## 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.

## Café Scientifique – Materials Science Seminar by Neil Branda on Wednesday, November 19, 2014

If you will attend the following seminar, please do come and say “Hello”!  The event is free, but registration is required!  For more information, visit the SFU Café Scientifique’s web site!

Time: 7:00 -8:30 pm

Date: Wednesday, November 19, 2014

Place: Boston Pizza, 1045 Columbia Street, New Westminster, BC

Title: It’s a Materials World – From Sticks and Stones to Nanotechnology, how materials have changed our world

Speaker: Neil Branda – Professor of Chemistry at Simon Fraser University, Executive Director of 4D LABS, and Chief Technology Officer of SWITCH Materials

Abstract:

Since the beginning, understanding how materials can be used for specific tasks has resulted in some of the biggest changes to civilizations. Modern society is becoming more and more dependent on the development and use of advanced materials. From the basics to the controversial, how materials have affected they way we live and play will be discussed.

Biography of Speaker:

Dr. Neil Branda is a professor of Chemistry and a Canada Research Chair at Simon Fraser University, the Executive Director of 4D LABS, a research centre for advanced materials and nano-scale devices, CTO of SWITCH Materials Inc., a company he founded to commercialize his molecular switching technology and Founder and Director of the NanoCommunity Canada Research Network, a community of nanotechnology researchers committed to sharing knowledge and working collaboratively to advance applications in medical diagnostics, therapeutics, renewable energy and advanced materials.

## 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 is a Bulk Property

It is important to note that optical rotation is usually discussed as a bulk property, because it’s usually measured as a bulk property by a polarimeter.  Any individual enantiomeric molecule can almost certainly rotate linearly polarized light.  However, in a bulk sample of a chiral substance, there is usually another molecule that can rotate light in the opposite direction.  This is due to the uniform distribution of the stereochemistry of a random sample of the molecules of one compound.  (In other words, the substance consists of different stereoisomers of one compound, and the proportions of the different stereoisomers are roughly equal.)  Because one molecule’s rotation of the light can be cancelled by another molecule’s optical rotation in the opposite direction, such a random sample of the compound would have no net optical rotation.  This type of cancellation will definitely occur in a racemic mixture.  However, if a substance is enantiomerically pure, then all of the molecules in that substance will rotate linearly polarized light in the same direction – this substance is optically active.

## Organic and Inorganic Chemistry Lesson of the Day – The Difference Between (+)/(-) and (R)/(S) in Stereochemical Notation

In a previous Chemistry Lesson of the Day, I introduced the concept of optical rotation (a.k.a. optical activity).  You may also be familiar with the Cahn-Ingold-Prelog priority rules for designating stereogenic centres as either (R) or (S).   There is no direct association between the (+)/(-) designation and the (R)/(S) designation.  In other words, an (R)-enantiomer can be dextrorotary or levorotary – it must be determined on a case-by-case basis.  The same holds true for an (S)-enantiomer.

(R)/(S) can be used to distinguish between enantiomers in one exception: If the stereoisomer has only 1 stereogenic centre, then this designation can also serve as a way to distinguish between 2 enantiomers.

Furthermore, note that the designation of optical rotation applies to a molecule, whereas the R/S designation applies to a particular stereogenic centre within a molecule.  Thus, a molecule with 2 stereogenic centres may have one (R) stereogenic centre and one (S) stereogenic centre.  However, a chiral compound consisting purely of one enantiomer can rotate linearly polarized light in only one direction, and that direction must be determined on a case-by-case basis by a polarimeter.

## Organic and Inorganic Chemistry Lesson of the Day – DO NOT USE THE PREFIXES (d-) and (l-) TO CLASSIFY ENANTIOMERS

In a recent Chemistry Lesson of the Day, I introduced the concept of optical rotation, and I mentioned the use of (+) and (-) to denote dextrorotary and levorotary compounds, respectively.

Some people use d- and l- instead of (+) and (-), respectively.  I strongly discourage this, because there is an old system of classifying stereogenic centres that uses the prefixes D- and L-, and the obvious similarity between the prefixes of the 2 systems causes much confusion.

This old system classifies stereogenic centres based on the similarities of their configurations to the 2 enantiomers of glyceraldehyde.  It is confusing, non-intuitive, and outdated, so I will not discuss its rationale or details on my blog.  (If you are interested, here is a good explanation from the University of Maine’s chemistry department.)

Also, note that D- and L- classify stereogenic centres, whereas d- and l- classify enantiomers – this just adds more confusion.

In short,

• DO NOT use d- and l- to classify enantiomers; use (+) and (-) instead.
• DO NOT use D- and L- to classify stereogenic centres; use the Cahn-Ingold-Prelog priority rules (R/S) instead.

