Re: Are all molecules optically active?

Paul,

By optically active, I assume that you mean that you are talking about a molecule which rotates/changes the polarisation of light.

A definition can be found at www.hyperdictionary.com

{Optically active}, {Optically inactive} (Chem. Physics), terms used of certain metameric substances which, while identical with each other in other respects, differ in this, viz., that they do or do not produce right-handed or left-handed circular polarization of light.

In this sense, not all molecules are optically active. In fact you need to have a very special type of molecule to see optical activity. The study of optical activity is usually performed by polarimetry:
britannica.com article

The idea of polarimetry is that you send in light with one polarisation and you measure to what degree this is turned into another. Light is usually considered to be either linearly polarised or circularly polarised where the linear polarisation can have some particular direction. (Obviously, circularly polarised light cannot have a direction since a circle has no definable direction, whereas an ellipse (a mixture of circular and linear in polarised light terms) has an axis... I digress).

You can find out more on polarisation of light from the Net Advance in Physics site.

Let us get back to the problem of optically active molecules. What we require is a molecule which will, say, when illuminated with linearly polarised light will rotate the plane of polarisation. If you take a simple molecule like carbon dioxide (CO₂) or water (H₂O) and put it into a polarimeter you will find that there is no change to the polarisation. (Actually scattering off the surface of water can generate a polarisation change, but that is a different effect to the one we are interested in - that relies on an interface between air and water.)

This lack of polarisation change is pretty much due to the fact that most molecules are, on average, polarisable in all directions and therefore don't have any preferred direction of passing light through them. This is usually true of liquids and gases.

When it comes to solids it is usually a different story. Crystalline materials are often optically active, but I won't go into that right now since we are interested in free molecules. Have a look at http://webphysics.davidson.edu/alumni/MiLee/JLab/Crystallogr aphy_WWW/birefringence.htm or http://sc ienceworld.wolfram.com/physics/Birefringence.html or http://hyperphysics.phy- astr.gsu.edu/hbase/phyopt/biref.html

So back to molecules... The really interesting optically active molecules are those which chiral. These are molecules which (usually) turn linearly polarised light into circularly (or elliptically) polarised light. The structure of these molecules is such that their mirror images are NOT identical. A simple example would be something like a carbon atom with a different atom covalently bonded to each of its four dangling bonds. An odd example would be something like CHFClBr which would have a tetrahedral- like structure. Have a look at http://www.sdsc.edu/GatherScatter/GSspring97/mccammon.html

The most common kind of chiral molecule however, is probably sugar and it is the right-handed form which is particularly common and, in fact, supports all life on this planet. Chirality is a very important in nature. Right-handed molecules are used in nearly all biological reactions. This has huge implications in the production of drugs. In fact the Nobel prize in Chemistry was won for this research.

The forms of the molecules (left-handed and right-handed) are called the two enatiomers (or stereo isomers). A mixture of the two enantiomers is called a racemic mixture and this mixture does not have any optical activity. http://chemistry.about.com/library/glossary/bldef5331.htm or http://mathworld.wolfram.com/Enantiomer.html

This whole field, usually called stereochemistry, is said to be started by Pasteur in 1848. He was able to separate the two enantiomers of sodium ammonium tartrate out by hand and discovered their optical activity. http://www.ualberta.ca/~csps/JPPS1 (1)/A.Mitchell/racemicview.htm

So, in conclusion, I hope that I have convinced you that actually most molecules are not optically active, but probably most of the interesting ones are. To find out... draw a molecule and then pretend that you are looking at it in a mirror. Can you rotate it until you put your mirrored molecule in the same position as the original? If not, then it is likely to be optically active.

You will notice that some of my references are to Eric Weisstein's Mathworld. Chirality is actually as big a topic in mathematics and physics as it is in chemistry and biology.

More information: http://ep.llnl.gov/msds/orgchem/stereochem.html