Q.Identify the functional groups in the following molecule :
Aspirin (structure shown: a benzene ring with a -COOH group and an adjacent -O-CO-CH3 (acetoxy/ester) group)
Concept understanding — Analgesics
What Are Analgesics?
Imagine you stub your toe. Your body sends pain signals racing from the injury site to your brain, where they register as that sharp, unpleasant sensation. An analgesic is a chemical that intercepts or dampens those signals — it relieves pain without putting you to sleep. That last part is crucial: analgesics are not general anaesthetics. You stay awake and aware; the pain just fades.
The word itself comes from Greek: an- (without) + -algesia (pain). So an analgesic is literally a "pain-remover."
The Two Families: Narcotic vs. Non-Narcotic
Pain comes in different flavours, and so do the drugs that fight it. Analgesics split into two broad classes based on how they work and how strong they are.
1. Narcotic Analgesics (Opioids)
These are the heavy hitters. They bind to specific receptors in the brain and spinal cord called opioid receptors, mimicking the body's own natural painkillers (endorphins). By locking onto these receptors, they block the transmission of pain signals and also alter the emotional response to pain — you care less about it.
The classic example is morphine, extracted from the opium poppy. It is used for severe pain: post-surgery, cancer, major injuries.
Narcotic analgesics are addictive. They produce euphoria and tolerance, meaning you need larger doses over time for the same effect. This is why they are strictly controlled substances. Never use them without a prescription.
Other members of this family: codeine, pethidine, and the semi-synthetic heroin (which is illegal and far more dangerous).
2. Non-Narcotic Analgesics
These are the everyday painkillers. They work differently — mostly by inhibiting an enzyme called cyclooxygenase (COX) . COX is needed to produce prostaglandins, chemical messengers that cause inflammation, fever, and sensitise nerve endings to pain. By blocking COX, these drugs reduce pain at its source (the injured tissue) rather than in the brain.
The classic example is aspirin (acetylsalicylic acid). It relieves mild to moderate pain (headache, toothache, muscle ache), reduces fever, and fights inflammation.
Aspirin also has a blood-thinning effect (it prevents platelet aggregation), which is why low-dose aspirin is sometimes prescribed to prevent heart attacks. But this same property can cause stomach bleeding — a major side effect.
Other members: paracetamol (acetaminophen) — which is a good painkiller and fever reducer but has almost no anti-inflammatory action — and ibuprofen, which is a stronger anti-inflammatory than aspirin.
The Key Distinction at a Glance
| Feature | Narcotic (e.g., Morphine) | Non-Narcotic (e.g., Aspirin) |
|---|---|---|
| Site of action | Brain & spinal cord (opioid receptors) | Peripheral tissues (blocks COX enzyme) |
| Pain severity | Severe, deep pain | Mild to moderate pain |
| Addiction risk | High | Very low (except psychological dependence) |
| Other effects | Euphoria, constipation, respiratory depression | Anti-inflammatory, anti-pyretic (fever-reducing) |
| Prescription | Strictly controlled | Often over-the-counter |
The Intuition in One Sentence
Think of pain as a fire alarm. Narcotic analgesics go to the central control room and turn down the volume of the alarm itself. Non-narcotic analgesics go to the fire and put out the flames — less heat, less smoke, less reason for the alarm to ring in the first place.
Analgesics are drugs that relieve pain without causing loss of consciousness. They are classified as narcotic (e.g., morphine, acting on the central nervous system, used for severe pain, addictive) and non-narcotic (e.g., aspirin, acting peripherally by inhibiting prostaglandin synthesis, used for mild pain, non-addictive).
Analgesics are one of the anchor topics of the NCERT Class 12 Chemistry chapter on Chemistry in Everyday Life, and this classification into narcotic and non-narcotic drugs is exactly what students look for when they search "analgesics classification class 12 chemistry" or "narcotic vs non-narcotic analgesics important questions" for CBSE board exams.
Aspirin's structure contains an aromatic (benzene) ring bearing a free carboxylic acid (-COOH) group and an ester/acetoxy (-O-CO-CH3) group.
Carboxylic acid group (-COOH) and ester group (-O-CO-CH3, an acetoxy group), both attached to a benzene ring.
Step 1. Recall aspirin's structure. Section 16.2.1 shows aspirin as the acetyl derivative of salicylic acid: a benzene ring carrying a -COOH group, with the adjacent (ortho) position bearing an -O-CO-CH3 group in place of salicylic acid's free -OH.
Step 2. Name each functional group. The -COOH group is a carboxylic acid group. The -O-CO-CH3 group is an ester group (specifically an acetate/acetoxy ester, since it is the acetic-acid ester of the original phenolic -OH).
Aspirin contains a carboxylic acid group (-COOH) and an ester (acetoxy, -O-CO-CH3) group on a benzene ring.
Reading the two substituents off aspirin's known structure and naming their functional-group class
- Calling the -O-CO-CH3 group a simple 'alcohol' or leaving it as -OH -- acetylation converts salicylic acid's phenolic -OH into an ESTER, which is exactly the chemical change that turns salicylic acid into aspirin.
- CBSE 2026Set 56/1/11 markMCQQ.Aspirin is obtained by acetylation of (A) Phenol (B) Salicylaldehyde (C) 2-Hydroxybenzoic acid (D) Benzoic acid
›Reveal solutionSolution
Aspirin (acetylsalicylic acid) is synthesized by acetylating the phenolic –OH group of salicylic acid (2-hydroxybenzoic acid) with acetic anhydride. The answer is (C).
