Todd:Chem3x11 ToddL3

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Chem3x11 Lecture 3

Being built (as of Sat, Apr 14).

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Key concepts

  • The preferred conformation of a substituted cyclohexane depends on what's attached to it
  • Two cyclohexanes can be fused to give decalins
  • Attachment of substituents to cyclohexanes can lock their conformations, giving complex, but very information-rich NMR spectra

Cyclohexanes with Two Substituents

cis and trans refer to Relative Stereochemistry. Nothing to do with Ring Flipping

We need to be clear on one thing from the outset. cis and trans isomers do not interconvert - they are separate structures that are configurational isomers (diastereomers). It's not like you can convert one to the other with a ring flip. Here are the cis and trans isomers of 1,2-dimethylcyclohexane.

Scheme 1: cis and trans Diastereomers of 1,2-Dimethylcyclohexane
Scheme 1: cis and trans Diastereomers of 1,2-Dimethylcyclohexane

Drawings like this allow us to see relative stereochemistry of those methyl groups, but we need to be happy thinking about the rings in 3D so we can think about what the preferred conformations might be. The easiest way to do this is to draw a cyclohexane ring, draw in one substituent, then draw in the other given the relative stereochemistry. Then perform a ring flip to get the other conformation. Here are the conformations for the cis isomer.

Scheme 2: Conformations of the cis Diastereomers of 1,2-Dimethylcyclohexane
Scheme 2: Conformations of the cis Diastereomers of 1,2-Dimethylcyclohexane

In both cases one methyl is equatorial and one is axial. The trans isomer is different.

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