Stereochemistry of Carbonyl Addition

Introduction

A planar carbonyl carbon presents two faces to an incoming reagent. The faces can look identical in a flat drawing yet lead to different three-dimensional products after the carbon changes from trigonal planar to tetrahedral. Re and Si provide a consistent way to name those starting faces. They do not, by themselves, specify the R or S configuration of the final product; that must be assigned after the new group is included.

Core explanation

At a trigonal carbonyl carbon, consider the three groups directly attached to that carbon in the planar arrangement: oxygen and two other substituents. Rank these three according to Cahn–Ingold–Prelog priorities, including the ordinary duplicate-atom treatment for a double bond when needed. Look at one face of the planar group. If the priority sequence 1 → 2 → 3 runs clockwise as viewed from that side, the face is Re. If it runs counterclockwise, that face is Si. Looking from the opposite side reverses the observed order, so the two sides of an appropriately substituted trigonal centre carry opposite labels.

The designation applies only when the three planar ligands can be distinguished. For acetone, the carbonyl carbon bears two identical methyl groups, so attack at its two faces cannot create a stereocentre at that carbon; the product propan-2-ol has two identical methyl groups. For butan-2-one, methyl and ethyl are different. Hydride addition produces butan-2-ol, whose OH-bearing carbon has OH, H, methyl and ethyl. It is stereogenic. In an achiral environment, equivalent-energy attack on the two enantiotopic faces generally yields a racemate, although a specific reaction may have additional selectivity effects that require evidence.

If the starting molecule already has a stereocentre, the carbonyl's faces may be diastereotopic rather than enantiotopic. Attack on them then gives diastereomers, whose transition-state energies need not be equal even with an achiral reagent. A nearby chiral auxiliary, catalyst or enzyme can likewise favour one face of a prochiral carbonyl. Steric approach, chelation and conformation can all influence the ratio. Simply identifying Re and Si names does not say which face reacts faster; that requires a model and substrate-specific conditions.

Re or Si describes the face before addition; R or S describes the final tetrahedral stereocentre. The letters are not automatically paired. The incoming group can have a priority that changes the final 1–4 ordering. For example, adding hydride introduces H, commonly low priority, whereas adding CN introduces a carbon-bearing group whose priority must be compared with existing carbon groups. Thus Re attack may yield R for one reagent and S for another. To determine the product label, draw the actual tetrahedral product and apply CIP priorities anew.

The Bürgi–Dunitz approach adds another layer. Nucleophiles often approach at an oblique angle relative to C=O rather than directly perpendicular to the drawing plane. The label Re or Si identifies the side of the planar centre, not an exact approach vector or transition-state geometry. A bulky substituent may shield part of a face; coordination of a metal to oxygen may present a different trajectory. Three-dimensional models are often essential for credible selectivity predictions.

Carbonyl addition can also make no new stereocentre even with distinct faces. Formaldehyde plus hydride gives methanol, whose carbon has three H atoms. A ketone with identical carbon substituents remains achiral at the alcohol carbon after hydride addition. Always count all four product groups before claiming an enantiomeric pair. Conversely, a formerly achiral aldehyde with an alkyl group can become chiral after addition of a different carbon nucleophile if the resulting carbon has four different groups.

Step-by-step reasoning

Draw the carbonyl flat and rank its three ligands. From the side being viewed, trace 1 → 2 → 3; clockwise is Re and counterclockwise is Si. Draw attack from each side using wedge and dash bonds in the tetrahedral products. List the four final groups and test whether a stereocentre exists. Apply CIP again to assign R/S if needed. Check whether the starting substrate and conditions are achiral before predicting a racemic mixture.

Visual explanation

Draw a trigonal carbonyl triangle with ligand priorities 1, 2 and 3. Put an eye symbol above the paper and an arrow tracing 1 → 2 → 3 clockwise, labelled Re. Draw a second eye below, where the same order appears counterclockwise, labelled Si. To the right, draw two tetrahedral addition products with Nu as wedge in one and dash in the other, then assign product configurations independently.

