## Organic and Inorganic Chemistry Lesson of the Day – Conformational Isomers (or Conformers)

Conformational isomerism is a special type of stereoisomerism that arises from the rotation of a single bond.  Specifically, 2 molecules are conformational isomers (or conformers) if they can be interconverted exclusively by the rotation of a single bond.  This type of isomerism differs from configurational stereoisomerism, whose isomers can only be interconverted by breaking certain bonds and reattaching* them to produce different 3-dimensional orientations.  Examples of configurational isomers include enantiomers, diastereomers, cis/trans isomers and meso isomers.

Different conformers are notable for having different stabilities, depending on the electrostatic interactions between the substituents along the single bond of interest.  I will talk about these differences in greater depth in future Chemistry Lessons of the Day.

*Such reattachment of the bonds must not result in different connectivities (or sequence of bonds); otherwise, that would result in structural isomers.

## Organic and Inorganic Chemistry Lesson of the Day – Stereoisomers

Two molecules are stereoisomers if they

• have the same molecular formula
• have the same sequence of bonds between each molecule’s constituent atoms
• have different 3-dimensional (spatial or geometric) orientations of the constituent atoms

Examples of stereoisomers include

It is important to emphasize that stereoisomers are defined for 2 or more molecules.  Consider 3 isomers, A, B and C.

• A and B may be stereoisomers.
• A and C may not be stereoisomers.  They may be structural isomers, which have the same atoms but different sequences of bonds.

## Organic and Inorganic Chemistry Lesson of the Day – Optical Rotation (a.k.a. Optical Activity)

A substance consisting of a chiral compound can rotate linearly polarized light – this property is known as optical rotation (more commonly called optical activity).  The direction in which the light is rotated is one way to distinguish between a pair of enantiomers, as they rotate linearly polarized light in opposite directions.

Imagine if you are an enantiomer, and linearly polarized light approaches you.

• If the light is rotated clockwise from your perspective, then you are a dextrorotary enantiomer.
• Otherwise, if the light is rotated counterclockwise from your perspective, then you are a levorotary enantiomer.

In a previous Chemistry Lesson of the Day, I introduced the concept of diastereomers, and I used threose as an example.  Let’s use threose to illustrate some notation about optical activity.

(-)-Threose

• Levorotary compounds are denoted by the prefix (-), followed by a hyphen, then followed by the name of the compound.  The above molecule is (-)-threose.
• Dextrorotary compounds are denoted by the prefix (+), followed by a hyphen, then followed by the name of the compound.  The enantiomer of (-)-threose is (+)-threose.

A compound’s optical rotation is determined by a polarimeter.

I strongly discourage the use of the prefixes (d)- and (l-) to distinguish between enantiomers.  Use (+) and (-) instead.

Beware of the difference between designating enantiomers as (+) or (-) and designating stereogenic centres as either (R) or (S).

It is important to note that optical rotation is usually referred to as a bulk property.

## Organic and Inorganic Chemistry Lesson of the Day – Cis/Trans Isomers Are Diastereomers

Recall that the definition of diastereomers is simply 2 molecules that are NOT enantiomers.  Diastereomers often have at least 2 stereogenic centres, and my previous lesson showed an example of how such diastereomers can arise.

However, while an enantiomer must have at least 1 stereogenic centre, there is nothing in the definition of a diastereomer that requires it to have any stereogenic centres.  In fact, a diastereomer does not have to be chiral.  A pair of cis/trans isomers are also diastereomers.  Recall the example of trans-1,2-dibromoethylene and cis-1,2-dibromoethylene:

Image courtesy of Roland1952 on Wikimedia.

These 2 molecules are stereoisomers – they have the same atoms and sequence/connectivity of bonds, but they differ in their spatial orientations.  They are NOT mirror images of each other, let alone non-superimposable mirror images.  Thus, by definition, they are diastereomers, even though they are not chiral.

