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.
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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:

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 – 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 – 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.

dibromoethylene

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.

cisplatin and transplatin

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.