Drug Classification & Functional Groups
In 1897, Felix Hoffmann at Bayer AG synthesised the first stable, pure acetylsalicylic acid (aspirin) because salicylic acid — the existing pain treatment — caused severe gastric irritation in his father. The functional group change from –OH to –OCOCH₃ reduced gastric side-effects while preserving the analgesic effect, making aspirin the world's first blockbuster pharmaceutical with over 40,000 tonnes produced annually today.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
Two common pain-relief medicines are sold in different tablet masses. A student says, "That probably just means one tablet has more filler than the other. The actual drug molecules cannot matter much."
- Why might the structure of a drug molecule affect how much of it is needed?
- What features in a structural formula would you inspect first if you wanted to compare two medicines chemically?
Know
- The main structural and functional medicine categories named in the course
- The key functional groups found in common drug molecules
- The meaning of the term pharmacophore
Understand
- How functional groups influence polarity, solubility, reactivity and binding
- Why small structural changes can alter pharmacological effect dramatically
- How skeletal formulas and structural formulas communicate chemical information efficiently
Can Do
- Classify medicines into functional and structural categories
- Identify functional groups in aspirin, paracetamol, ibuprofen and penicillin
- Link structural features to likely chemical and pharmacological behaviour
Core Content
Function and structure both matter
A medicine can be classified in more than one way. Chemists care about what it does biologically, but also about the structural features that help it do that job.
Functional categories of medicines: analgesic (paracetamol, ibuprofen, aspirin), antibiotic (penicillin), antiviral, antifungal, local anaesthetic (lignocaine), antacid. The same medicine can be classified both by what it does AND by its chemical structure and functional groups.
Pause — copy the highlighted medicine categories into your book.
These are functional categories because they group medicines by biological purpose. The same molecule can also be discussed structurally in terms of its functional groups and overall molecular framework.
Small substructures with big consequences
We just saw how medicines are classified functionally. That raises the question: what specific structural features within a molecule actually control those functional differences? This card answers it → the key functional groups and what each one does chemically.
Functional groups are chemically important because they influence intermolecular forces, acid-base behaviour, reactivity and how a molecule interacts with a biological target.
Key functional groups: –OH raises polarity and H-bonding; –COOH gives acidic behaviour and polarity; –NH₂ gives basic behaviour and H-bonding; –COO– (ester) is polar and can hydrolyse; –CONH– (amide) is strongly polar; aromatic ring contributes hydrophobic character and shape.
Pause — copy the highlighted functional groups and effects into your book before the check below.
Seeing the same molecule in two different languages
We just saw the key functional groups and their chemical effects. That raises the question: how do we actually spot these groups when looking at a drawn structure? This card answers it → how to read structural and skeletal formulas for the four key medicines.
Structural formulas show more explicitly where atoms are connected. Skeletal formulas compress that information and are widely used in organic and medicinal chemistry.
For HSC Chemistry, you should be able to interpret both styles for common medicines such as aspirin, paracetamol, ibuprofen and penicillin. The goal is not to memorise every atom by rote, but to recognise the important functional groups and overall framework.
Aspirin: aromatic ring + ester + carboxylic acid (acidic, hydrolysable). Paracetamol: aromatic ring + phenol + amide (polar, H-bonding). Ibuprofen: aromatic ring + carboxylic acid + branched hydrocarbon (mixed polar/non-polar). Penicillin: amide-containing framework tied directly to antibacterial action.
Pause — copy the highlighted medicine structure summary into your book.
The point of comparing medicine structures is not to memorise every bond. It is to notice the functional groups that control acidity, polarity, hydrogen bonding, and biological behaviour.
From polarity and solubility to target binding
We just saw which functional groups appear in each medicine. That raises the question: why do those groups actually matter pharmacologically? This card answers it → how functional groups influence solubility, reactivity and binding affinity.
Functional groups help determine more than just a molecule's name. They influence how it dissolves, whether it can hydrogen bond, how acidic or basic it is, and how it interacts with a biological target.
A molecule with polar groups such as –OH, –COOH or –CONH– can often form stronger interactions with water and with sites on proteins. Aromatic and hydrocarbon regions can add hydrophobic interactions and shape complementarity.
Polar groups (–OH, –COOH, –CONH–) increase water interactions and binding potential at protein sites. Aromatic and hydrocarbon regions add hydrophobic character and shape. Solubility, reactivity, binding affinity and biological effect can all change with a single functional group modification. Same therapeutic use does NOT imply identical structure or mechanism.
Pause — copy the highlighted structure-activity principle into your book before the check below.
The part of the molecule most responsible for activity
We just saw that functional groups drive pharmacological properties. That raises the question: is the whole molecule equally important, or is it just one part? This card answers it → the pharmacophore concept and why minor structural changes can have major effects.
When Bayer's chemists modified salicylic acid to make aspirin in 1897, they kept the –COOH group (the pharmacophore region that inhibits COX enzymes) while converting the irritating –OH group to –OCOCH₃. The analgesic activity was preserved; the stomach irritation was reduced. That is the pharmacophore concept in practice: one part of the molecule drives the biological effect, and you can modify other parts without destroying it — as long as you don't touch the critical region.
In practice, medicinal chemists ask which features must stay in place for activity to remain. Those might include a polar group, a hydrophobic ring, a specific spacing between atoms, or a combination of these features.
Pharmacophore = the key arrangement of structural features responsible for biological activity — the part of the molecule that must be preserved for the drug to work. Minor structural changes can alter the pharmacophore geometry or interaction pattern and therefore change potency or effect type.
