The Influence of Organic Solvents on Aromatic Adsorption at Platinum: Acetic Acid, Acetone, Acetonitrile, Dimethylacetamide, Dimethylsulfoxide, Sulfolane, and Tetrahydrofuran

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Abstract
The influence of nonaqueous solvents on the packing density and electrochemical reactivity of 2,2′,5,5′,‐tetrahydroxybiphenyl (THBP) chemisorbed at smooth polycrystalline Pt electrodes has been studied using thin layer electrochemical techniques. THBP was chosen as the model polyphenolic compound because its surface coverage is readily measured, and it displays electrochemical reactivity in the adsorbed state which is a function of its orientation. The nonaqueous solvents employed were: acetonitrile, ethyl acetate, acetone, acetic acid, dimethylacetamide, tetrahydrofuran, dimethylsulfoxide, and sulfolane. Adsorption of THBP from its aqueous solutions led to three different orientation states (flat, edge‐edge, and edge‐pendant) which depended upon the THBP concentration. The edge‐pendant state showed reversible quinone/diphenol reactivity similar to that in the unadsorbed state. Changes in the adsorption behavior and electrochemical reactivity of THBP was noted when 10 mM of organic solvent was added. The extent of change depended upon the adsorbability of the added solvent, being least noticeable with ethyl acetate (which is not chemisorbed on Pt) and most dramatic with dimethylsulfoxide (which is very strongly chemisorbed on Pt). The more strongly adsorbing of these solvents virtually eliminated adsorption of THBP in the flat orientation. The electrochemical reactivity of THBP adsorbed in the edge‐pendant orientation was sensitive to the nature of the nonaqueous solvent, probably because of solvent adsorbate interactions. These results provide revealing glimpses of the role played by oriented adsorbed intermediates and nonaqueous solvents in the stereospecificity of organic electrochemical reactions. In particular, the data appear to invalidate previous assumptions that the orientation of adsorbed aromatic intermediates is flat, regardless of solute concentration or nature of the solvent.

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