[novice]

On 1-isopropyl-4-methylcyclohexane, is the Carbon on the hexane ring (Carbon 4) bonded to the methyl group a stereocentre/chiral centre? I’m struggling with this as the isopropyl group para to the methyl group is tetrahedrally orientated, so that depending on the position of the two methyl and one hydrogen attached to the central carbon in the isopropyl group, the carbon 4 attached to the methyl group, may or may not be a stereocentre?



Thankyou once again!

[sjb]

I'm interested to hear why you feel that the relative position of the two methyl groups could possibly cause stereoisomerism.

Note also that "para" really only refers to 1,4-disubstitution in a benzene ring, so is not strictly correct here.

S

[novice]

I am trying to ascertain whether carbon 4 (red) is a stereocentre. the way we've been taught (most likely a simplified version) is that a stereocentre is attached to 4 different groups. Carbon 4 is attached to a CH3, a H and two other carbons as part of the ring. We have been taught to follow the ring from either side, until we find that if one side is different to the other, this constitutes 4 different groups. So basically, i want to know whether Carbon 4 is attached to 4 different groups and thus is a stereocentre/chiral centre?

Thankyou!

[sjb]

OK, if you imagine a mirror on the line between carbons 1 and 4 in the ring, you can see that the two halves coincide, so there is no stereocentre on those atoms (at a simple level).

Remember that single bonds (usually) imply free rotation, so in this case the fact that there is a conformation that has the two methyl groups in the isopropyl group "over one side" is in this case irrelevant.

S

[english]

This is a simple question.  All you need to pay close attention to is the definition of what an asymmetric (chiral) center is.

[novice]

Thats what i thought, but the question is:

How many stereoisomers are there of 1-isopropyl-4-methylcyclohexane?

And apparantly the answer is 2...so there must be one stereocentre, however that was the one carbon (carbon 4) that had any chance of being a chiral centre in my view...and as you guys have shown, it is not. So, is the answer 2 wrong? Does it indeed have no stereoisomers, or am i missing something?

[wilson]

Did you mistake these two drawings as enantiomers? They are the same molecule. Try 3D rotating them.

A bit off the subject, are single bonds always free to rotate?

[Yggdrasil]

Single bonds are not always free to rotate.  Sometimes, a structure will be drawn as having a single bond, but in reality, the bond will have some double bond character that will prevent rotation about the bond (e.g. the C-N bond in a amide).

Also, sterics can prevent rotation about some bonds.  For example, in the molecule shown here:

http://siggy.chem.ucla.edu/VOH/30A/Dinitrobiphenyl_(COOH)2.pdf#search='axial%20chirality'

sterics prevent the rotation about the single bond connecting the two aryl rings.

Chelation of metals can also affect rotation about single bonds, because specific conformations are needed to maintain the interaction.  Actually, I suppose any sort of interaction of the compound with some other compound can prevent rotation about single bonds.

[sjb]

Thats what i thought, but the question is:

How many stereoisomers are there of 1-isopropyl-4-methylcyclohexane?

And apparantly the answer is 2...so there must be one stereocentre, however that was the one carbon (carbon 4) that had any chance of being a chiral centre in my view...and as you guys have shown, it is not. So, is the answer 2 wrong? Does it indeed have no stereoisomers, or am i missing something?

Sorry it's taken a while to get back, I've been busy IRL.

However, the answer is correct, there are two stereoisomers of 1,4-disubstituted cyclohexanes, but this is more E/Z (or cis/trans isomerism) - geometric isomerism is what it used to be called, but I don't think IUPAC like that term anymore.

See e.g. Case study 2a.4 Di-substituted cycloalkanes at http://www.wpbschoolhouse.btinternet.co.uk/page07/isomerism2.htm

S

edits; not enough coffee, didn't make sense