Aromatic compounds

• Benzene:

  • Formula = C6H6
  • A colourless, sweet smelling, high flammable liquid
  • Found naturally in crude oil, is a compound of petrol, and also found in cigarette smoke
  • Classified as a carcinogen (can cause cancer)
• • A benzene molecule consists of a hexagonal ring of six carbon atoms, with each carbon atom joined to two other carbon & to one hydrogen atom.
  • Benzene is classed as an aromatic hydrocarbon or arene
• Kekule's model:

  • Kekule suggested that the structure of benzene was based on a six membered ring of carbon atoms joined by alternate single & double bonds
  • Not all chemists accepted the Kekule model of benzene as the structure is not able to explain all of its chemical & physical properties
• 1. The lack of reactivity of benzene --> benzene doesn't undergo electrophilic addition reactions. Benzene doesn't decolourise bromine under normal conditions
• 2) The length of the carbon - carbon bonds in benzene --> Using a technique called x-ray diffraction, it is possible to measure bond lengths in a molecule. It was found that all the bonds in benzene were 0.139nm in length. This bond length was between the length of a single bond, 0.153nm, and a double bond, 0.134nm.
• 3) Hydrogenation enthalpies --> If benzene did have Kekule's structure, then it would be expected to have an enthalpy change of hydrogenation that is three times that of cyclohexene. When cyclohexene is hydrogenated one double bond reacts with hydrogen. The enthalpy change of hydrogenation is -120KJmol^-1. The actual enthalpy change of hydrogenation of benzene is only -208KJmol^-1. The actual structure of benzene is therefore more stable than the theoretical Kekule model of benzene.
• Benzene

  • Planar
  • Cyclic
  • Hexagonal hydrocarbon
  • Contains 6 carbon atoms
  • Contains 6 hydrogen atoms
• Bonding of carbon atoms in benzene

  1. Each carbon atom uses 3 of its 4 available electrons

  2. Bonds to 2 other carbon atoms

  3. Bonds to 1 hydrogen atom
• Electrons in benzene

  • Each carbon atom has 1 electron in a p-orbital at right angles to the plane of the bonded carbon & hydrogen atoms
  • Adjacent p-orbital electrons overlap sideways, in both directions, above & below the plane of the carbon atoms to form a ring of electron density
• Delocalised pi bonds

  • The overlapping of the p-orbitals creates a system of pi bonds which spread over all 6 of the carbon atoms in the ring structure
  • The 6 electrons occupying this system of pi bonds are said to be delocalised
• Naming aromatic compounds:

  • Aromatic compounds with one substituent group are monosubstituted. In aromatic compounds, the benzene ring is often considered to be the parent chain
  • Alkyl groups, halogens and nitro groups are all considered the prefixes to benzene
  • When a benzene ring is attached to an alkyl chain with a functional group or to an alkyl chain with 7 or more carbon atoms, benzene is considered to be a substituent. Instead of benzene, the prefix phenyl is used in the name
• • Benzene and its derivatives undergo substitution reactions in which a hydrogen atom on the benzene ring is replaced by another atom or group of atoms
  • Benzene typically reacts with electrophiles and most of the reactions of benzene proceed by electrophilic substitution
• Nitration of benzene:

  • Benzene reacts slowly with nitric acid to form nitrobenzene
  • The reaction is catalysed by sulfuric acid & heated to 50 degrees Celsius to obtain a good rate of reaction
  • A water bath is used to maintain the steady temperature
  • In nitration, one of the hydrogen atoms on the benzene ring is replaced by a nitro (-NO2) group
  • If the temperature of the reaction rises above 50 degrees celsius, further substitution reactions may occur leading to the production of dinitrobenzene
• Reaction mechanism of nitration of benzene

  1. Electrophile involved is the nitronium ion, NO2+, produced by the reaction of concentrated nitric acid with concentrated sulfuric acid

  2. Electrophile, NO2+ accepts a pair of electrons from the benzene ring to form a dative covalent bond

  3. Organic intermediate formed is unstable & breaks down to form the organic product, nitrobenzene, and the H+ ion

  4. H+ ion formed reacts with the HSO4- ion to regenerate the catalyst, H2SO4
• Halogenation of benzene:

  • The halogens don;t react with benzene unless a catalyst called a halogen carrier is present
  • Common halogen carriers include AlCl3, FeCl3, AlBr3 and FeBr3 which can be generated in situ ( in the reaction vessel) from the metal and the halogen
• Bromination of benzene:

  • At room temperature & pressure and in the presence of a halogen carrier, benzene reacts with bromine in a electrophilic substitution reaction.
  • In bromination, one of the hydrogen atoms on the benzene ring is replaced by a bromine atom
• Reaction mechanism of bromination of benzene:

