[rleung]

Hi,

I have a question as to why the following reaction yields the following product.  With the way my textbook presents Friedel-Crafts alkylations, it almost seems that if there is chance for a rearrangement, the rearrangement will happen.  In this case, I would predict that the primary carbocation would rearrange into a more stable secondary, but that is not the case.  Thanks.

Ryan

[plu]

The stability of a six-membered ring over a five-membered ring is possibly the reason behind the observed product.  In addition, since the reaction is unimolecular, it is possible that the carbocation rearrangement (the hydride shift) does not occur due to kinetic reasons as well (the reaction simply goes too fast).

[Yggdrasil]

I agree.  In general, for Friedel-Crafts alkylation, the intramolecular rearrangement of the alkyl carbocation is kinetically favored over the intermolecular reaction of the carbocation with the aromatic system.  However, in this case, since both reactions are intramolecular, the assumption that the rearrangement will occur faster than the EAS is not as valid.

[rleung]

Thanks so much