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Organic Chemistry
Table of Contents
Unit 10 Overview: Organohalides
10.1 Names and Structures of Alkyl Halides
10.2 Preparing Alkyl Halides from Alkanes: Radical Halogenation
10.3 Preparing Alkyl Halides from Alkenes: Allylic Bromination
10.4 Stability of the Allyl Radical: Resonance Revisited
10.5 Preparing Alkyl Halides from Alcohols
10.6 Reactions of Alkyl Halides: Grignard Reagents
10.7 Organometallic Coupling Reactions
10.8 Oxidation and Reduction in Organic Chemistry

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10.4 Stability of the Allyl Radical: Resonance Revisited

Citation:

Allyl radicals are unique in organic chemistry due to their stability and reactivity. These conjugated systems consist of three sp2 hybridized carbons, allowing for electron delocalization across the π system. This resonance stabilization lowers the overall energy, making allyl radicals more stable than localized radicals.

Understanding allyl radicals is crucial for predicting reaction outcomes, especially in allylic bromination. The stability of radical intermediates influences product formation, with more stable radicals leading to major products. Factors like conjugation, hyperconjugation, and delocalization play key roles in determining radical stability and reactivity.

Stability and Reactivity of the Allyl Radical

Stability of allyl radical

  • Allyl radical is a conjugated π system consisting of three sp2 hybridized carbons
    • Two resonance structures can be drawn for the allyl radical
      • Unpaired electron can be delocalized across the entire π system (1,3-butadiene)
    • Overlap of the 2p orbitals allows for this delocalization
      • Leads to a more stable, lower energy configuration compared to a localized radical (methyl radical)
  • Resonance stabilization lowers the overall energy of the allyl radical
    • Estimated resonance stabilization energy of 14-16 kcal/mol compared to a non-resonance stabilized radical (ethyl radical)
  • Orbital overlap in the allyl radical occurs between the 2p orbitals of the three carbons
    • Forms a conjugated π system with the unpaired electron spread out across all three carbons
    • Results in a more stable, lower energy configuration than a localized radical (propyl radical)
  • Radical stability is enhanced through conjugation, which allows for electron delocalization

Electron delocalization in allyl radical

  • Molecular orbital theory can be used to describe the delocalization of the unpaired electron in the allyl radical
    • Three 2p orbitals from the sp2 hybridized carbons combine to form three molecular orbitals
      1. Bonding orbital (lowest energy)
      2. Non-bonding orbital (intermediate energy)
      3. Antibonding orbital (highest energy)
    • Unpaired electron resides in the non-bonding molecular orbital
      • Non-bonding orbital has a node at the central carbon and equal orbital contributions from the terminal carbons
  • Energy difference between the bonding and non-bonding orbitals is smaller than the energy difference between the non-bonding and antibonding orbitals
    • Contributes to the stability of the allyl radical by lowering the overall energy of the system
  • Delocalization of the unpaired electron across the three carbons results in a more stable radical compared to a localized radical (benzyl radical vs. phenyl radical)
    • Electron density is evenly distributed between the two terminal carbons, with a node at the central carbon

Products of allylic bromination

  • Allylic bromination reactions involve the addition of bromine to an alkene with allylic hydrogens (2-butene)
    • Reaction proceeds through an allylic radical intermediate
    • Bromine can add to either end of the allylic radical, leading to a mixture of products (3-bromo-1-butene and 1-bromo-2-butene)
  • For unsymmetrical alkenes, the more stable radical intermediate will be formed preferentially
    • Stability of the radical intermediate depends on the degree of substitution at each end of the allyl system
      • Tertiary radicals are more stable than secondary radicals, which are more stable than primary radicals
  • Major product of allylic bromination will be the one resulting from the more stable radical intermediate
    • For example, in the allylic bromination of 2-methyl-2-butene, the major product will be 1-bromo-2-methyl-2-butene
      • Results from the more stable tertiary allylic radical intermediate
  • Relative stabilities of the possible allylic radical intermediates can be used to predict the ratio of the products formed
    • A greater difference in stability between the possible radical intermediates will lead to a higher ratio of the major product to the minor product (3-methyl-1-butene vs. 2-methyl-1-butene)

