15.4 Arenes A2 Flashcards
Structure of benzene
- planar (bond angle 120°), symmetrical, non-polar
- delocalised ring of pi electrons gives benzene extra stability
4 physical properties of benzene
- Colourless liquid with characteristic smell
- Immiscible with water, miscible with organic solvents, benzene is a good solvent itself
- Arenas are flammable and burn with a smoky flame due to the high proportion of carbon
- Benzene vapour is highly toxic
2 reasons why aromatic compounds undergo electrophilic substitution reaction
- The benzene ring is an electron-rich area/has high electron density, so it attracts electrophiles.
- Although benzene ring is unsaturated, an ‘addition reaction’ is unlikely as it will destroy the stability of the benzene ring.
Substitution reactions with chlorine and with bromine
|»_space; Condition + product + observations
Benzene mixed with Cl2 or Br2 at:
- room temperature
- in the presence of CATALYST (halogen carriers): iron filings, FeCl3, FeBr3, AlCl3, AlBr3
- room temp alone, no reaction
Product:
- BROMObenzene, CHLORObenzene
- HCl, HBr (by-product)
Observations: steamy white fumes of HCl or HBr
*if substitute with Br2, decolourisation of brown bromine fumes
Substitution reactions with chlorine and with bromine
|»_space; Electrophilic substitution reaction mechanism
eg. with Br2 in the presence of FeBr3
Step 1: Halogen carrier (catalyst) polarises Br2 molecule. Then, Fe in halogen carrier accepts a lone pair of electrons from one of the Br atoms (dative covalent bonding) to generate Br+ (electrophile).
*FeBr3 + Br2 –> FeBr4- + Br+
Step 2: Br+ attacks benzene ring, forming the intermediate cation. Delocalised ring structure is temporarily disrupted.
Step 3: Expulsion of H+ from intermediate to form product and benzene ring is reestablished.
*FeBr4- + H+ –> FeBr3 (catalyst regenerated) + HBr (by-product)
Nitration - substitution reaction with conc. nitric acid (reagent)
» Condition + product
Condition:
- heat under reflux
- at 55°C to 60°C
- conc. H2SO4 as catalyst
- nitrating mixture = mixture of conc. HNO3 + conc. H2SO4
Product:
- NITRObenzene, pale yellow liquid with smell of almonds
- H2O (by-product)
Nitration - substitution reaction with conc. nitric acid
|»_space; Electrophilic substitution reaction mechanism
Step 1: Conc. HNO3 and conc. H2SO4 react to generate electrophile, NO2+ (nitronium ion)
*HNO3 + H2SO4 –> NO2+ + H3O+ +2HSO4-
Step 2: NO2+ attacks benzene ring, forming the intermediate cation. Delocalised ring structure is temporarily disrupted.
Step 3: Expulsion of H+ from intermediate to form product and benzene ring is reestablished.
*H+ + H3O+ + 2HSO4- –> 2H2SO4 (catalyst regenerated) + H2O (by-product)
*At higher temp >60°C, a mixture of products is formed as 2nd nitro group and 3rd nitro group can be substituted.
Hydrogenation of the benzene ring to form a cyclohexane ring
|»_space; Reagent + conditions + types of reaction
Reagent: Hydrogen gas
Conditions:
- nickel catalyst
- 20atm to 40atm
- 170°C to 350°C
Types of reaction: Addition and reduction
2 uses of cyclohexane
- As a non-polar solvent
2. Make nylon
Friedel-Crafts alkylation and acylation
|»_space; General info + name of mechanism
- substitutes a H atom in benzene ring for an alkyl group or acyl group (RC=O)
- results in INTRODUCTION of a SIDE-CHAIN into a benzene ring
- Electrophilic substitution reaction mechanism
Friedel-Crafts alkylation
|»_space; Reagent + conditions + product
Reagent: alkyl chloride (R-Cl, a halogenoalkane)
Conditions:
- room temperature
- anhydrous AlCl3 catalyst
Product:
- alkylbenzene
- HCl (by-product)
Friedel-Crafts acylation
|»_space; Reagent + conditions + product
Reagent: acyl chloride (RC=OCl)
eg. ethanoyl chloride
Conditions:
- heat under reflux
- 60°C
- anhydrous AlCl3 catalyst
Product:
- phenylketone
eg. phenylethanone - HCl (by-product)
