Unit 5 Notes - DSC 302 Chemistry Semester 5

General Concepts in Photochemistry

Photochemistry is the study of chemical reactions that proceed with the absorption of light (ultraviolet, visible, or infrared radiation) by atoms or molecules. Unlike thermal reactions that rely on heat energy to overcome activation barriers, photochemical reactions are initiated by photons, which excite electrons to higher energy states, leading to unique reactivity.

Energy Absorption and Electronic Transitions

When a molecule absorbs a photon of appropriate energy, an electron is promoted from a lower energy molecular orbital (typically the highest occupied molecular orbital, HOMO) to a higher energy molecular orbital (typically the lowest unoccupied molecular orbital, LUMO). This process is called an electronic transition.

Photoexcitation: The process where a molecule absorbs light energy and an electron jumps to a higher energy level, resulting in an electronically excited state.

Jablonski Diagram (Conceptual Overview)

The Jablonski diagram visually represents the electronic states of a molecule and the various photophysical processes that can occur after light absorption. Key processes include:

Absorption: The initial step where a molecule transitions from the ground singlet state (S₀) to a higher singlet excited state (S₁, S₂, etc.).
Vibrational Relaxation (VR): Rapid loss of vibrational energy within an electronic state, leading to the lowest vibrational level of that electronic state.
Internal Conversion (IC): Non-radiative transition between electronic states of the same spin multiplicity (e.g., S₂ to S₁). It is very fast.
Fluorescence: Radiative transition from the first excited singlet state (S₁) back to the ground singlet state (S₀), accompanied by the emission of a photon.
Intersystem Crossing (ISC): Non-radiative transition between electronic states of different spin multiplicity (e.g., S₁ to T₁, where T₁ is the first excited triplet state). This process involves a spin flip.
Phosphorescence: Radiative transition from the first excited triplet state (T₁) back to the ground singlet state (S₀), accompanied by the emission of a photon. It is typically much slower than fluorescence.

Franck-Condon Principle

Franck-Condon Principle: An electronic transition is most likely to occur without a change in the positions of the nuclei in the molecule and its environment. The nuclei are much heavier than electrons and therefore move much slower. Thus, during the instant of an electronic transition, the nuclear framework can be considered fixed.

This principle implies that vertical transitions (transitions that can be represented as a vertical line on a potential energy diagram, indicating no change in internuclear distance) are the most probable. After the vertical transition, the molecule will undergo vibrational relaxation to the lowest vibrational level of the excited electronic state.

Singlet and Triplet States

Electronic states are classified by their spin multiplicity, which is given by the formula 2S + 1, where S is the total spin angular momentum.

Singlet State (S)

In a singlet state, all electron spins are paired (S=0), meaning the excited electron has an opposite spin to its partner electron in the ground state orbital.
Spin multiplicity = 2(0) + 1 = 1.
The ground state of most organic molecules is a singlet state (S₀).
Excited singlet states (S₁, S₂, etc.) are formed directly upon light absorption.
Fluorescence occurs from S₁ to S₀.

Triplet State (T)

In a triplet state, two electrons have parallel spins (S=1), meaning the excited electron has the same spin orientation as its partner electron in the ground state orbital.
Spin multiplicity = 2(1) + 1 = 3.
Triplet states are generally lower in energy than the corresponding singlet excited states (i.e., T₁ < S₁) due to less electron-electron repulsion (Hund's rule).
Triplet states are typically formed from singlet excited states via intersystem crossing (ISC).
Phosphorescence occurs from T₁ to S₀.
Triplet states are generally more long-lived and reactive than singlet excited states because the spin forbidden nature of returning to the ground state gives them more time to react.

Norrish Type I and Type II Processes

These are common photochemical reactions of carbonyl compounds, particularly ketones.

Norrish Type I Reaction (α-Cleavage)

Norrish Type I Reaction: A photochemical reaction of a carbonyl compound involving homolytic cleavage of the bond between the carbonyl carbon and an adjacent carbon (the α-carbon). This is also known as α-cleavage.

