Chem 51A 11/30/09 Ch. 8. Elimination Reactions. Introduction to E2 Reactions

UCI Media

1 Dec 200936:53

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TLDRIn this lecture, the focus is on elimination reactions, particularly E2 and E1 mechanisms. The instructor explains the role of bases like sodium ethoxide in E2 reactions and contrasts this with SN2 and SN1 reactions. Key concepts include the stereochemistry of elimination, the role of strong bases, and the effects of substitution patterns on reaction pathways. Examples like tert-butyl bromide with sodium ethoxide and bromo propane with different bases are used to illustrate these concepts. The lecture emphasizes how steric hindrance and base strength influence whether elimination or substitution predominates.

Takeaways

📘 The discussion covers several chapters, primarily focusing on organic compounds and their reactions.
🔄 The concept of curved arrows in reaction mechanisms is introduced.
🧪 Stereochemistry plays a significant role in understanding reaction mechanisms, including issues related to stereochemical centers.
🧬 The lecture delves into different mechanisms of elimination reactions, particularly E2 eliminations.
⚛️ An E2 elimination requires a specific anti-periplanar relationship for the reaction to occur.
🔍 The lecture examines the conditions under which E2 and SN2 reactions take place, including the role of strong bases like sodium ethoxide.
⚖️ E2 reactions involve a transition state where a base removes a proton while a leaving group departs, leading to the formation of a double bond.
🔬 Steric hindrance and the substitution pattern of alkyl halides affect whether a reaction will proceed via an E2 or SN2 mechanism.
🌡️ The energy diagram of E2 reactions shows that the reaction is exergonic, with reactants starting high in energy and products ending lower.
⚗️ Primary alkyl halides typically undergo SN2 reactions, but bulky bases or substrates can favor E2 eliminations.

Q & A

What is the primary topic of discussion in the transcript?
-The primary topic of discussion is elimination reactions, specifically E2 and E1 elimination mechanisms, and their stereochemical implications.
What is the significance of sodium ethoxide in the reactions discussed?
-Sodium ethoxide is a strong base that facilitates E2 elimination reactions by deprotonating the substrate, which leads to the formation of a double bond and the elimination of a leaving group such as bromide.
What is an E2 elimination reaction?
-An E2 elimination reaction is a bimolecular elimination process where a base removes a proton from a substrate simultaneously as the leaving group departs, resulting in the formation of a double bond.
How does the transition state in an E2 reaction differ from that in an SN2 reaction?
-In an E2 reaction, the transition state involves the base pulling off a proton while the leaving group departs, forming a double bond. In contrast, the SN2 transition state involves the nucleophile attacking the electrophile while the leaving group leaves, forming a trigonal bipyramidal transition state.
Why is the stereochemistry important in E2 elimination reactions?
-Stereochemistry is important in E2 elimination reactions because the anti-periplanar geometry is required for the reaction to proceed. This means that the hydrogen being removed and the leaving group must be on opposite sides of the molecule for effective overlap of orbitals.
What factors influence whether a reaction proceeds via E2 or SN2 mechanisms?
-Factors influencing whether a reaction proceeds via E2 or SN2 mechanisms include the strength and steric hindrance of the base, the structure of the substrate (primary, secondary, tertiary), and the solvent used in the reaction.
What is the role of steric hindrance in determining the pathway of a reaction?
-Steric hindrance can prevent nucleophiles from effectively attacking the electrophile in SN2 reactions, making E2 elimination more favorable, especially with bulky bases and substrates.
How does the use of different bases affect the outcome of elimination reactions?
-Using bulkier or stronger bases, such as potassium tert-butoxide, tends to favor E2 elimination over SN2 substitution due to increased steric hindrance and greater base strength, which enhances deprotonation.
What happens when tert-butyl bromide reacts with sodium ethoxide?
-When tert-butyl bromide reacts with sodium ethoxide, an E2 elimination reaction occurs, resulting in the formation of isobutylene, ethanol, and sodium bromide.
What experimental evidence supports the predominance of E2 elimination over SN2 in certain reactions?
-Experimental evidence, such as the product distribution observed in reactions with various substrates and bases, supports the predominance of E2 elimination over SN2 substitution in reactions with tertiary alkyl halides and strong, bulky bases.

