Let's take a look at the exact mechanism of Friedel-Crafts acylation. Friedel-Crafts acylation involves an acyl halide, typically acid chloride, complexing with a Lewis acid catalyst to produce an electrophile. In this case, my electrophile is not going to be a carbocation like in alkylation. It's going to be an acylium ion. Now, what does an acylium ion look like? It resembles a carbon with a double bond to oxygen and an R group, carrying a positive charge. That is an acylium ion. Can you think of why that would be a good electrophile? It holds a full positive charge.
Now it is resonance stabilized, so there are two different ways that it can be drawn. One way is to take these electrons and move them into that carbon to form a triple bond. Another way to represent it is to move the positive charge up to the oxygen. It is important to note that both are representations of the acylium ion. The hybrid of these forms is a blend of both structures. However, the structure that helps us visualize the mechanism best is the first one, as it shows the positive charge on the carbon, which is the site of attack. Thus, I will draw the first resonance structure when discussing the acylium ion.
What are we aiming to produce at the end? Our goal is to synthesize ketones on the benzene ring. Let's delve into this mechanism. There is nothing particularly tricky about this mechanism. We are going to donate our chlorine to the Lewis acid catalyst which will give us an acylium ion: carbon double bond oxygen, single bond R positive, plus AlCl4 negative. This forms a potent electrophile for benzene to attack, capturing the carbon.
It is crucial to remember that this is not a carbocation intermediate reaction, meaning we do not have to contend with shifts. There are no rearrangements. The reason there are no rearrangements is that this is a resonance-stabilized electrophile; it does not desire to break resonance by migrating to the R group. It prefers to remain in its current state, which supports resonance. Hence, we don't need to worry about any rearrangement complications which simplifies our understanding significantly.
As we progress, we will discuss more details, but now, let’s draw out this full mechanism. All we do is make a bond. We don't have to break any bond; the full cation with an empty orbital can accept those electrons. Now, drawing our sigma complex: with the carbon double-bonded to OR positive. As the process continues, we shift the electrons, draw the double bond and move the cation around. Although this is correct, remember it is essential to include the hydrogen in the drawing. The hydrogen plays a key role in the elimination step. We are going to use the conjugate base, AlCl3 negative, to grab the electrons from that bond and perform a beta elimination. This step will yield a ketone and regenerate our Lewis acid catalyst while producing an acid as a byproduct.
Again, no rearrangements are possible here, making the mechanism straightforward. You always end up with your desired ketone on the benzene ring. That concludes this mechanism. Let's move on to the next one.
