Introduction Measuring the Weak Diiodine Basicity of Haloalkanes

1 Introduction Measuring the Weak Diiodine Basicity of Haloalkanes 

Haloalkanes are weak Lewis bases towards all Lewis acids. For this reason, they have attracted little attention in the construction of Lewis basicity scales. In this experiment, their basicity towards diiodine is studied by measuring the complexation constant of diiodine with lodocyclohexane in cyclohexane and comparing the result with those for other haloalkanes. 

Visible spectrometry is the method of choice for measuring diiodine basicity because the visible band of diiodine and the blue-shifted band of its complexes have rather high absorption coefficients ( 10001 moC cm ). Consequently, there are significant variations in absorbance on complexation, even when small quantities of complex are formed, and accurate complexation constants can be obtained for weak Lewis bases. A disadvantage of electronic spectrometry is the width of the absorption bands. Hence, even significant blue shifts on complexation cannot prevent the overlap of the visible bands of free and complexed diiodine. Therefore, there are two unknowns, the complexation constant and the absorption coefficient of the complex, in the equation relating the equilibrium constant to the absorbance and the initial concentrations of diiodine and base. 

A wide variety of methods has been devised for the treatment of experimental data. The simple and rigorous method of Rose-Drago is used here. 

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