No electrophilic aromatic alkylation to form a c c

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no electrophilic aromatic alkylation to form a C-C bond between and aromatic ring and a primary alkyl carbon. If you could get isobutyl chloride to react in the presence of AlCl 3 , it would form the tert -butyl cation, not the primary carbocation: Note that this gives you enough guidance to solve the problem even if you skip (or don’t understand) Hint 2. CH 3 H 3 C O CH 3 p -isobutylacetophenone ? R Cl CH 3 H 3 C AlCl 3 + Hint 1 tells you not to try this reaction Cl CH 3 H 3 C AlCl 3 Cl H 3 C H 3 C AlCl 3 H H 3 C CH 3 H 3 C H 3 C H 3 C H H H X
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Hint 2: you can buy 1-chloro-2-methyl-2-propene. This Hint tells you that the following chloride is commercially available: Hopefully, this gets you thinking about Problem 3b, and making allylic cations. More broadly, this should remind you that while you cannot make primary alkyl cations, you can make primary allylic and benzylic cations, because they are resonance stabilized. You are now prepared to analyze the synthesis problem. Focus on the product and the bonds that you need to form: The product tells you that you need to make one C-C bond between and aromatic ring and a carbonyl, and one C-C bond between an aromatic ring and a primary alkyl carbon. You only know one reaction for making the C-C bond between the aromatic ring and the carbonyl carbon: electrophilic aromatic acylation. You also know that you need a para (1,4) relationship between the two C-C bonds. Since carbonyl groups are meta directing and alkyl groups are ortho / para directing, you can now see that you need to do the electrophilic aromatic acylation last to get the para - substituted product: The question is now reduced to, “How do I make isobutylbenzene?” The Hints narrow down the range of possibilities you need to consider. Hint 1 reminds you that you cannot simply prepare isobutylbenzene by electrophilic aromatic alkylation. From lecture, you only know one way to get to a C-C bond between an aromatic ring and a primary alkyl carbon – sequential acylation and reduction of an aromatic ketone (using the Wolff-Kishner of Clemmensen reduction). A reasonable synthesis of isobutylbenzene could thus be: Cl CH 2 H 3 C CH 3 H 3 C O CH 3 CH 3 H 3 C O CH 3 CH 3 H 3 C AcCl, AlCl 3
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Hint 2 (and Problem 3) suggest an alternative approach to forming the C-C bond between the aromatic ring and a primary carbon – electrophilic aromatic allylation : Like the dimethylallyl cation in Problem 3, the 2-methylpropenyl cation intermediate can be formed because it is resonance stabilized, and it cannot undergo any rearrangements: Provided you still know the hydrogenation of alkenes from CH221, you are now only one step from isobutylbenzene: H 3 C CH 3 Cl O + AlCl 3 CH 3 H 3 C O H 2 NNH 2 , HO , or Zn/Hg, HCl, CH 3 H 3 C H 3 C CH 2 Cl + AlCl 3 CH 2 H 3 C CH 2 H 3 C H H CH 2 H 3 C H H CH 2 H 3 C H 2 /Pd CH 3 H 3 C
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  • Summer '10
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  • Benzene, H3C, Aromatic hydrocarbon

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