Mechanism: Upon excitation to a singlet (S₁) or triplet (T₁) state, the carbonyl compound undergoes homolytic cleavage of the C-C bond adjacent to the carbonyl group. This generates an acyl radical and an alkyl radical.
Substrates: Ketones.
Products: The resulting radicals can undergo various secondary reactions, including recombination (often leading to decarbonylation) or abstraction of hydrogen atoms from other molecules or from the solvent, leading to aldehydes, alkanes, or other ketones. Decarbonylation is a common outcome.
Example: Photolysis of acetone can lead to methyl radicals and an acetyl radical, which can then combine or disproportionate.

Norrish Type II Reaction (γ-Hydrogen Abstraction)

Norrish Type II Reaction: A photochemical reaction of a carbonyl compound that possesses at least one hydrogen atom at the γ-position relative to the carbonyl group. It involves intramolecular abstraction of this γ-hydrogen by the excited carbonyl oxygen.

Mechanism: The excited carbonyl oxygen (often in the n→π* excited state) abstracts a γ-hydrogen atom via a six-membered cyclic transition state, forming a 1,4-biradical. This biradical can then undergo either:
- Cleavage: To form an enol and an alkene.
- Cyclization: To form a cyclobutanol.
Substrates: Ketones and aldehydes with γ-hydrogens.
Products: Alkenes and enols (which tautomerize to ketones) or cyclobutanols.
Observation: This reaction is highly stereospecific for the formation of the biradical intermediate.

Paterno-Buchi Reaction

Paterno-Buchi Reaction: A photochemical [2+2] cycloaddition reaction between an electronically excited carbonyl compound (usually a ketone or aldehyde) and an alkene, forming an oxetane (a four-membered cyclic ether).

Mechanism: The excited carbonyl (often in the n→π* triplet state, T₁) reacts with the ground state alkene. The reaction proceeds via a 1,4-diradical intermediate, which subsequently closes to form the oxetane.
Stereochemistry: The reaction is generally non-stereospecific with respect to the alkene, indicating a diradical intermediate with free rotation.
Products: Oxetanes.
Application: A valuable method for synthesizing four-membered heterocyclic rings.

Barton Reaction (Nitrite Photolysis)

Barton Reaction: An intramolecular photochemical reaction involving the photolysis of an alkyl nitrite, leading to the formation of a δ-nitroso alcohol, which typically tautomerizes to an oxime. It is a useful method for selective functionalization of unactivated methyl or methylene groups remote from the nitrite group.

Mechanism:
- Step 1: Photolysis of the alkyl nitrite (R-O-N=O) leads to homolytic cleavage of the O-N bond, generating an alkoxy radical (RO•) and a nitric oxide radical (•NO).
- Step 2: The alkoxy radical undergoes an intramolecular 1,5-hydrogen atom transfer (H-abstraction) from a δ-carbon, forming a carbon-centered radical and an alcohol. This occurs via a six-membered transition state.
- Step 3: The carbon radical then combines with the nitric oxide radical (•NO) to form a C-nitroso compound at the δ-position.
- Step 4: The C-nitroso compound typically tautomerizes to an oxime.
Substrates: Alkyl nitrites with available δ-hydrogens.
Products: Oximes, particularly useful for functionalizing remote positions.
Exam Tip: Look for the 1,5-hydrogen transfer in the mechanism, which is key to its regioselectivity.

Photo-oxidation and Photo-reduction

Photo-oxidation

Photo-oxidation refers to oxidation reactions initiated by light, often involving molecular oxygen (O₂). These reactions are broadly classified into Type I and Type II mechanisms.

