituents is tilted further out of the ring in dioxane than in cyclohexane. This reduces the 1,3-diaxial interactions even further.

Of course, the entire analysis changes if there are groups on your substituents that can interact with the lone pairs.

For further information, see http://pubs.acs.org/doi/pdf/10.1021/ja01015a029 or http://www.iupac.org/publications/pac/pdf/1971/pdf/2503x0509.pdf for some of Professor Eliel's seminal work in the area.

[orgopete]

Explain this smaller change..

I thought electrons  around O were going to push electrons floating on H's of methyl group away more than just H electrons...of carbons but that was same answer she gave and it was wrong. 

Is that a typo or is that what was wrong? Smaller should correspond with less, no?

[Smrt guy]

1,3-diaxial interactions are nuclear repulsions, not electron cloud repulsions.  By switching from CH to O you take out the nuclear repulsion.  The electron cloud repulsion is still there (which is why you see some difference) but it has less effect despite the fact that the lone pair electrons tend to take up more space than those found in a bond (they do not have to remain between nuclei).

[Honclbrif]

"1,3-diaxial interactions are nuclear repulsions, not electron cloud repulsions."^{[citation needed]}

[mickelcorleone]

good luck