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  2. 2. INTRODUCTION• Benzene is a chemical that is a colourless or light yellow liquid at room temperature. It has a sweet odour and is highly flammable.• Natural sources of benzene include volcanoes and forest fires. Benzene is also a natural part of crude oil, gasoline, and cigarette smoke.
  3. 3. Michael Faraday • The word "benzene" derives historically from "gum benzoin", sometimes called "benjamin" an aromatic resin known to European pharmacists and perfumers since the 15th century as a product of southeast Asia. • Michael Faraday first isolated and identified benzene in 1825 from the oily residue derived from the production of illuminating gas, giving it the name bicarburet of hydrogen.
  5. 5. a) MONOSUBSTITUTION• 1) Benzene as the parent name: ethylbenzene chlorobenzene bromobenzene
  6. 6. 2)Common name:styrene cymene toluene
  7. 7. 3)Alkyl substituent is larger than ring: 3-phenyldecane5-phenylpentanoic acid
  8. 8. b) DISUBSTITUTION• 1)Ortho- disubstituted benzene has two substituent in a 1,2 positions:• Principle functional group is the benzene therefore root = benzene O-dichlorobenzene• There are two chlorine substituent therefore dichloro.• The substituent locants are 1 and 2 therefore ortho.
  9. 9. 2) Meta -disubstituted benzene has two substituents in a 1,3 positions:• Principle functional group is the benzne therefore root = benzene m-bromochlorobenzene• There is a chlorine substituent therefore chloro.• There is a bromine substituent therefore bromo.• The substituent locants are 1 and 3 therefore meta.
  10. 10. 3) Para-disubstituted benzene hastwo substituents in a 1,4 positions:
  11. 11. Polydisubstituted benzene derivatives:
  12. 12. c) POLYSUBSTITUTION• Benzene with more than 2 substituents are named by numbering the position of each substituent with lowest possible numbers:• Principle functional group is the aromatic amine therefore = aniline• There is a C1 substituent therefore methyl• There is a C2 substituent therefore ethyl• Numbering from the -NH2 (priority group at C1) gives the substituents the locants =2 and 3
  13. 13. 2-ethyl-3-methylaniline
  14. 14. Physical Properties of Benzene
  15. 15. 1.The Kekulé Structure of Benzene• German chemist Friedrich August Kekulé von Stradonitz• A structure of benzene, containing 3 cyclic conjugated double bonds which systematically called 1,3,5-cyclohexatriene
  16. 16. Cont. • The true structure of benzene is a resonance hybrid of the two Lewis structures, with the dashed lines of the hybrid indicating the position of the π bonds. hybrid
  17. 17. 2.Stability
  18. 18. Cont.• Benzene does not undergo addition reactions typical of other highly unsaturated compounds, including conjugated dienes.• Benzene does not react with Br2 to yield an addition product. Instead, in the presence of a Lewis acid, bromine substitutes for a hydrogen atom, yielding a product that retains the benzene ring.
  19. 19. 3.Boiling Point• The only attractions between neighbouring molecules are van der Waals dispersion forces.• Benzene boils at 80°C - o-dichlorobenzene m-dichlorobenzene p-dichlorobenzene b.p. 1810C b.p. 1730C b.p. 1700C
  20. 20. 4. The Criteria for Aromacity : Hückels Rule• 4 structural criteria must be satisfied for a compound to be aromatic:a) Cyclicb) Planarc) Completely Conjugatedd) Contain a particular number of π electrons
  21. 21. Cont. [4] A molecule must satisfy Hückel’s rule, which requires a particular number of  electrons. Hückels rule: •Benzene is aromatic and especially stable because it contains 6  electrons. •Cyclobutadiene is antiaromatic and especially unstable because it contains 4  electrons.
  22. 22. Cont.Note that Hückel’s rule refers to the number of  electrons, not the number of atoms in aparticular ring.
