16.7 Electrocyclic Reactions | Organic Chemistry

Chad's Prep

23 Feb 202122:33

EducationalLearning

32 Likes 10 Comments

TLDRThe video script delves into the intricacies of electrocyclic reactions, a type of pericyclic reaction in organic chemistry. It discusses how these reactions, which involve the formation or breaking of a ring, can occur under both thermal and photochemical conditions. The lesson focuses on the importance of understanding stereochemistry and the role of the highest occupied molecular orbital (HOMO) in these reactions. It explains the concepts of disrotatory and conrotatory processes, and how they are determined by the symmetry of the HOMO. The script uses the example of hexatriene to illustrate these principles and emphasizes the significance of counting pi electrons involved in the transition state, rather than the total number in the reactant. The video also touches on the broader principles that apply to reactions involving 4n and 4n+2 pi electrons, providing a clear framework for predicting the stereochemical outcomes of electrocyclic reactions.

Takeaways

🔬 Electrocyclic reactions are a type of pericyclic reaction involving the ring closure or ring opening of molecules, which can be reversible and dependent on factors like ring size and strain.
🌡️ Under thermal conditions, the highest occupied molecular orbital (HOMO) is psi 3, which is symmetric, allowing disrotatory reactions to be allowed and conrotatory to be forbidden.
🌞 In photochemical reactions, an electron is promoted from psi 3 to psi 4, making psi 4 the HOMO, which is anti-symmetric, thus allowing conrotatory reactions and forbidding disrotatory ones.
🧬 The stereochemistry of electrocyclic reactions can be predicted by considering the symmetry of the molecular orbitals and the direction of pi electron movement (clockwise or counterclockwise).
↔️ Electrocyclic reactions are influenced by whether the movement involves an odd or even number of pi electrons, which determines whether disrotatory or conrotatory reactions are favored.
🟢 For a 4n + 2 pi electron system, disrotatory electrocyclic reactions are allowed under thermal conditions and conrotatory are forbidden, with the opposite being true under photochemical conditions.
🟥 For a 4n pi electron system, the allowed and forbidden reactions are flipped compared to the 4n + 2 system.
🔄 The concept of disrotatory and conrotatory can be visualized by considering the direction of the movement of the ends of the molecular orbitals during the reaction.
📈 The stereochemistry outcome can often be correlated with the known behavior of the Diels-Alder reaction, which is analogous for 4n + 2 systems.
⚖️ Meso compounds result in the same product regardless of whether the methyl groups are represented as wedges or dashes, while chiral compounds yield two enantiomeric versions.
📚 Counting the pi electrons involved in the transition state is crucial for predicting the stereochemical outcome, rather than the total number of pi electrons in the reactant.

Q & A

What is an electrocyclic reaction?
-An electrocyclic reaction is a type of pericyclic reaction that involves the ring closure or ring opening of a molecule with the movement of π electrons. These reactions are typically reversible and can be influenced by thermal or photochemical conditions.
What are the two main conditions under which electrocyclic reactions occur?
-Electrocyclic reactions can occur under two main conditions: thermal conditions and photochemical conditions. The choice of condition affects the stereochemistry and the allowed or forbidden nature of disrotatory and conrotatory processes.
What is the difference between disrotatory and conrotatory in the context of electrocyclic reactions?
-Disrotatory refers to a movement where one end of the molecular orbital rotates in a clockwise direction and the other end rotates counterclockwise, leading to a change in the stereochemistry of the product. Conrotatory, on the other hand, involves both ends of the molecular orbital rotating in the same direction, either both clockwise or both counterclockwise.
How does the symmetry of the highest occupied molecular orbital (HOMO) influence the electrocyclic reaction?
-The symmetry of the HOMO plays a crucial role in determining whether disrotatory or conrotatory motion is allowed. For a symmetric HOMO, disrotatory motion is allowed, while for an anti-symmetric HOMO, conrotatory motion is allowed.
What is the significance of the number of π electrons moving in the transition state?
-The number of π electrons moving in the transition state is critical for predicting the stereochemical outcome of the reaction. For a 4n+2 number of π electrons, disrotatory motion is allowed under thermal conditions, while conrotatory motion is forbidden. Conversely, for a 4n number of π electrons, conrotatory motion is allowed under thermal conditions, and disrotatory motion is forbidden.
What is a meso compound in the context of electrocyclic reactions?
-A meso compound is a molecule that has an internal plane of symmetry, making it superposable on its mirror image. In electrocyclic reactions, the formation of a meso compound means that whether the product has wedges or dashes due to the rotation of the molecular orbital does not matter, as both configurations are equivalent.
How does the stereochemistry of the reactants influence the product in electrocyclic reactions?
-The stereochemistry of the reactants, specifically the orientation of substituents like methyl groups, influences the stereochemistry of the product. The relative positions of these groups (whether they are 'out' or 'in') determine the stereochemistry of the newly formed sigma bond and the overall stereochemistry of the product.
What is the relationship between electrocyclic reactions and Diels-Alder reactions in terms of stereochemistry?
-Electrocyclic reactions and Diels-Alder reactions share a similar principle regarding stereochemistry. Both involve the movement of π electrons, and the stereochemical outcome can often be predicted by considering whether the substituents are 'out' or 'in' and applying the same logic as in Diels-Alder reactions.
Why is it recommended to perform a ring opening by reversing a ring closure?
-It is recommended to perform a ring opening by reversing a ring closure because it simplifies the process of predicting the stereochemistry of the products. By considering the reverse reaction, students can more easily determine the correct relationship between the 'out' and 'in' orientations of substituents to match the desired product under given conditions.
How does the promotion of an electron under photochemical conditions affect the electrocyclic reaction?
-The promotion of an electron from a lower to a higher molecular orbital under photochemical conditions changes the symmetry of the HOMO. This switch from an anti-symmetric to a symmetric HOMO or vice versa alters the allowedness of disrotatory and conrotatory motions, thus affecting the stereochemical outcome of the reaction.
What is the general rule for predicting the stereochemistry of products in electrocyclic reactions involving 4n π electrons?
-For electrocyclic reactions involving 4n π electrons, the general rule is that conrotatory motion is allowed and disrotatory motion is forbidden under thermal conditions. Under photochemical conditions, the allowed and forbidden motions are flipped, with disrotatory motion being allowed and conrotatory motion being forbidden.

