16.8 Oxidation of Aromatic Compounds
3 min read•may 7, 2024
of is a crucial process in organic chemistry. It focuses on transforming the alkyl side chain while preserving the aromatic ring, thanks to its stability. This reaction produces derivatives, with primary being the most reactive.
Side-chain bromination and the stability of are key concepts in this topic. (NBS) selectively brominates the side chain through a radical mechanism. Benzyl radicals are uniquely stable due to resonance, making them important in various reactions.
Oxidation of Alkylbenzenes
Oxidation of aromatic side chains
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- Alkylbenzenes undergo oxidation of the alkyl side chain while leaving the aromatic ring intact due to the stability of the aromatic system
- Common include () and () which are strong enough to oxidize the side chain but not the ring
- The oxidation occurs through a series of steps, ultimately leading to the formation of a benzoic acid derivative
- The alkyl group is oxidized first to a primary alcohol, then further to an aldehyde, and finally to a carboxylic acid ( to benzoic acid)
- The number of carbons in the alkyl chain remains the same throughout the oxidation process as the oxidation only affects the functional group at the end of the chain
- The reaction is more facile for primary benzylic carbons compared to secondary or tertiary benzylic carbons
- This is due to the increased stability of the intermediate benzylic formed during the oxidation process as it can be stabilized by resonance with the aromatic ring
- Primary benzylic carbons form more stable carbocations than secondary or tertiary benzylic carbons ( vs )
- The is particularly susceptible to oxidation due to its proximity to the aromatic ring
Side-chain bromination mechanism
- N-Bromosuccinimide (NBS) is used as a selective brominating agent for the side-chain bromination of alkylbenzenes
- The reaction is initiated by a small amount of , such as or light, which homolytically cleave the N-Br bond in NBS
- The mechanism involves the formation of a (Br·) from the homolytic cleavage of the N-Br bond in NBS
- The bromine radical abstracts a hydrogen atom from the benzylic position of the alkylbenzene, forming a stable
- The benzylic radical then reacts with another molecule of NBS, leading to the formation of the benzylic bromide product and a
- The succinimide radical can then abstract a hydrogen atom from another alkylbenzene molecule, regenerating the bromine radical and continuing the chain reaction
- The reaction is highly selective for the benzylic position due to the increased stability of the benzylic radical intermediate which can be stabilized by resonance with the aromatic ring
Stability of benzyl radicals
- Benzyl radicals are more stable than most other carbon radicals due to
- The unpaired electron can be delocalized into the aromatic ring, resulting in multiple that spread out the electron density
- This delocalization lowers the overall energy of the radical making it more stable (benzyl radical vs methyl radical)
- In contrast, primary, secondary, and tertiary alkyl radicals do not benefit from resonance stabilization
- The stability of these radicals depends on and inductive effects from nearby alkyl groups
- The stability order for alkyl radicals is tertiary > secondary > primary, based on the number of electron-donating alkyl groups attached to the radical center (tert-butyl > isopropyl > ethyl)
- are also stabilized by resonance, as the unpaired electron can be delocalized across the adjacent double bond
- However, the degree of stabilization is less than that of benzyl radicals, as the allyl system has fewer resonance structures (3 vs 5)
- The increased stability of benzyl radicals makes them more easily formed and longer-lived compared to other carbon radicals
- This stability also contributes to the selectivity of reactions such as side-chain bromination with NBS which preferentially brominates at the benzylic position
Free Radical Reactions in Aromatic Oxidation
- play a significant role in the oxidation of
- These reactions often involve oxidizing agents that generate reactive oxygen species
- The process typically begins with the formation of a radical at the benzylic position
- Resonance structures of the resulting radical contribute to its stability and reactivity
Key Terms to Review (25)
Alkylbenzenes: Alkylbenzenes are a class of aromatic organic compounds consisting of a benzene ring with one or more alkyl substituents attached. They are important intermediates in the production of various chemicals and have widespread applications in the chemical industry.
Allyl Radicals: An allyl radical is a resonance-stabilized organic free radical species with a structure that includes a carbon-carbon double bond adjacent to a carbon-centered radical. These radicals are important intermediates in many organic reactions, particularly those involving the oxidation of aromatic compounds.
Aromatic Compounds: Aromatic compounds are a class of organic compounds characterized by the presence of one or more benzene rings in their structure. These compounds exhibit unique chemical and physical properties that set them apart from other organic molecules.
Benzoic Acid: Benzoic acid is an aromatic carboxylic acid with the chemical formula C6H5COOH. It is a white crystalline solid that is widely used as a food preservative and in the production of various organic compounds.