Outlines

00:00

📚 Overview of Reaction Mechanisms

This paragraph introduces the day's lecture content, covering various organic chemistry topics such as curved arrows, stereochemistry, and elimination reactions. It mentions SN2 reactions and sets the stage for discussing E2 eliminations and their stereochemical requirements. The paragraph also previews upcoming lessons and the importance of understanding molecular structures and reaction conditions.

05:04

🔍 Products of Sodium Ethoxide Reaction

The paragraph describes the E2 elimination reaction using tert-butyl bromide and sodium ethoxide. It explains why a displacement reaction doesn't occur and emphasizes the strength of sodium ethoxide as a base. The products, including ethanol and sodium bromide, are noted. The paragraph clarifies the bimolecular nature of E2 eliminations and compares it to SN2 reactions.

10:06

🧪 Transition State in E2 Reactions

This section focuses on the transition state of E2 reactions. It details the interaction between ethoxide and tert-butyl bromide, including the formation of a double bond and the expulsion of bromide. The importance of the transition state, characterized by partial charges and bond changes, is highlighted. The paragraph also explains the transition state symbol and the progression from reactants to products.

15:08

📉 Energy Diagram for E2 Reactions

The paragraph introduces the energy diagram for E2 reactions, showing the downhill progression in energy from reactants to products. It emphasizes the importance of strong bases like sodium ethoxide in facilitating the reaction. The energy diagram is used to illustrate the reaction coordinate and the relationship between reactants, transition states, and products.

20:10

🔬 Substitution Patterns in E2 and SN2 Reactions

This section compares the behavior of different alkyl halides in E2 and SN2 reactions. It explains that tertiary alkyl halides tend to undergo E2 eliminations, especially with strong bases like alkoxides. Secondary alkyl halides also favor elimination over substitution under these conditions. Examples of specific reactions, such as those involving cyclohexane derivatives, are provided.

25:13

🧮 Reaction Kinetics and Steric Effects

The paragraph discusses the kinetic factors influencing E2 versus SN2 reactions, including the substitution pattern of the alkyl halide. It highlights how steric hindrance can shift the reaction pathway towards elimination. The example of bromo propane with sodium ethoxide demonstrates the balance between substitution and elimination products, influenced by steric factors and base strength.

30:22

⚗️ Impact of Steric Hindrance on Reaction Pathways

This section explores how increased steric hindrance favors E2 elimination over SN2 substitution. Using examples like isobutyl bromide and bulkier bases like potassium tert-butoxide, it illustrates how these factors push the reaction towards elimination. The role of steric hindrance in controlling reaction outcomes is emphasized, showing how chemists can manipulate conditions to favor desired products.

35:26

🔄 Generation of Alkoxides

The final paragraph explains the generation of alkoxides from alcohols and alkali metals, producing alkoxides and hydrogen gas. It discusses the choice of metals, such as sodium or potassium, based on the steric hindrance of the alcohol. Practical considerations for laboratory safety and reaction conditions are mentioned, emphasizing the controlled use of reactive metals.

Mindmap

Keywords

💡Organic Compounds

Organic compounds are chemical compounds that contain carbon and hydrogen, often including other elements like oxygen, nitrogen, sulfur, and phosphorus. They are the focus of the video script, which discusses their reactions and properties. In the context of the video, organic compounds are involved in various types of reactions, such as substitution and elimination reactions.

💡Curvature Arrows

Curvature arrows, or curved arrows, are used in organic chemistry to represent the movement of electron pairs in a chemical reaction. They are essential for visualizing the mechanism of reactions, as seen in the script when discussing the formation of double bonds and the displacement of bromine in an elimination reaction.

