Notes on Aromatic Hydrocarbons / Arenes

Introduction to Organic Classes

• Aliphatic: Straight or branched chain organic substances
• Aromatic (Arenes): Includes one or more rings of six carbon atoms with delocalized bonding
• Benzene belongs to the aromatic class

Benzene Structure

• Formula: C6H6
• Basic Structure:
  • Hexagonal ring of six carbon atoms
  • Each carbon atom bonded to two other carbons and one hydrogen atom via single covalent bonds
  • One unused electron on each carbon in a p orbital, perpendicular to the ring plane
• Delocalization:
  • Six p electrons delocalized above and below the plane
  • All C-C bonds equal in length and energy
  • Delocalization increases thermodynamic stability

Thermodynamic Stability

• Enthalpy of hydrogenation used to show stability
  • Cyclohexene vs benzene energy differences
  • Delocalization energy accounts for benzene having less energy than expected

Naming Aromatic Compounds

• Simplest compounds are derivatives of benzene
• Substituents use numbers for position indication, alphabetical order, and di-, tri- prefixes for multiple identical substituents
• Phenyl Group (C6H5-): Benzene as a substituent

Reactions of Benzene

• Generally does not undergo addition reactions
• Involves substituting one hydrogen for another atom or group
• High electron density attracts electrophiles; reactions often electrophilic substitutions
• Carcinogenicity: Benzene is a carcinogen, banned in schools; methylbenzene is less toxic

Nitration of Benzene

• Important for synthesizing compounds (e.g., TNT, dyestuffs)
• Functional Group Change: Benzene to nitrobenzene
• Reagents: Concentrated nitric acid and sulfuric acid (catalyst)
• Mechanism: Electrophilic substitution

Friedel-Crafts Acylation

• Functional Group Change: Benzene to phenyl ketone
• Reagents: Acyl chloride, anhydrous aluminum chloride catalyst
• Conditions: Heat under reflux (50°C)
• Mechanism: Electrophilic substitution

Reduction of Nitroarenes to Amines

• Reagents: Sn and HCl or Fe and HCl
• Conditions: Heating
• Mechanism: Reduction
• Produces ionic salt C6H5NH3+Cl-, reacts with NaOH to form phenylamine

Effects of Delocalization on Side Groups

• Groups like OH, Cl, NH2 directly attached to benzene ring have extended delocalization
  • Alters properties and reactions
  • Chlorobenzene shows stronger C-Cl bond
  • Phenol has stronger C-O and weaker O-H bond, more acidic
  • Phenylamine less basic than aliphatic amines due to delocalization

Aromatic Synthetic Routes

• Emphasize reactions involving side chains rather than the benzene ring itself
• Various reactions including dehydration, esterification, and nucleophilic substitution are relevant

Key Takeaway

• Understanding benzene's structure and behavior is crucial for reactions involving aromatic compounds, focusing on electrophilic substitutions and the effects of delocalization.