The Reaction That Would Not Go to Completion
A final-year student attempts to synthesise ethyl acetate from ethanol and acetic acid using a sulfuric acid catalyst, refluxing for one hour, then distilling. The yield is disappointingly low, around 30%, and the distillate contains unreacted ethanol and acid. The student's supervisor suggests using an excess of one reactant and removing water as it forms. The student also plans to synthesise a substituted benzoic acid by nitration of benzoic acid, expecting the carboxyl group to direct the incoming nitro group to the meta position, and is surprised when the major product appears to be 3-nitrobenzoic acid but with a substantial amount of a side product. The laboratory has a rotary evaporator, a Dean-Stark apparatus, and standard glassware, but no chromatography beyond thin-layer chromatography.
1. Use Le Chatelier's principle and esterification mechanism to explain the low yield and design an improved procedure.
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Esterification of ethanol with acetic acid is an equilibrium reaction: ethanol + acetic acid ⇌ ethyl acetate + water, with an equilibrium constant near 4 at room temperature. With equimolar amounts, the maximum conversion is limited to roughly 67% at equilibrium, and the student's 30% suggests incomplete reaction and loss during work-up. Le Chatelier's principle says the system can be driven to the right by using an excess of one reactant (usually the cheaper ethanol) or by removing a product. Removing water as it forms is the most effective lever. A Dean-Stark apparatus with a solvent that forms an azeotrope with water (e.g., toluene) allows continuous removal of water, driving the equilibrium toward the ester. The mechanism involves protonation of the carbonyl, nucleophilic attack by ethanol, and loss of water; acid catalysis speeds both directions, so it does not change the equilibrium position, only the rate. An improved procedure: use excess ethanol (e.g., 2:1), add toluene, reflux with Dean-Stark until no more water collects, then distill the ester.
2. Evaluate the student's expectation about the nitration of benzoic acid and explain the likely side product.
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The carboxyl group is meta-directing because it is electron-withdrawing by both induction and resonance, so nitration of benzoic acid gives predominantly 3-nitrobenzoic acid, which matches the student's expectation. However, the carboxyl group is also deactivating, so the reaction is slow and requires stronger conditions (fuming nitric acid and concentrated sulfuric acid, often with heat). The side product is likely the result of over-nitration or of competing reactions: dinitration can occur under forcing conditions, giving 3,5-dinitrobenzoic acid, and if the conditions are too harsh, decarboxylation or oxidation side reactions are possible. The student should monitor by TLC, control temperature, and use the stoichiometric amount of nitrating mixture to favour mono-nitration. The substantial side product is a warning that the reaction conditions were too forcing, not that the directing effect was wrong.
3. The student proposes to separate the ester by simple distillation without removing water first. Predict the outcome and justify it.
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Simple distillation of the crude mixture without removing water first will not give a clean separation. Ethyl acetate forms a minimum-boiling azeotrope with water (about 8% water, boiling near 70 degrees Celsius), and ethanol also forms azeotropes with both water and ethyl acetate, so the distillate will be a mixture rather than pure ester. In addition, the equilibrium mixture still contains acetic acid and sulfuric acid; heating can cause charring and side reactions. The correct sequence is to drive the equilibrium with excess ethanol and water removal, then wash the organic layer with sodium carbonate solution to neutralise acid, dry it, and fractionally distil. The student should use the Dean-Stark apparatus first, then a separatory funnel wash, then distillation, and confirm purity by boiling point and TLC.