  23. 23. Electrophilic Aromatic substituentAn electrophile (E⁺) reacts with an aromatic ring and substitutesfor one of the hydrogenBenzene does not undergo addition reactions because additionwould yield a product that is not aromatic
  25. 25. Step 1 : Formation of arenium ionPositive ion X+ = electrophileExample :Step 2 : Loss of H +
  26. 26. Reactants : Benzene and halogens (Clorine or Bromine)Conditions : Lewis acid like FeCl₃ or FeBr₃Analogous reaction with I2 and F2 are not synthetically useful because I2 istoo unreactive and F2 is too violentlyElectrophile : Cl⁺ or Br⁺Example : + Br₂ no reaction (decolorization not observed) +
  27. 27. MECHANISM : Step 1 : Formation of Cl⁺ or Br⁺ Step 2 : Electrophilic substitution(The electrophile attacks the π electron system of the benzene ringto form a arenium ion)
  28. 28. Step 3 : Loss of proton to reform the aromatic ring bromobenzene Function of Lewis acid : Incerase the polarity of halogen molecules to produce positive halogen ions (Cl⁺ or Br⁺) = electrophile
  29. 29. Reactants : Benzene and concentrated HNO₃Conditions : Concentrated H₂SO₄Example : nitrobenzene
  30. 30. MECHANISM : Step 1 : Formation of Nitronium ion (NO₂⁺)- Sulfuric acid ionizes to produce a proton-Nitric acid accepts a proton from a stronger acid (H₂SO₄) andform a protonated nitric acid- The protonated nitric acid dissociates to form a nitronium ion(+NO2)
  31. 31. Step 2 : Electropjilic substitutionStep 3 : Loss of proton to re-form the aromatic ring
  32. 32. Reactants : Benzene and alkyl halideCondition : catalyst (Lewis acid like AlCl₃) +
  33. 33. Example :1) AlCl₃ tolouene2) Tert-butylbenzene
  34. 34. MECHANISM :Step 1 : Formation of carbocationStep 2 : Electrophilic substitution(electrophile attacks the π electron system of the benzene ring toform an arenium ion)
  35. 35. Step 3 : loss of proton to re-form the aromatic ring
  36. 36. Friedel-Craft AcylationReactancts: Aromatic rings and acid chlorideProduct: KetoneCondition: Strong Lewis acid AlCl3
  37. 37. Example:
  38. 38. Mechanism: Step 1: Dissociation of a chloride ion to form an acyl cation ("acylium ion") Step 2: The resulting acylium ion or a related adduct is subject to nucleophilic attack by the arene Step 3: Chloride anion (or AlCl4-) deprotonates the ring (an "arenium ion") to form HCl, and the AlCl3 catalyst is regenerated
  39. 39. Aromatic SulfonationOrganic reaction in which a hydrogen atom on an arene is replaced by a sulfonic acid functional group in an electrophilic aromatic substitution
  40. 40. Mechanism: Step 1: The p electrons of the aromatic C=C act as a nucleophile, attacking the electrophilic S, pushing charge out onto an electronegative O atom. This destroys the aromaticity giving the cyclohexadienyl cation intermediate. Step 2: Loss of the proton from the sp3 C bearing the sulfonyl- group reforms the C=C and the aromatic system. Step 3: Protonation of the conjugate base of the sulfonic acid by sulfuric acid produces the sulfonic acid.
  41. 41. Substituent Effect in Substituted Aromatic Benzene
  42. 42. Ortho-Para Orientation Meta Effect ofSubstituent Activating Reaction Rate Deactivating
  43. 43. Orientation Para-substitutionOrtho-substitution-2 substituents occupy Substituent occupy positions next to positions 1 and 4 each other. Meta-substitution - Substituent occupy positions 1 and 3
  44. 44. Activating Group & Deactivating Group
  45. 45. Ortho-Para Activator  Electron donating groups (EDG) with lone pairs on the atoms adjacent to the p system activate the aromatic ring by increasing the electron density on the ring through a resonance donating effect.  The resonance only allows electron density to be positioned at the ortho- and para- positions.  Hence these sites aremore nucleophilic, and the system tends to react with electrophiles at these ortho- and para- sites.
  46. 46. Examples:
  47. 47. Example:
  48. 48. Meta –Directing Deactivator
  49. 49. Meta-Directing -CORDeactivator increase -CO2R -SO3H -CHO -CO2H -CN -NO2 -NR2+
  50. 50. Explanation of meta- deactivatorsMeta directors slow the reaction by raising the energy of thecarbocation intermediate because they have (in one resonanceform, shown below) a positively charged atom attached to thering. Two positively charged atoms so close together is very highin energy (especially unstable).
  51. 51. SOME EXAMPLES OF "META DIRECTORS“ the acyl group in benzaldehyde the NO2 group in nitrobenzeneacyl , -CN , -SO3H , CF3 , and -NO2 are meta directors anddeactivate the ring toward electrophilic aromatic substitution.
  52. 52. HALOGENATION OF ALKYLBENZENE SIDE CHAINSide chain bromination at the benzylic position occurs when aklybenzene is treatedwith N-bromosuccinimide (NBS)
  53. 53. Mechanism of NBS (Radical) Reaction•Abstraction of a benzylic hydrogen atom generates an intermediate benzylic radical•Reacts with Br2 to yield product•Br· radical cycles back into reaction to carry chain•Br2 produced from reaction of HBr with NBS
  54. 54. Free radical also occurs between alkylbenzene side chain with halogen in thepresence of heat or light (hv) CH3 hv CH2Cl + Cl2
  55. 55. OXIDATION OF ALKYLBENZENE•Alkyl Benzene ring is inert to strong oxidizing agents such as KMnO4 and chromicreagent•side chains react readily with oxidizing agents are converted into carbonyl group –COOH (Benzoic acid) CH3 KMnO4 COOH CH2 CH3 KMnO4 COOHH3C CH3 KMnO4 COOH COOH