2009 final Q6C

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Question 6C on the Winter 2009 Final Exam states:

"Name this molecule." The question features a cyclopentane molecule with an OH group on the 1st carbon and a Cl on the 2nd carbon.


How can you tell that this molecule is cis?

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and What would trans for this specific molecule be like?

[ch1995]

To determine cis/trans, we would look at the atoms by priority described on pg. 16 of the organic chem textbook.

In the diagram of Q6C, we look at the carbons with non-hydrogen atoms attached. First, if we look at the Carbon at the top of the pentagon, there is an OH and H attached to it. From pg. 16, we see that OH > H, so OH is given the priority.

Then we look at the Carbon to the right of that with a Cl and H attached to it. Cl > H so Cl is given the priority.

We then compare the location of OH and Cl. As seen, they are both on the "top" half of the bond (both pointing upward), or the same side. Therefore the molecule is cis.

If they were pointing in different directions (OH pointing up and Cl pointing down or the other way around.), then they are located on different sides of the bond and the molecule is trans.

[Chem_Mod]

Compare the two structures below for cis-2-chloro-cyclopentanol and trans-2-chloro-cyclopentanol. In the structure that is cis, we see that the OH and the Cl groups are pointing up. Whereas in the trans structure, the OH points up and the Cl points down.