E/Z Configuration: 2011 Final Question 7B

[Mara Lockhart 3J]

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!

[Niharika Reddy 1D]

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.

[Chem_Mod]

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]

Okay, thank you so much!

[Christopher Phung 3G]

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?

[Chem_Mod]

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).