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Topic: Cis/Trans Isomers  (Read 15526 times)

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aficesk8ingal

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Cis/Trans Isomers
« on: September 13, 2004, 09:29:03 PM »
 ???Hi!!  I was wondering if someone could please help me understand the cis/trans isomerism of cyclic structures.  I understand that cis is on the same side of the bond, and trans is opposite.  However, do all cycloalkanes have cis/trans in their names?  For example, I wrote a structure to be names 1-bromo-2-ethyl-3,4-dimethylbutane.  Would I need to put the cis or trans out in front?  Also, say if I had two functional groups, such as a methane and a bromine.  If the groups are on different carbons, like 1 and 2, would this have this cis/trans?  Such as 1-bromo-2-ethylcyclohexane?  Sorry if this question is a little confusing.. I'm kinda confused just explaining what I'm confused with!  Any insights would be amazing!!!!!!!!!!!!!  Thanks sooo much!!!  ;D

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Re:Cis/Trans Isomers
« Reply #1 on: September 14, 2004, 12:27:44 AM »
Cis and trans are actually rather imprecise ways of naming compounds and lead to a lot of confusion.  Unfortunately it's part of the chemistry vernacular.

Your first example doesn't make sense because it isn't a cyclic structure so cis and trans don't mean much (they would be dependent on the way you drew the structure).  Are you sure that there isn't a "cyclo" in there somewhere?  The name you wrote should actually be 3-(bromomethyl)-4-methylhexane, by the way.

Cis and trans can apply to any two groups in a cyclic system, so yes, 1-bromo-2-ethylcyclohexane would have a cis and a trans isomer.

aficesk8ingal

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Re:Cis/Trans Isomers
« Reply #2 on: September 14, 2004, 02:31:52 PM »
Thanks for the reply!  Yes, actually I did mean to put a cyclo in that compound name.  I think that helped a little bit.  However, I am still a little confused about when the cis or trans is absolutely necessary in the compound name.  For example, if you had a ring with only one functional group, this couldn't be cis or trans correct?  Also, does it matter what carbons the functional groups are on for it to be cis or trans?  I have just been drawing my structures so that either the hydrogens are both up, or both across so that I would know what one it would classify as.  Thanks so much again for your *delete me*  I really appreciate it!  :)

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Re:Cis/Trans Isomers
« Reply #3 on: September 14, 2004, 03:38:27 PM »
You are correct that a cyclic compound with only one functional group does not have cis or trans isomers.

It doesn't matter where the functional groups are, just the relationship between their geometries.  You can even refer to groups as cis or trans when you have more than two groups, you just have to make it clear which groups you are talking about.  This is where the naming gets a little muddy.  Suppose you have 1,2,3-trimethyl-cyclopentane.  Then suppose that the methyls at 1 and 2 are cis to one another, but the ones at 2 and 3 are trans to one another.  You can say that 1 and 3 are trans or whatever, but naming this compounds can be confusing.  The correct way to name a compound like that is to use the (R) and (S) configurations of the stereocenters.  This gets more confusing when the material is racemic though.  The naming conventions aren't really perfect.


BTW, this is a little off topic, but in my example the methyl group at the 2 position is actually not a chiral center.  It is what is called a "chirotopic" carbon.  I guarantee that you don't need to know that for your class though.
« Last Edit: September 14, 2004, 03:39:01 PM by movies »

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Re:Cis/Trans Isomers
« Reply #4 on: September 14, 2004, 03:57:26 PM »
I always thought that you could have cis/trans isomers in terms of linear compounds as well.  Like if you had a straight chain hydrocarbon with two hydroxyl groups coming off of each other.  If they were on the same side, it would be a cis-isomer, and on opposite sides it would be a trans-isomer.  I didn't think any groups could fit inside the center of a cyclic compound such as benzene or cyclohexane.  (For cyclic compounds, I'm only familiar with ortho, para, and meta isomers).
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aficesk8ingal

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Re:Cis/Trans Isomers
« Reply #5 on: September 14, 2004, 08:13:47 PM »
Thanks every!!!!  I really appreciate all your help... u guys are amazing!  ;D

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Re:Cis/Trans Isomers
« Reply #6 on: September 14, 2004, 08:36:53 PM »
Sometimes you see people refer to acyclic structures as cis or trans, but this is entirely dependent on how the structure is drawn.  This makes it imprecise in that you can't necessarily recreate the structure from the name alone.  An example is the first part of the attachment.

The second part of the attachment is an example of cis/trans isomers in a cyclic system.

The "same side"/"opposite side" determination is relative to the "plane" of the ring.  If you were to flatten out the cyclohexane ring in the perspective drawing you would have the two methyls on the same side in the cis conformation, opposite in the trans conformation.  In a cyclic system you can't rotate around the bonds in the same way that you can in an acyclic system, so you don't have to worry about how it was drawn (there is only on way).

You can't have cis/trans isomers in benzene because it's flat.  The ortho/meta/para tags refer to the relative positions around the ring.  Ortho is 1,2; meta is 1,3; para is 1,4.  These don't change based on the relative stereochemistry at the substitutions.  

So no, groups can't fit inside the rings unless the ring is very large.
« Last Edit: September 14, 2004, 08:39:54 PM by movies »

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Re:Cis/Trans Isomers
« Reply #7 on: September 14, 2004, 11:08:16 PM »
The picture examples are perfect.  Thanks!  I understand it now.  Now if a double bond gets involved, however, things can change since there is no rotation around the bond, correct?
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Re:Cis/Trans Isomers
« Reply #8 on: September 15, 2004, 02:18:13 AM »
Cis and trans terminology with respect to double bonds is a little different than what has been discussed above, but essentially yes, there are cis and trans forms because the bond can't rotate.  The cis and trans terminology for double bonds is faulty because it doesn't work for tri- and tetra-substituted olefins.  The E/Z nomenclature was developed to account for these.  E/Z nomenclature is based on the priority of the groups, much like (R) and (S) for chiral centers.

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Re:Cis/Trans Isomers
« Reply #9 on: September 16, 2004, 10:54:32 AM »
Geodome, the two compounds you have drawn are the same.  You would need to change one of the methyl groups to something different.
« Last Edit: September 16, 2004, 11:01:28 AM by movies »

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Re:Cis/Trans Isomers
« Reply #10 on: September 16, 2004, 01:52:21 PM »
Geodome, the two compounds you have drawn are the same.  You would need to change one of the methyl groups to something different.

IUPAC adopts a better naming system in place of cis-trans, using the E-Z notation.

A substitutent is consider higher priority if it has a greater sum of individual atomic mass. Eg. CH3 (12+3X1) is of higher priority than H (1).

The letter Z is used when the higher priority substituents are on the same side of the C=C (or ring), whereas the letter E is used when the higher priority substituents are on different side of the C=C (or ring).

Below is the corrected picture:
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

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