Factors Affecting Radical Stability

  • Conjugation: Extended π systems increase radical stability through delocalization
  • Hyperconjugation: Interaction between σ bonds and adjacent p orbitals contributes to radical stability
  • Delocalization: Distribution of unpaired electron density over multiple atoms enhances radical stability

Key Terms to Review (30)

Delocalized: In the context of organic chemistry, delocalization refers to the spreading out of electrons across two or more adjacent atoms, usually seen in systems with conjugated double bonds or in ions with resonance structures. Delocalization increases the stability of a molecule by allowing electrons to be shared over several atoms rather than being confined to a single bond.
Molecular Orbital Theory: Molecular Orbital Theory is a model that describes the behavior of electrons in a molecule by considering the formation of molecular orbitals from the combination of atomic orbitals. This theory provides a more comprehensive understanding of chemical bonding compared to the earlier Valence Bond Theory.
2p Orbitals: 2p orbitals are one of the principal electron orbitals found in atoms, specifically in the second energy level (n=2). These orbitals play a crucial role in understanding the structure and bonding of organic molecules, particularly in the context of sp3 hybridization and the stability of allyl radicals.
Resonance: Resonance is a fundamental concept in organic chemistry that describes the ability of certain molecules to exist in multiple equivalent structures or resonance forms. This phenomenon arises from the delocalization of electrons within the molecule, leading to the stabilization of the overall structure and the distribution of electron density across multiple atoms.
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.
Conjugation: Conjugation refers to the overlap or sharing of atomic orbitals, resulting in the delocalization of electrons across a system of connected atoms. This concept is central to understanding resonance, the stability of certain molecules and ions, and the interpretation of various spectroscopic techniques in organic chemistry.
Delocalization: Delocalization refers to the dispersal or spreading out of electrons within a molecule, resulting in the stabilization of the overall structure. This concept is particularly important in understanding the behavior and properties of various organic compounds, including those involving resonance, aromatic systems, and conjugated pi systems.
Radical Stability: Radical stability refers to the relative stability of a radical species, which is an uncharged molecule or fragment that contains an unpaired electron. The stability of a radical influences its reactivity and the likelihood of it participating in various chemical reactions.
2-methyl-2-butene: 2-methyl-2-butene is an alkene with the molecular formula C₅H₁₀. It is a structural isomer of 2-butene, with a methyl group (CH₃) attached to the central carbon atom of the alkene. This term is important in the context of understanding the stability of alkenes and the stability of the allyl radical.
2-butene: 2-butene is an unsaturated hydrocarbon with the molecular formula C4H8. It is an alkene with a carbon-carbon double bond located at the second carbon position of the four-carbon chain. This structural feature of 2-butene is central to understanding its behavior and properties in the context of various organic chemistry topics.
1,3-Butadiene: 1,3-Butadiene is a simple conjugated diene, composed of four carbon atoms with two carbon-carbon double bonds separated by a single carbon-carbon bond. This structural feature gives 1,3-butadiene unique chemical properties and reactivity that are important in various organic chemistry topics.
Allylic Bromination: Allylic bromination is a chemical reaction where a bromine atom is added to the carbon atom adjacent to a carbon-carbon double bond in an alkene molecule. This process is used to prepare alkyl halides from alkenes.
Sp2 Hybridized: sp2 hybridization is a type of orbital hybridization where one s orbital and two p orbitals of an atom combine to form three equivalent sp2 hybrid orbitals. This hybridization pattern is commonly observed in organic chemistry and is crucial for understanding the stability of certain chemical structures and the interpretation of NMR spectra.
Methyl Radical: The methyl radical is a highly reactive organic species with the chemical formula CH3•. It is a free radical, meaning it has an unpaired electron, which makes it highly reactive and prone to participating in various chemical reactions.
