4.4 Stoichiometry in Medicine Production

In the manufacturing of medicines, the amount of the active ingredient is essential to produce the desired therapeutic effects. Stoichiometry provides the quantitative relationships between reactants and products in a chemical reaction, ensuring the accuracy of drug synthesis.

Any deviation from precise stoichiometric ratios can result in an incomplete reaction or contamination with unreacted reactants or unwanted by-products. Stoichiometry allows chemists to precisely control chemical reactions to produce drugs, ensuring their efficiency, effectiveness, and safe use.

Examples of Stoichiometry in Medicine:

• Antibiotics: In the preparation of antibiotics, stoichiometry ensures that each dose contains the correct amount of the active ingredient to effectively target bacteria without causing unnecessary side effects.
• Insulin: The use of insulin for managing diabetes relies on stoichiometry for the precise control of blood sugar levels. An incorrect dose can be ineffective or dangerous.
• Vaccines: Stoichiometry determines the exact concentration of viral antigens needed in the preparation of a vaccine to trigger an effective immune response.

Activity: Synthesis of Aspirin

This activity demonstrates how stoichiometry is applied in the practical synthesis of a common drug, aspirin (acetylsalicylic acid).

Materials Required:

• Salicylic acid (${\mathrm{C}}_7{\mathrm{H}}_6{\mathrm{O}}_3$)
• Acetic anhydride ($(\mathrm{C}{\mathrm{H}}_3\mathrm{CO}{)}_2\mathrm{O}$)
• Sulphuric acid (${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$)
• Sodium hydrogen carbonate ($\mathrm{NaHC}{\mathrm{O}}_3$)
• Ice bath
• Water
• Filtration setup

Chemical Reaction

The synthesis of aspirin from salicylic acid and acetic anhydride is a classic example of esterification.

${\mathrm{C}}_7{\mathrm{H}}_6{\mathrm{O}}_3+(\mathrm{C}{\mathrm{H}}_3\mathrm{CO}{)}_2\mathrm{O}\stackrel{{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4}{\to }{\mathrm{C}}_9{\mathrm{H}}_8{\mathrm{O}}_4+\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}$

Procedure

1. Mixing Reactants: Mix the reactants in stoichiometrically calculated amounts (a 1:1 molar ratio of salicylic acid to acetic anhydride) in a flask. Add a few drops of sulphuric acid, which acts as a catalyst.
2. Cooling: Place the flask in an ice bath to control the reaction temperature.
3. Reaction: Stir the mixture with a glass rod. Solid, insoluble aspirin will begin to precipitate.
4. Neutralization: When the reaction is complete, add a solution of sodium hydrogen carbonate ($\mathrm{NaHC}{\mathrm{O}}_3$) to neutralize the excess acetic anhydride and the sulphuric acid catalyst.
5. Isolation: Filter the solid aspirin from the solution. Wash the crystals with cold water and allow them to dry.
6. Yield Calculation:
   • Weigh the amount of dry aspirin formed. This is the actual yield.
   • Calculate the theoretical yield using the initial mass of the limiting reactant→ and the stoichiometric ratio from the balanced chemical equation.
7. Efficiency Assessment: Compare the actual yield obtained experimentally with the theoretical yield to assess the efficiency of the synthesis. The percent yield is calculated as:

$\text{Percent Yield}=\left(\frac{\text{Actual Yield}}{\text{Theoretical Yield}}\right)\times 100\%$

This activity helps in understanding how stoichiometry is fundamental to controlling the production and yield of medicines.

Possible Questions/Answers

Q: Why is stoichiometry crucial for drug manufacturing?

A: It ensures that reactants are used in the correct proportions to maximize product yield, minimize waste and by-products, and guarantee the final product's purity and correct dosage, which is critical for safety and effectiveness.

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Q: In the synthesis of aspirin, if you start with 10.0 g of salicylic acid (${\mathrm{C}}_7{\mathrm{H}}_6{\mathrm{O}}_3$, Molar Mass = 138.12 g/mol), what is the theoretical yield of aspirin (${\mathrm{C}}_9{\mathrm{H}}_8{\mathrm{O}}_4$, Molar Mass = 180.16 g/mol)?

A:

1. Given:

   • Mass of Salicylic Acid = 10.0 g
   • Molar Mass of Salicylic Acid = 138.12 g/mol
   • Molar Mass of Aspirin = 180.16 g/mol

2. Calculate moles of salicylic acid: $\text{moles}=\frac{\text{mass}}{\text{molar mass}}=\frac{10.0\text{g}}{138.12\text{g/mol}}=0.0724\text{mol}$

3. Apply stoichiometric ratio: From the balanced equation, the ratio of salicylic acid to aspirin is 1:1. Therefore, moles of aspirin produced will also be 0.0724 mol.

4. Calculate theoretical yield (mass) of aspirin: $\text{mass}=\text{moles}\times \text{molar mass}=0.0724\text{mol}\times 180.16\text{g/mol}\approx 13.04\text{g}$

   The theoretical yield of aspirin is 13.04 g.