Electrocyclic Reactions stereochemistry- thermal and photochemical condition-chemarticle.com
Introduction
Hello everyone, welcome to the chem article. Today we will discuss Electrocyclic Reactions. This is the second class of pericyclic reaction. We are already discussing what is the pericyclic reaction, an example of pericyclic reaction, symmetry of orbitals, molecular orbitals of butadiene, molecular orbitals of ethylene, properties of pericyclic reactions. We provide the pericyclic reactions class notes. Provide class notes for CSIR NET, GATE, IIT-JAM, M.sc Entrance, TIFR, BARC, NTPC, DRDO, etc all examinations.
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What are Electrocyclic Reactions?
Today we will discuss electrocyclic reactions and there several types of examples of electrocyclic reactions. The chemical reaction of electrocyclic reactions using heat or hv. Chemical reactions occur in the presence of thermal conditions or photochemical conditions.
The concept of electrons transitioning from HOMO to LUMO is already achieved in pericyclic reaction first class.
We know the condition of relative phase orbitals: the same phase orbitals overlap with each other and formation of bonding situations, and on the other hand, the different phase orbitals are repulsive to each other and formation of antibonding situations.
We consider cyclization of trans, cis, trans-2,4,6-octatriene in thermal condition to formation of cis 1, 2-dimethyl-cyclohexane-3,4-diene. If the whole cyclization reaction process occurs in the presence of a photochemical condition that case resultant value is trans 1,2-methylcyclohexane-3,4-diene. In case product yield is only trans isomer (photochemical condition). In thermal condition products yield is only cis isomer.
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| Pericyclic Reactions: electrocyclic reaction under thermal and photochemical condition-chemarticle.com- stereoselective product yield. Formation of cis and trans isomer under thermal and photochemical conditions. |
That is the fact of stereoselectivity of thermal condition, 99.9% yield of the cis isomer, and 0.1% of trans isomer product yield. That means the trans isomer is a thermodynamically more stable configuration than the cis isomer.
Molecular orbitals of trans, cis, trans-2,4,6-octatriene
In this case, a total of six MOs (Molecular Orbitals) are ψ1, ψ2, ψ3, ψ4, ψ5 and ψ6 made from six AOs (Atomic Orbitals). In this case total three bonding MOs are ψ1, ψ2, ψ3, and total three antibonding MOs are ψ4, ψ5, ψ6. This molecule has a total of six pi electrons, which are field up from lower energy molecular orbitals. Therefore six pi electrons filled in ψ1, ψ2, ψ3 orbitals. Where ψ3 is HOMO (Highest Occupied Molecular Orbitals) and ψ4 is LUMO (Lowest Unoccupied Molecular Orbitals).
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| Molecular Orbitals of 2,4,6-octatriene under Thermal and photochemical condition- chemarticle.com: HOMO, LUMO energy state. |
Electrocyclic Reactions in Thermal condition
To form a C-C sigma bond on cyclization, the terminal orbitals of the conjugated system overlap with each other. In cases, C2 and C7 the carbon atoms are carrying the terminal orbitals of the conjugated system. Where two methyl groups are substituted in carbon no-2 and carbon no-7. The conjugated system is rotated through 90 degrees and overlaps two orbitals. The change in hybridization is sp2 to sp3.
This rotation is two possibility
- Both conjugated terminal orbitals are rotated in the same direction. This rotation is called conrotatory.
- Both conjugated terminal orbitals are rotated in the opposite direction. This type of rotation is called disrotatory.
For Conrotatory movement, conjugated two-terminal orbitals are lobes Opposite phase - That is the formation of antibonding situation.
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| Electrocyclic Reactions under Thermal condition- chemarticle.com: Reaction mechanism of electrocyclic reactions-conrotatory, disrotatory- they leads to bonding formation. |
For disrotatory movement, Conjugated to Terminal orbitals lobes are the same phase - This is the formation of a bonding situation. And leading the cyclization reaction. To substitute methyl groups are different stereochemistry. In which case two methyl groups are the same site that means product formation cis isomer.
Electrocyclic Reactions in photochemical condition
In photochemical conditions, the molecular orbitals are small differences between HOMO and LUMO. In case molecules are irritated by hv and promotion of one electron into the next higher energy level. Therefore, they irritated the one electron from ψ3 to ψ4. Now, ψ4 has become HOMO.
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| Electrocyclic Reactions under photochemical condition- chemarticle.com: Reaction mechanism of electrocyclic reactions-conrotatory, disrotatory- they leads to bonding formation. |
Now conrotatory movement, Conjugated two-terminal orbitals lobes overlap to each other and they are at same phase- The formation of bonding situation. And lead to the formation of trans isomers.
Another example of electrocyclic reaction
Electrocyclic Reactions example of four pi system
Consider cyclization of Hexa -2,4- diene in thermal condition form trans isomer ( 3,4-dimethylcyclobutene). And on the other hand cyclization of Hexa -2,4 -diene in photochemical condition form cis isomer.
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| Molecular orbitals of Hexa-2,4-diene in thermal condition and photochemical condition- Chem Article: Electrocyclic Reaction example (four pi system) |
In case we see that stereochemistry is exactly opposite Compare two 2,4,6-octatriene.
Four pi system under Thermal condition
For thermal conditions, HOMO for diene is ψ2. Due to four pi electrons systems / four pi-electron conjugation systems. Four are electrons filled up from the lowest energy orbitals. Each orbital has two electrons that mean the ψ1 has two electrons and ψ2 has two electrons.
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| Mechanism of electrocyclic reactions in Thermal condition (four pi electrons system)-Chem Article: conrotatory, disrotatory. leads to bonding formation. |
Similarly, for conrotatory movement, Two conjugated terminal orbital lobes are Overlap each other. They are in the same phase- That means the formation of a bonding situation. And lead to the formation of trans isomers.
Four pi system under Photochemical condition
Molecular orbitals are irritated hv. And one electron transfers into the next higher level. That means HOMO(ψ2) one electron is irritated by hv and one electron is transferred to the next higher level( ψ3). Now, ψ3 has become HOMO. Due to electron transfer from ψ2 to ψ3.
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| Mechanism of electrocyclic reactions in Photochemical condition (four pi electrons system)-Chem Article: conrotatory, disrotatory. |
For conrotatory movement, Two Conjugated terminal orbitals lobes are opposite to each other - The formation of an anti-bonding situation.
On the other hand the disrotatory movement, The two conjugated Terminal orbitals lobes are the same phase-The formation of bonding situation. And lead to the formation of isomer products.
We summarised the generalization of their stereochemistry. For different types of electron systems, different types of stereochemical products are formed.
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| Generalizations of Thermal/Photochemical condition-chemarticle.com |
Conclusion
Today we are briefly discussing electrocyclic reactions. This is the first part of the Pericyclic reaction. We see electrocyclic reaction mechanisms and their orbitals electronic transition for photochemical conditions and Stabilization of thermal products yield. Percentage of stereoselective product yield. Thank you for reading.
Frequently asked question
Q.1. What is the importance of electrocyclic reactions?
