44E - Organic Chemistry (Mcmurry)

Question

Rank the following substances in order of increasing acidity:

Step-by-Step Solution

Answer

The substance with the largest value of pK_a is the least acidic. So, the order of increasing acidity is:

Acid and 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 ( $K_a$) for the acid-dissociation equilibrium.

Example:$\begin{array}{l}{\text{HA + H}}_2\text{O }\rightleftarrows {\text{ A}}^{-}{\text{ + H}}_{\text{3}}{\text{O}}^{-}\\ K_a\underset{}{}\text{=}\underset{}{}\frac{{\text{[H}}_{\text{3}}{\text{O}}^{+}{\text{][A}}^{-}\text{]}}{\left[HA\right]}\end{array}$

Relation between values and values

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 acid has a smaller $\mathbf{p}{K}_a$ , and a weaker acid has a larger  $\mathbf{p}{K}_a$

Rank the order of the following compound

The substance with the largest value of pK_a is the least acidic. So, the order of increasing acidity is shown below:

 

Least acidic    most acidic 

                                                Order of acidity