Real-world analogy

A flat printed card has a front and back; a clockwise route seen from the front appears counterclockwise from the back. This conveys how one planar object can have two named faces. The card does not model how a new chemical bond changes substituent priorities, so it cannot tell you the final R/S label without drawing the product.

Real-world example

Reduction of butan-2-one gives butan-2-ol. Its carbonyl carbon originally has oxygen, methyl and ethyl in a plane. Hydride can approach from either face, producing the R and S alcohols under ordinary achiral conditions. In an asymmetric reduction using a chiral catalyst, one face can be preferred and the alcohol can be enriched in one enantiomer. The name of that preferred face must be assigned from a specific oriented structure.

Why?

Addition changes the carbonyl carbon from planar to tetrahedral and can lock the direction of approach into a new stereogenic arrangement. Mirror-related faces of an achiral prochiral substrate lead to mirror-related products if the reagent and environment have no chirality. A chiral environment distinguishes those approaches energetically. Re/Si notation supplies an unambiguous language for the starting surface, while R/S supplies one for the resulting centre.

Common misconception

“Re face gives R product” is not a general rule. Product CIP priorities depend on the incoming group, and the geometry must be drawn after addition. Another mistake is claiming every carbonyl addition yields enantiomers. Acetone plus hydride gives achiral propan-2-ol, and formaldehyde gives an alcohol carbon with repeated hydrogen substituents. Test the final centre before assigning stereochemistry.

Worked example

Question: Why can NaBH4 reduction of butan-2-one yield two enantiomers, while the same reduction of propanone does not?

Reasoning: Butan-2-one's carbonyl carbon is planar and has methyl and ethyl substituents. Hydride can approach the two faces; after protonation the carbon bears H, OH, methyl and ethyl, four different groups. The two face-derived products are mirror-related R and S butan-2-ol in an achiral environment. Propanone has two methyl groups. After hydride addition its alcohol carbon bears H, OH and two identical methyl groups, so it is not stereogenic regardless of which side was attacked.

Answer: Butan-2-one is prochiral at carbonyl carbon and can form R/S butan-2-ol; propanone gives achiral propan-2-ol because the product has duplicate methyl groups.

Quick check

1. What does a clockwise 1 → 2 → 3 sequence of trigonal ligands define when viewed from one face? Answer: The Re face of that trigonal centre.

Exam focus

Write Re/Si with the correct capitalisation and assign from the specified viewing side. For product questions, draw wedges and dashes and recalculate CIP priorities. State racemic outcome only when enantiotopic faces react under an achiral influence and a stereocentre actually forms. Distinguish enantiomeric products from diastereomeric ones when the substrate already contains a stereogenic element.

Advanced insight

Facial selectivity is a comparison of transition-state free energies. Chelating Lewis acids can change preferred conformations, and enzymes create highly organised chiral pockets. A simple steric picture may help but can fail if electronic interactions or chelation dominate. Face nomenclature remains the same regardless of which pathway is faster, making it valuable when evaluating competing selectivity models.

Summary

Re and Si name the two faces of an appropriately substituted trigonal carbonyl according to CIP priorities viewed from each side. Carbonyl addition can convert a planar prochiral carbon into a tetrahedral stereocentre. Achiral conditions can yield enantiomers from enantiotopic faces, whereas chiral surroundings can favour one face. Final R/S configuration must be assigned on the product, not inferred from the starting face label alone.

Practice questions

1. What face is seen if ligand priorities 1 → 2 → 3 run counterclockwise? Answer: The Si face from that viewing side. 2. Why does hydride addition to acetone not create a stereocentre at carbonyl carbon? Answer: The product carbon still bears two identical methyl groups. 3. What kind of products can attack on two faces of a carbonyl in an already chiral molecule produce? Answer: Diastereomers, since the pre-existing chirality makes the faces diastereotopic. 4. Why must CIP ranking be repeated after nucleophilic addition? Answer: The incoming group becomes a fourth substituent and can change the priority order used to assign R or S. 5. Does attack on the Re face automatically give an R stereocentre? Answer: No. Assign R/S separately after including the incoming group and ranking all four product substituents.