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Isomerism Index

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Creative Chemistry Molecular Models

Diagram showing chiral moleculesOptical Isomerism

What is it?

Optical isomerism is an example of stereo-isomerism. It occurs when substances have the same molecular and structural formulae, but one cannot be superimposed on the other. Put simply, they are mirror images of each other (see the diagram on the right). No matter how hard you try, the molecule on the left will not turn into the molecule on the right – unless you break and make some bonds! Molecules like this are said to be chiral (pronounced ky-ral), and the different forms are called enantiomers.

Optical isomers can occur when there is an asymmetric carbon atom. An asymmetric carbon atom is one which is bonded to four different groups. The groups can be something hideously complex, or something comfortably simple like a hydrogen or chlorine atom. Remember:

  • there must be four groups, and
  • they must be different.

Optical isomers can rotate the plane of polarisation of plane-polarised light:

  • one enantiomer rotates the polarised light clockwise (to the right) and is the (+) enantiomer;
  • the other rotates the polarised light anticlockwise (to the left) and is called the (–) enantiomer.

A mixture containing equal concentrations of the (+) and (–) enantiomers is not optically active (it will not rotate the plane of polarisation). It is called a racemic mixture or racemate.

What is here?

I have put models of the optical isomers of 2-hydroxypropanoic acid (lactic acid) here. Lactic acid is a fairly common and simple example of optical isomerism. The (+) enantiomer of lactic acid is found in muscle. Sour milk contains a racemic mixture of the two enantiomers.

I have also put 2,3-dihydroxypropanal (glyceraldehyde) here. This is because glyceraldehyde is used as a standard by which other chiral molecules are compared. There are two enantiomers of glyceraldehyde, depending on the position of the –OH (hydroxyl) group on the molecule. These are known as D-glyceraldehyde and L-glyceraldehyde. The little capital letters D and L are deliberate. The positions of the hydroxyl groups on other chiral molecules can be compared with glyceraldehyde to see if they are the D-enantiomer or the L-enantiomer. This is very common in Biology (natural sugars are D-enantiomers and amino acids are L-enantiomers). However, knowing whether a molecule is the D or L-enantiomer does not tell us whether it is the (+) or (–) enantiomer – so be careful!

(+)-lactic acid

(–)-lactic acid

The dashed lines show bonds going into the screen; the wedges show bonds coming out of the screen.

(+) lactic acid

(-) lactic acid

 

D-glyceraldehyde

L-glyceraldehyde

Remember – these are reference molecules; the D and L signs do not tell you if the enantiomer is (+) or (–).

D-glyceraldehyde

L-glyceraldehyde

 
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The models are ball-and-stick, but you can right click to get a menu of options, such as animations and changing to space-filling models. You can also move the models using the left mouse button. Note that the molecules are mirror images of each other, and you cannot superimpose one on top of the other.
 

Molecular modelling applet courtesy of ChemAxon Ltd

 

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