Enantiotopic
Definition and meaning of Enantiotopic in chemistry.
Enantiotopic describes two atoms, groups, or faces within a molecule that are related to each other by an internal mirror plane (or other improper symmetry element) but not by any proper rotation. Substituting one versus the other converts an achiral molecule into a pair of enantiomers.
In more detail
Because a mirror operation relates them, enantiotopic groups are chemically identical in any achiral environment: they give identical NMR chemical shifts and react at identical rates with achiral reagents. A chiral reagent, catalyst, or enzyme, however, creates a chiral environment that can distinguish the two, reacting preferentially with one to give a single enantiomer. This distinguishes enantiotopic groups from homotopic groups (interchanged by a proper rotation, always equivalent) and diastereotopic groups (not symmetry-related at all, and distinguishable even in achiral environments, e.g., by NMR).
Key facts
| Field | Organic Chemistry |
|---|---|
| Substitution test | Swapping one group for another gives a pair of enantiomers |
| NMR behavior | Isochronous (identical chemical shift) in achiral media |
| Contrast terms | Homotopic (rotation-related); diastereotopic (no symmetry relation) |
In ethanol (CH3CH2OH), the two hydrogens on C1 (the CH2 group) are enantiotopic: replacing one with deuterium gives (R)-1-deuterioethanol, while replacing the other gives (S)-1-deuterioethanol. The two products are enantiomers, and enzymes such as alcohol dehydrogenase can act selectively on just one of these hydrogens.
Frequently asked questions
How do enantiotopic groups differ from diastereotopic groups?
Enantiotopic groups are related by a mirror plane, so swapping one for a substituent gives enantiomers, and the groups are indistinguishable by NMR or achiral reagents. Diastereotopic groups have no such symmetry relation; swapping them gives diastereomers, which have different physical properties and typically show different NMR shifts even in achiral solvents.
Can enzymes tell enantiotopic groups apart?
Yes. Because enzymes are built from chiral amino acids, their active sites are chiral environments that can bind a substrate in only one orientation, allowing them to act selectively on one of two enantiotopic atoms or groups even though ordinary achiral reagents cannot distinguish them.