Exercises: Drugs from natural sources

Many medicines originated as natural products.

1. a) Match each plant to the compound or drug derived from it: willow bark, cinchona bark, opium poppy. Options: morphine, quinine, salicin (precursor of aspirin).
2. b) State the medical use of each of the three.
3. c) Explain what is meant by a natural product.

Aspirin is acetylsalicylic acid.

1. a) Name the functional groups in aspirin that come from the carboxylic acid and ester families.
2. b) Name the two reactants used in the laboratory synthesis of aspirin from salicylic acid.
3. c) State two medical uses of aspirin.

Penicillin and taxol are both drugs of natural origin.

1. a) State the source of penicillin and its therapeutic use.
2. b) State the source of taxol and its therapeutic use.
3. c) Explain what is meant by an antibiotic.

Aspirin ($M_r = 180$) is made from salicylic acid ($M_r = 138$) and ethanoic anhydride ($M_r = 102$) in a 1:1 molar ratio. In a school laboratory $5.00\,\text{g}$ of salicylic acid is reacted with excess ethanoic anhydride and $4.80\,\text{g}$ of aspirin is obtained.

1. a) Calculate the amount of salicylic acid used, in mol.
2. b) Calculate the theoretical yield of aspirin, in g.
3. c) Calculate the percentage yield.
4. d) Suggest two reasons why the yield is less than $100\%$.

Salicylic acid from willow bark irritates the stomach, so it was chemically modified to aspirin.

1. a) Explain why the modification reduced the irritation.
2. b) Write the equation for the hydrolysis of aspirin in the body to salicylic acid and ethanoic acid, using structural formulas for the organic species.
3. c) A standard aspirin tablet contains $300\,\text{mg}$ of acetylsalicylic acid. Calculate the amount in mol in one tablet.
4. d) A low-dose tablet contains $75\,\text{mg}$. State how many low-dose tablets contain the same mass as one standard tablet.

Taxol (paclitaxel) was first extracted from the bark of the Pacific yew tree, which yields very little of the drug per tree.

1. a) Suggest why harvesting from the bark is not a sustainable source.
2. b) Explain what is meant by semi-synthesis and how it solved the supply problem.
3. c) Suppose $1.0\,\text{g}$ of taxol requires bark from three trees. Calculate the number of trees needed to treat 1,000 patients if each needs $2.5\,\text{g}$.
4. d) State one ethical issue in drug discovery from natural sources.

Evaluate the role of natural products in drug development, using willow bark to aspirin, cinchona to quinine and the Penicillium mould to penicillin.

1. a) For each example, identify the active compound and the way the natural source was used.
2. b) Explain why a pure, synthesised compound is preferred to a crude plant extract.
3. c) Explain why chemists modify the structure of a natural product to make a drug.
4. d) Discuss the importance of biodiversity to the discovery of future drugs.

A student prepares aspirin and tests its purity by measuring the melting point. Pure aspirin melts at about $136\,^{\circ}\text{C}$. The student's crude product melts over the range $118$–$126\,^{\circ}\text{C}$, and after recrystallisation it melts over $133$–$135\,^{\circ}\text{C}$.

1. a) Explain why the crude product has a lower and broader melting range.
2. b) Describe how recrystallisation purifies the product.
3. c) Describe a chemical test using iron(III) chloride that would detect unreacted salicylic acid, and state what is observed.
4. d) Explain why the melting range after recrystallisation still suggests that the sample may not be perfectly pure.

Penicillin acts by inhibiting the synthesis of bacterial cell walls. Resistance to antibiotics is a growing problem.

1. a) Explain why penicillin is harmless to human cells but harmful to bacteria.
2. b) Explain how misuse of antibiotics, such as stopping a course early, promotes resistance.
3. c) The table gives the percentage of bacterial isolates resistant to an antibiotic in a hospital over four years: 2019: 8%; 2020: 12%; 2021: 19%; 2022: 27%. Describe the trend and calculate the average yearly percentage-point increase.
4. d) Discuss two strategies to address the problem of antibiotic resistance.