16.9 Reduction of Aromatic Compounds
2 min read•may 7, 2024
Aromatic compounds can be reduced to through . This process uses hydrogen gas and metal catalysts under high pressure and temperature. It's tougher than reducing alkenes due to the stability of aromatic rings.
can be converted to through a two-step process. First, the ketone is reduced to an alcohol. Then, it's dehydrated and hydrogenated to form the alkylbenzene. This showcases the versatility of aromatic reduction methods.
Reduction of Aromatic Compounds
Process of catalytic hydrogenation
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- Reduces aromatic rings to cyclohexanes using hydrogen gas () and a metal catalyst (platinum (), palladium (), or nickel ())
- Carried out under high pressure and elevated temperature conditions
- Mechanism involves:
- adsorbs onto the metal catalyst surface weakening the bond
- Aromatic ring adsorbs onto the catalyst surface
- Hydrogen atoms transfer to the aromatic ring breaking the aromaticity
- Process continues until the aromatic ring is fully reduced to a cyclohexane
- Requires harsher conditions compared to alkenes with higher temperatures, pressures, and longer reaction times
Conversion of aryl alkyl ketones
- Reduces to alkylbenzenes using a two-step process:
- Ketone is reduced to a secondary alcohol using a reducing agent ( () or ())
- Secondary alcohol undergoes dehydration and hydrogenation to form the alkylbenzene
- Mechanism involves:
- Reducing agent ( or ) delivers a hydride () to the carbonyl carbon forming an intermediate
- Alkoxide is protonated by the solvent (usually an alcohol) to form the secondary alcohol
- Dehydration of the secondary alcohol forms a -like intermediate (an alkene with an aromatic ring)
- Catalytic hydrogenation of the alkene yields the final alkylbenzene product
- In some cases, can occur, directly converting the ketone to an alkylbenzene without isolating the alcohol intermediate
Reactivity of aromatics vs alkenes
- Alkene double bonds are more reactive than aromatic rings in catalytic hydrogenation
- Alkenes readily undergo hydrogenation under milder conditions (lower temperature and pressure)
- Reaction rates for alkene hydrogenation are generally faster than those for aromatic rings
- Aromatic rings have lower reactivity due to:
- makes them less prone to reaction
- Delocalized electrons are less available for interaction with the catalyst surface
- Breaking the aromaticity requires more energy than breaking a simple bond in an alkene
- Consequences of the reactivity difference:
- Allows for selective hydrogenation of alkenes in the presence of aromatic rings
- Reducing aromatic rings requires harsher conditions and longer reaction times
Alternative Reduction Methods
- : Uses alkali metals in liquid ammonia to partially reduce aromatic rings to 1,4-cyclohexadienes
- : Employs diimide (NH=NH) as a reducing agent for selective reduction of alkenes and some aromatic compounds
- : Utilizes hydrogen donors like cyclohexene or formic acid instead of H2 gas for reduction of aromatic compounds
Key Terms to Review (13)
Alkoxide: An alkoxide is a functional group consisting of an alkyl group (R-) bonded to an oxygen atom (O-). Alkoxides are important intermediates in many organic chemistry reactions, including Grignard reactions, elimination reactions, and carbonyl condensation reactions.
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.
Aryl Alkyl Ketones: Aryl alkyl ketones are a class of organic compounds that contain a carbonyl group (C=O) bonded to an aromatic ring (aryl group) and an alkyl group. These compounds are important intermediates in various organic reactions and have diverse applications in the chemical industry.
Birch Reduction: The Birch reduction is a chemical reaction that involves the reduction of aromatic compounds to alkenes or alkynes using sodium or lithium metal in liquid ammonia. It is a useful method for the selective reduction of aromatic rings while preserving other functional groups.
Catalytic Hydrogenation: Catalytic hydrogenation is a chemical process where hydrogen gas is used to reduce unsaturated organic compounds, such as alkenes, aromatic rings, and carbonyl groups, in the presence of a metal catalyst. This reaction allows for the selective and controlled addition of hydrogen to these functional groups, leading to the formation of new, more saturated compounds.
Cyclohexanes: Cyclohexanes are a class of saturated, alicyclic hydrocarbons with a six-membered carbon ring. They are closely related to the reduction of aromatic compounds and the intramolecular olefin metathesis reaction, two important topics in organic chemistry.
Diimide Reduction: Diimide reduction is a method of reducing aromatic compounds by using diimide, a reactive intermediate species generated in situ, to selectively hydrogenate the aromatic ring. This process allows for the controlled and chemoselective reduction of aromatic systems without affecting other functional groups.
Hydrogenolysis: Hydrogenolysis is a chemical reaction where a carbon-heteroatom bond, such as a carbon-oxygen or carbon-nitrogen bond, is cleaved by the addition of hydrogen. This process is commonly used in organic chemistry for the selective removal of protecting groups and the reduction of various functional groups.
Lithium Aluminum Hydride: Lithium aluminum hydride (LiAlH4) is a powerful reducing agent used in organic chemistry for the selective reduction of various functional groups. It is a white, crystalline solid that reacts violently with water and other protic solvents, making it an important reagent in many synthetic transformations.
Resonance Stability: Resonance stability refers to the increased stability of certain molecules or ions due to the delocalization of electrons across multiple atoms or bonds. This phenomenon arises from the ability of a molecule to be represented by multiple valid Lewis structures, which contribute to the overall stability of the system.
Sodium Borohydride: Sodium borohydride is a powerful reducing agent commonly used in organic chemistry reactions to reduce carbonyl compounds to alcohols. It is a versatile reagent that finds applications in various topics, including the reduction of aromatic compounds, the preparation of alcohols, the synthesis of ethers, and the nucleophilic addition of hydride to carbonyl groups.
Styrene: Styrene is an aromatic hydrocarbon compound with the chemical formula C6H5CH=CH2. It is a colorless liquid with a sweet odor and is widely used in the production of polystyrene and other polymers. Styrene is particularly relevant in the context of the reduction of aromatic compounds and chain-growth polymerization.
Transfer Hydrogenation: Transfer hydrogenation is a type of reduction reaction where hydrogen is transferred from a hydrogen donor molecule to an unsaturated organic compound, typically an alkene or carbonyl group, without the direct involvement of molecular hydrogen gas. This process allows for the selective reduction of functional groups while maintaining the integrity of other sensitive moieties within the molecule.