E/Z Configuration: 2011 Final Question 7B

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Mara Lockhart 3J
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E/Z Configuration: 2011 Final Question 7B

Postby Mara Lockhart 3J » Thu Mar 12, 2015 10:58 am

For question 7B on the 2011 final, we are asked to give the IUPAC name of the structure (as seen in the attachment). I feel comfortable with naming the structure, up until deciding if it is E or Z. I through the molecule was E, because COOH gets the priority on one C and CH2 get the priority on the other. The answer is Z and I have no idea why. Please help!
Attachments
IUPAC Naming.png
Trans or Cis

Niharika Reddy 1D
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Re: E or Z Confusion

Postby Niharika Reddy 1D » Thu Mar 12, 2015 11:34 am

On the C that is attached to the carboxyl group and chlorine, Cl gets priority. We look at just the C in the COOH group, not the whole group collectively when determining priority, so since Cl has a greater atomic number than C, Cl receives priority. Both groups with priority are on the same side of the double bond so we use Z instead of E.

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Re: E or Z Confusion

Postby Chem_Mod » Thu Mar 12, 2015 12:22 pm

Niharika is correct. Use the atomic mass of the first atom, not of the whole group. Only if the first atoms tie, you move on to the second atoms, etc.

Mara Lockhart 3J
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Re: E/Z Configuration: 2011 Final Question 7B

Postby Mara Lockhart 3J » Thu Mar 12, 2015 8:01 pm

Okay, thank you so much!

Christopher Phung 3G
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Re: E/Z Configuration: 2011 Final Question 7B

Postby Christopher Phung 3G » Fri Mar 13, 2015 12:56 am

I'm really lost in all this.
I don't recall anything about priority!
How are cis and trans isomers named? I get that trans isomers are on opposite sides, and that cis are on the same side, but there seems to be more to it than just that.
I was working on this same problem and didn't think about whether or not it was an E or a Z isomer. I can't think of any other way you can draw this isomer and still make it a 2-chloropent-2-enoic acid molecule.
How should I look out for these isomers? And how would I name them after I've found an E or a Z isomer?

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Re: E/Z Configuration: 2011 Final Question 7B

Postby Chem_Mod » Fri Mar 13, 2015 1:08 am

Hi Christopher, cis and trans do indeed mean "same side" and "across" but only works for simple molecules where it is obvious what groups to compare.

If a double bond has four DIFFERENT substituents there is no obvious choice of groups, so E/Z is required. We use the priority system to assign the two groups which will be compared. If they are across we have (E), same side is (Z).


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