Aromatic compounds can be reduced to cyclohexanes through catalytic hydrogenation. 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.

Aryl alkyl ketones can be converted to alkylbenzenes 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

• Reduces aromatic rings to cyclohexanes using hydrogen gas ($H_2$) and a metal catalyst (platinum ($Pt$), palladium ($Pd$), or nickel ($Ni$))
• Carried out under high pressure and elevated temperature conditions
• Mechanism involves:
  1. $H_2$ adsorbs onto the metal catalyst surface weakening the $H-H$ bond
  2. Aromatic ring adsorbs onto the catalyst surface
  3. Hydrogen atoms transfer to the aromatic ring breaking the aromaticity
  4. 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:
  1. Ketone is reduced to a secondary alcohol using a reducing agent (lithium aluminum hydride ($LiAlH_4$) or sodium borohydride ($NaBH_4$))
  2. Secondary alcohol undergoes dehydration and hydrogenation to form the alkylbenzene
• Mechanism involves:
  1. Reducing agent ($LiAlH_4$ or $NaBH_4$) delivers a hydride ($H^-$) to the carbonyl carbon forming an alkoxide intermediate
  2. Alkoxide is protonated by the solvent (usually an alcohol) to form the secondary alcohol
  3. Dehydration of the secondary alcohol forms a styrene-like intermediate (an alkene with an aromatic ring)
  4. Catalytic hydrogenation of the alkene yields the final alkylbenzene product
• In some cases, hydrogenolysis 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:
  • Resonance stability makes them less prone to reaction
  • Delocalized $\pi$ electrons are less available for interaction with the catalyst surface
  • Breaking the aromaticity requires more energy than breaking a simple $\pi$ 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

• Birch reduction: Uses alkali metals in liquid ammonia to partially reduce aromatic rings to 1,4-cyclohexadienes
• Diimide reduction: Employs diimide (NH=NH) as a reducing agent for selective reduction of alkenes and some aromatic compounds
• Transfer hydrogenation: Utilizes hydrogen donors like cyclohexene or formic acid instead of H2 gas for reduction of aromatic compounds