Outlines

00:00

🔬 Introduction to Electrocyclic Reactions

This paragraph introduces electrocyclic reactions as the topic of the lesson, following cycloaddtion reactions. It explains that electrocyclic reactions involve either ring closure or ring opening and are reversible, with the equilibrium influenced by ring size and strain. The paragraph also touches on thermal and photochemical conditions, disrotatory and conrotatory processes, symmetry considerations, and the challenge of teaching and learning this topic. It emphasizes that electrocyclic reactions involve the highest occupied molecular orbital (HOMO) of a single reactant, contrasting with cycloadditions which involve two reactants.

05:01

🌡️ Stereochemistry and Thermal Conditions in Electrocyclic Reactions

The second paragraph delves into the specifics of stereochemistry in electrocyclic reactions under thermal conditions. It discusses how the symmetry of the HOMO (psi 3 in this case) affects the reaction, leading to disrotatory motion being allowed and conrotatory being forbidden. The paragraph uses the example of hexatriene with methyl groups to illustrate the formation of wedge and dash bonds and how these lead to cis products. It also draws parallels with Diels-Alder reactions, noting that for six pi electrons in motion, a similar stereochemical outcome is expected.

10:01

🌞 Photochemical Conditions and Electrocyclic Reactions

This paragraph explores the impact of photochemical conditions on electrocyclic reactions. It explains that promoting an electron from psi 3 to psi 4 changes the HOMO to psi 4, which is anti-symmetric. This switch results in conrotatory motion being allowed and disrotatory being forbidden under photochemical conditions. The paragraph also discusses the formation of new sigma bonds and how the methyl groups' positions are affected, leading to different stereochemical outcomes compared to thermal conditions. It emphasizes the importance of counting pi electrons involved in the transition state rather than those in the reactant.

15:02

🔁 Reversibility and Electron Counting in Electrocyclic Reactions

The fourth paragraph focuses on the reversibility of electrocyclic reactions and the importance of correctly counting pi electrons in the transition state. It advises students to approach ring-opening reactions by reversing the ring closure process and to be cautious of counting errors that can lead to incorrect predictions. The paragraph reinforces the generalization that for 4n plus 2 pi electrons, disrotatory motion is allowed under thermal conditions and forbidden under photochemical conditions, with the opposite being true for 4n pi electrons.

20:02

🔬 Further Generalizations and Predictions in Electrocyclic Reactions

The final paragraph provides further insights into predicting the products of electrocyclic reactions by understanding the symmetry of the molecular orbitals and the number of pi electrons involved. It explains that symmetric molecular orbitals require disrotatory motion for constructive overlap, while anti-symmetric orbitals require conrotatory motion. The paragraph also discusses how to predict the stereochemistry of products by comparing to known reactions like Diels-Alder and by examining the molecular orbitals directly. It concludes by encouraging students to use their knowledge of cycloadditions to aid in understanding electrocyclic reactions.

Mindmap

Keywords

💡Electrocyclic Reaction

An electrocyclic reaction is a type of pericyclic reaction involving the conversion of a conjugated pi system into a cyclic compound or vice versa. It is central to the video's theme as it is the main topic being discussed. The script describes how electrocyclic reactions can be either ring closures or ring openings and are subject to thermal or photochemical conditions.

💡Pericyclic Reactions

Pericyclic reactions are a class of organic reactions that involve the concerted rearrangement of a cyclic array of electrons. The video is focused on one specific type of pericyclic reaction, the electrocyclic reaction. The term sets the context for the broader category of reactions being discussed.

