Designing a Green Synthesis Route
A class must choose between two routes to make a substituted benzoate ester for a fragrance project. Route 1 uses benzoyl chloride and an alcohol in pyridine, giving 92 % yield in 20 minutes but producing a stoichiometric amount of pyridinium chloride waste and requiring fume-hood handling of a lachrymator. Route 2 uses the carboxylic acid and alcohol with a catalytic acid, giving 65 % yield after 3 hours of reflux, with water as the only by-product. The school has one fume hood, 40 minutes of practical time per lesson, and a waste-disposal budget that charges per kilogram of chlorinated or amine waste. The ester must be pure enough for a blind smell test.
1. Compare the two routes on atom economy, safety and practical feasibility for this school, and recommend one.
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Route 1 is fast (20 min, fits a lesson) and high-yielding (92 %), but it consumes benzoyl chloride stoichiometrically and generates pyridinium chloride, an amine-containing waste charged by mass; benzoyl chloride is a lachrymator requiring fume-hood work, and only one hood is available for the class. Its atom economy is poor because the Cl and much of the pyridine end up as waste. Route 2 has water as the only by-product, uses a catalytic acid, and avoids lachrymators, so it is safer and cheaper in waste terms. Its drawbacks are lower yield (65 %) and a 3-hour reflux, which exceeds the 40-minute lesson and requires carrying the apparatus over multiple periods. Recommendation: Route 2 on safety and waste grounds, adapting the timetable by setting up reflux in one lesson and isolating in the next, provided the apparatus can be left safely. If time is the binding constraint, Route 1 could be run for a small number of students in the single hood with strict controls, but Route 2 is the better overall choice.
2. The 65 % yield of Route 2 is described as 'wasteful'. Explain, using equilibrium ideas, one change that would raise the yield without changing the route.
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Esterification is an equilibrium: acid + alcohol ⇌ ester + water. At equilibrium some reactants remain, which is why the yield is below 100 %. Removing water as it forms (e.g. with a Dean–Stark trap or a drying agent) lowers the product concentration, so Q < Kc and the position shifts toward the ester, raising the yield. Alternatively, using an excess of the cheaper alcohol pushes the equilibrium right. Neither change alters the route or adds hazardous reagents; both simply exploit Le Chatelier's principle. The 'wasteful' label is therefore partly a process-design issue rather than an inherent flaw of the route.