Benzene

• The benzene ring has six carbon atoms arranged in a hexagonal shape, with alternating single and double bonds between them.
• Each carbon atom in the benzene ring forms three sigma (σ) covalent bonds with adjacent carbons and one pi (π) bond with another carbon atom.
• Benzene is a colorless liquid at room temperature that evaporates easily into the air.
• Benzene can be found naturally in crude oil and gasoline, as well as in some fruits and vegetables.
• Benzene is used to make many products such as plastics, dyes, detergents, pesticides, and drugs.
• Aromatic compounds have a cyclic or 'ring' structure and a delocalized pi (π) system of electrons
• Benzene has a molecular formula of C6H6 and a relative molecular mass of 78.06
• Benzene is usually shown in its skeletal formula and has a planar shape with internal bond angles of 120o
• Benzene is a colorless, flammable liquid with a sweet smell found in crude oil and petroleum
• The structure of benzene was first proposed as cyclohexa-1,3,5-triene with alternating single and double bonds by Kekulé
• Benzene has hybrid bonds between the carbon atoms, shown as a ring inside the hexagon, formed from one a orbital electron and 2 p orbital electrons (sp2 hybrid bonds)
• The unbonded p orbitals from the carbon atoms overlap above and below the carbon ring to form a delocalized π electron system
• Benzene does not decolourise bromine water and does not undergo addition reactions due to the stability of the π electron system
• Benzene undergoes electrophilic substitution reactions that do not disrupt the delocalised electron ring
• The bond length between all carbon atoms in benzene is equal at 139 pm, less than a carbon-carbon single bond but greater than a carbon-carbon double bond
• The enthalpy change for the hydrogenation of benzene is 208 kJ mol-1, 152 kJ mol-1 less than expected, showing the delocalisation or 'resonance' enthalpy of benzene
• Benzene reacts by electrophilic substitution where an electrophile substitutes an atom or group for one (or more) of the hydrogen atoms in the benzene ring
• An electrophile is a species that attacks an area of high electron density – the π system in this case
• Nitration of benzene is the substitution of a nitronium group to form nitrobenzene, a yellow oil with an almond-like odour
• Reagents for nitration of benzene: concentrated nitric and sulfuric acids (HNO3 and H2SO4)
• Conditions for nitration of benzene: 50°C (higher temperatures lead to further nitration)
• Reaction type for nitration of benzene: electrophilic substitution
• The mechanism for nitration of benzene has 2 parts:

  • Firstly, the two acids react to form a nitronium ion, NO2+
  • The nitronium ion formed is the electrophile which reacts with the benzene to form nitrobenzene
• The first stage of the mechanism for the reaction shows the movement of electrons towards the positive electrophile
• A temporary Wheland intermediate is formed during the reaction, with a disrupted electron ring and both hydrogen and the nitronium group bonded to the same carbon atom, which immediately breaks down to form nitrobenzene and a hydrogen ion
• Halogenation is the substitution of a halogen into the benzene ring
• Chlorination is the substitution of chlorine for hydrogen to form chlorobenzene, a common solvent
• Reagents for chlorination: chlorine (Cl2)
• Conditions for chlorination:

  • Dark (out of direct sunlight) to avoid homolytic fission and radicalisation of chlorine
  • Catalyst: aluminum chloride (AlCl3) or iron(III) chloride (FeCl3)
• Reaction type for chlorination: electrophilic substitution
• Equation for the overall reaction:

  • Catalyst breaks the Cl-Cl bond to form the aluminium tetrachloride ion AlCl4- and the electrophile Cl+
  • Electrophile reacts with benzene to form chlorobenzene
  • Aluminium chloride catalyst is regenerated at the end of the reaction
• Reagents: bromine Br

  conditions

  • room temperature, in the dark (out of direct sunlight)
  • aluminum bromide AlBr3 or iron(III) bromide FeBr3catalyst

  Reaction type:electrophilic substitution

  The equation for the overall reaction is:

  The mechanism is similar to the chlorination reaction above.
• Friedel-Crafts reaction is the substitution of an alkyl group into the benzene ring
• Named after 19th-century scientists who developed the process
• Example: substitution of a methyl group to form methylbenzene, can be applied to other alkyl chains
• Ethylbenzene is used to make polystyrene (polyphenylethene)
• Reagents: chloromethane
• Conditions: reflux in anhydrous conditions, aluminum chloride catalyst (AlCl3)
• Reaction type: electrophilic substitution
• In the first part of the reaction, the catalyst breaks the CH3-Cl bond to form the aluminium tetrachloride ion AlCl4- and the electrophile CH3+