Benzoyl Peroxide: Benzoyl peroxide is an organic compound that is widely used as an oxidizing agent, a bleaching agent, and a treatment for acne. It is a white, crystalline solid that decomposes when exposed to heat or light, producing oxygen and other byproducts.
Benzyl Radicals: Benzyl radicals are reactive species derived from toluene or other alkylbenzenes by the homolytic cleavage of the benzylic C-H bond. They are important intermediates in various organic reactions, particularly in the oxidation of aromatic compounds.
Benzylic Carbons: Benzylic carbons are the carbon atoms that are directly attached to the aromatic ring in benzene-containing compounds. These carbons have a unique reactivity due to the stabilizing effect of the adjacent aromatic system, making them susceptible to various oxidation and substitution reactions.
Benzylic Position: The benzylic position refers to the carbon atom adjacent to an aromatic ring, particularly a benzene ring. This position is of great importance in the context of oxidation reactions involving aromatic compounds.
Benzylic Radical: A benzylic radical is a reactive intermediate species that forms when a hydrogen atom is removed from the carbon atom adjacent to an aromatic ring. This carbon is known as the benzylic carbon, and the resulting radical is stabilized by the resonance effects of the nearby aromatic system.
Bromine Radical: A bromine radical is a highly reactive chemical species containing a single, unpaired electron. Bromine radicals play a crucial role in the context of preparing alkyl halides from alkenes through allylic bromination, as well as in the oxidation of aromatic compounds.
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.
Chromic Acid: Chromic acid, also known as chromium(VI) oxide, is a strong oxidizing agent commonly used in organic chemistry. It is a bright orange-red crystalline compound with the chemical formula CrO3, which can be used to oxidize various organic compounds.
Cumene: Cumene, also known as isopropylbenzene, is an aromatic hydrocarbon compound with the chemical formula C₆H₅CH(CH₃)₂. It is an important industrial chemical used in the production of various compounds and materials.
Ethylbenzene: Ethylbenzene is an aromatic hydrocarbon compound with the chemical formula C6H5CH2CH3. It is a colorless, flammable liquid with a distinctive petrol-like odor. Ethylbenzene is an important industrial chemical with applications in the production of other organic compounds and the synthesis of various materials.
Free Radical Reactions: Free radical reactions are a type of chemical reaction involving highly reactive, unpaired electron-containing species called free radicals. These reactions are particularly important in the context of the oxidation of aromatic compounds, where free radicals can initiate and propagate chain reactions that lead to the breakdown or modification of aromatic structures.
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.
N-bromosuccinimide: N-bromosuccinimide (NBS) is a versatile organic reagent used in various chemical reactions, particularly in the context of alkene functionalization, aromatic substitution, and oxidation of aromatic compounds. It serves as a source of electrophilic bromine and is commonly employed in reactions such as halohydrin formation, allylic bromination, and aromatic substitution.
Oxidation: Oxidation is a fundamental chemical process in which a substance loses electrons, resulting in an increase in its oxidation state. This term is central to understanding various reactions and transformations in organic chemistry, from the hydration of alkenes to the oxidation of alcohols and aldehydes.
Oxidizing Agents: Oxidizing agents, also known as oxidants, are substances that have the ability to oxidize other substances by removing electrons from them. They play a crucial role in various organic chemistry reactions, including the oxidative cleavage of alkynes, the oxidation of aromatic compounds, the oxidation of alcohols, the preparation of aldehydes and ketones, and the preparation of carboxylic acids.
Potassium Permanganate: Potassium permanganate is a strong oxidizing agent with the chemical formula KMnO4. It is a dark purple crystalline solid that is widely used in various chemical reactions and processes due to its powerful oxidizing properties.
Radical Initiators: Radical initiators are chemical species that can initiate radical chain reactions, which are essential for the oxidation of aromatic compounds. These reactive species generate free radicals that drive the oxidation process, leading to the formation of new products.
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.
Resonance Structures: Resonance structures are a set of contributing structures that describe the delocalization of electrons in a molecule. They represent the different ways in which the atoms in a molecule can be bonded to satisfy the octet rule and create the most stable arrangement of electrons.
Succinimide Radical: The succinimide radical is a reactive species derived from the cyclic imide compound succinimide, which is commonly encountered in the context of aromatic compound oxidation reactions. This radical plays a crucial role in various chemical transformations and is an important intermediate in organic synthesis.
Toluene: Toluene is an aromatic hydrocarbon compound with the chemical formula C6H5CH3. It is a colorless, flammable liquid with a distinctive sweet odor, and is widely used as a solvent and in the production of various chemical compounds.