Overview
This lecture covers the photochemistry of carbonyl compounds, focusing on the Norrish Type I reaction, its mechanism, effects of molecular structure, and how reaction conditions affect product formation.
Introduction to Photochemistry
- Photochemistry studies chemical reactions initiated by light (radiation).
- Three main classes: organic compounds, carbonyl compounds, and aromatic compounds.
- Photochemical reactions occur when molecules absorb light and become excited.
Norrish Type I Reaction (α-Cleavage)
- Norrish Type I reaction involves the cleavage of a carbon–carbon bond next to a carbonyl group (α-cleavage) under light.
- The reaction yields two radicals upon absorbing a photon.
- This process is more likely when the C–C bond at the α-position is weak (low dissociation energy).
Mechanism of Norrish Type I Reaction
- Step 1: Absorption of light excites the carbonyl to a higher energy state.
- Step 2: The excited state causes α-cleavage, producing two radicals.
- If the reaction is at room temperature, radicals may recombine to form a dimer (coupling product).
- If the reaction runs at higher temperatures (>100°C), a radical may lose a small group (like CO or alkyl), forming an alkene or new carbonyl compound.
Influence of Molecular Symmetry and Structure
- Symmetrical ketones typically yield identical radicals; asymmetrical ketones can give different radicals.
- The stability of the generated radicals influences which pathway is favored (secondary radicals are more stable than primary).
Examples and Product Prediction
- Acetone (a symmetrical ketone) undergoes Norrish Type I to produce two identical methyl radicals and one acetyl radical.
- In cyclic ketones, α-cleavage can happen at different positions, providing varied products.
- The position and type of substituent (like hydrogen vs. alkyl) affect product stability.
Reaction Conditions and Outcomes
- Room temperature favors radical coupling (dimerization).
- High temperature favors elimination (formation of alkenes or loss of CO).
- The presence of hydrogen at the α-position may lead to specific elimination products.
Key Terms & Definitions
- Photochemistry — Chemistry of reactions initiated by absorption of light.
- Carbonyl Compound — Organic molecule containing the C=O group.
- Norrish Type I Reaction — Photochemical α-cleavage of a carbonyl compound forming two radicals.
- α-Cleavage — Breaking the C–C bond adjacent to a carbonyl group.
- Radical — Atom or molecule with an unpaired electron, highly reactive.
- Symmetrical Ketone — Ketone with identical groups on both sides of the carbonyl carbon.
- Asymmetrical Ketone — Ketone with different groups on either side of the carbonyl carbon.
- Dimerization — Combination of two radicals to form a larger molecule.
Action Items / Next Steps
- Practice drawing the Norrish Type I reaction mechanism for both symmetrical and asymmetrical ketones.
- Review examples of Norrish Type I reactions with product prediction under varying conditions.
- Prepare for questions on the effect of molecular structure and temperature on photochemical product distribution.