Effect of Mass Transport on Reaction Selectivity

The manner in which mass transport can affect reaction selectivity depends on the kind of reactions involved.24 Three general types of selectivity have been defined and mass transport effects each in a different way. 

Type I selectivity is that occurring when two simultaneous reactions are taking place.33 An example of this type is the hydrogenation of the C4 acetylenes 

Type II selectivity involves the differentiation between two parallel reactions in which different products are formed by separate paths from the same starting material.33 This type of selectivity is encountered in the hydrogenation of crotonaldehyde to either butyraldehyde or 2 buten-l-ol (Eqn. 5.8). When both reactions are of the same kinetic order changes in mass transport will influence them both to the same extent and there will be no effect on reaction selectivity. When the reactions are of different kinetic orders, that one with the higher order will be more affected by mass transport limitation. 

Type III selectivity is that found with serial reactions such as the hydrogenation of an acetylene first to an alkene and then to the alkane (Eqn. 5.9).33 In such reactions the transport of the organic substrate through the 

Rylander, Catalysis in Organic Synthesis (W. H. Jones, Ed.) Academic Press, New York, 1980, p. 155. 

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