Aromatic chemistry

• Aliphatic organic molecules = based on carbon chains
• Aromatic organic molecules = molecules with a benzene based structure
• Aromatics are also known as arenes
• Many natural fragrances contain a substituted benzene ring
• Michael Faraday discovered benzene in the 1830s. Lother von Meyer was the first to suggest benzene was compromised of a ring of carbon atoms. In 1858 Friedrich August Von Stradonitz proposed that the Valence of a carbon atoms is four. In 1865 he discovered the ring structure of benzene
• The valence of an atom is the fixed number of other atoms or groups it can bond with
• all carbon - carbon bonds in benzene have the same length. They are intermediate between a carbon-carbon single bond and a carbon-carbon double bond
• In benzene, all six carbons are equivalent so there is no priority rule when naming them
• benzene is planar with bond angles of 120 degrees
• cyclohexatriene would form two disubstituted compounds, benzene would form three
• cyclohexatriene has double bonds (is unsaturated) and so decolourises bromine (it is a reactive alkene)
• Benzene has no double bonds. and so there is no visible change with bromine
• Both benzene and cyclohexene are unsaturated compounds
• Benzene normally undergoes substitution reaction, whereas cyclohexene normally undergoes addition reactions
• cyclohexatriene does not exist and is described as hypothetical
• Benzene is more stable than cyclohexatriene because it's delocalised electron cloud is stable. Benzene's hydrogenation value is higher
• benzene never goes through addition reactions
• The main reactions of benzene involve the replacement of one or more of the six hydrogen atoms by a functional group. The hydrogen atoms is said to be substituted by the functional group. Hence, all such reactions are substitution reactions
• Benzene's ring is an area of electron density because of the delocalised electron cloud, and is therefore attacked by electrophiles
• The aromatic ring is very stable. It needs energy to be put in to break the ring before the system is destroyed. This is known as the delocalised energy. It means the ring almost always remains intact in the reactions of arenes
• Because the aromatic ring is very stable and is an area of high electron density, most reactions of aromatic systems are electrophilic substitution reactions
• In electrophilic substitution reactions involving benzene, the electrophile is attracted towards the benzene ring and the delocalised electron cloud donates a pair of electrons to the electrophile. An intermediate is formed that contains a partially delocalised positive charge. In order to restore the stability of the delocalised electron cloud a carbon - hydrogen bond is broken and a pair of electrons in the bond restore the delocalised electron cloud
• when benzene is refluxed on a water bath at 60 degrees with a mixture of concentrated sulphuric acid and concentrated nitric acid, nitrobenzene is formed
• In the nitration if benzene, sulfuric acid is a catalyst
• Nitrobenzene is often converted to phenylamine by the reduction of the NO2 group to an NH2 group. This reduction is achieved using iron or tin with a moderately concentrated HCL as the reducing agent OR with hydrogen gas and nickel acting as the catalyst
• Phenylamine is used in the dye industry
• Nitration of arenes is important in the manufacture of explosives
• 2,4,6 - trinitromethylbenzene or trinitrotoluene is also known as TNT
• Friedel-Crafts acylation of benzene use aluminium chloride as a catalyst and a hydrogen atom on the benzene ring is substituted for an acyl group
• Acyl chlorides are the sources of the acyl group in the acylation of benzene. They react with aluminium chloride to form an acyl carbocation
• Many synthetic routes need chemists to increase the number of carbon atoms in a molecule. This can be achieved by:

  • reaction of an aromatic compound with an acyl chloride
  • reaction of an aldehyde with hydrogen cyanide
• During recrystallisation the minimum amount of hot water is used to dissolve the crude product to saturate it
• During recrystallisation, the flask is cooled because solubility is higher if warm
• During recrystallisation, the crystals are compressed in the funnel to ensure air passes through the sample, not just around it
• During recrystallisation, a little cold water is poured through the crystals to remove soluble impurities
• Water can be an impurity. Crystals should be allowed to dry before their melting point is tested
• The enthalpy of hydrogenation is the enthalpy change when one mole of a compound reacts fully with an excess of H2
• Hydrogenation is an exothermic reaction. When hydrogenation of benzene is carried out, it is less exothermic than expected
• Kekule's benzene model does not undergo addition reactions like expected
• The valence bond theory is that if two atoms have orbitals which contain unpaired electrons, then the orbitals will overlap with each other, forming a covalent bond