smc-3684-20-reaction conditions

The atom economy ( AE ) of a reaction is a measure of the mass of atoms in the starting materials that are incorporated into the desired product. Higher AE means lower mass of waste products.

Urea can be produced in a variety of ways. One way is to react ammonia (high toxicity) with phosgene (high toxicity). Another way is to react ammonia with dimethyl carbonate (DMC, low toxicity). The chemical equations and AE for these two processes are provided.

Which of these two processes is preferable for urea production? Justify your answer with reference to the information provided. (3 marks)

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→ The atom economy for the reaction utilizing dimethyl carbonate (DMC) is 48.4%, which is significantly higher compared to the 35.9% achieved in the reaction using phosgene.

→ Consequently, the process involving DMC generates a lower mass of waste products and relies on a less hazardous starting material.

→ Additionally, the DMC-based synthesis requires only 2 moles of ammonia, compared to the 4 moles needed for the phosgene process.

→ These factors highlight the synthesis of urea via DMC as the preferable method for urea production as it offers advantages in terms of atom economy, reduced toxicity of reactants, and minimized use of harmful chemicals.

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→ The atom economy for the reaction utilising dimethyl carbonate (DMC) is 48.4%, which is significantly higher to the 35.9% achieved in the phosgene reaction.

→ Consequently, the process involving DMC generates a lower mass of waste products and relies on a less hazardous starting material.

→ Additionally, the DMC-based synthesis requires only 2 moles of ammonia, compared to the 4 moles needed for the phosgene process.

Analyse how a student could design a chemical synthesis process to be undertaken in the school laboratory. In your response, use a specific process relating to the synthesis of an organic compound, including a chemical equation, and refer to:

selection of reagent(s)
reaction conditions
any potential hazards and any safety precautions to minimise the risk
yield and purity of the product(s). (8 marks)

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Selecting reagents

→ The student could synthesise ethyl ethanoate through esterification between acetic acid and ethanol.

→ Both readily available in the school laboratory and are relatively safe.

Acetic acid + ethanol ⇌ Ethyl ethanoate + water

→ Concentrated sulfuric acid should be used as the acid catalyst as it is a strong acid that is also readily available.

Reaction conditions

→ Increasing the temperature of the system increases the reaction rate because it increases the average kinetic energy of the reactant molecules, and thus increases the likelihood of successful collisions, producing more product.

→ Additionally, the addition of reactants would increase the likelihood of successful collisions, thus increasing the reaction rate.

→ The reaction should also be undertaken under reflux allowing vaporised molecules to condense and return back to the reaction vessel, increasing the amount of reactants, and thus increasing the rate of reaction.

→ Concentrated sulfuric acid should also be utilised as a catalyst in order to speed up the reaction and lower the activation energy.

Potential hazards and safety precautions

→ The acetic acid and sulfuric acid used is corrosive and may cause skin and eye burns, therefore, appropriate lab coat and safety glasses should be utilised.

→ The organic reactants are highly flammable and may cause fires. The reaction mixture should be heated on a hot plate or heating mantle instead of a bunsen burner.

→ Refluxing may cause pressure build-up, therefore, ensure the reflux condenser is open.

→ Superheating and bumping may occur in apparatus. Boiling chips should be utilised to provide nucleation sites allowing liquids to boil smoothly.

Yield and purity

→ Concentrated sulfuric acid, used as a catalyst, also acts as a dehydrating agent that removes water from the system and improves yield.

→ When the reaction reaches equilibrium, the ester can be separated from the mixture by adding excess sodium carbonate solution in order to neutralise the acid.

→ Transfer to a separation funnel to separate the organic layer (containing the ester) from the aqueous layer.

→ Then use fractional distillation to separate the ester from the organic layer.