Type I Photo-oxidation (Radical Mechanism): Involves direct interaction of the excited substrate (S*) with another molecule (often a sensitizer or the substrate itself) to generate radicals, which then react with ground state oxygen.
- Mechanism: Excited sensitizer (Sens*) abstracts a hydrogen atom or transfers an electron to a substrate (RH or R-H). The resulting radicals (R•) then react with ground state O₂ to form peroxy radicals (ROO•) and ultimately hydroperoxides or other oxidized products.
- Reagents: Sensitizer (e.g., ketones, quinones) and substrate with abstractable hydrogens.
Type II Photo-oxidation (Singlet Oxygen Mechanism): Involves the generation of highly reactive singlet oxygen (¹O₂) by an excited sensitizer, which then reacts with the substrate.
- Generation of Singlet Oxygen: A sensitizer (Sens) absorbs light and undergoes intersystem crossing to its triplet state (³Sens*). ³Sens* then transfers its energy to ground state triplet oxygen (³O₂), promoting it to the more reactive singlet oxygen (¹O₂).
- Reactions of Singlet Oxygen: Singlet oxygen is an electrophile and reacts with electron-rich substrates:
  - Ene Reaction (Alder-ene): Reaction with alkenes containing allylic hydrogens to form allylic hydroperoxides.
  - [2+2] Cycloaddition: Reaction with electron-rich alkenes (e.g., enamines, vinyl ethers) to form 1,2-dioxetanes.
  - Diels-Alder like [4+2] Cycloaddition: Reaction with conjugated dienes to form endoperoxides.
- Sensitizers: Rose Bengal, Methylene Blue, Porphyrins, etc.

Photo-reduction

Photo-reduction refers to reduction reactions initiated by light, where a molecule gains electrons or hydrogen atoms.

Mechanism: Photo-reduction often proceeds via either electron transfer or hydrogen atom abstraction by the excited state of the molecule.
- Electron Transfer: An excited molecule can accept an electron from a suitable donor or donate an electron to an acceptor.
- Hydrogen Abstraction: An excited molecule (e.g., n→π* excited carbonyl) can abstract a hydrogen atom from a suitable donor (e.g., solvent, alcohol) to form a ketyl radical, which can further dimerize or be reduced.
Example: Photoreduction of ketones in the presence of a hydrogen donor (e.g., isopropanol) can lead to pinacols (1,2-diols) through dimerization of the intermediate ketyl radicals.

Photochemical Rearrangements

Light can induce unique molecular rearrangements that are often forbidden or difficult to achieve under thermal conditions due to different orbital symmetry requirements in excited states compared to ground states.

Photo Fries Rearrangement

Photo Fries Rearrangement: A photochemical rearrangement of aryl esters or aryl amides where the acyl or carbamoyl group migrates from the phenolic oxygen or anilidic nitrogen to an ortho or para position on the aromatic ring.

Mechanism:
- Step 1: Photolysis of the aryl ester/amide leads to homolytic cleavage of the C-O (for ester) or C-N (for amide) bond, generating an acyl radical and an aryloxy radical (or anilino radical).
- Step 2: These radicals remain in a solvent cage.
- Step 3: Recombination of the radicals within the solvent cage occurs at the ortho or para positions of the aromatic ring.
Substrates: Phenyl esters, phenyl carbamates, anilides.
Products: Ortho- and para-acylated phenols or anilines.
Important Observation: Unlike the thermal Fries rearrangement which often requires a Lewis acid catalyst and proceeds via carbocation intermediates, the photo Fries is a radical process and can occur intermolecularly as well (if the radicals escape the solvent cage).

Di-π Methane Rearrangement

Di-π Methane Rearrangement: A photochemical rearrangement of a 1,4-diene or related systems containing two π-systems separated by a single sp³-hybridized carbon atom (the methane carbon), resulting in the formation of a vinylcyclopropane derivative.

Mechanism:

Step 1: Excitation of one of the π-systems.
Step 2: Formation of a 1,4-diradical intermediate where the excited π-system interacts with the methane carbon and the other π-system.

Step 3:

Unit 5: Photochemistry of Organic Compounds: A Comprehensive Study Guide