## 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 – Racemic Mixtures

A racemic mixture is a mixture that contains equal amounts of both enantiomers of a chiral molecule.  (By amount, I mean the usual unit of quantity in chemistry – the mole.  Of course, since enantiomers are isomers, their molar masses are equal, so a racemic mixture would contain equal masses of both enantiomers, too.)

In synthesizing enantiomers, if a set of reactants combine to form a racemic mixture, then the reactants are called non-stereoselective or non-stereospecific.

in 1895, Otto Wallach proposed that a racemic crystal is more dense than a crystal with purely one of the enantiomers; this is known as Wallach’s rule.  Brock et al. (1991) substantiated this with crystallograhpic data.

Reference:

Brock, C. P., Schweizer, W. B., & Dunitz, J. D. (1991). On the validity of Wallach’s rule: on the density and stability of racemic crystals compared with their chiral counterparts. Journal of the American Chemical Society, 113(26), 9811-9820.

## Organic and Inorganic Chemistry Lesson of the Day – Stereogenic Centre

A stereogenic centre (often called a stereocentre) is an atom that satisfies 2 conditions:

1. it is bonded to at least 3 substituents.
2. interchanging any 2 of the substituents would result in a stereoisomer.

If a molecule has only 1 stereogenic centre, then it definitely has a non-superimposable mirror image (i.e. this molecule is chiral and is an enantiomer).  However, depending on its stereochemistry, it is possible for a molecule with 2 or more stereogenic centres to be achiral; such molecules are called meso isomers (or meso compounds), and I will discuss them in a later lesson.

In organic chemistry, the stereogenic centre is usually a carbon atom that is attached to 4 substituents in a tetrahedral geometry.  In inorganic chemistry, the stereogenic centre is usually the metal centre of a coordination complex.

In organic chemistry, stereogenic centres with substituents in a tetrahedral geometry are common.  Inorganic coordination complexes can also have a tetrahedral geometry.  A stereoisomer with $n$ tetrahedral stereogenic centres can have at most $2^n$ stereoisomers.  The “at most” caveat is important; as mentioned above, it is possible for a molecule with 2 or more stereogenic centres to have a spatial arrangement that results in having a superimposable mirror image; such isomers are meso isomers.   I will discuss meso isomers in more detail in a later lesson.

## Inorganic Chemistry Lesson of the Day – 2 Different Ways for Chirality to Arise in Coordination Complexes

In a previous Chemistry Lesson of the Day, I introduced chirality and enantiomers in organic chemistry; recall that chirality in organic chemistry often arises from an asymmetric carbon that is attached to 4 different substituents.  Chirality is also observed in coordination complexes in inorganic chemistry.  There are 2 ways for chirality to be observed in coordination complexes:

1.   The metal centre has an asymmetric arrangement of ligands around it.

• This type of chirality can be observed in octahedral complexes and tetrahedral complexes, but not square planar complexes.  (Recall that square planar complexes have a plane formed by the metal and its 4 ligands.  This plane can serve as a plane of reflection, and any mirror image of a square planar complex across this plane is clearly superimposable onto itself, so it cannot have chirality just by having 4 different ligands alone.)

2.   The metal centre has a chiral ligand (i.e. the ligand itself has a non-superimposable mirror image).

• Following the sub-bullet under Point #1, a square planar complex can be chiral if it has a chiral ligand.

## Organic and Inorganic Chemistry Lesson of the Day – Chirality and Enantiomers

In chemistry, chirality is a property of a molecule such that the molecule has a non-superimposable mirror image.  In other words, a molecule is chiral if, upon reflection by any plane, it cannot be superimposed onto itself.

Chirality is a property of the 3-dimensional orientation of a molecule, and molecules exhibiting chirality are stereoisomers.  Specifically, two molecules are enantiomers of each other if they are non-superimposable mirror images of each other.  In organic chemistry, chirality commonly arises out of an asymmetric carbon atom, which is a carbon that is attached to 4 different substituents.  Chirality in inorganic chemistry is more complicated, and I will discuss this in a later lesson.

It is important to note that enantiomers are defined as pairs.  This will be later emphasized in the lesson on diastereomers.

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

Cis/Trans isomerism is a type of stereoisomerism in which the relative positions of 2 functional groups differ between the isomers.  An isomer is cis if the 2 functional groups of interest are closer to each other, and trans if they are farther from each other.  You may find these definitions to be non-rigorous based on the subjectivity of “closer” and “farther”, but cis/trans isomers have only 2 possible relative positions for these functional groups, so “closer” and “farther” are actually obvious to identify.  It’s easier to illustrate this with some examples.

Let’s start with an organic molecule.

Image courtesy of Roland1952 on Wikimedia.