💡Stereochemical Issues

Stereochemistry is the study of the three-dimensional arrangement of atoms in a molecule. In the video, stereochemical issues refer to the challenges and considerations regarding the spatial orientation of atoms during chemical reactions, particularly in the context of elimination reactions.

💡SN2 Reactions

SN2 reactions, or substitution nucleophilic bimolecular reactions, are a type of chemical reaction in which a nucleophile replaces a leaving group in a single concerted step. The video script discusses these reactions as part of the broader conversation on reaction mechanisms, specifically in comparison to elimination reactions.

💡E2 Eliminations

E2 eliminations, or elimination bimolecular reactions, involve the removal of a proton and a leaving group from adjacent carbons in the presence of a strong base, forming a double bond. The script explains the E2 mechanism and its requirements, such as the anti-periplanar geometry necessary for the reaction to proceed.

💡Stereochemical Requirements

Stereochemical requirements refer to the specific spatial arrangements needed for a reaction to occur. In the script, this concept is discussed in relation to E2 eliminations, which require an anti-periplanar alignment of the base, the hydrogen, and the leaving group for the reaction to be successful.

💡Alkyl Halides

Alkyl halides are organic compounds in which an alkyl group is covalently bonded to a halogen. The video script uses tert-butyl bromide as an example of an alkyl halide that undergoes an E2 elimination reaction when treated with sodium ethoxide.

💡Nucleophiles

Nucleophiles are chemical species that donate an electron pair to an electrophile in a reaction. In the context of the video, sodium ethoxide acts as a nucleophile in the E2 elimination reaction, pulling off a proton and facilitating the formation of a double bond.

💡pKa

pKa is a measure of the acidity of a solution, specifically the pH at which half of the acid has been deprotonated. The script discusses the pKa values of water and ethanol to illustrate the relative basicity of ethoxide, which is crucial for its role as a nucleophile in the E2 reaction.

💡Steric Hindrance

Steric hindrance refers to the effect where the size or shape of a molecule interferes with its reactivity. The script explains how steric hindrance can influence the outcome of reactions, favoring E2 elimination over SN2 substitution in certain cases due to the difficulty of the nucleophile approaching the substrate.

💡Alkoxide

An alkoxide is an organic compound with a negatively charged oxygen atom that is bonded to an alkyl group. In the script, alkoxides like sodium ethoxide and potassium tert-butoxide are used as strong bases to facilitate elimination reactions, with their reactivity influenced by their pKa values and steric factors.

Highlights

Introduction to organic compounds, focusing on curved arrows and their implications in reactions.

Discussion on different stereochemical issues in organic chemistry.

Explanation of SN2 reactions with emphasis on reaction mechanisms and stereochemistry.

Introduction to elimination reactions, specifically E2 eliminations.

Detailed explanation of the anti-periplanar requirement in E2 eliminations.

Comparison of SN1 and SN2 reactions in terms of molecular involvement in the transition state.

Introduction of sodium ethoxide as a strong base for elimination reactions.

Discussion on the pKa values of ethoxide and their relevance to reaction mechanisms.

Step-by-step breakdown of the E2 elimination reaction mechanism using tert-butyl bromide and sodium ethoxide.

Illustration of the transition state in E2 reactions and the associated energy changes.

Explanation of how steric hindrance affects the preference for E2 or SN2 pathways.

Comparative analysis of the reactivity of different alkyl halides in E2 and SN2 reactions.

Impact of base strength and steric hindrance on the reaction pathway, using potassium tert-butoxide as an example.

Discussion on generating alkoxides using alcohols and alkali metals, highlighting practical considerations in the laboratory.

Final emphasis on how organic chemists can control reaction conditions to favor desired products.

Transcripts

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Related Tags

Elimination Reactions Organic Chemistry Stereochemistry E2 Mechanism E1 Mechanism Strong Bases Reaction Pathways Substitution Patterns Alkyl Halides Reaction Conditions