[katherinetran25]

How are these compounds (attached image) diastereomers of each other?

They do not contain a chirality centre, and diastereomers must have more than one chirality centre. I would have thought that they are geometric isomers (the first image being trans, and the second being cis).

[sjb]

and

Is there anything at https://en.wikipedia.org/w/index.php?title=Diastereomer&oldid=692947693 that contradicts your position?

[katherinetran25]

Not that I can see...

But I'm still confused how they are considered diastereomers: "Diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more (but not all) of the equivalent (related) stereocenters and are not mirror images of each other"

These compounds don't have stereocenters since there are no carbons that are attached to 4 different substituents in either of the molecules.

and

Is there anything at https://en.wikipedia.org/w/index.php?title=Diastereomer&oldid=692947693 that contradicts your position?

and

Is there anything at https://en.wikipedia.org/w/index.php?title=Diastereomer&oldid=692947693 that contradicts your position?

[Dan]

These compounds don't have stereocenters since there are no carbons that are attached to 4 different substituents in either of the molecules.

What is the definition of diastereoisomerism?

Useful website: http://www.chem.qmul.ac.uk/iupac/stereo/

[orgopete]

I'm with Katherine on this one, IUPAC and others not withstanding. As I understand those arguing this example, cis and trans-2-butene are also diastereomers. However, in the common way in which these terms are used, cis and trans-2-butene are geometric isomers.

I understand that they perhaps could or should also be referred to as diastereomers. If that is the point, then okay. Otherwise, I'd prefer to refer to these as geometric isomers, such as cis/trans or E/Z. What am I missing?

[Doc Oc]

I agree with orgopete, I've literally never heard anyone describe E/Z isomers as diastereomers.

[sjb]

I think the point IUPAC are trying to make is that there is an asymmetric point half-way (approx) along the double bond and if you consider that, the cis and trans (or E/Z etc) isomers are not super-imposable, much like the central atom in allenes, for instance.

[Dan]

Geometrical isomerism is a type of diastereoisomerism by definition. My view is that until the official definition of the term is changed, it should be used as defined. If evolution of language results in a revision of the definition, that's fine, but until then E- and Z-but-2-ene are diastereomers.

A graphical representation of isomer classification from wikipedia:

[orgopete]

I'm not wanting to belabor the point here nor am I advocating any change in definitions. I assume Dan's illustration correctly shows diastereomers. The original poster asked whether the problem described geometric isomers. It would seem the simple answer was yes. If geometric isomers and isomers with two chirality centers are both diastereomers, I could be accepting of that definition. I should also point out that it now requires terminology to separate these two classes.

Please pardon my referring back to Wikipedia,

Diastereomers (sometimes called diastereoisomers) are a type of a stereoisomer.[1] Diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more (but not all) of the equivalent (related) stereocenters and are not mirror images of each other.[2] When two diastereoisomers differ from each other at only one stereocenter they are epimers. Each stereocenter gives rise to two different configurations and thus increases the number of stereoisomers by a factor of two

This definition seems to agree with the original poster (it also excludes rotamers and conformers).

I avoided going back to the IUPAC Gold definition as I often find them to be confusing. I don't "know" the definition of diastereomers. My experience in using the terms agrees with Katherine.

[Dan]

Please pardon my referring back to Wikipedia,

Diastereomers (sometimes called diastereoisomers) are a type of a stereoisomer.[1] Diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more (but not all) of the equivalent (related) stereocenters and are not mirror images of each other.[2] When two diastereoisomers differ from each other at only one stereocenter they are epimers. Each stereocenter gives rise to two different configurations and thus increases the number of stereoisomers by a factor of two

This definition seems to agree with the original poster (it also excludes rotamers and conformers).

The Wikipedia quote is correct; "stereocentre" does not mean asymmetric centre/chiral centre, it is a more general term that also applies to double bonds (see: IUPAC definition of stereogenic unit).
and

A stereocenter or stereogenic center is any point in a molecule, though not necessarily an atom, bearing groups, such that an interchanging of any two groups leads to a stereoisomer

Changing the configuration of a double bond results in a stereoisomer, so a double bond is a stereocentre and it follows that (since they are not enantiomeric) geometrical isomers are diastereomers.

Diastereomers (IUPAC) are any set stereoisomers not related as enantiomers, which includes geometric isomers.

I think that's relevant to the original question: "How are these compounds diastereomers of each other?"

The Wikipedia "Diastereoisomerism occurs when..." quote is a bit misleading, because while it is true, it is not true that diastereoisomerism only occurs when... The IUPAC definition (of diastereomers as any stereoisomers that are not enantiomeric) is broader and includes conformers and rotamers (which are stereoisomers according to IUPAC).

I agree that in common usage, stereocentre is usually used in the context of asymmetric centre/chiral centre, and that diastereomer is usually used in the context of molecules containing >2 asymmetric centres. There's nothing wrong with that, it is correct, but the terms stereocentre and diastereomer can also apply to alkenes that have no asymmetric centres.

 

[Dan]

To clarify:

Stereoisomers are split into two subclasses: Enantiomers and diastereomers

Diastereomers are then further subdivided into various subclasses, one of which is geometric isomers (E/Z isomers), others include: epimers, anomers, rotamers, conformers etc.

[spirochete]

I do not like the wikipedia definition of diastereomer. IUPAC as well as many textbooks describe diastereomers as being any stereoisomer that is not an enantiomer. I think this makes much more sense.

The distinction between geometric isomers and diastereomers with chiral centers is somewhat arbitrary. They are both stereoisomers that are not enantiomers. Enantiomers are deserving of a special name because they have the same physical and chemical properties in most environments, which is remarkable and interesting considering that they are actually different molecules.  In contrast, diastereomers do have different properties.

It is important to specifically point out cis/trans as an example of diastereomerism to students, however, because it often blows their minds. These are the same students that think "chirality" and "stereoisomerism" are the same thing.