## Organic and Inorganic Chemistry Lesson of the Day – Meso Isomers

A molecule is a meso isomer if it

Meso isomers have an internal plane of symmetry, which arises from 2 identically substituted but oppositely oriented stereogenic centres.  (By “oppositely oriented”, I mean the stereochemical orientation as defined by the Cahn-Ingold-Prelog priority system.  For example, in a meso isomer with 2 tetrahedral stereogenic centres, one stereogenic centre needs to be “R”, and the other stereogenic centre needs to be “S”. )  This symmetry results in the superimposability of a meso isomer’s mirror image.

By definition, a meso isomer and an enantiomer from the same stereoisomer are a pair of diastereomers.

Having at least 2 stereogenic centres is a necessary but not sufficient condition for a molecule to have meso isomers.  Recall that a molecule with $n$ tetrahedral stereogenic centres has at most $2^n$ stereoisomers; such a molecule would have less than $2^n$ stereoisomers if it has meso isomers.

Meso isomers are also called meso compounds.

Here is an example of a meso isomer; notice the internal plane of symmetry – the horizontal line that divides the 2 stereogenic carbons:

(2R,3S)-tartaric acid

Image courtesy of Project Osprey from Wikimedia (with a slight modification).

## Organic and Inorganic Chemistry Lesson of the Day – Diastereomers

I previously introduced the concept of chirality and how it is a property of any molecule with only 1 stereogenic centre.  (A molecule with $n$ stereogenic centres may or may not be chiral, depending on its stereochemistry.)  I also defined 2 stereoisomers as enantiomers if they are non-superimposable mirror images of each other.  (Recall that chirality in inorganic chemistry can arise in 2 different ways.)

It is possible for 2 stereoisomers to NOT be enantiomers; in fact, such stereoisomers are called diastereomers.  Yes, I recognize that defining something as the negation of something else is unusual.  If you have learned set theory or probability (as I did in my mathematical statistics classes) then consider the set of all pairs of the stereoisomers of one compound – this is the sample space.  The enantiomers form a set within this sample space, and the diastereomers are the complement of the enantiomers.

It is important to note that, while diastereomers are not mirror images of each other, they are still non-superimposable.  Diastereomers often (but not always) arise from stereoisomers with 2 or more stereogenic centres; here is an example of how they can arise.  (A pair of cis/trans-isomers are also diastereomers, despite not having any stereogenic centres.)

1) Consider a stereoisomer with 2 tetrahedral stereogenic centres and no meso isomers*.  This isomer has $2^{n = 2}$ stereoisomers, where $n = 2$ denotes the number of stereogenic centres.

2) Find one pair of enantiomers based on one of the stereogenic centres.

3) Find the other pair enantiomers based on the other stereogenic centre.

4) Take any one molecule from Step #2 and any one molecule from Step #3.  These cannot be mirror images of each other.  (One molecule cannot have 2 different mirror images of itself.)  These 2 molecules are diastereomers.

Think back to my above description of enantiomers as a proper subset within the sample space of the pairs of one set of stereoisomers.  You can now see why I emphasized that the sample space consists of pairs, since multiple different pairs of stereoisomers can form enantiomers.  In my example above, Steps #2 and #3 produced 2 subsets of enantiomers.  It should be clear by now that enantiomers and diastereomers are defined as pairs.  To further illustrate this point,

a) call the 2 molecules in Step#2 A and B.

b) call the 2 molecules in Step #3 C and D.

A and B are enantiomers.  A and C are diastereomers.  Thus, it is entirely possible for one molecule to be an enantiomer with a second molecule and a diastereomer with a third molecule.

Here is an example of 2 diastereomers.  Notice that they have the same chemical formula but different 3-dimensional orientations – i.e. they are stereoisomers.  These stereoisomers are not mirror images of each other, but they are non-superimposable – i.e. they are diastereomers.

(-)-Threose

(-)-Erythrose

Images courtesy of Popnose, DMacks and Edgar181 on Wikimedia.  For brevity, I direct you to the Wikipedia entry for diastereomers showing these 4 images in one panel.

In a later Chemistry Lesson of the Day on optical rotation (a.k.a. optical activity), I will explain what the (-) symbol means in the names of those 2 diastereomers.

*I will discuss meso isomers in a separate lesson.