The decisive result is not simply that two sites are substituted, but that the two substitutions produce different connectivities. Because connectivity specifies which atoms are linked and how, different products show that the original positions were constitutionally nonequivalent. This makes the test a structural criterion rather than an observation based only on position or appearance.
The distinction depends on what changes when one position is replaced. Constitutional heterotopy is associated with different constitutional isomers, whereas homotopic and stereotopic relationships represent other forms of positional comparison. Using the replacement test helps classify the relationship according to constitutional connectivity instead of treating every nonequivalent-looking site as constitutionally different.
Molecular symmetry provides a framework for evaluating whether positions can be treated as equivalent during structural analysis. Constitutional heterotopy focuses the analysis on connectivity: if separate replacement at two positions leads to different connectivities, symmetry considerations must accommodate that constitutional distinction. This helps connect molecular structure with the way positions are classified.
Select the two positions being compared, introduce the same substituent at the first position, and then perform the corresponding replacement at the second position. Compare the resulting structures by their atom-to-atom connectivities. If the products differ constitutionally, the original positions satisfy the criterion for constitutional heterotopy.
The relationship can help predict whether nuclei associated with two positions may experience different chemical environments in NMR spectroscopy. It does not replace spectral measurement, but it provides structural reasoning for anticipating nonequivalent environments. Consequently, constitutional analysis can guide interpretation of NMR observations by linking possible differences in signals to molecular connectivity.
Different connectivity relationships can make substitution at one position structurally distinct from substitution at another. Constitutional heterotopy therefore clarifies why alternative reaction sites may lead to different constitutional products rather than interchangeable outcomes. In chemistry, this provides a framework for relating the identity of a reacting position to the structures formed during product analysis.