Chemicals in Medicines: Pharmacological Classification
Medicines are chemicals of low molecular masses that interact with macromolecular targets to produce a biological response. When this response is therapeutic and useful, these chemicals are called medicines and are used for diagnosis, prevention, or treatment of diseases. If taken in doses higher than recommended, most medicines become harmful poisons. Understanding how these drugs interact with enzymes and receptors is a core concept tested in competitive examinations.
Drugs are classified based on their pharmacological effect, drug action, chemical structure, or molecular target. For competitive exams, memorizing the specific categories and prototype examples is essential. For instance, analgesics reduce pain without causing impairment of consciousness, mental confusion, or paralysis, whereas antipyretics lower body temperature in cases of fever. Common dual-function drugs include aspirin and paracetamol, both of which act as effective antipyretics and analgesics.
Analgesics are broadly sub-divided into two main classes: narcotic analgesics and non-narcotic analgesics. Narcotic analgesics are chiefly opium derivatives like morphine, codeine, and heroin. While extremely effective for relieving severe post-operative pain or cardiac pain, they are addictive and can cause central nervous system depression. Non-narcotic analgesics, such as aspirin (acetylsalicylic acid), inhibit prostaglandin synthesis that stimulates pain receptors and also exhibit anti-blood clotting action, making them useful for heart attack prevention.
Antimicrobials, Antibiotics, and Antiseptics
Infectious diseases caused by microorganisms such as bacteria, fungi, viruses, or parasites are managed using antimicrobials. These agents can be bactericidal (killing microbes) or bacteriostatic (inhibiting their growth). Antibiotics are chemical substances produced wholly or partly by microorganisms that inhibit the growth of or even destroy other microorganisms at low concentrations. Paul Ehrlich pioneered this field, leading to the discovery of salvarsan (arsenobenzamine) for syphilis treatment, structurally similar to azodyes.
Antibiotics are classified as broad-spectrum or narrow-spectrum based on the range of bacteria they target. Narrow-spectrum antibiotics, such as penicillin G, are effective mainly against Gram-positive or Gram-negative bacteria. Broad-spectrum antibiotics, including chloramphenicol, ampicillin, and amoxicillin, target a wide range of Gram-positive and Gram-negative bacteria and are prescribed when the specific causative pathogen is unidentified. Chloramphenicol is rapidly absorbed in the digestive tract and is administered orally for typhoid, dysentery, and acute fever.
“Antiseptics and disinfectants are both antimicrobial agents, but their application sites differ fundamentally: antiseptics are applied safely to living tissues like wounds and cuts, whereas disinfectants are applied exclusively to inanimate objects like floors, drainage systems, and instruments due to their toxicity.”
Common antiseptic formulations include dethol, which is a mixture of chloroxylenol and terpineol, and tincture of iodine, a 2-3% solution of iodine in alcohol-water mixtures. Bithionol is often added to soaps to act as an antiseptic to reduce the odor caused by bacterial decomposition of organic matter on the skin. Boric acid in dilute aqueous solution is a weak antiseptic used widely for eyes.
Tranquilizers and Antifertility Drugs
Tranquilizers are chemical compounds used for the treatment of stress, anxiety, and severe mental illnesses. They form an essential component of sleeping pills and act on the central nervous system by affecting message transfer from nerve to receptor. There are various types of tranquilizers:
- Noradrenaline plays a role in mood changes; if a person has low levels due to depression, signal transmission fails, and they become depressed. Antidepressant drugs like iproniazid or phenelzine inhibit the enzyme that degrades noradrenaline.
- Valium (diazepam) and serotonin analogues function as mild tranquilizers suitable for relieving severe tension.
- Barbituric acid derivatives (such as veronal, amytal, luminal, and seconal) are powerful neurological depressants, acting as hypnotics or sleep-inducing agents.
Antifertility drugs or oral contraceptives control population growth and birth rates. These are synthetic hormonal compositions containing a mixture of synthetic estrogen and progesterone derivatives. Progesterone suppresses ovulation, while estrogen derivatives control the menstrual cycle. Common examples of birth control pills include norethindrone (a progesterone derivative) and ethynylestradiol (novestrol, an estrogen derivative).
