45E - Organic Chemistry (Mcmurry)

Question

Which, if any, of the substances in Problem 2-44 is a strong enough acidto react almost completely with NaOH? (The $\mathbf{p}{K}_a$ of  is 15.74.)

Step-by-Step Solution

Answer

In order to react completely with NaOH, an acid must have a  $\text{p}{K}_a$ at least 3 units smaller than  $\text{p}{K}_a$ of $H_{2}O$ . Thus all substances in the previous problem except acetone react completely with NaOH.

Acid-Base reactions

Acids are substances that can donate protons, and bases are substances that can accept protons. Conjugate bases are formed from acids that have donated a proton, while conjugate acids are formed from bases that have accepted a proton.

Acid and bases are important to many chemical processes, maintaining a stable internal environment in the human body, producing salt and water with the help of a neutralization process in which acid and base react to form a salt and water.

Strength of Acid and bases

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.

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 $p{K}_a$ , and the weaker acid has a larger $p{K}_a$ .

 

Reactivity of following substances with NaOH

In order to react completely with NaOH, an acid must have a   $p{K}_a$ at least 3 units smaller than  $\text{p}{K}_a$ of $H_{2}O$ . Thus all substances in the previous problem except acetone react completely with NaOH.

Acetone does not react with NaOH because the $p{K}_a$ value of acetone is greater than water, due to which extent of the reaction is not in the forward direction. Therefore, the reaction of acetone with NaOH is not possible.

The strong acid reacts completely with the strong base and neutralizes each other.

The lower the $p{K}_a$ value, the stronger the acid becomes.