Help on Problems 1.28 and 1.29  [ENDORSED]

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ntruong2H
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Help on Problems 1.28 and 1.29

Postby ntruong2H » Sat Mar 11, 2017 2:33 pm

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For 1.28, why do we have to specify E or Z? I understand that E=trans and Z=cis; however, I do not get why or when we have to use these? For instance in our course reader, on page 102, there is a similar example to problem 1.28 and we did not have to specify E or Z? Thank you.

The question also applies to 1.29, as I only know how/when to use E vs Z when comparing two different substituents (as in the examples on page 103 in the course reader). Also, I was wondering why my answer (written in pencil) is wrong? (The correct answer is written in blue)
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@Chem_Mod Could this please be clarified? Thank you!

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Re: Help on Problems 1.28 and 1.29  [ENDORSED]

Postby Chem_Mod » Sat Mar 11, 2017 4:13 pm

For the first part: E & Z are not necessarily the same as cis and trans, but they are similar. Cis/trans is used to designate orientation around a double bond when there are only two substituents that are important. E/Z is used for more complicated situations i.e when there are multiple substituents around a double bond. The reason we use these is to specify the sterochemical orientation around a double bond.

As for your answer, your molecule is the E form of the molecule. The question asks for the Z form, where the substituents of highest priority are on the same (z-ame) side.

ntruong2H
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Re: Help on Problems 1.28 and 1.29

Postby ntruong2H » Sat Mar 11, 2017 6:37 pm

Chem_Mod wrote:For the first part: E & Z are not necessarily the same as cis and trans, but they are similar. Cis/trans is used to designate orientation around a double bond when there are only two substituents that are important. E/Z is used for more complicated situations i.e when there are multiple substituents around a double bond. The reason we use these is to specify the sterochemical orientation around a double bond.

As for your answer, your molecule is the E form of the molecule. The question asks for the Z form, where the substituents of highest priority are on the same (z-ame) side.


I see. Thank you! So when do we need to specify the different isomers? As in, on our example on page 102 in the course reader, should the correct name also involve E or Z? Also, when there are multiple double bonds in the structure, which double bond do we base our focus on when trying to figure out the orientation of the substituents?

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Re: Help on Problems 1.28 and 1.29

Postby Chem_Mod » Sat Mar 11, 2017 9:03 pm

Whenever there is more than one conformation possible, you need to specify the E/Z/Cis/Trans. If you have multiple alkenes, you would need to specify the geometry around each. If a molecule had double bonds on carbons 2 and 4, it would look something like (2Z, 4E).


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