Chapter 7 : Stereochemistry

Stereochemistry Terminology

Like conformational analysis (which is a sub-branch of stereochemistry), stereochemistry has a language all of its own associated with it.
A list and short description of key terms in the language of stereochemistry is provided below, more detailed descriptions are given within the appropriate chapter pages.

Stereo : the prefix stereo means three-dimensionl, i.e. 3D.

Stereochemistry is all about the 3D spatial aspects of chemistry.

Stereoisomers are molecules that differ only in the arrangement of bonds in 3D space.

Mirror image : a mirror image is an image or object that has the same form as another, but has "reversed" structure. All objects have mirror images. Don't over complicate this !

Superimposable : if two objects are superimposable, it means that they are the same and you can't tell them apart, i.e. they are the same thing.

Non-superimposable : if two objects are non-superimposable, then it means you can tell them apart, you can distinguish them. Sometimes the differences are very obvious (e.g. hands and feet), but it others it is more subtle (e.g. left hand and right hand).

Superimposable mirror image : this means that an object and it's mirror image are identical and can't be distinguished.

Non-superimposable mirror image : this means that an object and it's mirror image are different and can be distinguished.

If we restrict our thoughts to 2D for a second, and think about letters of the alphabet, some letters are the superimposable on their mirror images (e.g. A or O) while others are not (e.g. S, Z).
To further appreciate this, print some words on a piece of paper, then look at them in a mirror. Do they still look the same (superimposable) or are they different (non-superimposable) ?
Words like "MOM" are the same as the mirror image but "DAD" is not. 

That's why the word "AMBULANCE" on the front hood of the vehicle looks "odd" unless you are looking at it in your rear view mirror. Note that when you look carefully, you can see some letters are reversed while some look the same.

Chiral from the Greek word for "handedness". An object that has a non-superimposable mirror image has the property of "handedness" and is said to be "chiral" and one that has a superimposable mirror image is "achiral" (doesn't have ""handedness"). This applies to your left and right hands. They are essentially mirror images but they are not superimposable (try it !)

Enantiomers : a pair of molecules that are non-superimposable mirror images of each other. This phrase is the standard expression for defining enantiomers (which is why we use it here), but to simplify a little, it means we have a pair of molecules that are so similar that they are mirror images of each other, but we can always tell them apart.

The most common type of "chirality" is observed when a carbon atom has four different groups attached to it (so it must be sp3 hybridised). This carbon atom is then described as a chirality center.  Older terms are chiral, asymmetric or stereogenic center. This later term can also be contracted to a stereocenter.

Enantiomers have the same chemical and physical properties (melting points, boiling points, heat of combustion etc.), except for their interaction with plane polarised light or with other chiral molecules (reagents, solvents, catalysts etc). Think about how your feet feel if you put them in the wrong shoes - both your feet and shoes are chiral.

Diastereomers are stereoisomers that are not enantiomers.

The differing interaction with plane polarised light gives rise to optical activity. Enantiomers cause the plane of polarised light to rotate in opposite directions, but to the same extent (clockwise = +ve, counterclockwise = -ve). This can be measured using a polarimeter. An achiral molecule is optically inactive.

A 50:50 mixture of a pair of enantiomers is called a racemic mixture. This is optically inactive since the rotations produced by each of the enantiomers must cancel each other out.

If there is more of one enantiomer than the other, then the optical purity of a sample can be determined by measuring the rotation and comparing it to that of a pure enantiomer. This can be used to establish the enantiomeric excess (ee) of the mixture.

© Dr. Ian Hunt, Department of Chemistry