Find Equivalent Protons
Two protons are chemically equivalent when a symmetry operation or fast conformational change interchanges them — and equivalent protons share a single NMR signal. One position is highlighted; click every hydrogen-bearing atom that shares its environment. The answer key colors the full equivalent set.
Common Questions
When are two protons equivalent in NMR?
Two protons are equivalent when swapping them gives the same molecule (homotopic) or its mirror image (enantiotopic). Equivalent protons share one NMR signal. Fast rotation about single bonds and rapid ring flips average positions, making many protons that look different on paper equivalent at room temperature.
What is the difference between homotopic, enantiotopic, and diastereotopic protons?
Homotopic protons interchange by a rotation and are always equivalent. Enantiotopic protons interchange only by a mirror reflection and are equivalent in an achiral solvent (one signal). Diastereotopic protons cannot be interchanged by any symmetry operation and are inequivalent, so they can show separate signals.
How can I tell if two hydrogens are equivalent?
Use the replacement test: replace each hydrogen in turn with a test group and compare. Identical products mean homotopic; enantiomers mean enantiotopic (equivalent in an achiral solvent); diastereomers mean diastereotopic (inequivalent). Symmetry elements of the whole molecule — mirror planes and rotation axes — are the fastest way to spot equivalent sets.
Are the two CH₂ hydrogens next to a stereocenter equivalent?
Often not. A CH₂ group adjacent to a stereocenter is usually diastereotopic: its two hydrogens sit in different environments and are inequivalent, so they can give separate NMR signals. Without a nearby stereocenter or restricted rotation, a freely rotating CH₂ typically has two equivalent hydrogens.
When do two positions count as equivalent?
Two positions are equivalent when the molecule's symmetry or its fast internal motions make them interchangeable: rotation about single bonds, ring flips, and mirror planes all merge positions into one environment. Enantiotopic positions count as equivalent here too (the solvent is achiral), so the answer merges them into one set.