1,3,5-hexatriene

Definition

1,3,5-hexatriene is a conjugated polyene with six carbon atoms and three carbon-carbon double bonds arranged in an alternating pattern. It is a key term that is important in understanding the stability of conjugated dienes, the molecular orbitals of conjugated pi systems, the stereochemistry of thermal electrocyclic reactions, and the photochemical electrocyclic reactions.

5 Must Know Facts For Your Next Test

1. 1,3,5-hexatriene is a conjugated polyene, meaning it has a system of alternating carbon-carbon double and single bonds.
2. The stability of 1,3,5-hexatriene can be explained using molecular orbital theory, which describes how the bonding and antibonding orbitals of the pi system interact.
3. The stereochemistry of thermal electrocyclic reactions involving 1,3,5-hexatriene follows the Woodward-Hoffmann rules, which predict the stereochemical outcome based on the symmetry of the molecular orbitals.
4. Photochemical electrocyclic reactions of 1,3,5-hexatriene can result in the formation of cyclic products through the concerted movement of pi electrons, also governed by the Woodward-Hoffmann rules.
5. The conjugation and pi-electron delocalization in 1,3,5-hexatriene contribute to its stability and reactivity, making it an important structural motif in organic chemistry.

Review Questions

• Explain how the molecular orbital theory can be used to understand the stability of 1,3,5-hexatriene.
  • The stability of 1,3,5-hexatriene can be understood using molecular orbital theory. The three carbon-carbon double bonds in the molecule form a conjugated pi system, where the pi electrons are delocalized across the entire structure. This delocalization of electrons leads to the stabilization of the molecule, as the energy of the bonding molecular orbitals is lowered, and the energy of the antibonding molecular orbitals is raised. The extent of this stabilization can be predicted using the principles of molecular orbital theory, which take into account the number of pi electrons and the symmetry of the pi system.
• Describe the stereochemistry of thermal electrocyclic reactions involving 1,3,5-hexatriene, and how the Woodward-Hoffmann rules can be used to predict the outcome.
  • The stereochemistry of thermal electrocyclic reactions involving 1,3,5-hexatriene is governed by the Woodward-Hoffmann rules. These rules state that a conrotatory cyclization (where the terminal carbon atoms move in the same direction) is favored in a thermal electrocyclic reaction. This is because the conrotatory motion of the pi electrons allows for the optimal overlap of the bonding molecular orbitals, leading to a more stable transition state. In the case of 1,3,5-hexatriene, the thermal electrocyclic reaction would be expected to proceed via a conrotatory pathway, resulting in the formation of a cyclic product with a specific stereochemistry that can be predicted using the Woodward-Hoffmann rules.
• Compare and contrast the stereochemical outcomes of thermal and photochemical electrocyclic reactions involving 1,3,5-hexatriene, and explain how the Woodward-Hoffmann rules can be used to rationalize the differences.
  • The Woodward-Hoffmann rules can be used to predict and explain the differences in the stereochemical outcomes of thermal and photochemical electrocyclic reactions involving 1,3,5-hexatriene. In a thermal electrocyclic reaction, the conrotatory cyclization is favored, as it allows for the optimal overlap of the bonding molecular orbitals. However, in a photochemical electrocyclic reaction, the stereochemistry of the product is determined by the symmetry of the excited state molecular orbitals. The Woodward-Hoffmann rules state that a disrotatory cyclization (where the terminal carbon atoms move in opposite directions) is favored in a photochemical electrocyclic reaction, as this motion allows for the optimal overlap of the bonding molecular orbitals in the excited state. Therefore, the thermal and photochemical electrocyclic reactions of 1,3,5-hexatriene would be expected to proceed with different stereochemical outcomes, which can be rationalized using the principles of the Woodward-Hoffmann rules.

Related terms

Conjugated Diene: A conjugated diene is a organic compound with two carbon-carbon double bonds separated by a single carbon-carbon bond, forming a system of alternating double and single bonds.

Molecular Orbitals: Molecular orbitals are mathematical functions that describe the wave-like behavior of an electron in a molecule. They are used to predict the stability and reactivity of molecules.

Electrocyclic Reaction: An electrocyclic reaction is a type of pericyclic reaction where a cyclic product is formed from an acyclic reactant, or vice versa, through the concerted movement of pi electrons.