EAS: Friedel-Crafts Acylation Mechanism

Let's take a look at the exact mechanism of Friedel-Crafts Acylation. So Friedel-Crafts Acylation is going to involve an acyl halide typically acid chloride complexing with a Lewis acid catalyst to produce an electrophile but in this case my active electrophile is not going to be a carbocation like alkylation, it's going to be an acylium ion. Now what does an acylium ion look like? It looks like this, you've got a carbon with a double bond to O and an R group with a positive charge. So that is an acylium ion. Can you think of why that would be a good electrophile? Full positive charge. Now it is resonant stabilised so there's two different ways that it can be drawn. That's one way, another way would be to take these electrons and move them into that carbon to become a triple bond and the other way to represent it would be to now move the positive up to the O. So just so you guys know these are both acylium ions obviously the hybrid is going to be a blend of both of these it's going to look like you know some mixture of those contributing structures but the one that's easiest for us to use that helps us to visualise the mechanism the best is this first one because the first one shows the positive on the carbon that's we're going to be attacking. So when I draw the acylium ion I'm going to draw the first resonant structure. Awesome, so notice that what are we trying to get at the end? We're trying to make ketones.

So we're, our end product will be a ketone on the benzene ring. Let's go ahead and take a look at this mechanism. Okay guys, so really there's nothing tricky about this mechanism. What we're going to do is we are going to donate our chlorine to the Lewis acid catalyst and what that's going to give me is an acylium ion. I'm going to get carbon, double bond O, single bond R positive plus my ALCL4 negative. So as you can imagine this is a wonderful electrophile for a benzene to react with and it's going to grab the carbon. Now something to keep in mind guys is that this is not a carbocation intermediate reaction so that means we don't have to worry about shifts so there's no rearrangements. You might say well why, that's not how I spell it let's try it again, arrangement. Guys, the reason that it's not going to rearrange is because this is a resonant stabilised electrophile so it doesn't want to break resonance by moving to the R group it just wants to stay where it is because where it is it can resonate so you don't have to worry about that so you don't worry about any of the complications of rearrangement which is great and we're going to discuss more later why that's so important. So let's go ahead and draw this full mechanism, that's it I'm just going to make a bond. Do I have to break a bond? No I have a full cation there, empty orbital, so I don't have to break a bond I can accept those electrons.

Now I'm going to draw my sigma complex which would look like this, now with C, double bond O, R positive and you guys know what to do. Move it over, draw your double bond, draw my cation here, move it over again and I get my cation up here. Now actually I mean this is right but always remember you need to draw that hydrogen. The hydrogen is important because it's part of the elimination step so let's just draw the hydrogen facing up. What do you think we're going to use, by the way I didn't draw the hydrogens in the other ones it's not necessary you can just draw the hydrogen in the last one if you want because hydrogens can always be implied in a bond-like structure. So what's going to happen in this last step? Yeah guys, you just use your conjugate base, your ALCL4, A L C L oops that's not what I meant let's erase, CLALCL3 negative and as always I'm just going to grab the electrons from that bond and do my beta elimination. What that's going to give me is now a ketone plus I'm going to get my Lewis acid catalyst that's regenerated plus I'm going to get my acid as a byproduct. Once again guys no rearrangements are possible here so there's less to think about, you're always just going to get your ketone on the benzene ring and that's it. Alright, so that's it for this mechanism. Let's move on to the next one.