  • Benzene is too stable to react with a non-polar bromine molecule
  • The electrophile is the bromonium ion Br+ which is generated when the halogen carrier catalyst reacts with bromine in the first stage of the mechanism
  • The bromonium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond. The organic intermediate is unstable and breaks down to form the organic product, bromobenzene and a H+ ion
  • The H+ ion reacts with the FeBr4- ion to regenerate the FeBr3 catalyst
• Chlorination of benzene:

  • Chlorine will react with benzene in the same way as bromine and following the same mechanism.
  • The halogen carrier used is FeCl3, AlCl3 or iron metal & chlroine which reacts to make FeCl3
• Alkylation of benzene:

  • Alkylation of benzene is the substitution of a hydrogen atom in the benzene ring by an alkyl group.
  • The reaction is carried out by reacting benzene with a haloalkane in the presence of AlCl3, which acts as a halogen carrier catalyst, generating the electrophile
  • Alkylation increases the number of carbon atoms in a compound by forming carbon-carbon bonds
  • Also called a Friedel - Crafts alkylation
• Acylation reactions:

  • When benzene reacts with an acyl chloride in the presence of an AlCl3 catalyst an aromatic ketone is formed
  • Example of electrophilic substitution
  • The reaction forms carbon-carbon bonds & is useful in organic synthesis
  • Ethanoyl chloride, CH3COCl is the first member of the acyl chloride homologous series
  • Phenylenthanone is produced in the reaction between benzene & ethanoyl chloride & used in the perfume industry
• Alkenes


  Decolourise bromine by an electrophilic addition reaction
• Electrophilic addition reaction of alkenes with bromine

  Bromine adds across the double bond in cyclohexane
• Pi bond in alkenes

  • Contains localised electrons above and below the plane of the two carbon atoms in the double bond
  • Produces an area of high electron density
• Localised electrons in the pi bond

  Induce a dipole in the non-polar bromine molecule
• Induced dipole in bromine

  Makes one bromine atom slightly positive & the other bromine atom slightly negative
• Slightly positive bromine atom


  Enables the bromine molecule to act like an electrophile
• • Benzene doesn't react with bromine unless a halogen carrier is present
  • Benzene has delocalised pi electrons spread above and below the plane of the carbon atoms in the ring structure
  • The electron density around any two carbon atoms in the benzene ring is less than that in a carbon - carbon double bond in an alkene
  • When a non-polar molecule such as bromine approaches the benzene ring there is insufficient pi electron density around any two carbon atoms to polarise the bromine molecule. This prevents any reaction taking place.
• • Phenols are a type of organic chemical containing a hydroxyl, -OH functional group directly bonded to an aromatic ring
  • 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
  • Phenol is classified as a weak acid
  • Phenol is more acidic than alcohols but less acidic than carboxylic acids
• • Ethanol doesn't react with sodium hydroxide ( a strong base) or sodium carbonate ( a weak base)
  • Phenols & carboxylic acids react with solution of strong bases such as aqueous sodium hydroxide
  • Only carboxylic acids are strong enough acids to react with weak base, sodium carbonate
  • A reaction with sodium carbonate can be used to distinguish between a phenol & a carboxylic acid - the carboxylic acid reacts with sodium carbonate to produce CO2, which is evolved as a gas
• Bromination of phenol:

  • Phenol reacts with an aqueous solution of bromine ( bromine water) to form a white precipitate of 2,4,6 -tribromophenol
  • The reaction decolourises the bromine water ( orange to colourless)
  • With phenol a halogen carrier catalyst is not required & the reaction so carried out at room temperature
• Reaction of phenol with sodium hydroxide:

  • Phenol reacts with sodium hydroxide to form the salt, sodium phenoxide and water in a neutralisation reaction
• Nitration of phenol:

  • Phenol reacts readily with dilute nitric acid at room temperature. A mixture of 2 - nitrophenol and 4-nitrophenol is formed
• Phenol


  Organic compound with a hydroxyl group (-OH) attached to a benzene ring
• Benzene


  Aromatic hydrocarbon compound with a 6-membered ring of carbon atoms
• Bromine and nitric acid


  React more readily with phenol than they do with benzene
• Phenol


  Is nitrated with dilute nitric acid rather than needing concentrated nitric and sulfuric acids as with benzene
• Increased reactivity of phenol

  • Caused by a lone pair of electrons from the oxygen p-orbital of the -OH group being donated into the pi system of phenol
  • The electron density of the benzene ring in phenol increases
  • The increased electron density attracts its electrophiles more strongly than with benzene
• The aromatic ring in phenol is more susceptible to attack from electrophiles than in benzene
• For bromine


  The electron density in the phenol ring structure is sufficient to polarise bromine molecules & so no halogen carrier catalyst needed
• Further substitution:

  • Many substituted aromatic compounds can undergo a second substitution - distribution
  • Some of these reactions take place more readily than benzene itself whereas other reactions take place less easily & require extreme conditions