Homotopic groups and faces

Let us consider dichloromethane (LV), which contains one pair of chlorine atoms 

Applying the substitution criterion leads of course to the same conclusion of homotopism. For example, substituting each H in turn by another group such as 2H yields two molecules which are superimposable, i.e., nondistinguishable. 

The molecule of toluene (LVII) provides a slightly more complex case since here the time factor must be taken into account. Indeed, the three hydrogen atoms of the 

The molecular environment within a molecule can be defined relative to faces of the molecule instead of groups. For example, dichloroethylene (LVII) has two faces. Since there is no way for an observer or attacking reagent to distinguish between these two faces, they are said to be equivalent. 

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