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To determine cis and trans, you have to look at whether both of the groups are "up" or "down" in terms of the plane of the molecule. For example, in this example, both of the propyl groups are above the hydrogens that are bonded to those carbons, so this is a cis-molecule. I have attached a picture of what a trans chair conformation would look like. See how one Cl is above the H and one is below- that's what makes it trans instead of cis.
- ring_f9.gif (1.52 KiB) Viewed 750 times
For the conformation of cyclohexanes some basic guidelines are that the conformation is a chair and in a chair axial groups must alternate pointing up and down. In general for chair conformations larger substituents are placed equatorially because there is more room and less strain. However in the case of having substituents on C1 and C2 if we were to place both propyl groups equatorially we would have to have one H above the propyl and the other below. This would give us trans-1,2 - dipropylcyclohexane. To create cis-1,2-dipropylhexane both propyl group must be above (or below) their respective hydrogen, that is they have to be on the same side of the carbon ring. So we must place one of the propyl group in an axial position and the other in an equatorial.
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