Question

A mixture of benzoic acid and phenol can be separated using (a) Dilute \(\mathrm{HCl}\) (b) Aqueous \(\mathrm{NaOH}\) (c) aqueous \(\mathrm{NaHCO}_{3}\) (d) both (b) and (c)

Short answer

Answer: Both aqueous NaOH and aqueous NaHCO3 can effectively separate a mixture of benzoic acid and phenol.

Step-by-step solution

Analyze the properties of benzoic acid and phenol

Benzoic acid is a carboxylic acid with the formula \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COOH}\). It's relatively acidic because the carboxyl group is electron-withdrawing. Phenol, on the other hand, has the formula \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{OH}\). It's a weak aromatic acid since the hydroxyl group is bonded directly to the aromatic ring with electron-donating properties. It's crucial to keep their acidity differences in mind in order to apply the correct reagent for separation.

Test the reaction of benzoic acid and phenol with dilute HCl

Dilute \(\mathrm{HCl}\) will protonate both benzoic acid and phenol to form their respective chloride salts \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COO}^-\mathrm{H}_{3}\mathrm{O}^+\) and \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{O}^-\mathrm{H}_{3}\mathrm{O}^+\). Since both compounds will react similarly with dilute \(\mathrm{HCl}\), it's not suitable for separating them. So, option (a) is not the correct answer.

Test the reaction of benzoic acid and phenol with aqueous NaOH

Aqueous \(\mathrm{NaOH}\) (sodium hydroxide) is a strong base. It reacts with benzoic acid to form sodium benzoate (\(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COO}^-\mathrm{Na}^+\)), which is soluble in water. However, phenol being a weak acid, doesn't completely react with sodium hydroxide to form a significantly water-soluble salt. Thus, treating the mixture with aqueous \(\mathrm{NaOH}\) will form a water-soluble benzoate salt and barely soluble phenolate salt, making it possible to separate benzoic acid and phenol. Therefore, option (b) seems to be a correct choice.

Test the reaction of benzoic acid and phenol with aqueous NaHCO3

Aqueous \(\mathrm{NaHCO}_{3}\) (sodium bicarbonate) is a weak base. It reacts with relatively strong acids, like benzoic acid, forming soluble sodium benzoate (\(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COO}^-\mathrm{Na}^+\)) and carbon dioxide gas. However, it doesn't react with phenol, being a weak acid, to form any water-soluble salt. So, treating the mixture with aqueous \(\mathrm{NaHCO}_{3}\) will form a water-soluble benzoate salt while phenol remains unreacted. Therefore, option (c) is also a correct choice. This leads us to the final answer:

Identify the correct answer

The correct answer is (d) both aqueous \(\mathrm{NaOH}\) and aqueous \(\mathrm{NaHCO}_{3}\), as both methods can effectively separate benzoic acid and phenol.

Key concepts

Carboxylic Acid Reactivity

Carboxylic acids, such as benzoic acid (\(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COOH}\)), exhibit distinct reactivity due to the presence of the carboxyl group (\(-COOH\)). This functional group consists of a carbonyl ((\(C=O\))) and a hydroxyl ((\(-OH\))) group. The electron-withdrawing nature of the carbonyl increases the acidity of the hydroxyl hydrogen, making carboxylic acids more acidic than alcohols.

When reacted with bases, carboxylic acids can form carboxylate salts. For instance, benzoic acid reacts with sodium hydroxide (NaOH) to form the highly water-soluble sodium benzoate (\(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COO}^-\mathrm{Na}^+\)). These reactions are often exploited in separation techniques to isolate carboxylic acids from non-acidic or less acidic substances.

Weak Aromatic Acid Properties

Phenol (\(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{OH}\)), exemplifies weak aromatic acids. These weak acids have an \(OH\) group bonded to an aromatic system, such as a benzene ring. The aromatic ring's electron-donating characteristics decrease the acidity of the \(OH\) group's hydrogen and, consequently, phenol displays limited reactivity with weak bases.

Aromatic acids like phenol do not readily lose their hydrogen ion in the presence of a weak base like sodium bicarbonate (\(\mathrm{NaHCO}_{3}\)). This contrasting behavior between carboxylic and weak aromatic acids forms the basis of certain separation techniques, leveraging their different acid strengths to facilitate their isolation in mixtures.

Acid-Base Separation Techniques

Acid-base separation techniques take advantage of the difference in reactivity between acids of varying strengths to separate components in a mixture. A critical aspect is choosing the right reagent to react selectively with one component over another.

Application in Separation

When dealing with a mixture of benzoic acid and phenol, we employ reagents such as NaOH and \(\mathrm{NaHCO}_{3}\). NaOH, being a strong base, will react with the benzoic acid to form a soluble salt, while leaving phenol largely unreacted because of its low acidity. Conversely, \(\mathrm{NaHCO}_{3}\), a weaker base, only reacts with the more acidic benzoic acid. By manipulating these reactions, we can effectively separate benzoic acid from phenol, as the formed salts can be washed away while the unreacted substances can be retrieved from the mixture.

The choice of reagent directly influences the purity and yield of the substances obtained after separation. Understanding these concepts ensures that one is well-equipped to tackle problems involving the separation of mixed compounds not just on paper, but in practical laboratory environments as well.