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Definitions: Prochiral


A molecule is prochiral if the addition of a new group or an exchange of one group on the molecule would create a new stereocenter and, therefore, a chiral molecule. A prochiral atom must be bonded to three different groups before any change is made.

For example, consider the following molecule.


This molecule is prochiral by addition. Groups can be added to a molecule if a double bond is present before any change is made. This can be done by many different chemical processes. The steps for the addition process are discussed below.

Step 1: Check to make sure that the atom being considered is bonded to exactly three different groups before any change is made. In the molecule above, the carbon marked with an asterisk is bonded to a methyl group, a tert-butyl group, and an oxygen. This atom fits the criterion of being bonded to exactly three different groups.

Step 2: Change the double bonds in the molecule of interest into single bonds. This is shown below, with the new single bond colored in blue.




However, now both atoms where the bond had been broken are not attached to enough groups. (Recall a carbon should have four bonds, and an oxygen should have two bonds. In this molecule, the carbon now only has three bonds, and the oxygen only has one bond.) Step 3 will remedy this.

Step 3: Add a new group, usually a deuterium, to both atoms that were originally joined by the double bond. (Deuterium (heavy hydrogen) is normally used as the new group, as it is not commonly found in organic molecules.) A deuterium is added to both the carbon and the oxygen, which gives them each the appropriate number of bonds. This is shown below, with the new deuteriums colored in red.




After this addition, it can be seen that the carbon marked with an asterisk has four different groups, creating a new stereocenter. Because this addition creates a new stereocenter, the carbon marked with an asterisk is prochiral.

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