help with organic chem Q please :)

etymon

Hi
Just wondering if someone could explain this answer to me:

Which of the following isomers has the lowest heat of combustion?

1. cis-1,2-dimethylcyclohexane
2. trans-1,2-dimethylcyclohexane
3. cis-1,3-dimethylcyclohexane
4. trans-1,3-dimethylcyclohexane

The answer is 3 but for reasons that are unclear to me. Firstly, I though trans isomers were more stable thus less reactive thus might have a lower heat of combustion; secondly why 1,3 dimethyl and not 1,2 - what is the difference?
Please help if you have any idea Thanks

Marvinthefish

I'd suggest drawing out all four structures in the chair representation and add the methyl groups. Or even better, make models so you can easily where the methyl groups are in space.


I drew them out and one looks like it would definitely have a lower heat of combustion. Why? You have to imagine how millions of the these molecules will interact with each other : bumping into each other and maybe stacking on top of one another as they whizz around the flask...

etymon

Thanks Marvin

I did that and figured out what I was doing wrong; I was mixing cis and trans with axial/equatorial... duh....

bazza1981

With 3 both the methyl's will occupy the equatorial position, which is the most stable confirmation
number 2 will also have this confirmation, but the reason 3 has a lower heat of combustion is as it has no steric interactions like 2 as the methyl is one carbon further away.
1 and 4 both have an eq and ax methyl.