Organic Chemistry · Orbitals and Organic Chemistry: Pericyclic Reactions
Photochemical Electrocyclic Reactions
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In 30 seconds
A photochemical electrocyclic reaction is the same bond reorganization as the thermal version — a conjugated polyene closes to a cycloalkene or opens back — but driven by light instead of heat. The profound consequence is that the stereochemical outcome flips: the allowed mode of motion under photochemical conditions is the exact opposite of the thermal one. Where thermal butadiene closes conrotatorily, the photochemical reaction A reaction driven by absorption of light (photons) rather than heat Full entry → is disrotatory; where thermal hexatriene closes disrotatorily, the photochemical reaction is conrotatory.
The reason is the frontier-orbital logic of Topic 1. Absorption of a photon promotes an electron from the HOMO into the LUMO. The reacting orbital is now the formerly empty LUMO, whose symmetry is opposite to the ground-state HOMO (the S/A labels alternate up the ladder). Since the allowed rotation mode is dictated by that symmetry, light reverses the rules. Photochemical electrocyclic reactions are not a curiosity: they operate in nature (the first step of vitamin D₃ biosynthesis) and give chemists an alternative stereoisomer that heat cannot reach.
Why this matters
- Complementary stereochemistry: thermal and photochemical routes from the same starting material give different stereoisomers of the same product — a powerful way to reach either isomer on demand.
- Vitamin D₃ biosynthesis: in human skin, UV light drives the photochemical electrocyclic ring opening of 7-dehydrocholesterol (a steroid with a conjugated diene in the B ring) to previtamin D₃, which then isomerizes to vitamin D₃. This is a genuine, everyday example of a photochemical electrocyclic reaction.
- Synthesis under mild conditions: photochemical reactions can proceed at low temperature, which matters for thermally sensitive substrates.
- Exams: the "flip the rules" concept is a classic test of whether students understand why the rules exist rather than just memorizing them.
The college version
Core Concepts
What photoexcitation changes
In the ground state, butadiene's HOMO is ψ2 (antisymmetric, A) and its LUMO is ψ3 (symmetric, S). Absorption of a photon promotes one electron from ψ2 into ψ3. The excited molecule now has a singly occupied orbital (a SOMO Singly occupied molecular orbital in the excited state Full entry →) of S symmetry where its frontier electron lives — the symmetry the reaction must obey. Because S replaces A as the controlling orbital, the allowed mode reverses: the thermal conrotatory path (required for the A HOMO) is replaced by the disrotatory path (required for the S orbital).
The photochemical rules
For a photochemical electrocyclic reaction, count the π electrons in the conjugated system:
- 4n π electrons (4, 8, 12, ...) → disrotatory motion (opposite of thermal).
- 4n + 2 π electrons (2, 6, 10, ...) → conrotatory motion (opposite of thermal).
So photochemical closure of butadiene (4 π electrons) is disrotatory, and photochemical closure of hexatriene (6 π electrons) is conrotatory. The mnemonic is simple: photochemical = thermal rules reversed.
Stereochemical consequences
Because the mode flips, the product stereochemistry flips too (Topic 3's mapping with the modes exchanged):
- Photochemical closure of (2E,4E)-2,4-hexadiene (disrotatory) gives cis-3,4-dimethylcyclobutene — the isomer thermal reaction produces from the (2E,4Z) diene instead.
- Photochemical closure of (2Z,4E,6Z)-2,4,6-octatriene (conrotatory) gives trans-5,6-dimethyl-1,3-cyclohexadiene, while the thermal reaction gives the cis isomer.
The same starting material under heat vs light yields different stereoisomers — a dramatic demonstration that chemists control structure by choosing the energy source.
What counts as "light"
For a photochemical reaction to occur, the photon must supply at least the HOMO→LUMO excitation energy of the π system. For simple dienes and trienes, that gap corresponds to ultraviolet wavelengths (roughly 200–300 nm; butadiene absorbs near 217 nm, hexatriene near 258 nm). Ordinary visible light cannot excite most unsubstituted polyenes. In practice, UV lamps emitting around 254 nm (mercury lamps) or 300 nm (sunlamps) drive these reactions; always follow standard UV safety precautions (protect eyes and skin, use UV-rated shielding) because UV is not visible and causes burns and eye damage.
How It Works / Step-by-Step Process
- Count the π electrons in the conjugated chain.
- Apply the photochemical rules: 4n → disrotatory; 4n + 2 → conrotatory (thermal rules reversed).
- Check that the light source's wavelength is short enough to be absorbed (roughly λ ≤ 300 nm for simple dienes/trienes).
- Rotate the ends by the photochemical mode and connect the termini with a new σ bond.
- Read the stereochemistry: same face → cis; opposite faces → trans (Topic 3's tracking method).
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Photochemical = hotter reaction | Photochemical = photon absorption | Light and heat are different energy sources with opposite stereochemical outcomes |
| Photochemical and thermal rules are the same | They are exact opposites | Excitation moves the frontier electron into an orbital of opposite symmetry |
| Any light source works | The photon must be absorbed | Only wavelengths matching the π→π* gap (UV for simple polyenes) drive the reaction |
| Light gives the same product as heat, just faster | Light gives a different stereoisomer | The reversed mode changes which faces the substituents land on |
| UV light is harmless because it's invisible | UV causes burns and eye damage | Always shield skin and eyes; use UV-rated glass/shielding |
| SOMO is the same as HOMO | SOMO is the excited-state frontier orbital | After excitation the reacting orbital is the former LUMO, now singly occupied |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Think of the molecule as a swing set where one electron usually sits on the low swing (HOMO). Shining light gives that electron a big push so it jumps to the high swing (LUMO) — and now the rules of the game change, because the high swing sways the opposite way. The molecule still closes into a ring, but its flags (substituents) end up on the opposite sides compared with the heat version. Heat turns the doorknobs one way; light turns them the other way!
