Arenes - Benzene


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Revision of Benzene

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Arenes - Benzene

  1. 1. Arenes Benzne
  2. 2. Contents <ul><li>Arenes and Benzene </li></ul><ul><li>Kukl é model + Problems with it </li></ul><ul><ul><li>Low reactivity </li></ul></ul><ul><ul><li>Carbon-Carbon bond lengths </li></ul></ul><ul><ul><li>Hydrogenation of Benzene </li></ul></ul><ul><li>Delocalised model </li></ul><ul><li>Reactions </li></ul><ul><ul><li>Electrophilic substitution </li></ul></ul><ul><ul><li>Halogenation of Benzene </li></ul></ul><ul><li>Definitions </li></ul><ul><li>Summary </li></ul><ul><ul><li>Reactions </li></ul></ul><ul><ul><li>Electrophilic substitution </li></ul></ul>
  3. 3. Arenes and Benzene <ul><li>Arenes are aromatic hydrocarbons containing one or more benzene rings </li></ul><ul><li>Aromatic comes from the Latin ‘aroma’ meaning fragrance </li></ul><ul><li>Arenes occur in crude oil and coal </li></ul><ul><li>Benzene has a molecular formula of C ₆ H ₆ and has a molecular weight of 78 </li></ul><ul><li>The ring is classified as an aromatic compound </li></ul><ul><li>It is a carcinogen which can cause cancer or anaemia </li></ul><ul><li>It is a colourless liquid with a sweet smell and highly flammable </li></ul><ul><li>Benzene is used in detergents, explosives, pharmaceuticals and dyes </li></ul>Back to Contents
  4. 4. Kukl é model <ul><li>Kukl é was the first person to come up with the idea of a ring </li></ul><ul><li>He suggested that it had a planar, cyclic structure </li></ul><ul><li>He also said there were alternating double and single bonds </li></ul>Back to Contents
  5. 5. Problems: Low Reactivity <ul><li>If there were C=C bonds then it would act like an alkene </li></ul><ul><li>It did not react with bromine water though which is the test for double bonds </li></ul><ul><li>Benzene also takes part is substitution reactions not addition reactions at expected with double bonds </li></ul><ul><li>It was thought that the bonds create an equilibrium which is represented in the resonance hybrid </li></ul>Back to Contents
  6. 6. Problems: Carbon-Carbon Bond Lengths <ul><li>Kukl é supposed there were 3 C–C bonds and 3 C=C bonds </li></ul><ul><li>When X-ray studies revealed that C–C bonds and C=C bonds had different lengths </li></ul><ul><li>However, it was found that all the bond lengths in benzene were equal and between the two lengths (0.139nm) </li></ul><ul><li>So benzene looked like this: </li></ul>Back to Contents
  7. 7. Problems: Hydrogenation of Benzene <ul><li>Benzene was compared with cyclohexene </li></ul><ul><li>When looking at the enthalpy change of cyclohexene it was -120 kJmol ⁻ ¹ </li></ul><ul><li>Thus benzene should have been 3 times this </li></ul><ul><li>However it was found to be -208 kJmol ⁻ ¹ rather than -360 kJmol ⁻ ¹ </li></ul><ul><li>This shows that it is more stable than thought </li></ul><ul><li>This explains why it is less reactive than it was previously thought </li></ul>Back to Contents
  8. 8. Delocalised model <ul><li>The theory suggests that there are 3 localised bonds and 6 delocalised electrons </li></ul><ul><li>This forms p-orbitals and these overlap to create pi ( π ) bonds </li></ul><ul><li>This would give the planar structure and no double bonds </li></ul><ul><li>The structure is stable and resists attempts to break it down by addition reactions </li></ul><ul><li>But substitution of hydrogens would not affect the delocalised theory </li></ul>Back to Contents
  9. 9. Reactions: Electrophilic substitution <ul><li>Reagents: conc. Nitric acid and conc. Sulphuric acid (catalyst) </li></ul><ul><li>Conditions: reflux at 55C </li></ul><ul><li>Equation: </li></ul><ul><li>Mechanism: </li></ul><ul><li>Restoring the Catalyst: </li></ul>Back to Contents
  10. 10. Reactions: Halogenation of Benzene <ul><li>Reagents: Chlorine and a halogen carrier (catalyst) </li></ul><ul><li>Conditions: reflux and halogen carrier </li></ul><ul><li>Equation: </li></ul><ul><li>Mechanism: </li></ul><ul><li>Halogen Carriers: iron, iron (III) chloride, iron (III) bromide, aluminium chloride </li></ul><ul><li>Chlorine is non polar so anhydrous aluminium chloride can act as a catalyst </li></ul>Back to Contents
  11. 11. Definitions <ul><li>Delocalised electrons – shared between more than two atoms </li></ul><ul><li>Addition reaction – a reactant is added to an unsaturated molecule to make a saturated molecule </li></ul><ul><li>Substitution reaction – an atom or group of atoms is replaced with a different atom or group of atoms </li></ul><ul><li>Electrophile – an atom (or group of atoms) that is attracted to an electron-rich centre, where it accepts a pair of electrons to form a new covalent bond </li></ul><ul><li>Electrophilic substitution – a type of substitution reaction in which an electrophile is attracted to an electron-rich centre or atom, where it accepts a pair of electrons to form a new covalent bond </li></ul><ul><li>Reaction mechanism – a series of steps that, together, make up the overall reaction </li></ul><ul><li>Curly arrow – a symbol used in reaction mechanisms to show the movement of an electron pair in the breaking or formation of covalent bonds </li></ul>Back to Contents
  12. 12. Summary - Reactions Br ₂ FeBr ₃ Conc. HNO ₃ Conc. H ₂ SO ₄ 50 ° C Cl ₂ AlCl ₃ Back to Contents Nitrobenzne Chlorobenzene Bromobenzene Benzene
  13. 13. Summary – Electrophilic Substitution Back to Contents