The Discovery of Benzene Benzene was discovered in 1825 by the English chemist Michael Faraday Royal Institution Faraday called this new hydrocarbon bicarburet of hydrogen Faraday isolated benzene from a compressed illuminating gas that had been made by ID: 1040055
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1. Benzene andAromatic Compounds
2. The Discovery of Benzene Benzene was discovered in 1825 by the English chemist Michael Faraday (Royal Institution)Faraday called this new hydrocarbon “bicarburet of hydrogen”.Faraday isolated benzene from a compressed illuminating gas that had been made by pyrolyzing whale oil.Micheal Faraday(1791 – 1867) English organic chemistEilhardt Mitscherlich(1794 – 1863) German organic chemist In 1834 the German chemist Eilhardt Mitscherlich (University of Berlin) synthesised benzene by heating benzoic acid with calcium oxide.
3. Friedrich August Kekulé (1829 – 1896) German organic chemistStructure of Benzene
4. Hybridization of Benzene
5. C-C bond is 1.54 Å C=C bond is 1.34 Å But, All Benzene C to C bonds are 1.39 ÅExample:If each double bond was independent!HClWhy is Benzene so Unreactive to Addition Reactions?
6. Why is Benzene so Unreactive to Addition Reactions?
7. =Aromatic Properties: Planar Cyclic Conjugated Undergoes substitution reactions that retain its planer conjugation – No Electrophilic Addition Reactions!Why is Benzene so Unreactive to Addition Reactions?
8. HClCl2H2O, H+BH3H2O2, HO-H2PtKMnO4NOReactionNOReactionNOReactionNOReactionNOReactionNOReaction Undergoes substitution reactions that retain its planer conjugation – No Electrophilic Addition Reactions!Loss of Conjugation!AromaticityWhy is Benzene so Unreactive to Addition Reactions?
9. A Substitution Reaction of BenzeneUndergoes substitution reactions that retain its planer conjugation – No Electrophilic Addition Reactions!+RetainsConjugation!
10. Aromatic Properties: Planar Cyclic Conjugated Undergoes Substitution Reactions that retain its Planer Conjugation – No Electrophilic Addition Rxs! Hückel 4n +2 electrons=Huckel’s Rule: The 4n+2p Electron Rule
11. 11The AnnulenesAnnulenes are monocyclic compounds with alternating double and single bondsAnnulenes are named using a number in brackets that indicates the ring sizeBenzene is [6]annulene and cyclooctatetraene is [8]annuleneAn annulene is aromatic if it has 4n+2p electrons and a planar carbon skeletonThe [14]and [18]annulenes are aromatic (4n+2, where n= 3,4)The [16] annulene is not aromatic
12. 12Benzenoid Aromatic CompoundsPolycyclic benzenoid are aromatic compounds have two or more benzene rings fused together
13. Aromatic Compound NomenclatureCommon Names
14. 1. Name the substituent and then the parent, benzene:Aromatic Compound NomenclatureIUPAC NamesIf the alkyl chain has more carbons, then the benzene ring becomes a substituent phenyl (Ph- , C6H6- , φ-):ChlorobenzenePropylbenzeneNitrobenzeneflourobenzene
15. para (1,4-)3. When two substituent are present, use these isomeric designations:Aromatic Compound NomenclatureIUPAC Names1,2-dibromobenzeneo-dibromobenzene1,3-dibromobenzenem-dibromobenzene1,4-dibromobenzenep-dibromobenzene3-iodophenolm-iodophenolortho (1,2-)ortho (1,2-)meta(1,3-) meta(1,3-)
16. If more than two substituents, number the ring using the lowest possible numbers.When more than two substituents are present and the substituents are different, list them in alphabetical order .Aromatic Compound NomenclatureIUPAC Names4-bromo-1,2-dimethylbenzene2-chloro-1,4-dinitrobenzene2,4,6-trinitrotoluene2,6-dibromophenol3-chlorobenzoic acidm-chlorobenzoic acid
17. Reactions of Aromatic Compounds
18. Electrophilic Aromatic SubstitutionArene (Ar-H) is the generic term for an aromatic hydrocarbonThe aryl group (Ar) is derived by removal of a hydrogen atom from an areneAromatic compounds undergo electrophilic aromatic substitution (EAS)The electrophile has a full or partial positive charge
