E/Z vs. cis/trans

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Mayvera Doaran 2H
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E/Z vs. cis/trans

Postby Mayvera Doaran 2H » Thu Mar 10, 2016 10:40 am

Is there a difference between E/Z and cis/trans? I know for E/Z, priority goes to the larger molecule when naming. What my TA told me was that for cis/trans, you must look at the longest carbon chain rather than the larger molecule. For instance, when deciding priority between Cl or CH2CH2CH3, CH2CH2CH3 would get priority when naming with cis/trans. Is that true?

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Re: E/Z vs. cis/trans

Postby Chem_Mod » Thu Mar 10, 2016 12:17 pm

No. Officially, cis/trans only work when you have two identical group on two sides of the double bond. But Dr. Lavelle allows you to use interchangeably with E/Z. Thus, you follow the priority rule as when you determine E/Z.

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Re: E/Z vs. cis/trans

Postby Chem_Mod » Thu Mar 10, 2016 12:37 pm

When molecules take on higher levels of complexity, it is not always entirely clear how to assign simple cis/trans designations because perhaps all 4 possible groups attached to the carbons of a C=C double bond are different. For the molecule in the image below, we might choose to name it as "cis" and it would make perfect sense as long as we understood to consider the more massive groups on either side of the double bond for such naming conventions - which is not entirely accurate.

There is technically no direct connection between cis/Z and trans/E. The cis/trans and E/Z conventions are based on different naming rules.

In (Z)-1-bromo-2-chloro-1-fluoroethane (see attached image below), we name the molecule based on the highest priority groups (following the Cahn-Ingold-Prelog priority rules naming system). The more massive substituents on each carbon are on the same side.

In (E)-2-bromobut-2-ene (see attached image below), according to the rules of cis/trans naming, we would name it as a cis, because the longest continuous carbon chain making up the substituents has both substituents on the same side. However, this is actually named not as a Z, but as an E in the E/Z system because the highest priority groups on each side of the double bond (CH3 > H and Br > CH3) are across from one another.

However, for our simpler molecules, we may use E/Z and trans/cis interchangeably. You should be aware this is not entirely rigorous.
Attachments
E2brbut2ene.png
E2brbut2ene.png (10.8 KiB) Viewed 484 times
1br2cl1fethene.png


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