Friedel-Crafts Alkylation Practice Question 1

Friedel-Crafts alkylation produces a carbocation as an active electrophile that attacks the aromatic compound. In this reaction this step will look like this:

formation of a carbocationic electrophile in a friedel-crafts alkylation reaction

An important aspect of this step is that the newly formed carbocation is unstable. As an high-energy species, this carbocation will rearrange (if possible) into a more stable carbocation. In this case, we can do a hydride shift that would form a tertiary carbocation.

If a carbocationic rearrangement can yield a more stable carbocation, it will happen before any other step in the mechanism!

Thus, the hydride shift will occur before the carbocation can attack the aromatic ring. This is common trick I see on the tests, so make sure to look out for those carbocationic rearrangements!

After the carbocation has rearranged to a more stable version, the second step can proceed:

electrophilic attack by the carbocation from step 1 on the aromatic ring in the friedel-crafts alkylation reaction

So, now you can see that the correct answer to this question is option B.

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