I suppose you are referring to the feasibility of a nucleophilic chemical reaction in which one reagent (nucleophile) is not completely soluble in/miscible with the substrate.
Two cases may arise:
1. Some nucleophile has been dissolved and the excess amount is in the neat (solid or liquid) phase. In this case the reaction would proceed like a homogeneous phase reaction and the undissolved nucleophile will gradually dissolve and will be consumed with the passage of the reaction.
2. The nucleophile is completely insoluble in /miscible with the substrate. Still there remains the chance of heterogenous phase reaction occurring at the interface. Though the reaction rate is reduced drastically when compared to a homogeneous phase reaction under identical conditions, heterogeneous phase reactions have the advantage of ease of separation of product/s. Often the reaction is slowed down to such an extent that practically no product would be recovered in a given amount of time. In such cases, use of an alternative solvent or a suitable chemical that enables efficient passage of substances between two phases (known as phase transfer catalyst) offers a way out.