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SC CHEM 334 - Mechanisms and Friedal Crafts Reactions

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Chem 334 1nd Edition Lecture 6 Outline of Last Lecture I. Reactions of side chainsOutline of Current Lecture II. Halogenation MechanismIII. Nitration MechanismIV. Friedal Crafts ReactionsA. AlkylationB. AcylationCurrent LectureSome General Reaction Mechanisms:Halogenation:Remember that a benzene ring will not react with Cl2 alone. It needs a Lewis acid(LA) catalyst to act as an electron acceptor. Cl Cl2 FeCl3Cl – Cl : Fe – Cl  Cl – Cl – Fe – Cl  Cl FeCl4  + Cl – FeCl3  L.B LAFeCl3 is not consumed in this reaction. It can go on and continue to promote the reaction. In this reaction, step A is the slow, ratedetermining step. Cl FeCl4  + Cl – FeCl3  Step B is the fast step, because you gain aromaticity. Br2 works the same way.These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute. HClCl.. Cl + - Cl + -.. Cl Cl-A chlorinium ion is a very good electrophile and π electrons attack this electrophileStep BStep AClHCl-The positively charged intermediate is resonance stabilized. The positive charge (attached to the figure after Step A) can only be placed on the ortho or para positions.Nitration: NO2 H2SO4/HNO3/ΔIn this reaction, the nitrogen ion is a super electrophile. O O H – O – S – O – H H – O – NO2 HSO4  +N  H2O  O OFriedal Crafts Alkylation:Friedel Crafts alkylation is the alkylation of an aromatic ring with an alkyl halide using a strong Lewis acid catalyst. R R – Cl AlCl3Mechanism: Cl : AlCl3  Cl + Al-Cl3  + -AlCl4  + Cl – AlCl3 Limitations:1) You can only use sp3 hybridized alkyl halides2) The carbocation intermediates can rearrange. Primary alkyl halides don’t work (except methyl and ethyl groups).3) If the electron withdrawing group is already on the ring, it doesn’t workFriedal Crafts Acylation:Friedel Crafts acylation is the acylation of aromatic rings with an acyl chloride (or an acid anhydride) using a strong Lewis acid catalyst. The limitations of Friendal Crafts acylation are similar to Friedel Crafts. However, this reaction is more advantageous than the alkylation because the ketone product is less reactive than the original product, due to the electron-withdrawing effect of the carbonyl group. This means that multiple acylations will not occur. Another advantage of this reaction is that - N- NO2 O OH..+-....H-....there are no carbocation rearrangements, since the carbonium ion is stabilized by a resonance structure in which the positive charge is on the oxygen.Mechanism: O O R Cl R AlCl3 O O O O O + O + + R Cl AlCl3  R Cl AlCl3  R R  R  Acylium ion is stabilized by resonanceHCCl-Al3


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SC CHEM 334 - Mechanisms and Friedal Crafts Reactions

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