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Organic Chemistry
Table of Contents
Unit 7 Overview: Alkenes
7.1 Industrial Preparation and Use of Alkenes
7.2 Calculating the Degree of Unsaturation
7.3 Naming Alkenes
7.4 Cis–Trans Isomerism in Alkenes
7.5 Alkene Stereochemistry and the E,Z Designation
7.6 Stability of Alkenes
7.7 Electrophilic Addition Reactions of Alkenes
7.8 Orientation of Electrophilic Additions: Markovnikov’s Rule
7.9 Carbocation Structure and Stability
7.10 The Hammond Postulate
7.11 Evidence for the Mechanism of Electrophilic Additions: Carbocation Rearrangements

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7.11 Evidence for the Mechanism of Electrophilic Additions: Carbocation Rearrangements

Citation:

Electrophilic additions to alkenes involve a two-step process with a carbocation intermediate. This mechanism allows for rearrangements, providing key evidence for its stepwise nature. Understanding these reactions is crucial for predicting products and interpreting experimental results.

Carbocation stability plays a vital role in determining reaction outcomes. Rearrangements like hydride and alkyl shifts can occur, leading to more stable intermediates. Recognizing these possibilities is essential for accurately predicting the final products of electrophilic addition reactions.

Evidence for the Mechanism of Electrophilic Additions

Evidence for stepwise addition mechanism

  • Electrophilic addition reactions add an electrophile to an alkene forming a new carbon-carbon bond
  • Reaction proceeds through a carbocation intermediate formed in the first step of the mechanism
    • Electrophile adds to one carbon of the alkene forming a carbocation
    • Carbocation is then attacked by a nucleophile in the second step forming the final product
  • Carbocation rearrangements provide evidence for the stepwise nature of the mechanism
    • Rearrangements would not be possible if the reaction occurred in a single step
    • Observation of rearranged products suggests a carbocation intermediate is formed which can undergo rearrangement before the final product is formed

Types of carbocation rearrangements

  • Hydride shifts migrate a hydrogen atom with its bonding electron pair from an adjacent carbon to the carbocation center
    • Results in the formation of a more stable carbocation
    • Driven by the formation of a more substituted and therefore more stable carbocation (tertiary > secondary > primary)
  • Alkyl group shifts migrate an alkyl group such as methyl or ethyl from an adjacent carbon to the carbocation center
    • Also results in the formation of a more stable carbocation
    • Driven by the formation of a more substituted and therefore more stable carbocation (tertiary > secondary > primary)
  • Both types of rearrangements occur rapidly and reversibly with the more stable carbocation being favored at equilibrium

Products of rearranged electrophilic additions

  • When predicting products of electrophilic addition reactions consider the possibility of carbocation rearrangements
  • If a less stable carbocation is initially formed it may undergo rearrangement to form a more stable carbocation before the final product is formed
    • Secondary carbocation may undergo a hydride shift to form a more stable tertiary carbocation
  • Final product determined by the structure of the most stable carbocation intermediate
    • Nucleophile will attack the most stable carbocation leading to the formation of the final product
  • To predict products consider the following steps:
    1. Determine the structure of the initially formed carbocation
    2. Consider possible rearrangements that could lead to a more stable carbocation
    3. Determine the structure of the most stable carbocation intermediate
    4. Predict the final product based on the attack of the nucleophile on the most stable carbocation

Factors Influencing Carbocation Stability and Reaction Outcome

  • Carbocation stability is a key factor in determining the reaction pathway and final products
    • More stable carbocations are favored in rearrangements and product formation
  • Hyperconjugation contributes to carbocation stability through electron donation from adjacent C-H bonds
  • Markovnikov's rule predicts the major product based on the formation of the most stable carbocation intermediate
  • Stereochemistry of the product is influenced by the carbocation intermediate and the approach of the nucleophile
  • Reaction kinetics can provide insight into the stepwise nature of the mechanism and the relative stability of intermediates

Key Terms to Review (20)

