2. Figure Number: UN
Title: Conrotatory Ring Closure
Caption: Ring closure which occurs when both orbitals rotate in the same direction to achieve overlap is called conrotatory.
Notes: Conrotatory ring closure occurs when the top lobe of one orbital has the same phase as the bottom lobe of the other orbital involved in forming a sigma bond.

3. Figure Number: UN
Title: Disrotatory Ring Closure
Caption: Ring closure which occurs when the orbitals rotate in opposite directions to achieve overlap is called disrotatory.
Notes: Disrotatory ring closure occurs when the top lobe of one orbital has the same phase as the top lobe of the other orbital involved in forming a sigma bond.

4. Figure Number: UN
Title: Ring Closure With Symmetric HOMO
Caption: Molecules with symmetric HOMOs give disrotatory ring-closure products.
Notes: Molecules with symmetric HOMOs have the top lobe of one orbital in the same phase as the top lobe of the other orbital.

5. Figure Number: UN
Title: Ring Closure with Antisymmetric HOMO
Caption: Molecules with antisymmetric HOMOs give conrotatory ring-closure products.
Notes: Molecules with antisymmetric HOMOs have the top lobe of one orbital in the same phase as the bottom lobe of the other orbital.

6. Figure Number: UN
Title: (2E,4Z,6E)-Octatriene Ring Closure
Caption: (2E,4Z,6E)-Octatriene ring closure is disrotatory, yielding cis-5,6-dimethyl-1,3-cyclohexadiene.
Notes: The HOMO of (2E,4Z,6E)-octatriene is symmetric because MOs of linear conjugated pi systems alternate in symmetry starting with the lowest-energy MO being symmetric. (2E,4Z,6E)-Octatriene has six MOs (from six atomic p orbitals overlapping), half of which (three) are filled in the ground state. The third-lowest-energy orbital has to be the HOMO, and it has to be symmetric rather than antisymmetric.

7. Figure Number: UN
Title: (2E,4Z,6Z)-Octatriene Ring Closure
Caption: (2E,4Z,6Z)-Octatriene ring closure is disrotatory, yielding trans-5,6-dimethyl-1,3-cyclohexadiene.
Notes: The HOMO of (2E,4Z,6Z)-octatriene is symmetric because MOs of linear conjugated pi systems alternate in symmetry starting with the lowest-energy MO being symmetric. (2E,4Z,6Z)-Octatriene has six MOs (from six atomic p orbitals overlapping), half of which (three) are filled in the ground state. The third-lowest-energy orbital has to be the HOMO, and it has to be symmetric rather than antisymmetric.

8. Figure Number: UN
Title: Photochemically induced (2E,4Z,6Z)-Octatriene Ring Closure
Caption: Photochemically induced (2E,4Z,6Z)-octatriene ring closure is conrotatory, yielding cis-5,6-dimethyl-1,3-cyclohexadiene.
Notes: The HOMO of (2E,4Z,6Z)-octatriene which has been excited by light is antisymmetric because MOs of linear conjugated pi systems alternate in symmetry starting with the lowest-energy MO being symmetric. (2E,4Z,6Z)-Octatriene has six MOs (from six atomic p orbitals overlapping), half of which (three) are filled in the ground state. The third-lowest-energy orbital has to be the HOMO in the ground state, and the fourth-lowest-energy orbital has to be the HOMO of the photochemically excited state. This orbital has to be antisymmetric.

9. Figure Number: UN
Title: (2E,4Z)-Hexadiene Ring Closure
Caption: (2E,4Z)-Hexadiene undergoes conrotatory ring closure to yield cis-3,4-dimethylcyclobutene.
Notes: The HOMO of (2E,4Z)-hexadiene has to be antisymmetric because this compound has to have four pi MOs, two of which are filled. The HOMO has to be the second-lowest-energy orbital. Since the lowest-energy orbital has to be symmetric, the HOMO has to be antisymmetric.

10. Figure Number: UN
Title: (2E,4E)-Hexadiene Ring Closure
Caption: (2E,4E)-Hexadiene undergoes conrotatory ring closure to yield trans-3,4-dimethylcyclobutene.
Notes: The HOMO of (2E,4E)-hexadiene has to be antisymmetric because this compound has to have four pi MOs, two of which are filled. The HOMO has to be the second-lowest-energy orbital. Since the lowest-energy orbital has to be symmetric, the HOMO has to be antisymmetric.

11. Figure Number: UN
Title: Suprafacial vs. Antarafacial Bond Formation
Caption: In a cycloaddition reaction, bond formation is called suprafacial if both sigma bonds form on the same side of the pi system, and antarafacial if the sigma bonds form on the opposite side of the pi system.
Notes: Antarafacial cycloadditions result in strained transition states for small rings, as one of the fragments undergoing cycloaddition must simultaneously bond to the top face of one side of its partner and the bottom face of the other side of the partner, forming a strained-ring transition state. Generally, a seven-membered or larger ring needs to be formed by a cycloaddition reaction in order to observe antarafacial ring formation.

12. Figure Number: 29-05
Title: Figure 29.5
Caption: Frontier molecular-orbital analysis of a [4 + 2] cycloaddition reaction.
Notes: The HOMO of either of the reactants used with the LUMO of the other gives the same results as switching the reactant which contributes the HOMO with the reactant which contributes the LUMO. Both cases require suprafacial overlap for bond formation. In the normal situation, the four-electron system contributes the HOMO and the two-electron system contributes the LUMO, but this situation can be reversed.

13. Figure Number: 29-06
Title: Figure 29.6
Caption: Frontier MO analysis of a [2 + 2] cycloaddition reaction under thermal and photochemical conditions.
Notes: Under thermal conditions, this cycloaddition would have to be antarafacial, which is impossible for a [2 + 2] cycloaddition (forms a four-membered ring). Under photochemical conditions, this reaction allows suprafacial ring formation.

14. Figure Number:
Title: Table Woodward-Hoffmann Rules for Electrocyclic Reactions
Caption:
Notes:

15. Figure Number:
Title: Table Configuration of the Product of an Electrocyclic Reaction
Caption:
Notes:

16. Figure Number:
Title: Table Woodward-Hoffmann Rules for Cycloaddition Reactions
Caption:
Notes:

17. Figure Number:
Title: Table Woodward-Hoffmann Rules for Sigmatropic Rearrangements
Caption:
Notes: