Alkylation of benzene

Question

I was solving a question where I had to determine a way to convert benzene into n-propyl benzene.

I figured out a way by first performing Friedel-Craft acylation with propanoyl chloride and then performing Clemmensen reduction to form the desired compound.

On checking the solution, there were 2 alternative ways portrayed:

One was the same method I had used.

The other involved reaction of benzene with n-chloropropane in the presence of $\ce{FeBr3}$.

However wouldn't there be carbocationic rearrangements taking place that convert the carbocation to isopropyl carbocation?

Please tell me what prevents the rearrangements from happening in the second case.

Answer 1

Initially one would assume that cumene (isopropyl benzene) would be the major product. However, kinetic control does have a say in this reaction, according to Gilman and Means.¹

At - 2°, n-propyl bromide with benzene and aluminum chloride gave n-propylbenzene, identified as its sulfonamide. Genvresse (6) obtained both n-propylbenzene and isopropylbenzene by conducting the reaction at reflux temperature. Konowalow (7) found that below $0^\circ$, n-propyl chloride gave n-propylbenzene, while from $0^\circ$ to reflux temperature it gave mixtures of n-propylbenzene and isopropylbenzene. More recently it has been shown that at $-6^\circ$, 60% of the monopropylbenzene was n-propylbenzene, and 40% was isopropylbenzene. At $35^\circ$, this ratio was reversed.

a low temperature would facilitate the production of larger amounts of n-propylbenzene due to kinetic control. However, this is still not a very efficient method of preparing n-propylbenzene, and acylation followed by Clemmensen/Wolff-Kishner reduction would be more suitable here.

References:

1. Gilman, Henry, R. N. Meals, “Rearrangements in the Friedel-Crafts Alkylation of Benzene” Journal of Organic Chemistry 1943, 08(2), 126–146. doi:10.1021/jo01190a003.

Answer 2

Aniruddha Deb gave an excellent answer to your question. Yet, there is another important point of differences on two Friedel-Craft processes: Alkylation vs acylation. I admit that rearrangement is the major drawback on alkylation process (using alkyl halides with a catalyst). However, another fact is you cannot stop the reaction after monoalkylation. The reaction would proceed to give di- and trialkylated products if steric effect did not stop giving more substitution. The reason for this is because alkylation activates the benzene nucleus to electrophilic substitution. For example, monoalkyl benzene is more active than benzene.

In contrary to direct alkylation, the alkylation by the acylation first followed by reduction method gives you much cleaner product. The reason for this is monoacylation deactivate the ring (recall o,p-directive and m-directive groups). As a result the reaction stop after monoacylation. You got only one product.

Finally, the difference between two reduction conditions: 1) Clemmensen Reduction: This is done in acidic conditions, hence should avoid if your compound contains acid-sensitive groups. 2) Wolff-Kishner Reduction: This is done in basic conditions, hence should avoid if your compound contains base-sensitive groups.