Q46E - Organic Chemistry (Mcmurry)

Question

Carboxylic acids (RCOOH; $\mathbf{p}{K}_a$=5) are approximately 1011 times more

acidic than alcohols (ROH; $\mathbf{p}{K}_a$ =16). In other words, a carboxylate ion $\left({{\mathbf{RCO}}_2}^{-}\right)$

is more stable than an alkoxide ion $\left({\mathbf{RO}}^{-}\right)$. Explain, using

resonance.

Step-by-Step Solution

Answer

Carboxylic acids are more acidic than alcohols because the negative charge of carboxylate ion is stabilized by resonance. Whereas alkoxide anion are not resonance stabilized.

Step-by-step solution Step 1: Resonance

Resonance is a method of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis structure. 

A molecule or ion with such delocalized electron is represented by several contributing structure called resonance structure.

Step 2: 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: 

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

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

                                                              $\text{p}K\text{a = -log }{K}_a$

The stronger acid has a smaller $\text{p}{\mathrm{K}}_{\mathrm{a}}$ and a weaker acid has a larger $\text{p}{\mathrm{K}}_{\mathrm{a}}$ .

Step 3: Stability of carboxylate ion and alkoxide anion by resonance

                                           

 

           

                 Alkoxide ion                                                                              Carboxylate ion

Carboxylic acids are more acidic than alcohols because the negative charge of carboxylate ion is stabilized by resonance. This resonance stabilization favors formation of carboxylate anion over alkoxide anions, and increases ${\mathrm{K}}_{\mathrm{a}}$, for carboxylic acids. Whereas alkoxide anion are not resonance stabilized.