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.