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
of a group requires that one of the existing groups on an atom be exchanged for a different group. To see this more clearly, the hydrogens on the carbon marked with an asterisk should be drawn in, as shown below.
atom must be 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 two hydrogens. This atom fits the criterion of being bonded to exactly three different groups. However, since there must be a total of four groups on each atom, one group is represented twice. In this case, there are two
can be determined by replacing
one of these "repetitious" groups with a different one. Deuterium (heavy hydrogen) is normally used as the new group, as it is not commonly found in organic molecules. If the "repetitious" hydrogen colored in red in the structure above is replaced
by a deuterium, as shown below, the carbon marked with an asterisk has four different groups, creating a new stereocenter.
Because this replacement
creates a new stereocenter, the carbon marked with an asterisk is prochiral