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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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7.8 Orientation of Electrophilic Additions: Markovnikov’s Rule

Citation:

Electrophilic addition reactions are key in organic synthesis. They follow Markovnikov's rule, which predicts product formation based on carbocation stability. Understanding this rule helps chemists control reaction outcomes and design efficient synthetic routes.

Carbocation stability is crucial in determining reaction orientation. More stable carbocations form preferentially, leading to major products. This knowledge allows chemists to choose appropriate alkenes for specific product synthesis and anticipate potential rearrangements during reactions.

Electrophilic Addition Reactions and Markovnikov's Rule

Markovnikov's rule for alkene reactions

  • States that in an electrophilic addition reaction, the more stable carbocation intermediate will form preferentially
    • More highly substituted carbocation is more stable due to hyperconjugation and inductive effects (tertiary > secondary > primary > methyl)
    • Resonance stabilization can also contribute to carbocation stability
  • Predicts the major product of the reaction based on the more stable carbocation intermediate formed
    • Example: addition of HBr to 2-methylbut-2-ene yields 2-bromo-2-methylbutane as the major product (forms more stable tertiary carbocation)

Carbocation stability and reaction orientation

  • Stability of carbocation intermediates determines the regiochemistry (orientation) of electrophilic addition reactions
    • More stable carbocation intermediates are formed preferentially, leading to the major product
  • Carbocation stability increases with increasing substitution
    1. Tertiary carbocations: most stable, hyperconjugation and inductive effects from three alkyl groups
    2. Secondary carbocations: more stable than primary, hyperconjugation and inductive effects from two alkyl groups
    3. Primary carbocations: less stable, only one alkyl group providing hyperconjugation and inductive effects
    4. Methyl carbocations: least stable, no hyperconjugation, only inductive effects from hydrogen atoms
  • Major product of an electrophilic addition reaction results from the more stable carbocation intermediate
    • Example: addition of HCl to 2-methylpropene yields 2-chloro-2-methylpropane (tertiary carbocation) as the major product
  • Hammond's postulate relates the structure of the transition state to the stability of reaction intermediates

Alkene synthesis through electrophilic addition

  • To synthesize a specific product, choose an alkene that will form the more stable carbocation intermediate leading to the desired product
    • Consider the orientation of the electrophile and the stability of the resulting carbocation
  • For the addition of HX (X = Cl, Br, I), the halogen adds to the more substituted carbon of the alkene
    • To obtain a specific halogenated product, use an alkene that places the double bond in the necessary position
  • For the acid-catalyzed hydration of alkenes, the hydroxyl group (OH) adds to the more substituted carbon of the alkene
    • To obtain a specific alcohol product, use an alkene that places the double bond in the necessary position
  • Consider any rearrangements that may occur (hydride or alkyl shifts) which can lead to unexpected products
    • These rearrangements typically occur to form more stable carbocation intermediates

Exceptions and alternative mechanisms

  • Anti-Markovnikov addition can occur through free radical addition mechanisms
  • Free radical addition reactions can lead to different product distributions compared to electrophilic additions
  • Stereochemistry of the addition products may vary depending on the reaction mechanism and conditions

Key Terms to Review (26)

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.
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.
Regiospecific: In organic chemistry, regiospecificity refers to a chemical reaction that produces one predominant structural isomer when there could be several possible outcomes. This selectivity occurs because the reactant's structure directs the formation of the product to a specific region of a molecule.
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.
Resonance Stabilization: Resonance stabilization is a phenomenon where the delocalization of electrons in a molecule or ion leads to a more stable configuration compared to a single Lewis structure. This concept is crucial in understanding the behavior and properties of various organic compounds, including their acidity, basicity, reactivity, and stability.
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.
HCl: HCl, or hydrochloric acid, is a strong, corrosive acid that plays a crucial role in various chemical processes and reactions. It is composed of hydrogen (H) and chlorine (Cl) atoms, and its unique properties make it an important component in numerous organic chemistry topics, including acid-base chemistry, electrophilic additions, alcohol reactions, ester chemistry, and peptide sequencing.
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.
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.
2-chloro-2-methylpropane: 2-chloro-2-methylpropane, also known as tert-butyl chloride, is an organic compound with the chemical formula (CH3)3CCl. It is a polar molecule that is used in various organic reactions, particularly in the context of addition reactions and Markovnikov's rule.
HBr: HBr, or hydrobromic acid, is a strong acid composed of hydrogen (H) and bromine (Br). It is an important reagent in organic chemistry, commonly used in various reactions and processes, including the addition of HBr to alkenes, the preparation of alkyl halides from alcohols, and electrophilic additions to conjugated dienes.
2-methylpropene: 2-methylpropene, also known as isobutylene, is a branched-chain alkene with the molecular formula C₄H₈. It is an important organic compound that is relevant in the context of several topics in organic chemistry, including the addition of HBr to ethylene, the stability of alkenes, Markovnikov's rule, and the Hammond postulate.
Hammond's Postulate: Hammond's postulate is a fundamental concept in organic chemistry that describes the relationship between the structure and reactivity of reaction intermediates. It provides a framework for understanding the stability and reactivity of various intermediates that can form during the course of a chemical reaction.
Inductive Effects: Inductive effects refer to the ability of substituents or functional groups to influence the distribution of electron density within a molecule through space. This phenomenon can have significant implications on the stability, reactivity, and orientation of various organic reactions.
Anti-Markovnikov Addition: Anti-Markovnikov addition is a type of electrophilic addition reaction that occurs when a hydrogen-containing molecule, such as water or hydrogen halide, adds to an alkene or alkyne in a way that places the hydrogen on the less substituted carbon. This is in contrast to the Markovnikov addition, which places the hydrogen on the more substituted carbon.
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.
2-bromo-2-methylbutane: 2-bromo-2-methylbutane is an organic compound with the chemical formula CH3C(CH3)2CH2Br. It is a type of alkyl halide, where a bromine atom is attached to a carbon atom in a branched alkane structure.
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.
Free Radical Addition: Free radical addition is a type of organic reaction where a free radical, an unstable molecule with an unpaired electron, adds to a carbon-carbon double bond to form a new carbon-centered radical. This process is particularly important in the context of understanding the orientation of electrophilic additions, as described by Markovnikov's rule.
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.
Methyl Carbocation: A methyl carbocation is a positively charged carbon atom that has three single bonds and one empty orbital, resulting from the loss of a hydride ion (H-) from a methyl group. These reactive intermediates play a crucial role in the context of electrophilic addition reactions and Markovnikov's rule.
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.
2-methylbut-2-ene: 2-methylbut-2-ene is an organic compound with the molecular formula C₅H₁₀. It is an alkene with a methyl group (CH₃) attached to the second carbon of the four-carbon chain. This term is particularly relevant in the context of understanding Markovnikov's rule for the orientation of electrophilic additions and the hydration of alkenes through the process of oxymercuration.
Regiochemistry: Regiochemistry refers to the study of the regioselectivity or regional selectivity of a chemical reaction, which is the tendency of a reaction to occur at a specific site or region of a molecule. It is a crucial concept in understanding the orientation and outcome of various organic reactions, particularly in the context of electrophilic additions and the preparation of alcohols.
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.