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Topic: How can someone identify cis and trans here?  (Read 1694 times)

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Offline INeedSerotonin

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How can someone identify cis and trans here?
« on: December 20, 2019, 05:41:38 PM »


The exercise says "It is possible to say that gossyplure (this substance) has two cis double bonds. Assign true or false."

I have no idea how to solve this. The answer is "false. The first (left to right) double bond is cis, but the last (left to right) is trans."

How can I even identify that with this image?  ???

Thanks

Offline Borek

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Re: How can someone identify cis and trans here?
« Reply #1 on: December 20, 2019, 05:47:44 PM »
How are trans and cis defined?
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Offline AWK

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Offline INeedSerotonin

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Re: How can someone identify cis and trans here?
« Reply #3 on: December 21, 2019, 08:11:17 AM »
I've learnt that cis is when we have the same element on the same side of the double bond, and trans is when we have to "cross" the double bond to find the same element.



Is this the correct reasoning to solve this exercise? I had never seen it before. This is very interesting. I suppose benzene is all cis too, according to this reasoning.

Offline AWK

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Re: How can someone identify cis and trans here?
« Reply #4 on: December 21, 2019, 08:51:59 AM »
Trans geometry is only possible with an eight-membered ring. There is no need to discuss cis/trans geometry for smaller rings.
https://wiki2.org/en/Trans-Cyclooctene
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Offline MNIO

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Re: How can someone identify cis and trans here?
« Reply #5 on: December 26, 2019, 12:45:30 PM »
cis and trans depends on which side the "highest priority group" is on.  It has nothing to do with the "same atoms"

you have these double bonds

LEFT double bond..
      R - CH2      CH2 - R'
              \       /
               C = C
               /      \
             H        H

the left C in that double bond is bonded to a CH2-R and an H
the priority goes by the highest atomic mass.  In the case of a tie, you look at atoms the C or H is bonded to
    C in CH2-R has atomic mass = 12
    H bonded to the C= has atomic mass = 1
    12 > 1 so the CH2-R gets the priority

           has priority
           ↓
     R - CH2      CH2 - R'
              \       /
               C = C
               /      \
             H        H
over this ↑

the C on the right is the same way.  C in CH2-R' has > atomic mass than the H so that has priority

       priority groups on SAME SIDE = cis
           ↓          ↓
     R - CH2      CH2 - R'
              \       /
               C = C
               /      \
             H        H

**********
the RIGHT side C = C goes like this

             H        CH2 - R'
              \       /
               C = C
               /      \
       R - CH2      H

same deal.. the left C in C=C has H and CH2-R groups attached... the C in CH2-R has higher atomic mass than the H so that gets the priority.  The right C in C=C is bonded to CH2-R' and an H.  the CH2-R' gets the priority and we have this setup
                     priority
                       ↓
             H        CH2 - R'
              \       /
               C = C
               /      \
       R - CH2      H
             ↑
         priority

in this case, the priority groups are on OPPOSITE sides and this is a "trans" bond

**********
to demonstrate priority groups, check out this double bond
       CH3      CH3
         \        /
          C = C
          /      \
        H        CH2-CH3

you might think that's "cis" because the CH3's are on the same side.. but that's WRONG.
the priority group on the left is CH3
on the the right =C there are 2 groups attached.. -CH3 and -CH2-CH3
you can see that both atoms attached to the =C are C's
so we turn to the next atoms in line
    on the top.. 3 H's
    on the bottom.. 2 H's + 1 C
that 1 C makes the CH2-CH3 the priority group (it has higher molar mass than the H
giving is this setup

    priority
      ↓
      CH3      CH3
         \        /
          C = C
          /      \
        H        CH2-CH3
                   ↑
                priority

and the priority groups are on OPPOSITE sides making this a "trans" double bond.

Offline MNIO

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Re: How can someone identify cis and trans here?
« Reply #6 on: December 26, 2019, 01:47:44 PM »
benzene doesn't have cis and trans isomers.  it has conjugated double bonds and is a planar molecule

Offline Borek

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Re: How can someone identify cis and trans here?
« Reply #7 on: December 26, 2019, 02:44:13 PM »
cis and trans depends on which side the "highest priority group" is on.

To some extent - but in general, no.

Quote
the priority goes by the highest atomic mass.

You have made that up. Yes, in E-Z notation we use Cahn–Ingold–Prelog priority rules, part of which is comparison of atomic masses. No, cis-trans notation is not equivalent to E-Z notation and should be avoided whenever there are more than two substituents on both sides of the double bond.

Quote
to demonstrate priority groups, check out this double bond
       CH3      CH3
         \        /
          C = C
          /      \
        H        CH2-CH3

you might think that's "cis" because the CH3's are on the same side.. but that's WRONG.

Yes, that's wrong. It is better to avoid naming this molecule using cis-trans notation.
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Offline MNIO

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Re: How can someone identify cis and trans here?
« Reply #8 on: December 26, 2019, 04:08:20 PM »
Borek... sounds wonderful that we would all be using the E-Z system... but that's just not the case.  Nor are we all using current IUPAC nomenclature.  And some of us still use normality.. right?

https://pubchem.ncbi.nlm.nih.gov/compound/cis-3-Methyl-2-pentene
https://pubchem.ncbi.nlm.nih.gov/compound/trans-3-Methyl-2-pentene
https://www.chemicalbook.com/ChemicalProductProperty_EN_CB4239583.htm
etc.
note the structure and the nomenclature.

yes.. books like Morrison and Boyd use cis and trans for di-substituted double bonds and E-Z for anything else.  No not everyone follows those guidelines.

