Surface on the Course of a Reaction

The steric factors influencing the course of a reaction can be modified if the reactant is spread as monomolecular film. An example of this was found by Marsden and Rideal (35) in 1938. They studied the oxidation of monolayers of erucic and brassidic acids, containing, respectively, cis and trans double bonds. The oxidant was acid permanganate in the underlying aqueous solution. The oxidation products from both acids appeared to be identical, corresponding most closely with the properties of the cis dihydroxy acid. This material, m.p. 100°C., occurs in the bulk oxidation from erucic acid only the trans unsaturated acid gives an isomer of m.p. 130°C. There is thus some evidence for a directive oxidation due to orientation at an interface. The comparison of the products of the surface reaction was made by spreading films of the various dihydroxy compounds. 

The influence of a surface on the course of a polymerization reaction was discussed by Carothers (69) in 1936. He pointed out that in bulk solution the amino acids or their esters polymerize mainly to the cyclic dimers, the diketopiperazines. In living cells, however, these cyclic compounds are evidently not formed reaction is exclusively intermolecu-lar leading to the formation of linear polyamides. Carothers suggested that adsorption at a fat-water interface may well be responsible for this, since here the reactant monomer molecules will be oriented to lie flat in the plane of the surface. As an example, the molecule NH2 CH2-CO-NH-CHs COOH is anchored to the interface through the polar groups at each end and in the middle of the molecule. Under these conditions the difficulty of bending may prevent cyclization. 

Although these possibilities for altering the course of polymerization reactions have not yet been applied commercially, some preliminary practical investigations have been begun in the U.S.S.R. Bresler, Judin, and Talmud (70) found that the elasticity and viscosity of mixed films could conveniently be used to follow the course of the polymerization. Reaction between the diamine cadaverine and films of stearic aldehyde led to the production of a linear polymer without elastic properties. 

Tetraamiiiodiphenyl and stearic aldehyde, however, gave rise to a solid, brittle monolayer, extensively cross-linked. 

Another example of the presence of a surface altering the course of a reaction is provided by the studies of Wasteneys and Borsook (08). It has already been mentioned (Section VI) that the presence and nature of an oil-water interface modified considerably the acid- and basecombining properties of the complex derivatives whose formation from peptides and an enzyme was catalyzed by an emulsion of oil. 

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