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C ~ ~ R WIGER GE
Caiilwnia Sfate Univsrsily Carson. CA 90747
More on Effects Controlling Carboxylic Acidity Lowell M. Schwartz Universih of Massachusens Boston. MA 02125 I t has been nearly a decade since Calder and Barton (I) published an article in this column explaining that the relative strengths of carboxylic acids are due not to energetic differences brought about by charge redistribution within the solute species but rather to entropic differences attributable to order-disorder effects of the solvent water molecules clustered around those species. Yet authors of some recent introductory organic textbooks continue to offer the same erroneous explanation that was discredited in 1971. Perhaps the reason is that a satisfactory explanation of these entropic effects reouires an author to either assume orior exoosure of the students to thermodynamic concepts or to introduce these concepts in the text a t this point and for this purpose. If this is true, it is likely that an author wishing neither to assume nor to teach thermodynamics nor to omit the discussion of acid strengths entirely from his text would opt to explain it in the old way. The objective here is to show that experimental data gathered during the past decade offers future authors an escape from this dilemma: an explanation of relative acid strengths in the gas phase where, in the absence of solvent molecules, energetic factors, indeed, are controlling. T h e table shows the relative strengths of halogen-substituted acetic and propionic acids. pK. refers to the equilibrium constant for the proton transfer from the acid HA to H 2 0 either in aqueous solution or in the gas phase
+
HA(aq) HzO(1) = A-(aq) HAW
+ H30t(aq)
+ HzO(g) = A%) + HaOt(g)
(1)
