8.7:
¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons
In a molecule, protons in identical electronic environments have the same chemical shift and are chemically equivalent.
Equivalence is determined by the replacement test, where each of the protons being examined is replaced by a substituent. If identical molecules are obtained, the protons are equivalent or homotopic.
In ethane, replacement of any proton by chlorine yields chloroethane, because all six protons are rendered homotopic by rapid rotation about the carbon-carbon bond.
Homotopic protons have rotational symmetry and yield a single NMR signal.
In chloroethane, however, replacement of one alpha- or one beta-hydrogen gives 1,1-dichloroethane and 1,2-dichloroethane, respectively.
These constitutional isomers are obtained by the replacement of heterotopic protons, which are non-equivalent and have distinct NMR signals.
8.7:
¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons
Protons in identical electronic environments within a molecule are chemically equivalent and have the same chemical shift. The replacement test is a useful tool to identify chemical equivalence and predict NMR spectra. A substituent replaces each of the protons being examined and the resulting molecules are compared. If the same molecule is obtained, the protons are equivalent or homotopic. Replacement of any hydrogens in ethane by chlorine yields chloroethane because all six protons are rendered homotopic by the rapid rotation of the carbon-carbon bond. Homotopic protons are interchangeable by rotation about an axis of symmetry and yield a single NMR signal.
In chloroethane, however, replacing the alpha- and beta-hydrogens gives 1,1-dichloroethane and 1,2-dichloroethane, respectively. Here, the protons attached to the alpha and beta carbons are non-equivalent with respect to each other and yield distinct NMR signals. Such protons are chemically non-equivalent and called constitutionally heterotopic or just heterotopic.