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
replacement.
Replacement 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.
A
prochiral 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 hydrogens.
Prochirality 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.
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