My answer is accurate.  It's how many of us and many references determine and use "cis" and "trans" which was the question. 


Online sjb

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Re: How can someone identify cis and trans here?
« Reply #10 on: December 27, 2019, 08:19:57 AM »
As suggested upthread, this compound demonstrates the ambiguities in simple cis/trans naming. From the Aldrich link, it would appear they name it cis because the highest groups on either side of the double bond are cis to each other. You could conceivably name it trans because the methyl groups are trans.


Offline MNIO

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Re: How can someone identify cis and trans here?
« Reply #11 on: December 27, 2019, 11:39:03 AM »
sjb.... I told you how cis and trans are named in 3-methylpent-2-ene.

this is cis
    CH3         CH2CH3
         \        /
          C = C                the methyl on the left is cis to the ethyl on the right
          /      \
        H        CH3

this is trans
    CH3      CH3
         \        /
          C = C                 the methyl on the left is trans to the ethyl on the right
          /      \
        H        CH2-CH3

you will never find the opposite naming anywhere in literature

trans
https://www.fishersci.com/shop/products/trans-3-methyl-2-pentene-tci-america-2/M03121ML
cas # 616-12-6

cis
https://www.fishersci.com/shop/products/cis-3-methyl-2-pentene-tci-america/M03115ML#?keyword=cis+-3-Methyl-2-pentene
cas # 922-62-3

This discussion is the reason why E-Z came to be.

Offline MNIO

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Re: How can someone identify cis and trans here?
« Reply #12 on: December 27, 2019, 11:56:37 AM »
notice this compound
http://www.chemexper.com/chemicals/supplier/cas/10574-36-4+Z-3-Methyl-2-hexene.html
cas # 10574-36-4

    CH3         CH2CH2CH3
         \        /
          C = C                this is called "(Z)-3-methyl-2-hexene" and "cis-3-methyl-2-hexene"
          /      \                by various multiple chemical suppliers.  not trans
        H        CH3

and this
http://www.chemspider.com/Chemical-Structure.4509522.html
cas # 17618-77-8

    CH3         CH3
         \        /
          C = C                this is called "(E)-3-methyl-2-hexene" and "trans-3-methyl-2-hexene"
          /      \             
        H        CH2CH2CH3

it's not "cis".  You'll never find that called "cis" in literature.

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Whether you like it or not, that's the way these molecules are named. If you don't like "cis / trans" use E-Z

Offline Borek

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Re: How can someone identify cis and trans here?
« Reply #13 on: December 27, 2019, 01:34:34 PM »
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996))
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Offline MNIO

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Re: How can someone identify cis and trans here?
« Reply #14 on: December 28, 2019, 02:21:27 PM »
Borek..
  (1) You copied and pasted a 1996 "unauthenticated" something and a 1993 publication
        by Blackwell publications that calls itself a "guide" and refers to the IUPAC rules and
        states that E/Z has largely replaced cis / trans.  I'm not arguing with that.
  (2) The original question was "how can someone identify cis and trans".  right?  It wasn't
        "has cis / trans been replaced with E/Z" was it?
  (3) IUPAC does in fact discourage the use of "cis" and "trans" in favor of E-Z in their preferred names
        see "Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names, 2013" by
        Favre and Powel (that is the IUPAC BLUE BOOK published by IUPAC with their rules in it.  The horses
        mouth).  Chapter P-9 has the relevant rules.  But again, the question wasn't which is the preferred
        system was it?

continuing with the actual IUPAC blue book.
        here's an image from P-92.1.1(e)
       
         notice the 4 ligands
         P-92.1.2 states that CIP rules are used to prioritize a,b,c,d
         P-92.1.3 defines the rules to determine order of precedence.. each to be applied exhaustively before
            the next is applied.  in brief they are:
              (1) higher atomic number has priority
              (2) higher atomic mass has priority
              (3) when considering double bonds with tetraligand atoms "seqcis" = "Z" > "seqtrans"="E"
              (4) refers to chiral centers
              (5) states that R > S
          P-92.4 explains "rule (3)" in detail and states again that "seqcis" = "Z" and "seqtrans" = "E"
             and states that Z and E are PIN (Preferred IUPAC Nomenclature) and states that
             Z and E are derived from the German words "Zusammen" and "Entgagen" (same and opposite)
         here's an image you might find interesting from P-92.6

         where IUPAC determined by rule (1) that CH3 > H and by rule (5) R>S and called this thing "TRANS"
         not "E" in the PIN
         The rest of the chapter is focused giving examples of those 5 sequencing (or priority) rules.
  (4) IUPAC isn't always consistent.  They seem to equate "cis/trans" with "E/Z" and USUALLY declare
       E/Z to be the PIN.  But not always.
  (5) You can see by my examples, there is a world packed full of chemical manufacturers, suppliers,
        marketing folks, researchers, chemical users, government agencies, etc that use "cis/trans"
        nomenclature even when there are 3 or more ligands (non-H atoms) attached to the double
        bonded C's.  Argue if you like... but all those MILLIONS of people do indeed use cis/trans. 
        And they determine which is which exactly as I described.

back to the original question.. "how do you determine cis vs trans".  My answer is accurate.


« Last Edit: December 28, 2019, 02:42:33 PM by MNIO »

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