Methylcyclohexane

I'm not a chemistry major, so I don't quite understand why the arrangements of the carbons and their neighbors are not tetrahedral in nature, so I am assuming they ARE tetrahedral in nature, which would make all the 6 carbons in the ring identical, although neighboring carbons would lean to opposite side as its neighbors.

Based on this premise, all possible connecting points for the methyl would be identical in form, and the answer would be that there is only one form for it.

However, it would seem that in actuality, there are a few varying distortions of the tetrahedral shape, and that within these slightly altered shapes, some carbons in the ring are distinctly different than others.

So for the chemistry major, the answer would appear to be greater than one. Very interesting.

Originally posted by Ponderable
* As pointed out if we take the boundary condition: Methlycyclohexane, there is only one.

* If we look at different steric conformations, which are separatble by matrix techniques and have small but distinguishible different energy contents we do have:

# Chair shaped with equatorial methyl
# chair shaped with axial methyl
# boat shaped with equato ...[text shortened]... quick that you "see" only an average signal when using NMR or IR to observe the conformations.

So basically you are saying the answer is 4?
(But statistically the most stable will predominate and therefore in practice the answer is 1?)

Evidently I was a bit unclear:

* There is only one isomer. (In terms of definition of isomers)

* There are four conformations in which this molecule can exist. These of course are extrema, which means, that all possible conformations between these can occur (but these can't be characterized or "nailed down". But you can isolate the four conformationsusing special techniques (not being taught at high school) and you can even estimate the energy differences.

Originally posted by geepamoogle
I'm not a chemistry major, so I don't quite understand why the arrangements of the carbons and their neighbors are not tetrahedral in nature, so I am assuming they ARE tetrahedral in nature, which would make all the 6 carbons in the ring identical, although neighboring carbons would lean to opposite side as its neighbors.

Based on this premise, all po ...[text shortened]... So for the chemistry major, the answer would appear to be greater than one. Very interesting.

I was a chemical biology major. There's only 1.

Originally posted by AThousandYoung
I was a chemical biology major. There's only 1.

Hmm, I was rethinking this, and while it is true that, in my estimation, which carbon in the ring the methyl attaches to makes no difference, which of the two hydrogens it replaces CAN make a difference, meaning there may be two distinct shapes.

The cyclo-carbon ring can be imagined to largely be associated with and center on a particular plane, although the three-dimensional nature of it will, of course, place 3 of the carbons slightly above it, and 3 slightly below it.

Now, for each carbon, it will link to a hydrogen that is moreorless in the same plane (perhaps on the opposite side heightwise), and one on the outside directly away from the plane.

Depending on which hydrogen is replaced with a methyl group, you'll have two slightly different shapes, one flatter than the other.

It may be, however, that any given molecule may form and reform and change between the two frequently enough that there is little distinction of note in all practicality.

Originally posted by geepamoogle
Hmm, I was rethinking this, and while it is true that, in my estimation, which carbon in the ring the methyl attaches to makes no difference, which of the two hydrogens it replaces CAN make a difference, meaning there may be two distinct shapes.

The cyclo-carbon ring can be imagined to largely be associated with and center on a particular plane, altho between the two frequently enough that there is little distinction of note in all practicality.

You're referring to the axial and equatorial positions on the ring. Yes, the ring flops back and forth. No matter where the methyl group attaches, it's position becomes axial immediately.

This is a question about isomers, and everyone keeps talking about comformations. Different conformations are not different isomers any more than dancing is like having an arm growing out of your face.

If person A is sitting, and person B is standing, and person C is extending one leg out in a side kick position - they're all shaped like normal human beings. They have different conformations but are not different isomers. But the guy with balls hanging off his chin, or hands for feet - his body is a different isomer.

This might help things - how many isomers of dimethylcyclopentane are there? Dimethylcyclopentane is isomeric to methylcyclohexane, which we've been discussing, but is not an isomer of methylcyclohexane. Does that make sense?

Originally posted by AThousandYoung
You're referring to the axial and equatorial positions on the ring. Yes, the ring flops back and forth. No matter where the methyl group attaches, it's position becomes axial immediately.

This is a question about isomers, and everyone keeps talking about conformations. Different conformations are not different isomers any more than dancing is like having an arm growing out of your face.

I see. That makes sense, I suppose.

Originally posted by AThousandYoung
This might help things - how many isomers of dimethylcyclopentane are there? Dimethylcyclopentane is isomeric to methylcyclohexane, which we've been discussing, but is not an isomer of methylcyclohexane. Does that make sense?

7

and it had been stated previously that there is only one isomer of Methylcyclohexane.

Originally posted by Ponderable
7

and it had been stated previously that there is only one isomer of Methylcyclohexane.

But dimethylcyclopentane is isomeric with methylcyclohexane. Do you agree?

Originally posted by AThousandYoung
You're referring to the axial and equatorial positions on the ring. Yes, the ring flops back and forth. No matter where the methyl group attaches, it's position becomes axial immediately.

This is a question about isomers, and everyone keeps talking about comformations. Different conformations are not different isomers any more than dancing is li ...[text shortened]... the guy with balls hanging off his chin, or hands for feet - his body is a different isomer.

No matter where the methyl group attaches, it's position becomes equatorial immediately.

Oops, mistake. There's the correct answer.

Originally posted by AThousandYoung
But dimethylcyclopentane is isomeric with methylcyclohexane. Do you agree?

Dimethylcyclopentane has a different number of atoms to Methylcyclohexane, thus the two CAN'T be isomeric to each other.

Isomers share the same number (and type) of atoms, but have a different arrangement of these.

Methylcyclohexane is C7H13. Dimethylcyclopentane is C7 H14.

But Methylcyclohexane is isomeric to Ethylcyclopentane or to Heptene, or to the different methylhexenes, or to diverse ethylpentenes.

The OP implied that he was interested in conformers. Conformers are isomers which undergo isomerisation very fast. So at room temperature you only have one methylcyclohexane with distinct properties. But not all the molecules do have the same conformation. You can even calculate the various proportions of conformations using the enbergies, I gave further up.

Originally posted by AThousandYoung
Originally posted by AThousandYoung
You're referring to the axial and equatorial positions on the ring. Yes, the ring flops back and forth. No matter where the methyl group attaches, it's position becomes axial immediately.


No matter where the methyl group attaches, it's position becomes [b]equatorial immediately.

Oops, mistake. There's the correct answer.[/b]

In fact the energy differnec between equatorial and axial is about 7 kJ/mol, thus at room temperature I expect only one NMR signal due to the fast interchange. This signal will split if you go to temperatures near to the freezing point of Methylcyclohexane.

Originally posted by Ponderable
Methylcyclohexane is C7H13. Dimethylcyclopentane is C7 H14.

Cyclohexane has the formula C6H12. Each methyl replaces an H with a CH3, making methyl cyclohexane C7H14.

Pentahexane has the formula C5H10, and with 2 hydrogens replaced by methyls (CH3), you also get the formula C7H14.

So the two are isomers of each other. The original question, however, indicates the number of isomers for ring of 6 carbons with a single methyl attached, for which only one isomer exists which qualifies..