Question from DAT Destroyer Orgo #53

[snlassen]

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Just an alkylation question from the Orgo section of DAT Destroyer 2011, #53. If you're doing a Friedel-crafts alkylation of benzene with 1-chloropropane, I just assumed it would add the carbon chain in linear fashion to benzene (benzene attaching to carbon 1). But according to the solutions section, it looks like carbon 2 of propane is what attaches to benzene. Can anyone help me understand why? I know rearrangements can occur when carbocations are formed, but I thought that in the case of a primary alkyl halide, no carbocation could be formed, and that benzene just reacts directly with the lewis-acid-lewis-base compound.... Thanks!

 

[ijk90825]

AlCl3 takes cholorine and makes the 1-chloropropane form a primary carbocation, and hydride shift occurs and secondary carboncation forms and attaches to the benzene.

 

[snlassen]

Ok, so a primary carbocation does form in this case, but only momentarily before the hydride shift occurs? I guess I thought that the carbocation couldn't even form at all given that it was primary. But that makes sense given the answer to the question. Thanks!

 

[ijk90825]

yeah cuz the role of AlCl3 is to take the chlorine from the R-Cl group, therefore carbocation forms, and hydride shift occurs

 

[OUDent88]

For friedle-crafts, the rule about rearrangement will apply to anything 3 carbons long or more because those will all be able to form a more stable product.