Chemicals in Food: Preservatives and Artificial Sweeteners
Chemicals are added to food for various purposes: to preserve them, enhance their appeal, or add nutritive value. Food additives include food colors, flavors, sweeteners, fat emulsifiers, antioxidants, and preservatives. Food preservatives prevent spoilage due to microbial growth, with classic examples including table salt, sugar, vegetable oils, and chemical compounds like sodium benzoate ($\text{C}_6\text{H}_5\text{COONa}$), propanoic acid salts, and sulfur dioxide.
Natural sweeteners like sucrose raise energy intake, leading to the development of artificial sweeteners for diabetic patients and calorie-conscious individuals. These sweeteners are many times sweeter than sucrose and pass through the body unchanged:
- Aspartame: Roughly 100 times sweeter than cane sugar; stable only at cold temperatures, restricting its use to cold foods and soft drinks.
- Saccharin: Ortho-sulfobenzimide, roughly 550 times sweeter than sucrose. It is excreted unchanged from the body in urine and is non-caloric.
- Sucralose: Trichlorosucrose, 600 times sweeter than sugar, heat-stable, and suitable for cooking and baking.
- Alitame: Roughly 2000 times sweeter than sucrose; high potency requires strict control during food sweetening.
Cleansing Agents: Soaps and Synthetic Detergents
Cleansing agents are substances used to remove grease and cleanse surfaces. The two primary types of cleansing agents are soaps and synthetic detergents, both of which lower the surface tension of water to wet materials efficiently. Soaps are sodium or potassium salts of long-chain fatty acids, such as palmitic, stearic, or oleic acid, produced via the saponification of fats and oils using sodium hydroxide.
Soaps have limitations: they do not work effectively in hard water. Hard water contains calcium ($\text{Ca}^{2+}$) and magnesium ($\text{Mg}^{2+}$) ions that react with sodium or potassium soaps to precipitate out insoluble calcium or magnesium salts (scum), wasting soap and hindering cleaning action. Synthetic detergents overcome this disadvantage because their corresponding calcium and magnesium salts are soluble in water.
Synthetic detergents are categorized into three main classes:
- Anionic Detergents: Sodium salts of sulfonated long-chain alcohols or hydrocarbons. Example: Sodium lauryl sulfate. These are commonly used in household dishwashing liquids and laundry detergents.
- Cationic Detergents: Quaternary ammonium salts of amines with acetates, chlorides, or bromides. Example: Cetyltrimethylammonium bromide. They possess strong germicidal properties and are used in hair conditioners.
- Non-ionic Detergents: Formed by reacting stearic acid with polyethylene glycol. Example: Pentaerythritol monostearate. These act as effective liquid dishwashing detergents without ionizing in aqueous solutions.
Key Points to Remember
- Aspirin: Acts simultaneously as an antipyretic, analgesic, and anti-blood clotting agent by inhibiting prostaglandin synthesis.
- Broad-spectrum antibiotics: Chloramphenicol, ampicillin, and amoxicillin combat a wide variety of pathogenic bacteria.
- Antiseptics vs Disinfectants: Bithionol is added to soaps as an antiseptic, whereas chlorine ($\text{0.2 to 0.4 ppm}$) acts as a municipal water disinfectant.
- Tranquilizers: Barbiturates induce sleep and act as central nervous system depressants.
- Artificial Sweeteners: Alitame is ~2000x sweeter than sugar; Aspartame is thermally unstable and restricted to cold foods.
- Saponification: Alkaline hydrolysis of triesters in fats using $\text{NaOH}$ yields glycerol and soap.
- Detergent Biodegradability: Straight-chain hydrocarbon tails in detergents are biodegradable, whereas branched tails cause persistent environmental foam and water pollution.
Quick Revision Summary
- Medicines target specific macromolecules like enzymes and receptors to elicit therapeutic biological responses.
- Analgesics relieve pain; antipyretics reduce body temperature during fevers.
- Antibiotics can be narrow-spectrum (Penicillin G) or broad-spectrum (Chloramphenicol).
- Antiseptics are safe for living tissues; disinfectants are meant for inanimate surfaces.
- Barbiturates are hypnotic tranquilizers; antidepressant drugs inhibit noradrenaline degradation enzymes.
- Oral contraceptives like norethindrone and ethynylestradiol regulate female ovulation cycles.
- Food preservatives such as sodium benzoate prevent microbial spoilage.
- Soaps precipitate in hard water due to insoluble calcium and magnesium salt formation.
- Synthetic detergents function effectively in both soft and hard water environments.
- Anionic, cationic, and non-ionic detergents serve distinct industrial and domestic cleaning roles.