Worked example
Example 1: Photochemical closure of (2E,4E)-2,4-hexadiene
The diene has 4 π electrons, so photochemical closure is disrotatory (thermal would be conrotatory). The (2E,4E) isomer has its terminal methyls on opposite faces of the chain zigzag. Disrotation rotates the ends in opposite directions, which — for this starting geometry — delivers the methyls to the same face of the forming cyclobutene ring. Product: cis-3,4-dimethylcyclobutene. Note the contrast: the thermal reaction (Topic 3) converts (2E,4E)-2,4-hexadiene to the trans cyclobutene; the photochemical reaction converts the same diene to the cis isomer. Same starting material, different stereoisomer, controlled entirely by heat vs light.
Example 2: The photon energy requirement for butadiene
To close butadiene photochemically, a photon must first excite the π→π* transition. The excitation energy is E = hc/λ, where h = 6.626 × 10-34 J·s is Planck's constant, c = 2.998 × 108 m/s is the speed of light, and λ is the wavelength absorbed. Using butadiene's observed absorption maximum, λmax = 217 nm = 2.17 × 10-7 m:
E = hcλ = (6.626 × 10-34 J·s)(2.998 × 108 m/s)2.17 × 10-7 m = 9.15 × 10-19 J
Unit check: J·s × (m/s) ÷ m = J. Converting to kilojoules per mole with Avogadro's number NA = 6.022 × 1023 mol-1:
E = (9.15 × 10-19 J)(6.022 × 1023 mol-1) = 5.51 × 105 J/mol = 551 kJ/mol
So light of 217 nm (or shorter) carries enough energy to excite butadiene; a 254 nm mercury-lamp photon (471 kJ/mol) falls slightly short of the peak but still excites the low-energy tail of the absorption band. For hexatriene (λmax ≈ 258 nm), the required energy is E = hc/(2.58 × 10-7 m) = 7.70 × 10-19 J = 464 kJ/mol, which a 254 nm lamp easily supplies.
Example 3: Photochemical closure of (2Z,4E,6Z)-2,4,6-octatriene
This triene has 6 π electrons, so photochemical closure is conrotatory (thermal would be disrotatory). The (2Z,4E,6Z) isomer has its terminal methyls on the same face of the chain. Conrotation moves both ends the same way, which delivers the methyls to opposite faces of the new six-membered ring. Product: trans-5,6-dimethyl-1,3-cyclohexadiene — the stereoisomer thermal reaction gives from the (2E,4E,6E) isomer instead. Once again: flip the conditions, flip the product.
Key takeaways
- Photochemical electrocyclic reactions obey the reversed Woodward–Hoffmann rules: 4n → disrotatory; 4n + 2 → conrotatory.
- Photoexcitation promotes an electron HOMO → LUMO; the new frontier orbital has opposite symmetry, which is why the mode flips.
- Butadiene (4 e⁻) photochemically closes disrotatorily; hexatriene (6 e⁻) photochemically closes conrotatorily.
- Photochemical closure of (2E,4E)-2,4-hexadiene gives cis-3,4-dimethylcyclobutene; of (2Z,4E,6Z)-2,4,6-octatriene gives trans-5,6-dimethyl-1,3-cyclohexadiene.
- Vitamin D₃ synthesis begins with a photochemical electrocyclic ring opening of 7-dehydrocholesterol in skin.
- The photon must match the π→π* energy gap: unsubstituted dienes/trienes need UV light (~200–300 nm), not visible light.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
What is the photochemical rule for a 4-π-electron system, and why does it differ from the thermal rule?
Show answer
4n π electrons → disrotatory photochemically (thermal: conrotatory). The photon promotes an electron into the LUMO, whose symmetry is opposite to the ground-state HOMO, so the mode that achieves bonding overlap reverses.
Photochemical closure of (2E,4E)-2,4-hexadiene gives which stereoisomer?
Show answer
cis-3,4-dimethylcyclobutene — disrotatory closure of the (2E,4E) diene brings its opposite-face methyls onto the same face of the ring.
Why does vitamin D₃ biosynthesis require UV light?
Show answer
The photochemical ring opening of 7-dehydrocholesterol requires a photon energetic enough to excite its conjugated π system; sunlight supplies the UV component that drives this step in skin.
What energy (in kJ/mol) does a 254 nm photon carry, and is that enough to excite hexatriene (λmax ≈ 258 nm)?
Show answer
E = hc/λ= (6.626 × 10-34)(2.998 × 108)/(2.54 × 10-7) = 7.82 × 10-19 J = 471 kJ/mol. Hexatriene needs ~464 kJ/mol (from λmax ≈ 258 nm), so 254 nm light has enough energy.
How does photoexcitation change which orbital controls the reaction?
Show answer
The electron promoted HOMO → LUMO becomes the frontier electron (SOMO); its symmetry, opposite to the ground-state HOMO's, now dictates the allowed rotation mode.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- photochemical reaction
- A reaction driven by absorption of light (photons) rather than heat
- excited state
- The molecule's configuration after an electron is promoted HOMO → LUMO
- SOMO
- Singly occupied molecular orbital in the excited state
- π→π* transition
- Promotion of a π electron into an antibonding π orbital
- UV (ultraviolet) light
- Radiation at roughly 200–400 nm
- π→π transition
- Promotion of a π electron into an antibonding π orbital
Sources & references
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