The molecule on the left is trans-1,2-dibromoethylene, and the molecule on the right is cis-1,2-dibromoethylene.  The 2 functional groups of interest are the 2 bromides, and the isomerism arises from the 2 different ways that these bromides can be positioned relative to each other.  (Notice that the 2 bromides are bonded to different carbon atoms, thus the “1,2-” designation in its name.)  Relative to the other bromide, one bromide can either be on the same of the double bond (“closer”) or on the opposite side of the double bond (“farther”).  To view the isomerism from another perspective, the double bond serves as the plane of separation, and the bromides can be on different sides of that plane (trans) or the same sides of the plane (cis).  Cis/Trans isomerism often arises in organic chemistry because of a bond with restricted rotation, and such restriction is often due to a double bond or a ring structure.  Such a bond often serves as the plane of separation on which the relative positions of the 2 functional groups can be established.

Let’s now consider a coordination complex in inorganic chemistry.

Image courtesy of Anypodetos on Wikimedia.

Cisplatin and transplatin are both 4-coordinated complexes with a square planar geometry.  Their ligands are 2 chlorides and 2 ammonias.  When looking at the pictures above, it’s obvious that there are only 2 relative positions for one chloride to take compared to the other chloride – they can be either closer to each other (cis) or farther apart (trans).

Cis/Trans isomerism can also arise in 6-coordinated octahedral complexes in inorganic chemistry.

## Inorganic Chemistry Lesson of the Day: 5-Coordinated Complexes

There are 2 common geometries for 5-coordinated complexes:

• Square pyramid: The metal centre is coordinated to 4 ligands in a plane and a 5th ligand above the plane.
• Trigonal bipyramid: The metal centre is coordinated to 3 ligands in a plane and 2 lignads above and below the plane.

## Inorganic Chemistry Lesson of the Day: 2-Coordinated Complexes

Some coordination complexes have just 2 ligands attached to the metal centre.  These complexes have a linear geometry; this allows the greatest separation of the electron clouds in the metal-ligand bonds, which minimizes electron repulsion.

## Inorganic Chemistry Lesson of the Day: 4-Coordinated Complexes

My last lesson stated that the most common coordination number for coordination complexes is 6.  The next most common coordination number is 4, and complexes with this type of coordination adopt either the tetrahedral or the square planar geometry.  The tetrahedron is far more common than the square plane for 4-coordinated complexes, and the type of geometry depends a lot on the size and bonding strength of the ligands.  If the ligands are too big, then a tetrahedral geometry provides greater separation between ligands and minimizes electron repulsion.  If the ligands are too small, then there is room for 2 extra ligands to bond to the metal centre to form a 6-coordinated complex, and an octahedral geometry is adopted instead.

The square planar geometry is usually adopted by 4-coordinated complexes with metal ions that have a d8 electronic configuration.  Examples of such ions include Ni2+, Pd2+, Pt2+, and Au3+.

## Inorganic Chemistry Lesson of the Day: 6-Coordinated Complexes

The most common coordination number for inorganic coordination complexes is 6, and these complexes will most commonly adopt an octahedral geometry.  This geometry is especially common for coordination complexes with a first-row transition metal ion as the Lewis-acid centre.  It consists of 4 ligands forming a plane, and 2 ligands above and below the plane.  The “octa-” prefix in “octahedral” refers to the 8 faces that this geometry has.

Two alternative geometries of 6-coordinated complexes are the trigonal prism and hexagonal plane; these are far less common than the octahedron.

## Inorganic Chemistry Lesson of the Day – Coordination Complexes

A coordination complex is a compound that consists of Lewis bases bonded to a Lewis acid in its centre.  The charge of the complex can be neutral, positive, or negative; if the complex has a positive or a negative charge, then it is called a complex ion.  The Lewis acid is almost always a metal atom or a metal ion.  The Lewis bases are called ligands, and they are often covalently bonded to the Lewis acid.  Common ligands include carbon monoxide, water, and ammonia; what unifies them is the existence of at least one lone pair of electrons in their outermost energy level, and this lone pair of electrons is donated to the Lewis acid.

Some key terminology:

• The donor atom is the atom within the ligand that is attached to the Lewis acid centre.
• The coordination number is the number of donor atoms in the coordination complex.
• The denticity of a ligand is the number of bonds that it forms with the Lewis acid centre.
• If a ligand forms 1 bond with the Lewis acid centre, then it is monodentate (sometimes called unidentate).
• If a ligand forms multiple bonds with the Lewis acid centre, then the coordination complex is polydentate.  For example, a bidentate ligand forms 2 bonds with the Lewis acid centre.

In later Inorganic Chemistry Lessons of the Day, I will only refer to coordination complexes with metal atoms or metal ions as the Lewis acid centres.