REACTION MECHANISM AND KINETICS

1.1. Free Radical Halogenation

Free Radical Halogenation

Example: Conversion of alkanes to alkyl halides (used in making anesthetic agents).

Proceeds via initiation, propagation, and termination steps involving radicals.

Free Radical Halogenation

Mechanism Type: Involves free radicals formed by homolytic bond cleavage; proceeds in three steps: initiation, propagation, and termination.

Example:
Methane + Cl₂ → Chloromethane (CH₃Cl) + HCl
(in the presence of UV light)
This reaction is used in the production of inhaled anesthetic agents like halothane and chloroform.

Medical Relevance: Halogenated alkanes are starting materials for anesthetic gases.

 Electrophilic Addition to Alkenes

Example: Adding HBr to ethene → bromoethane; important in pharmaceutical synthesis.

Electrophilic Addition to Alkenes

Mechanism Type: The π bond of an alkene attacks an electrophile, followed by nucleophilic addition.

Example:
Ethene + HBr → Bromoethane
CH₂=CH₂ + HBr → CH₃–CH₂Br

• Medical Relevance: Alkyl halides like bromoethane are intermediates in drug synthesis (e.g., alkylating agents in chemotherapy).

 Electrophilic Aromatic Substitution (EAS)

Example: Nitration of benzene → nitrobenzene; key to creating many drugs, like painkillers.

Electrophilic Aromatic Substitution (EAS)

Mechanism Type: An electrophile replaces a hydrogen atom on an aromatic ring via formation of an arenium ion intermediate.

Example:
Benzene + HNO₃ (with H₂SO₄ catalyst) → Nitrobenzene + H₂O

Medical Relevance: Nitrobenzene is a precursor to aniline, used in making paracetamol (acetaminophen) and other analgesics.

 Enzyme-Catalyzed Mechanisms

Enzymes act as biological catalysts with specific mechanisms for transforming substrates into products.

Example: Pepsin in the stomach breaks down proteins via acid-catalyzed hydrolysis.

Enzyme-Catalyzed Mechanisms

Mechanism Type: Enzymes speed up reactions by stabilizing transition states; highly specific and efficient.

Example:
Pepsin hydrolyzing proteins in the stomach
Proteins + H₂O → Peptides (via pepsin, in acidic pH)

Medical Relevance: Understanding enzyme action is key to digestive health and the design of enzyme inhibitors in drugs (e.g., protease inhibitors for HIV).