💡Thermal Conditions

Thermal conditions refer to reactions that occur with the input of heat. In the context of the video, it is one of the conditions under which electrocyclic reactions can take place. The script explains that under thermal conditions, certain stereochemical outcomes (like disrotatory closures) are favored.

💡Photochemical Conditions

Photochemical conditions involve the absorption of light to facilitate a chemical reaction. The video discusses how these conditions affect electrocyclic reactions differently from thermal conditions. For instance, it is mentioned that under photochemical conditions, conrotatory closures are allowed, contrasting with thermal conditions.

💡Disrotatory

Disrotatory refers to a type of electrocyclic reaction where the p-orbitals on either side of the forming bond rotate in opposite directions. The video explains that disrotatory closures are allowed under thermal conditions when the highest occupied molecular orbital (HOMO) is symmetric, as with psi3.

💡Conrotatory

Conrotatory is the opposite of disrotatory, where the p-orbitals rotate in the same direction. The script details that conrotatory closures are forbidden under thermal conditions with a symmetric HOMO but are allowed under photochemical conditions with an anti-symmetric HOMO, such as psi4.

💡Stereochemistry

Stereochemistry is the aspect of chemistry that deals with the three-dimensional arrangement of atoms in a molecule. The video emphasizes the importance of stereochemistry in predicting the outcomes of electrocyclic reactions, particularly in determining whether products will be cis or trans, and whether they will result in meso or chiral compounds.

💡Meso Compound

A meso compound is a type of molecule that appears to be chiral but is not, due to a plane of symmetry. The video uses the term to illustrate that certain electrocyclic reactions can lead to meso compounds, which do not exhibit optical activity despite having multiple stereocenters.

💡Chiral Compound

Chiral compounds are molecules that do not have an internal plane of symmetry and cannot be superimposed on their mirror image. The script discusses how electrocyclic reactions under certain conditions can lead to the formation of chiral compounds, resulting in two enantiomers.

💡Molecular Orbital

Molecular orbitals are mathematical descriptions of the spatial regions where electrons are likely to be found in a molecule. The video uses the concept of molecular orbitals to predict the stereochemical outcomes of electrocyclic reactions, particularly focusing on the symmetry of the highest occupied molecular orbital (HOMO).

💡Sigma Bond

A sigma bond is the simplest type of covalent bond in which the atomic orbitals combine to produce a region of high electron density along the line connecting the nuclei of the atoms. The video mentions sigma bonds in the context of their formation during electrocyclic reactions, which is crucial for the ring closure process.

Highlights

Electrocyclic reactions are reversible and can be either ring closure or ring opening.

The size of the ring and ring strain determine the favored product between the open chain or the ring-closed form.

Cyclic movement of pi electrons forms a ring in a six-membered ring example.

Stereochemistry is affected by the presence of methyl groups in the reaction.

Thermal and photochemical conditions affect the stereochemistry outcome.

Electrocyclic reactions involve the highest occupied molecular orbital (HOMO) of the reactant.

Symmetry of molecular orbitals (symmetric or anti-symmetric) is crucial for predicting product stereochemistry.

Disrotatory and conrotatory refer to the direction of pi electron rotation and are determined by the symmetry of the HOMO.

Under thermal conditions, disrotatory is allowed and conrotatory is forbidden for reactions involving 4n+2 pi electrons.

Conrotatory is allowed and disrotatory is forbidden under photochemical conditions for the same type of reactions.

For electrocyclic reactions, the stereochemistry can be predicted by considering the movement of pi electrons and their interaction with substituents.

Meso compounds do not have a plane of symmetry, resulting in the same compound regardless of the stereochemistry of the substituents.

Chiral compounds formed in electrocyclic reactions can yield two enantiomeric versions.

Counting pi electrons in the transition state is essential for predicting the correct stereochemistry.

Ring opening reactions should be approached by reversing the ring closure to simplify electron counting.

General principles for 4n and 4n+2 pi electrons in electrocyclic reactions can be applied to predict thermal and photochemical behavior.

Comparing electrocyclic reactions to Diels-Alder reactions can provide a conceptual framework for predicting stereochemistry.

Molecular orbital diagrams can be used to visualize and predict the stereochemical outcome of electrocyclic reactions.

Transcripts

Browse More Related Video

16.6 Cycloaddition Reactions | Organic Chemistry

16.2c Pi Molecular Orbitals of 1,3,5-Hexatriene | Organic Chemistry

1.4 Molecular Orbital Theory | Organic Chemistry

16.5 Diels-Alder Reactions | Organic Chemistry

Chem 125. Advanced Organic Chemistry. 13. Cycloadditions and Sigmatropic Rearrangments.

16.1 Conjugated Systems and Heats of Hydrogenation | Organic Chemistry

Related Tags

Chemistry Electrocyclic Reactions Pericyclic Reactions Thermal Conditions Photochemical Conditions Stereochemistry Orbital Symmetry Organic Chemistry Ring Closure Educational