Aromatic Compounds

• Why was the

  Kékulé ModelofBenzenedisproven?1) Lack of Reactivity:• It doesn't undergo addition reactions.• It doesn't decolourise bromine.• Therefore, no C=C bonds are present in Benzene.2) Thermodynamic Stability:• Predicted Hydrogenation enthalpy was -360 kJ mol⁻¹.• It was actually -208 kJ mol⁻¹.• 152 kJ mol⁻¹ less energy was needed, hence Benzene was more stable than the Kékulé model.3) Bond Length:• C-C bonds in Benzene are smaller than C-C bonds in alkanes, but larger than C=C bonds in alkenes.• Kékulé predicted 3 short C=C & 3 long C-C bonds.• Bond lengths ended up being the same in Benzene.

  
• What is the

  ModernDelocalised ModelofBenzene?• C₆H₆ is a planar, cyclic & hexagonal hydrocarbon.

  • Each C uses 3 of 4 e⁻ to bond to 1 H and 2 C atoms.

  • Each C has 1 e⁻ in a p-orbital at right angles to the plane of the bonded C & H atoms.

  • Adjacent p-orbital e⁻'s overlap sideways, above & below the plane of C atoms to form a ring of e⁻ density.

  • This creates a system of π-bonds which spread over all 6 C atoms in the ring structure, with delocalised e⁻'s.

  • e⁻'s in π-systems are less susceptible to electropilic attack than in standard π-bonds between C atoms.

  
• What happens in the

  Nitration of Benzene?• C₆H₆ reacts slowly with HNO₃ to form nitrobenzene.

  • The reaction is catalysed by sulfuric acid at 50°C.

  • In nitration, one of the hydrogen atoms on the benzene ring is replaced by a nitro, —NO₂, group.

  • If temperature rises above 50°C, further substitution may occur leading to production of dinitrobenzene.

  • This is an example of Electrophilic Substitution.

  
• What happens in the

  Halogenation of Benzene?• Halogens only react with C₆H₆ in presence of halogen carriers such as AlCl₃ or FeBr₃.

  • This is because C₆H₆ is too stable to react with non-polar halogen molecules.

  • Cl⁺/Br⁺ ions must be formed first.

  • This is an example of Electrophilic Substitution.

  
• What happens in the

  Alkylation of Benzene?• C₆H₆ reacts with a haloalkane in presence of an AlCl₃ catalyst, which generates the electrophile.

  • H atom in C₆H₆ ring is substituted by an alkyl group.

  • Alkylation increases the number of carbon atoms in a compound by forming carbon—carbon bonds.

  • This makes it useful in organic synthesis.

  • It's sometimes called a Friedel-Crafts alkylation.

  • This is an example of Electrophilic Substitution.

  
• What happens in the

  Acylation of Benzene?• C₆H₆ reacts with an acyl chloride in presence of an AlCl₃ catalyst, to form an aromatic ketone.

  • Acylation also increases the number of carbon atoms in a compound by forming carbon—carbon bonds.

  • This makes it useful in organic synthesis.

  • This is an example of Electrophilic Substitution.

  
• Why are

  Alkenes more reactive than Arenes?• Alkenes decolourise bromine in electophilic addition.

  • Alkenes contain localised πe⁻'s above & below the plane of the 2 C atoms in the double bond.

  • This produces an area of high e⁻ density.

  • Localised e⁻'s in the π-bond induce a dipole in the non-polar Br₂, making one δ⁺ Br and δ⁻ Br.

  • The δ⁺ Br atom allows the Br₂ to act as an electrophile.

  • C₆H₆ only reacts with Br₂ in presence of halogen carriers due to delocalised πe⁻'s above & below the plane of the C atoms in the ring structure.

  • When a non-polar molecule such as Br₂ approaches the C₆H₆ ring there is insufficient πe⁻density around any 2 C atoms to polarise the Br₂ molecule.

  • This prevents any reaction taking place.

  
• What are

  Phenols?• C₆H₆ rings bonded directly with OH⁻ groups.

  • Phenol is less soluble in water than alcohols due to the presence of the non-polar benzene ring.

  • When dissolved in water, phenol partially dissociates forming the phenoxide ion and a proton.

  • Because of this ability to partially dissociate to produce protons, phenol is classified as a weak acid.

  • Phenol is more acidic than alcohols but less acidic than carboxylic acids (seen in Ka comparisons).

  • Ethanol does not react with sodium hydroxide (a strong base) or sodium carbonate (a weak base).

  • Phenols and carboxylic acids react with solutions of strong bases such as aqueous sodium hydroxide.

  • Only carboxylic acids are strong enough acids to react with the weak base, sodium carbonate.

  • A reaction with sodium carbonate can be used to distinguish between a phenol and a carboxylic acid.

  • The carboxylic acid reacts with sodium carbonate to produce carbon dioxide, which is evolved as a gas.

  
• What happens in the

  Bromination of Phenol?• Phenol reacts with Br₂ (aq) to form a white precipitate of 2,4,6-tribromophenol, decolourising it.

  • With phenol, a halogen carrier catalyst is not required and the reaction is carried out at room temperature.

  • This is an example of Electrophilic Substitution.

  
• What happens in the

  Nitration of Phenol?• Phenol reacts readily with dilute HNO₃ to form a mixture of 2-nitrophenol and 4-nitrophenol.

  • This reaction occurs at room temperature.

  • This is an example of Electrophilic Substitution.

  
• Why does

  Electrophilic Substitution occur more readily in Phenol than Benzene?• Br₂ & HNO₃ react more readily with phenol than C₆H₆.

  • Phenol is nitrated with dilute HNO₃ rather than needing concentrated HNO₃ & H₂SO₄ as with C₆H₆.

  • The increased reactivity is caused by activation due to a lone pair of e⁻'s from the oxygen p-orbital of the OH⁻ group being donated into the π-system of phenol.

  • e⁻ density of the C₆H₆ ring in phenol is increased.

  • ↑ e⁻ density attracts electrophiles more strongly than C₆H₆.

  • The aromatic ring in phenol is therefore more susceptible to attack from electrophiles than in C₆H₆.

  • For Br₂, the e⁻ density in the phenol ring structure is sufficient to polarise bromine molecules, so no halogen carrier catalyst is required.
• What are theDirecting EffectsofElectron-donating & Electron-withdrawing groups?• Many different groups can be attached to a C₆H₆ ring.

  • Different groups can have a directing effect on any second substituent on the benzene ring.

  • All 2-/4-directing groups (ortho/para-directors) are activating groups, with the exception of the halogens.

  • 3-directing groups (meta directors) are deactivating.