Add the pharmacophore definition to your notes before the check below.
This helps explain why small changes can matter so much. A modification that looks minor on paper may alter the geometry or interaction pattern of the pharmacophore and therefore change potency or even the type of biological effect.
Classify the key groups before you talk about function
This kind of interpretation is the bridge between plain structure reading and medicinal reasoning. First identify the groups, then explain what those groups are likely to do chemically and biologically.
The carboxylic acid group (-COOH) is responsible. At physiological pH (~7.4 in blood, ~8 in small intestine), the -COOH group donates its proton to form the carboxylate anion (-COO-). This ionised form is more water-soluble and readily absorbed across the intestinal membrane into the bloodstream.
Complete the Learn phase to unlock Practice.
Activities
For each molecule or medicine type, classify it and state the structural evidence you would use.
1. A molecule contains a carboxylic acid group and is used to relieve pain and inflammation.
2. A medicine is classified as an antibiotic and contains an amide-containing framework important for activity.
3. A basic substance is taken to neutralise stomach acid rather than bind to a protein target in the usual drug-receptor sense.
Name the key groups and connect them to likely chemical properties.
1. Paracetamol contains a phenolic –OH and an amide. What property changes would you expect from those groups?
2. Ibuprofen contains a carboxylic acid and a large hydrocarbon region. How does that create mixed character in the molecule?
3. Why can a small change to a functional group alter pharmacological effect more than you might expect?
Check Your Understanding
Understand Band 3
1. Which functional group is represented by –COOH?
Understand Band 4
2. Which medicine category is defined mainly by biological function rather than a single functional group?
Apply Band 4
3. Which pair of functional groups is identified in paracetamol in this course?
Analyse Band 5
4. Why can two medicines used for pain relief be sold in different masses per tablet?
Analyse Band 5
5. Which statement best describes a pharmacophore?
Apply Band 4
1. Identify three functional groups that may be present in medicine molecules and explain one chemical property associated with each. (4 marks)
Analyse Band 5
2. Explain how functional groups influence chemical and pharmacological properties such as polarity, solubility, reactivity and binding affinity. (5 marks)
Evaluate Band 5–6
3. Evaluate the statement: "The difference between a 200 mg ibuprofen tablet and a 500 mg paracetamol tablet is mostly just a packaging choice." In your answer, refer to functional groups, pharmacophores and pharmacological potency. (5 marks)
Show All Answers
Activity 1
1. This would likely fit the analgesic category, and the carboxylic acid is a key structural clue that can affect polarity and acid-base behaviour.
2. Penicillin is a likely example. Its amide-containing framework is part of the structure tied closely to antibacterial activity.
3. This describes an antacid. It is chemically different from many organic drugs because its main role is acid neutralisation rather than classic receptor binding through an organic pharmacophore.
Activity 2
1. The phenolic –OH and amide increase polarity and hydrogen bonding capacity, which can influence solubility and binding interactions.
2. The carboxylic acid contributes a polar, acidic region, while the large hydrocarbon region contributes non-polar character, so the molecule has mixed behaviour.
3. A small functional-group change can alter polarity, geometry, reactivity or the key interactions needed for binding, so biological effect can change much more than expected.
Multiple Choice
1. B — –COOH is the carboxylic acid group.
2. D — analgesic is a biological or functional category, not a single structural group.
3. A — paracetamol is treated here as containing a phenol and an amide.
4. C — structural differences can change binding and potency, so equal masses are not expected.
5. B — a pharmacophore is the key feature set responsible for biological activity.
Short Answer Model Answers
Q1 (4 marks): One possible answer is alcohol or phenol, carboxylic acid and amide. An –OH group can increase polarity and hydrogen bonding. A carboxylic acid group can give acidic behaviour and increase polarity. An amide group is strongly polar and can contribute to important intermolecular or binding interactions.
Q2 (5 marks): Functional groups influence a drug molecule's polarity, ability to form hydrogen bonds, acid-base behaviour and chemical reactivity. These factors affect solubility in water and other environments, which influences how the molecule is transported and absorbed. Functional groups also help determine how well the molecule binds to a biological target, because polarity, shape and interaction sites affect binding affinity. As a result, changes in functional groups can alter both chemical properties and pharmacological action.
Q3 (5 marks): The statement is too simplistic. Packaging choice may matter commercially, but the main chemical reason two medicines can be sold in different masses is that their molecular structures are different. Ibuprofen and paracetamol contain different functional groups and therefore have different polarity, acid-base behaviour, binding features and pharmacophores. Those structural differences affect pharmacological potency and the amount of drug needed to achieve the intended effect. Overall, tablet mass should be understood as a consequence of medicinal chemistry and dosing requirements, not just presentation or packaging.
Return to the story of Felix Hoffmann's 1897 aspirin synthesis at Bayer. Now that you understand functional groups and pharmacophores, explain the chemistry behind his decision.
- Why did converting the –OH group to –OCOCH₃ reduce gastric irritation — and what does that change tell you about how functional groups affect biological interaction with stomach lining?
- Why is the –COOH group in aspirin considered the pharmacophore — and why could Hoffmann modify the –OH without losing pain-relief activity?
- Write one sentence explaining why tablet mass alone cannot tell you whether two drugs have equivalent pain-relief potency.
Review
List four functional categories of medicines and one example drug for each.
Name three functional groups in common pain-relief drugs and state one chemical effect of each.
What functional groups are in aspirin? What do they tell you about its chemistry?
Define pharmacophore. Why can a small structural change have a large effect on drug potency?
Why does ibuprofen have mixed polar and non-polar character?