Propyl Radical: The propyl radical is a reactive organic species consisting of a three-carbon alkyl group with an unpaired electron. It is an important intermediate in various chemical reactions and plays a crucial role in the context of understanding the stability of the allyl radical and the concept of resonance.
Non-Bonding Orbital: A non-bonding orbital is an atomic orbital that does not participate in the formation of a chemical bond. It is an orbital that is occupied by electrons but does not contribute to the overall bonding interactions between atoms.
1-bromo-2-methyl-2-butene: 1-bromo-2-methyl-2-butene is an organic compound with the molecular formula C$_{5}$H$_{9}$Br. It is a bromoalkene, containing a carbon-carbon double bond and a bromine substituent. This term is particularly relevant in the context of understanding the stability of the allyl radical, as discussed in Section 10.4 'Stability of the Allyl Radical: Resonance Revisited'.
3-methyl-1-butene: 3-methyl-1-butene is an organic compound with the molecular formula C5H10. It is an alkene with a methyl group (CH3) attached to the third carbon of the four-carbon chain. This structural feature gives rise to the term '3-methyl-1-butene' and influences its chemical properties and reactivity, particularly in the context of understanding the stability of the allyl radical.
Ethyl Radical: The ethyl radical is a reactive organic species consisting of a carbon atom with three hydrogen atoms and one unpaired electron. It is an important intermediate in various chemical reactions, particularly in the context of the stability of the allyl radical and the concept of resonance.
3-bromo-1-butene: 3-bromo-1-butene is an organic compound with a bromine atom attached to the third carbon of a four-carbon alkene chain. This term is relevant in the context of understanding the stability of the allyl radical and the electrophilic additions to conjugated dienes, which involve the formation of allylic carbocations.
2-methyl-1-butene: 2-methyl-1-butene is an organic compound with the chemical formula CH3CH2C(CH3)=CH2. It is an alkene, a type of hydrocarbon with a carbon-carbon double bond, and is classified as a branched-chain alkene due to the presence of a methyl group (CH3) substituent on the second carbon atom.
Benzyl Radical: The benzyl radical is a reactive species formed by the removal of a hydrogen atom from the methyl group of toluene or other benzyl-containing compounds. It is an important intermediate in various organic reactions and plays a crucial role in understanding the stability of allyl radicals.
Tertiary Radicals: Tertiary radicals are a type of organic radical species where the unpaired electron is located on a carbon atom that is bonded to three other carbon atoms. This structural feature confers enhanced stability to the radical compared to primary or secondary radicals, making tertiary radicals an important consideration in organic reaction mechanisms and the study of radical stability.
Primary Radicals: Primary radicals are organic free radicals in which the unpaired electron is located on a carbon atom that is bonded to only one other carbon atom. This structural feature confers a relatively high degree of stability to primary radicals compared to other types of radicals.
Allyl Radical: The allyl radical is a resonance-stabilized organic radical species with the formula CH2=CH-CH2•. It is an important intermediate in many organic reactions and its stability is a key concept in understanding reactivity patterns.
Bonding Orbital: A bonding orbital is a molecular orbital that results from the constructive interference of atomic orbitals, leading to an increased electron density between the bonded atoms. This stabilizes the molecule and facilitates chemical bonding.
1-bromo-2-butene: 1-bromo-2-butene is an organic compound with the formula CH3CH=CHCH2Br. It is a halogenated alkene that is important in the context of understanding the stability of the allyl radical and the electrophilic additions to conjugated dienes, specifically the formation of allylic carbocations.
Secondary Radicals: Secondary radicals are organic free radical species in which the unpaired electron is located on a carbon atom that is bonded to two other carbon atoms. These radicals are more stable than primary radicals due to increased electron delocalization and hyperconjugation effects.
Antibonding Orbital: An antibonding orbital is a type of molecular orbital that has higher energy than the bonding orbital and results in a decrease in bond strength between atoms. It is a crucial concept in understanding the stability of the allyl radical.
Phenyl Radical: The phenyl radical is a highly reactive organic species consisting of a benzene ring with a single unpaired electron. It is an important intermediate in many organic reactions and plays a crucial role in understanding the stability of allyl radicals.