Aspirin is one of the most widely used drugs in the world, and understanding its synthesis reveals why the starting material must have both a carboxylic acid group and a phenolic hydroxyl group in the right positions.
The acetylation reaction replaces an active hydrogen (typically on an –OH or –NH₂ group) with an acetyl group, −COCHX3. When we acetylate a phenol, we convert Ar−OH into Ar−O−CO−CHX3, forming an ester. The key is identifying which compound has the structural features needed to become aspirin.
Aspirin's structure is 2-acetoxybenzoic acid: a benzene ring with a carboxylic acid group at position 1 and an acetoxy ester group (from acetylation) at position 2. Working backward, the precursor must have a carboxylic acid at position 1 and a free hydroxyl group at position 2.
Let me examine each option:
-
Phenol (CX6HX5OH) has only a hydroxyl group attached to benzene, with no carboxylic acid. Acetylating phenol gives phenyl acetate, not aspirin.
-
Salicylaldehyde has an aldehyde group (−CHO) at position 1 and a hydroxyl at position 2. While acetylation would modify the –OH, the aldehyde is not a carboxylic acid, so this cannot yield aspirin.
-
2-Hydroxybenzoic acid (salicylic acid) has a carboxylic acid (−COOH) at position 1 and a phenolic hydroxyl (−OH) at position 2. When treated with acetic anhydride or acetyl chloride, the –OH undergoes acetylation:
CX6HX4(OH)(COOH)+(CHX3CO)X2OCX6HX4(OCOCHX3)(COOH)+CHX3COOH
This produces acetylsalicylic acid—aspirin.
- Benzoic acid (CX6HX5COOH) has only a carboxylic acid group with no hydroxyl to acetylate. It cannot form aspirin.
Salicylic acid+Acetic anhydrideHX+Aspirin+Acetic acid
The reaction is typically catalyzed by a small amount of acid (often sulfuric or phosphoric acid) and is a classic example of esterification of a phenolic group. The carboxylic acid group remains intact because carboxylic acids are much less nucleophilic than phenols under these conditions.
TipRemember: salicylic acid = 2-hydroxybenzoic acid. The "salicyl-" prefix always indicates the ortho relationship between –OH and –COOH on a benzene ring.
✓Final answerThe correct option is (C) 2-Hydroxybenzoic acid (salicylic acid).
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- CBSE 2025Set ANNUAL1 markQ.Write the name of compound obtained from the acetylation of salicylic acid.
›Reveal solutionSolution
Acetylation (esterification) of the phenolic -OH group of salicylic acid gives aspirin, a widely used analgesic-antipyretic-anti-inflammatory drug.
Salicylic acid has both a phenolic -OH and a -COOH group. Reacting it with acetic anhydride (or acetyl chloride) acetylates the phenolic -OH specifically, converting it into an acetate ester:
Salicylic acid + (CH3CO)2O -> Acetylsalicylic acid (Aspirin) + CH3COOH
Aspirin is one of the most widely used analgesic (pain-relieving), antipyretic (fever-reducing), and anti-inflammatory drugs, and is also used in low doses as an antiplatelet/blood-thinning agent.
✓Final answerAspirin (acetylsalicylic acid).
- CBSE 2024Set ANNUAL1 markMCQQ.Aspirin is :(a) chlorobenzoic acid(b) acetyl salicylic acid(c) anthranilic acid(d) benzoyl salicylic acid
›Reveal solutionSolution
Aspirin is the trade name for acetylsalicylic acid, made by acetylating the phenolic -OH of salicylic acid with acetic anhydride; it is used as a mild analgesic, antipyretic, and anti-inflammatory drug.
Aspirin is prepared by treating salicylic acid (2-hydroxybenzoic acid) with acetic anhydride, which acetylates the phenolic hydroxyl group (converting -OH to -OCOCH3) while leaving the carboxylic acid group untouched: C6H4(OH)(COOH)+(CH3CO)2O→C6H4(OCOCH3)(COOH)+CH3COOH The product, 2-(acetyloxy)benzoic acid, is commonly called acetylsalicylic acid or aspirin. It is used as an analgesic (pain reliever), antipyretic (fever reducer), and anti-inflammatory agent, and in low doses as a blood-thinning (antiplatelet) medication. It is not chlorobenzoic acid, anthranilic acid (2-aminobenzoic acid), or benzoyl salicylic acid.
✓Final answerThe correct answer is (b) acetyl salicylic acid — aspirin is the acetate ester of salicylic acid's phenolic -OH group.
- CBSE 2017Set ANNUAL1 markQ.How does aspirin act as an analgesic ?
›Reveal solutionSolution
Aspirin blocks the enzyme that makes prostaglandins, the molecules that signal pain and inflammation.
Aspirin (acetylsalicylic acid) acts as an analgesic by inhibiting the cyclooxygenase (COX) enzyme involved in the biosynthesis of prostaglandins — hormone-like substances that mediate pain, inflammation and fever at the site of tissue injury. By blocking prostaglandin synthesis, aspirin reduces the pain and inflammatory response, in addition to its antipyretic (fever-reducing) and anti-blood-clotting (antiplatelet) actions.
✓Final answerBy inhibiting the COX enzyme and hence prostaglandin synthesis, reducing the pain/inflammation signal.
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