Benzoic acid and quinine are dissolved in a solution of dichloromethane. (benzoic acid pKa = 4.2; quinine pKa = 8.5)
a) Besides the normal neutral forms for benzoic acid and quinine, will there be any other chemical form(s) present in the dichloromethane solution?
a) What species will they be? Draw these along with the neutral forms of benzoic acid and quinine (HINT: write it as a reaction between the neutral species and the conjugate species)
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In order to determine whether there will be any other chemical forms present in the dichloromethane solution of benzoic acid and quinine, we need to consider their pKa values.
A compound can exist in different forms depending on the pH of the solution it is in. The pKa value represents the acidity of a compound and determines the equilibrium between its neutral and ionized forms. Substances with pKa values below the pH of the solution tend to exist in their ionized form, while substances with pKa values above the pH of the solution exist mainly in their neutral form.
The pKa of benzoic acid is 4.2, which is lower than the pH of most solutions (typically around neutral, i.e., pH 7). This means that benzoic acid is predominantly in its neutral form in dichloromethane solution.
On the other hand, the pKa of quinine is 8.5, which is higher than the pH of most solutions. This indicates that quinine tends to exist mostly in its ionized form in dichloromethane solution.
When a compound exists in both its neutral and ionized forms, it can be represented using a chemical equilibrium equation. In this case, we can represent the equilibrium for benzoic acid as follows:
Benzoic acid ⇌ Benzoate ion + H+
Similarly, for quinine, we can represent the equilibrium as:
Quinine ⇌ Quinine ion + H+
These equations show the reaction between the neutral form (benzoic acid or quinine) and its conjugate base (benzoate ion or quinine ion) which occurs based on the given pKa values.
To summarize, in the dichloromethane solution, benzoic acid will predominantly exist in its neutral form, while quinine will mainly exist in its ionized form.