54E - Organic Chemistry (Mcmurry)

Question

Assume that you have two unlabeled bottles, one of which contains

phenol ( $\mathbf{p}{K}_a$9.9) and one of which contains acetic acid ($\mathbf{p}{K}_a$ =4.76).

In light of your answer to Problem 2-52, suggest a simple way to determine

what is in each bottle.

Step-by-Step Solution

Answer

Sodium bicarbonate reacts with acetic acid to produce carbonic acid, which breaks down to form  . Thus bubbles of  indicate the presence of an acid stronger than carbonic acid. In this case, it is acetic acid, as the  $\mathbf{p}{K}_a$value indicates.

Phenol does not react with sodium bicarbonate.

Acid-Base reactions

Acids 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 essential 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.

Relation between acidic strength and p K a values

Acid strengths are normally using  values rather than  values, where the  is the negative common logarithm of  (acid dissociation constant):

 $\text{p}K\text{a = -log }{K}_a$

 The lower the  $\mathbf{p}{K}_a$value, the stronger the acid will be.

The higher the  $\mathbf{p}{K}_a$value, the weaker the acid will be.

Identifying the compound in each bottle

Sodium bicarbonate reacts with acetic acid to produce carbonic acid, which breaks down to form  . Thus bubbles of  indicate the presence of an acid stronger than carbonic acid. In this case, it is acetic acid, as the  value indicates. The $\text{p}{K}_a$ value of acetic acid ( $\text{p}{K}_a$= 4.76) is less than that of carbonic acid ( $\text{p}{K}_a$= 6.37). So the extent of the reaction is in a forward direction.

 

                                       $C{H}_{3}COOH+NaHC{O}_3\to C{H}_{3}COONa+H_{2}O+C{O}_2$

                               Acetic acid  Sodium         Sodium         water   Carbon

                                                Bicarbonate     acetate                    dioxide

 

Phenol does not react with sodium bicarbonate. The  $\text{p}{K}_a$value of phenol ( $\text{p}{K}_a$= 9.9) is more than that of carbonic acid ( $\text{p}{K}_a$= 6.37). So the extent of the reaction is not in the forward direction. So, the reaction of phenol with sodium bicarbonate is not feasible.

 

                                                       Reaction of phenol