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Optical isomerism

What is it?

Optical isomerism occurs when substances have the same molecular formula and structural formula, but one cannot be superimposed on the other. Put simply, they are mirror images of each other.

Molecules like this are said to be chiral (pronounced ky-ral). 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. It forms a chiral centre of the molecule.

The four groups can be something hideously complex, or something comfortably simple like a hydrogen atom or a chlorine atom. Remember:

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

Optical isomers can rotate the plane of plane-polarised light. When a solution is viewed from above:

  • the (–) enantiomer rotates the plane anticlockwise (to the left)
  • the (+) enantiomer rotates the plane clockwise (to the right).

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.

A molecular model and its mirror image

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!

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Hands

Hands are chiral objects. They have the same shape but are mirror images of each other.

What is here?

You can see models of the optical isomers of 2-hydroxypropanoic acid (lactic acid). Mammalian cells produce the (+) enantiomer. Different species of bacteria can produce the (+) enantiomer, the (–) enantiomer, or both enantiomers. Sour milk contains the (–) enantiomer. Synthetic lactic acid is usually a racemic mixture.

You can also see models of butan-2-ol, CH3CH2CH(OH)CH3. The carbon atom to which the hydroxyl (—OH) group is attached is an asymmetric carbon atom. It has four different groups attached to it.

—OH —H —CH3 —CH2CH3

This carbon atom is the chiral centre of the molecule. The model of the (+) enantiomer is stationary so that you can try to rotate the model of the (–) enantiomer so they exactly superimpose.

mouse_icon.gifDouble tap to stop and restart each animation.
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Most natural amino acids are (–) enantiomers, and natural sugars are (+) enantiomers. You may see the letters l and d instead:

  • (–) enantiomers are laevoratory, l
  • (+) enantiomers are dextroratory, d.

chemdoodle.png atom labels
ball and stick model
space-filling model
perspective view

(–)–lactic acid

(+)–lactic acid

(–)–butan-2-ol

Click to stop the animation, then move the model around to see if you can get it to exactly match (+) enantiomer.

(+)–butan-2-ol

This model cannot be animated, but you can move it around in 3D using the mouse, if you want to do this.