Photochemical Reactions in JEE Main Chemistry

Photochemical reactions are initiated by light and follow different pathways from thermal reactions. JEE Main tests photochemistry primarily through organic free-radical halogenation questions, but also through fundamental laws and the comparison of photochemical vs thermal product selectivity. This guide gives you the full picture.

Fundamental Laws of Photochemistry

Grotthuss-Draper Law: only light that is absorbed by a substance can cause photochemical change (self-evident but foundational — a transparent substance cannot undergo photochemistry, no matter how intense the light). Stark-Einstein Law of Photochemical Equivalence: each absorbed photon activates exactly one molecule (in the primary photochemical process). One Einstein (one mole of photons) activates one mole of molecules. Energy per photon E = hν = hc/λ. JEE tests: if 10 kJ of UV light at λ = 300 nm is absorbed, how many molecules are activated? Answer: (10×10³ J) / (hc/λ) per molecule = (10×10³ × 300×10⁻⁹) / (6.626×10⁻³⁴ × 3×10⁸) = 1.51 × 10²² molecules. Quantum yield Φ = number of molecules reacted / number of photons absorbed. Φ can be < 1 (quenching), = 1 (ideal), or >> 1 (chain reactions — some reactions have Φ up to 10⁶).

The H₂ + Cl₂ → 2HCl reaction in light has Φ ~ 10⁶ because each Cl• radical initiates a chain reaction converting millions of HCl molecules per photon absorbed. JEE uses quantum yield to distinguish chain reactions from simple photochemical processes. Take a free organic chemistry mock. For free radical mechanisms see our reaction mechanisms guide.

Photohalogenation of Alkanes

Chlorination of alkanes (hν or high temperature, free-radical mechanism): produces a mixture of products because Cl• is relatively non-selective. Selectivity: 3° C-H > 2° C-H > 1° C-H (3° radical more stable). But Cl• reactivity is so high that selectivity differences are small. Bromination of alkanes (Br₂/hν): Br• is much more selective than Cl• (Br• is less reactive, more selective): strongly prefers 3° > 2° >> 1°. This selectivity difference (Cl vs Br) is tested directly in JEE: bromination gives predominantly the most substituted product (tertiary monobromide); chlorination gives a statistical mixture closer to 1:1. Example: neopentane + Cl₂/hν → neopentyl chloride only (no secondary or tertiary C-H bonds); 2-methylbutane + Br₂/hν → 2-bromo-2-methylbutane predominantly (3° position).

Photochemical vs Thermal Reactions: Key Differences

Photochemical reactions: initiated by light (photons), often proceed through excited electronic states, can occur at low temperature, frequently involve free-radical intermediates, may give products not accessible by thermal routes. Thermal reactions: initiated by heat (kinetic energy), proceed through ground-state potential energy surface, follow Arrhenius kinetics. Classic contrast: [2+2] cycloaddition (alkene + alkene → cyclobutane) is photochemical (symmetry-forbidden thermally by Woodward-Hoffmann rules, allowed photochemically); [4+2] Diels-Alder is thermal. JEE Main level: know that photochemical conditions can give different regiochemistry and stereochemistry from thermal conditions.

Important Photochemical Reactions to Know

1. Photosynthesis: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂ (occurs in chloroplasts, uses absorbed light energy to drive an endothermic reaction — the classic photochemical reaction). 2. Silver halide photography: 2AgBr → 2Ag + Br₂ (light decomposes AgBr to Ag metal, creating the latent image). 3. Ozone layer chemistry: O₂ + hν (UV) → 2O• (photodissociation); O• + O₂ → O₃ (ozone formation); CFC photodissociation (CF₂Cl₂ + hν → CF₂Cl• + Cl•) → Cl• catalyses O₃ destruction. 4. Fries rearrangement under photochemical conditions gives ortho product preferentially; under thermal conditions gives para. JEE tests the photochemical Fries rearrangement product identity. For the full organic reaction repertoire see our named reactions guide and our organic reactions mechanism guide.