Final 2012 Q5: configuration of a cyclooctenone

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geekatherine
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Final 2012 Q5: configuration of a cyclooctenone

Postby geekatherine » Thu Mar 13, 2014 9:29 pm

"Give the IUPAC name for molecule B."

The answer is Z-cyclooct-4-enone. I'm having trouble understanding why this molecule is cis. My understanding was that for cyclic molecules, we determine cis and trans based on whether the substituents are both "up" or "down" (cis) or one "up" and one "down" (trans).

704278444
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Re: Final 2012 Q5

Postby 704278444 » Thu Mar 13, 2014 10:34 pm

You also determine whether a molecule is cis or trans by looking at the bonds beside the double bonds. In molecule B, you see that both sides of the double bonds have atom on the same side, thus, it is cis.

geekatherine
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Re: Final 2012 Q5

Postby geekatherine » Thu Mar 13, 2014 10:43 pm

In that case, for Final 2013 Q6A, shouldn't the molecule be cis? I'm having a hard time determining when we need to identify cis and trans for cycloalkenes in general.

y3chem
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Re: Final 2012 Q5

Postby y3chem » Sun Mar 16, 2014 10:06 pm

it is quite tough to determine when to put cis and trans when dealing with cycloalkenes. Usually we put cis or trans down when there is are substituents that is pointing inward and/or outward from the chain (3D-wise).
both inward or both outward--> cis
one in and one out --> trans

http://chemwiki.ucdavis.edu/Organic_Chemistry/Hydrocarbons/Alkanes/Properties_of_Alkanes/Cycloalkanes/Rings%3A_cis%2F%2Ftrans_and_axial%2F%2Fequatorial_relationships

here is a post that talks about determining cis/trans in cycloalkenes.

Victoria Liang 3L
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Re: Final 2012 Q5

Postby Victoria Liang 3L » Sat Mar 14, 2015 1:24 am

Shouldn't this molecule be cis no matter what? Because if the double bond was trans, this molecule wouldn't be possible as stated on a similar question on this post:
viewtopic.php?f=86&t=3763

And because this molecule has only one substituent, wouldnt Z-cyclooct-4-enone be the same as cyclooct-4-enone?

Victoria Liang 3L
Posts: 11
Joined: Fri Sep 26, 2014 2:02 pm

Re: Final 2012 Q5

Postby Victoria Liang 3L » Sat Mar 14, 2015 1:29 am

Never mind, I miss read the post; it only said "Cyclopropene, cyclobutene, cyclopentene, cyclohexene"
However, I'm still a little confused on how to determine cis or trans when theres no 3D model, like in this question for Q5 on 2012 final.

Chem_Mod
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Re: Final 2012 Q5

Postby Chem_Mod » Sat Mar 14, 2015 1:50 am

The (Z) refers to the configuration of the double bond, not to substituents which are up or down. Note that there are no substituents anyway.

E/Z can only describe a double bond. Up/down substituents on a ring will be described by cis/trans.

In cyclohexene, the double bond may only be (Z), since (E) is too strained to exist. Since there is only one possibility, there's no need to write (Z).

In cyclooctene, the ring is actually large and flexible enough to allow the (E) form, so we need to specify which form.


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