47E - Organic Chemistry (Mcmurry)

Question

Is tert-butoxide anion a strong enough base to react significantly with

water? In other words, can a solution of potassium tert-butoxide be

prepared in water? The pK_a of tert-butyl alcohol is approximately 18.

Step-by-Step Solution

Answer

 $\text{p}{K}_a$= 15.7                                                                        $\text{p}{K}_a$= 18

Stronger acid                                                         weaker acid

The tert-butoxide anion is a strong enough base to react significantly with water. So a solution of potassium tert-butoxide cannot be prepared in water.

Acid-Base Strength

Acids differ in their ability to donate H⁺. Stronger acids, such as HCl, react almost completely with water, whereas weaker acids, such as acetic acid (CH₃COOH), react only slightly. The exact strength of a given acid HA in water solution is described using the acidity constant ( ) for the acid-dissociation equilibrium.

$\begin{array}{l}{\text{HA + H}}_2\text{O }\rightleftarrows {\text{ A}}^{-}{\text{ + H3O}}^{-}\\ K_a\underset{}{}\text{=}\underset{}{}\frac{{\text{[H3O}}^{+}{\text{][A}}^{-}\text{]}}{\left[HA\right]}\end{array}$

Identify the stronger base with the help of value

Acid strengths are normally using  $\text{p}{K}_a$values rather than $K_a$  values, where the  $\text{p}{K}_a$is the negative common logarithm of $K_a$:

 $\text{p}K\text{a = -log }{K}_a$

The stronger base has a larger  $\text{p}{K}_a$, and the weaker base has a smaller $\text{p}{K}_a$ value.

Reaction of tert-butoxide anion with water

The reaction takes place as written because water is a stronger acid than tert-butyl alcohol. Thus, a solution of potassium tert-butoxide in water can’t be prepared.

tert-butoxide anion                                                  Potassium tert-butoxide

 $\text{p}{K}_a$= 15.7                                                                        $\text{p}{K}_a$= 18

Stronger acid                                                                   weaker acid

 

The tert-butoxide anion is a strong enough base to react significantly with water. So a solution of potassium tert-butoxide cannot be prepared in water.