[madscientist]

I would appreciate some guidance with one of the quiz questions.  I have a question that
asks for the 'main product' of an elimination reaction on a haloalkane.  I have narrowed
it down to three possibilities:

.  cis-alkene
.  trans-alkene
.  mixture of trans-alkene and cis-alkene

Most of my research says that the trans-alkene is more likely to be the product than the
cis-alkene because trans- is the more stable product.

I am at the point of choosing between "trans-alkene" or "mixture of trans-alkene and cis-
alkene".  The "trans-alkene" option implies that I know that "trans-" is more stable.  
However, the "mixture" implies that I do not need to know which is more stable.  A
mixture of "trans-" and "cis-" is more likely than just one or the other.

I know it is hard to ask for help without giving away the answer.  Let me put it this way:

Does "mixture of trans-alkene and cis-alkene" mean a racemic homogeneous mixture of
the two?  

Thanks.

madscientist :albert:

[movies]

Careful with your terminology.  Racemic refers specifically to a 50:50 mixture of enantiomers.  Cis and trans isomers are not enantiomers but rather diastereomers.

At any rate, I think that the question would be eroneous if the correct answer were that the "major product" is a mixture.  For that to make sense, I suppose it would have to be a 1:1 mixture of the cis and trans isomers, otherwise whichever isomer was more prevalent in the mixture of isomers would be the major product!

[madscientist]

Thanks movies,

much appreciated

[laotree]

enantiomers and diastereomers are the terms in chirality, have nothing to do with trans or cis. Trans and cis isomers are not diastereomers!!!!!!!!

RR/SS enantiomers; RR/RS diastereomers.

Trans-alkene might be the major because it is termodynamically more stable. But, the reaction condition, starting material etc are very important in trans or cis.

[Winga]

enantiomers and diastereomers are the terms in chirality, have nothing to do with trans or cis. Trans and cis isomers are not diastereomers!!!!!!!!

RR/SS enantiomers; RR/RS diastereomers.

Trans-alkene might be the major because it is termodynamically more stable. But, the reaction condition, starting material etc are very important in trans or cis.

'Diastereomers are stereoisomers that are not enantiomers or mirror images of each other', quoted from Wikipedia.

Trans/Cis isomers fulfill the above criterion.

[laotree]

Trans or cis double bonds (regioisomers, I guess) are not chiral. Diastereomers must have chiral centers. I never heard chemists talk about cis/trans isomers as diastereomers.

I might be wrong, Movies has very strong background in chemistry.

http://www.usm.maine.edu/~newton/Chy251_253/Lectures/Chirality/Chirality2.html

[Winga]

'So what is the definition of diastereomers? Diastereomers are stereosiomers that are not enantiomers.'
http://www.usm.maine.edu/~newton/Chy251_253/Lectures/Chirality/Chirality2.html

It also doesn't mention diastereomers must have stereocenters (chiral centers).

If you agree with that cis/trans isomers are stereoisomers, and you know that they are not enantiomers, therefore...you answer it.

[laotree]

I never saw anybody treat double bond cis/trans as diasteromers in my 20 years of natural product and medicinal chemistry experience.

But it does mean I will be right in this case. We are discussing chemistry, I wish more people would join.

If the cis/trans is in a ring, like cyclohexane, they are diasteromers.

Are these two structures diasteromers? They could fit into your concept of 'Diastereomers are stereoisomers that are not enantiomers or mirror images of each other'

[Winga]

'Stereoisomerism is the arrangement of atoms in molecules whose connectivity remains the same but their arrangement in space is different in each isomer.'
(quoted from wikipedia)

Do these two molecules have same connectivity of arrangement of atoms?

[laotree]

I gave a very bad example, I am confused totally. Movies will give a right answer.

[Winga]

Actually, I was also confused with this problem when I first time heard about it.

[movies]

Sorry to have missed so much of this discussion, I was out sick yesterday.

I subscribed to the definition that diastereomers are two chiral molecules which are not enantiomers until I saw a professor from the University of Chicago refer to cis/trans isomers as diastereomers.  I thought he was wrong, so after his lecture I went back to my lab and looked up the definition.

According to the book "Stereochemistry of Organic Compounds" diastereomers are "Stereoisomers not related as mirror images."  The definition on the IUPAC website concurs, defining diastereoisomerism as "Stereoisomerism other than enantiomerism.  Diastereoisomers are stereoisomers not related as mirror images."

So, these definitions would seem to include cis/trans isomers, provided that cis/trans isomers are stereoisomers of one another.  Again citing the same two references, the definition of stereoisomer is "Isomers of of identical constitution but differing in the arrangement of their atoms in space" and stereoisomerism as "Isomerism due to differences in the spatial arrangement of atoms without any differences in connectivity or bond multiplicity between the isomers."  Cis/trans isomers certainly fulfill these criteria.

[laotree]

What can I say? Learn something!!!!!!!!

[madscientist]

Thankyou so much to all of you that helped me with this question, very very much appreciated!!

cheers,

madscientist :albert: