Draw the products of the reaction of benzoic acid with NaOH (aq) and identify the conjugate acid and base in the products. Determine the direction of the equilibrium for this reaction and use pKa values to justify your answer. Is the organic product of this reaction water soluble? Why or why not? Using the chemicals provided, benzoic acid, Ethyl p-amino benzoate, Fluor none, Sodium Hydroxide, Hydrochloric Acid, Anhydrous Sodium Sulfate, Diethyl Ether , how might you convert this product back to the original species?
The Correct Answer and Explanation is:
Correct Answer Summary:
Reaction:
Benzoic acid (C₆H₅COOH) reacts with aqueous sodium hydroxide (NaOH) to produce sodium benzoate (C₆H₅COO⁻ Na⁺) and water (H₂O).
Products:
- Organic Product: Sodium benzoate (C₆H₅COO⁻ Na⁺)
- Inorganic Product: Water (H₂O)
Conjugate Acid/Base Pairs:
- Conjugate Acid: Water (H₂O)
- Conjugate Base: Benzoate ion (C₆H₅COO⁻)
Equilibrium Direction:
- The reaction favors the products.
- Justification: Benzoic acid has a pKa ≈ 4.2, and water (as a conjugate acid of OH⁻) has a pKa ≈ 15.7. Equilibrium favors formation of the weaker acid (higher pKa), i.e., water, making the reaction go essentially to completion.
Solubility:
- Sodium benzoate is water-soluble.
- Reason: It is an ionic salt formed from a carboxylate anion (C₆H₅COO⁻) and sodium cation (Na⁺), both of which interact well with water molecules via ion-dipole interactions.
Conversion Back to Benzoic Acid:
- Add Hydrochloric Acid (HCl) to the sodium benzoate solution.
- Reaction:
C₆H₅COO⁻ Na⁺ + HCl → C₆H₅COOH (benzoic acid) + NaCl - The acidic proton from HCl reprotonates the benzoate ion, regenerating benzoic acid.
Isolation:
- Acidify the aqueous sodium benzoate with HCl to pH ~2.
- Benzoic acid precipitates out (less soluble in water due to loss of ionic character).
- Extract with diethyl ether (benzoic acid is more soluble in ether than in water).
- Dry the organic layer over anhydrous sodium sulfate.
- Evaporate ether to obtain pure benzoic acid.
Benzoic acid is a weak organic acid (C₆H₅COOH) that reacts with aqueous sodium hydroxide (NaOH), a strong base, in a typical acid-base neutralization reaction. The reaction forms sodium benzoate (C₆H₅COO⁻ Na⁺), a salt, and water (H₂O). This transformation involves deprotonation of the carboxylic acid group by hydroxide ions.
In this reaction, benzoic acid (acid) donates a proton to hydroxide (base), forming benzoate ion (C₆H₅COO⁻, conjugate base) and water (conjugate acid). The direction of equilibrium is strongly driven toward the formation of products because the base (OH⁻) is strong, and the resulting conjugate acid (water) is very weak. This is evident from the pKa values: benzoic acid has a pKa of ~4.2, while water’s conjugate acid/base system has a pKa of 15.7. Since equilibrium favors the formation of the weaker acid (higher pKa), the reaction proceeds nearly to completion toward the products.
Sodium benzoate is water-soluble due to its ionic nature. The negatively charged carboxylate group interacts favorably with polar water molecules via ion-dipole interactions, enhancing its solubility.
To regenerate benzoic acid from sodium benzoate, acidification is required. Adding HCl introduces H⁺ ions that protonate the benzoate ion, forming benzoic acid. Since benzoic acid is poorly soluble in water, it precipitates out. Extraction with diethyl ether allows for separation due to benzoic acid’s higher solubility in ether than in water. The ether layer is dried with anhydrous sodium sulfate to remove moisture, then evaporated to yield pure benzoic acid.
This reversible acid-base chemistry is foundational in organic synthesis and purification strategies.
