To figure out which hydrogen is pro-R, you use a simple thought experiment called the substitution test. Pick one of the two identical-looking hydrogens on a prochiral carbon, mentally replace it with deuterium (the heavier isotope of hydrogen), and then assign R or S configuration to the resulting chiral center. If the result is R, that hydrogen is the pro-R hydrogen. Here’s exactly how to do it, step by step.
What Makes a Carbon Prochiral
A prochiral carbon is a tetrahedral carbon that isn’t currently a chiral center but would become one if you swapped out just one of its attached groups. The most common case: a carbon bonded to two hydrogens and two other different groups. Those two hydrogens look identical at first glance, but they actually occupy distinct spatial positions. Replacing either one with something different (like deuterium) would give the carbon four unique substituents, making it a true stereocenter.
The Substitution Test
This is the standard method for assigning pro-R and pro-S labels. It works on any prochiral carbon, whether the molecule is simple like ethanol or complex like a sugar phosphate.
- Step 1: Identify the prochiral carbon, the one carrying two identical groups (usually two hydrogens).
- Step 2: Pick one of the two hydrogens and mentally replace it with deuterium (D).
- Step 3: Assign R or S configuration to the new chiral center using the standard priority rules. The key detail: deuterium outranks regular hydrogen in priority because it has a higher atomic mass.
- Step 4: If the resulting configuration is R, that original hydrogen is the pro-R hydrogen. If it’s S, it’s the pro-S hydrogen.
You only need to test one hydrogen. Once you know one is pro-R, the other is automatically pro-S.
Why Deuterium Outranks Hydrogen
The priority system used in stereochemistry (Cahn-Ingold-Prelog rules) ranks atoms first by atomic number, then by atomic mass when the atomic number is the same. Deuterium and hydrogen both have one proton, so they share atomic number 1. But deuterium has a neutron that hydrogen lacks, giving it an atomic mass of 2 versus 1. That mass difference is enough to break the tie: D ranks higher than H. This is what makes the substitution test work. Replacing H with D gives the carbon four groups of genuinely different priority, turning it into a stereocenter you can assign as R or S.
Assigning R vs. S After Substitution
Once you’ve mentally swapped in deuterium, you have a standard stereocenter. To assign its configuration:
- Rank the four substituents from highest priority (1) to lowest priority (4) using atomic number and mass.
- Orient the molecule so the lowest-priority group (4) points away from you, into the page.
- Trace a path from group 1 to group 2 to group 3. If that path is clockwise, the configuration is R. If counterclockwise, it’s S.
In ethanol, for example, the prochiral carbon carries an OH group, a methyl group, and two hydrogens. Replace one hydrogen with deuterium and the priorities become: OH (highest), CH₃, D, H (lowest). Orienting the molecule with H pointing away and tracing from OH to CH₃ to D, a clockwise path means R. So the hydrogen you replaced is the pro-R hydrogen.
Ethanol as a Worked Example
Ethanol (CH₃CH₂OH) has a prochiral carbon at C-1, the carbon bonded to the hydroxyl group. That carbon carries OH, CH₃, and two hydrogens. If you replace the hydrogen that yields an R configuration when swapped for deuterium, that’s your pro-R hydrogen. Replace the other, and you get an S center, making it the pro-S hydrogen. The two resulting molecules (with deuterium in place of one hydrogen or the other) are mirror images of each other, meaning the original hydrogens are called enantiotopic.
Enantiotopic vs. Diastereotopic Hydrogens
Not all prochiral hydrogen pairs behave the same way. If the molecule has no other stereocenters, replacing one hydrogen with deuterium creates a molecule that is the mirror image (enantiomer) of the molecule created by replacing the other hydrogen. These hydrogens are enantiotopic. In NMR spectroscopy, enantiotopic hydrogens look identical; they show up as a single signal in achiral solvents.
If the molecule already has a stereocenter elsewhere, the two substitution products won’t be mirror images. They’ll be diastereomers instead. The prochiral hydrogens in this case are called diastereotopic. Glyceraldehyde-3-phosphate is a classic example: it already has one chiral center, so the pro-R and pro-S hydrogens on its prochiral carbon produce R,R and S,R diastereomers. Diastereotopic hydrogens do show different signals in NMR, which makes them chemically distinguishable even without enzymes.
Why It Matters in Biochemistry
Enzymes can tell pro-R and pro-S hydrogens apart. An enzyme’s active site is chiral, so it interacts differently with the two spatial positions even though the hydrogens themselves are chemically identical. Alcohol dehydrogenase, for instance, specifically removes one of the two prochiral hydrogens from the cofactor NADH during reactions, leaving the other untouched. Studies on dehydrogenase and diaphorase enzymes from Clostridium bacteria showed that both enzymes transferred specifically the pro-R hydrogen of NADH during catalysis. This kind of stereospecificity is the rule in enzyme chemistry, not the exception. Virtually every enzyme that handles a prochiral center discriminates between the two faces or the two hydrogens with near-perfect selectivity.
Understanding pro-R and pro-S designations lets biochemists predict which hydrogen an enzyme will add or remove, trace metabolic pathways using isotope labeling, and design drugs that interact with the correct face of a prochiral molecule.