[jena]

Hi,

I have a question that states:

Which isomer is the most stable?

a. cis-1,2-dimethylcyclohexane
b. trans-1,2-dimethylcyclohexane
c.  cis-1,3-dimethylcyclohexane
d. cis-1,4-dimethylcyclohexane
e. not a-d.

My answer: b.

I'm not too sure about this question though because my teacher said to look for the most stable form of the cyclohexane, and then find if there is any tension. So based off what is said that I think that it would be the trans since the methyl were away from each other. Is that right to come to a conclusion like that.

Thank You

[dexangeles]

first thing you need to do is view this in it's chair comformation.  this is the only way you will be able to distinguish which one is the most stable amongst all four.

try this website and it might help
http://neon.cm.utexas.edu/bauld/teach/cyclohex.html

by the way...regarding your title "enantiomers"
I believe that a-d are actually diasteriomers and not enantiomers
as far as I remember, enantiomers are non-superimposable mirror images, while diasteriomers are non-enantiomers
well, a-d are not mirror images of each oter
a compound will always have only one enantiomer, so all 4 can't be

[jena]

Hi,

Sorry about the title . I think I have an idea of what's going on. Is the answer going to be cis-1,4-dimethylcyclohexane, because its in the equatorial positon, and there is less steric hinderance, in comparison of the other options. Is this correct?

Thank You

[dexangeles]

hint:having both methyl groups in equatorial position would make it more stable

a. cis-1,2-dimethylcyclohexane ---> 1 equatorial and 1 axial substituent

b. trans-1,2-dimethylcyclohexane ---> both methyl will be on equatorial

c.  cis-1,3-dimethylcyclohexane ---> both methyl will be on equatorial

d. cis-1,4-dimethylcyclohexane ---> 1 equatorial and 1 axial substituent

note:forget the flipping of the chair conformation for now

between b and c, my answer would be c
although b has both substituents also on the equatorial position, it has more steric hindrance between the substituents than the substituents of c

[movies]

A note: only two of the options are actually diastereomers of one another (a and b).  To be diastereomers, the two compounds must have the same atom connectivity, and differ only in stereochemistry.  Furthermore, as stated, they two molecules must not be enantiomers to be diastereomers.

Check out the isomer heirarchy on the downloads page for clarification.

http://www.chemicalforums.com/dls/Isomers2.jpg