Q. 15 P - Organic Chemistry (Mcmurry)

Question

Ammonia, NH₃, has =36, and acetone has =19. Will the following reaction take place to a significant extent?

Step-by-Step Solution

Answer

The above reaction will take place to virtual completion due to large difference in a value.

Step 1: 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 (${\mathit{K}}_{\mathbf{a}}$) for the acid-dissociation equilibrium.

Example:

                                      $$\begin{gathered} {\text{HA + H}}_2\text{O }\rightleftarrows {\text{ A}}^{-}{\text{ + H}}_{\text{3}}{\text{O}}^{-} \\ {\text{K}}_a\text{ = }\frac{{\text{[H}}_{\text{3}}{\text{O}}^{+}{\text{][A}}^{-}\text{]}}{\left[HA\right]} \end{gathered}$$

Acid strengths normally uses $\text{p}{K}_a$ 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 $\text{p}{K}_a$ and a weaker acid has a larger $\text{p}{K}_a$.

Step 2: Determine the significant extent of reaction

Stronger Acid -  $\text{p}{K}_a=19$

weakker Acid -   $\text{p}{K}_a =36$

A reaction takes place if the stronger acid is the reactant. The above reaction will take place to virtual completion due to large difference in a value.