19. A General Mechanism for Electrophilic Aromatic Substitution: Arenium Ion IntermediatesBenzene reacts with an electrophile using two of its p electronsThis first step is like an addition to an ordinary double bondUnlike an addition reaction, the benzene ring reacts further so that it may regenerate the very stable aromatic systemIn step 1 of the mechanism, the electrophile reacts with two p electrons from the aromatic ring to form an arenium ionThe arenium ion is stabilized by resonance which delocalizes the charge In step 2, a proton is removed and the aromatic system is regenerated
20. Halogenation of BenzeneHalogenation of benzene requires the presence of a Lewis acid.Fluorination occurs so rapidly it is hard to stop at monofluorination of the ring. A special apparatus is used to perform this reaction.Iodine is so unreactive that an alternative method must be used.
21. Nitration of BenzeneNitration of benzene occurs with a mixture of concentrated nitric and sulfuric acidsThe electrophile for the reaction is the nitronium ion (NO2+)Sulfonation of BenzeneSulfonation occurs most rapidly using fuming sulfuric acid (concentrated sulfuric acid that contains SO3) The reaction also occurs in conc. sulfuric acid, which generates small quantities of SO3, as shown in step 1 below
22. Friedel-Crafts AlkylationAn aromatic ring can be alkylated by an alkyl halide in the presence of a Lewis acid The Lewis acid serves to generate a carbocation electrophile
23. Synthetic Applications of Friedel-Crafts Acylations: The Clemmensen ReductionPrimary alkyl halides often yield rearranged products in Friedel-Crafts alkylation which is a major limitation of this reaction.Unbranched alkylbenzenes can be obtained in good yield by acylation followed by Clemmensen reduction.Clemmensen Reduction reduces phenyl ketones to the methylene (CH2) group.
24. Effects of Substituents on Reactivity and OrientationThe nature of groups already on an aromatic ring affect both the reactivity and orientation of future substitutionActivating groups cause the aromatic ring to be more reactive than benzeneDeactivating groups cause the aromatic ring to be less reactive than benzeneOrtho-para directors direct future substitution to the ortho and para positionsMeta directors direct future substitution to the meta positionActivating Groups: Ortho-Para DirectorsAll activating groups are also ortho-para directorsThe halides are also ortho-para directors but are mildly deactivatingThe methyl group of toluene is an ortho-para director Toluene reacts more readily than benzene, e.g. at a lower temperatures than benzene
25. The methyl group of toluene is an ortho-para directorAmino and hydroxyl groups are also activating and ortho-para directorsThese groups are so activating that catalysts are often not necessary Alkyl groups and heteroatoms with one or more unshared electron pairs directly bonded to the aromatic ring will be ortho-para directors
26. Deactivating Groups: Meta DirectorsStrong electron-withdrawing groups such as nitro, carboxyl, and sulfonate are deactivators and meta directorsHalo Substitutents: Deactivating Ortho-Para DirectorsChloro and bromo groups are weakly deactivating but are also ortho, para directorsIn electrophilic substitution of chlorobenzene, the ortho and para products are major:
27. Classification of Substitutents
28. Oxidation of the Side ChainAlkyl and unsaturated side chains of aromatic rings can be oxidized to the carboxylic acid using hot KMnO4
29. Synthetic ApplicationsWhen designing a synthesis of substituted benzenes, the order in which the substituents are introduced is crucialExample: Synthesize ortho-, meta-, and para-nitrobenzoic acid from toluene