Methylene group: A methylene group is a functional group consisting of two hydrogen atoms bound to a carbon atom, which is then connected to other parts of a molecule. In the context of alkenes, it often refers to the CH2 unit that can be part of the alkene's structure.
Electrophilic addition reaction: An electrophilic addition reaction is a chemical process in which an electrophile reacts with a nucleophile, typically an alkene or alkyne, forming a new sigma bond by adding across the double or triple bond. This reaction is key in organic synthesis, resulting in the addition of atoms or groups to the carbon atoms involved in the multiple bond.
Sigmatropic rearrangement: Sigmatropic rearrangement is a type of pericyclic reaction in organic chemistry where a sigma bond (σ-bond) adjacent to a pi system (π-system) migrates from one position to another, simultaneously affecting the positions of π-electrons. This migration is guided by specific orbital symmetries that allow the reaction to proceed without the need for external reagents.
Anti stereochemistry: Anti stereochemistry describes the spatial arrangement in a chemical reaction where two substituents are positioned on opposite sides of a double bond or ring structure after the reaction. It is particularly relevant in the halogenation of alkenes, resulting in products where the added atoms are located across from each other.
Stereochemistry: Stereochemistry is the study of the three-dimensional arrangement of atoms in molecules and how this arrangement affects the chemical and physical properties of the substance. It examines the spatial orientation of atoms and their relationship to one another, which is crucial in understanding many organic chemistry concepts.
Hyperconjugation: Hyperconjugation is a type of conjugation in organic chemistry where the sigma bonds of alkyl groups (such as methyl or ethyl) interact with adjacent pi bonds, leading to increased stability of the molecule. This stabilizing effect is particularly important in understanding the stability of carbocations and the orientation of electrophilic additions.
Carbocation: A carbocation is a positively charged carbon atom that is part of an organic molecule. These reactive intermediates play a crucial role in various organic reactions, including electrophilic additions, nucleophilic substitutions, and elimination reactions.
Ethyl: Ethyl is a two-carbon alkyl group with the chemical formula -CH2CH3. It is a common substituent group in organic chemistry and plays a crucial role in understanding various topics, including alkanes, alkyl groups, naming conventions, and carbocation rearrangements.
Methyl: The methyl group is a simple alkyl group consisting of a single carbon atom bonded to three hydrogen atoms. It is denoted by the formula -CH3 and is the most basic and common alkyl group found in organic chemistry. The methyl group plays a crucial role in various organic reactions and structural features across several key topics in this course.
Reaction Kinetics: Reaction kinetics is the study of the rates and mechanisms of chemical reactions. It examines the factors that influence the speed and efficiency of a reaction, such as temperature, pressure, and the presence of catalysts. This concept is crucial in understanding organic reactions, as the rate and pathway of a reaction can have a significant impact on the products formed and the overall efficiency of the process.
Nucleophile: A nucleophile is a species that donates a pair of electrons to form a covalent bond with another atom or molecule. Nucleophiles are central to understanding many organic reactions, including polar reactions, electrophilic addition reactions, and nucleophilic substitution reactions.
Electrophilic Addition: Electrophilic addition is a type of organic reaction where an electrophile, a species that is attracted to electrons, adds to the carbon-carbon double bond of an alkene. This results in the formation of a new carbon-carbon single bond and the incorporation of the electrophile into the molecule.
Markovnikov's Rule: Markovnikov's rule is a principle in organic chemistry that describes the orientation of addition reactions involving unsaturated compounds, such as alkenes. It states that in the addition of a hydrogen halide (HX) to an alkene, the hydrogen atom of the HX bond attaches to the carbon atom of the alkene that can best stabilize the resulting carbocation intermediate.
Tertiary Carbocation: A tertiary carbocation is a positively charged carbon atom that has three alkyl groups attached to it, making it a highly stable intermediate in organic reactions. This term is crucial in understanding various topics related to electrophilic additions, carbocation stability, and reaction mechanisms.
Rearrangement: Rearrangement refers to the process in organic chemistry where the structure of a molecule is reorganized, often through the formation of a carbocation intermediate, resulting in the generation of a new product with a different arrangement of atoms compared to the original reactant.
Primary Carbocation: A primary carbocation is a positively charged carbon atom that has three single-bonded substituents and one hydrogen atom attached to it. These carbocations are the least stable type of carbocation due to the limited ability to delocalize the positive charge.
Carbocation Stability: Carbocations are positively charged carbon atoms that are formed as intermediates in many organic reactions. The stability of a carbocation is a crucial factor in determining the mechanism and outcome of these reactions. Carbocation stability is a key concept that connects various topics in organic chemistry, including electrophilic additions, the SN1 reaction, and the reactivity of conjugated dienes.
Secondary Carbocation: A secondary carbocation is a positively charged carbon atom that has two alkyl groups attached to it. These types of carbocations are more stable than primary carbocations due to the ability of the alkyl groups to stabilize the positive charge through hyperconjugation.
Alkyl Shift: An alkyl shift is a type of carbocation rearrangement that occurs during electrophilic addition reactions. It involves the migration of an alkyl group (such as methyl, ethyl, or isopropyl) from one carbon atom to an adjacent carbocation center, stabilizing the intermediate and altering the product formation.
Hydride Shift: A hydride shift is a type of rearrangement reaction in organic chemistry where a hydride ion (H-) moves from one carbon atom to an adjacent carbon atom within a molecule. This process is often observed in the context of carbocation intermediates and plays a crucial role in various organic reactions.