Ch. 18 - Reactions of Aromatics: EAS and BeyondWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins
Electrophilic Aromatic Substitution
Benzene Reactions
EAS: Halogenation Mechanism
EAS: Nitration Mechanism
EAS: Friedel-Crafts Alkylation Mechanism
EAS: Friedel-Crafts Acylation Mechanism
EAS: Any Carbocation Mechanism
Electron Withdrawing Groups
EAS: Ortho vs. Para Positions
Acylation of Aniline
Limitations of Friedel-Crafts Alkyation
Advantages of Friedel-Crafts Acylation
Blocking Groups - Sulfonic Acid
EAS: Synergistic and Competitive Groups
Side-Chain Halogenation
Side-Chain Oxidation
Birch Reduction
EAS: Sequence Groups
EAS: Retrosynthesis
Diazo Replacement Reactions
Diazo Sequence Groups
Diazo Retrosynthesis
Nucleophilic Aromatic Substitution
Additional Practice
EAS: Sulfonation Mechanism
EAS: Gatterman–Koch Reaction
EAS: Total Benzene Isomers
EAS: Polycyclic Aromatic Hydrocarbons
EAS: Directing Effects
Resonance Theory of EAS Directing Effects
EAS: Badass Activity Chart
Activated Benzene and Polysubstitutions
Clemmensen Reduction
EAS: Dueling Benzenes
Hydrogenation of Benzene
EAS: Missing Reagent
EAS: Synthesis
Diazonization of Aniline
Diazo Coupling Reactions
SNAr vs. Benzyne
Aromatic Missing Reagent
Aromatic Synthesis
Aromatic Retrosynthesis
EAS on 5-membered Heterocycles

Friedel-Crafts Acylation has several advantages that make it much more synthetically useful than alkylation. There are 3 in particular that I want you to know. 

Concept #1: Advantages


Now let's discuss how Friedal-Crafts Acylation is so much more effective than Friedal-Crafts Alkylation. So Friedal-Crafts Acylcation has several advantages that are going to make it much more synthetically useful than alkylation and a few of these we've already discussed basically that acylation reactions are going to deactivate the ring to further reactions favouring mono substitution which is a big deal in organic chemistry synthesis. We want to make sure that we're only adding one group at a time not two, not four. Also we learned that acylation reactions are not susceptible to carbocation rearrangement because the acylium ion can't resonate. Perfect, so here I just want to show you as an example for how two synthesis could go completely different directions depending on which one you use. So let's say that we're trying to add a three carbon chain to the benzene ring. First we're going to use acylation. Acylation, what I'm going to wind up getting is an acylium ion that looks like this. It's going to be carbon with double bond O positive and a three carbon chain. Everyone cool with that? So I'm going to wind up attacking and after all our arenium ions don't worry about it too much we're going to wind up getting a product that looks like this. It's going to have a three carbon chain, it's going to be a ketone with a three carbon chain and that's it we're not going to get a second reaction we're not going to get a rearrangement that's it. Now notice what happens when we try to use alkylation. Alkyaltion may seem like the more, the better choice because we don't want the ketone we just want the chain.

So our first thought would be let's just use alkylation because alkylation doesn't give us that ketone but guys alkylation is going to give so many more problems because look at this. What happens when the bond gives its electrons to the aluminum? Remember guys that primary carbocations are unstable right so that means that a rearrangement is actually going to happen right in the mechanism right away this H is going to wind up doing a 1, 2 hydrite shift and making a carbocation that looks like this so all of a sudden my carbocation doesn't look like a three carbon chain that's a street chain it looks like a branch chain it's attaching in the middle. So what I'm going to wind up getting is I'm going to wind up getting now an isopropyl benzene but that's not all I'm going to get, I might actually get another isopropyl or I might even get another one, who knows? I'm going to get a poly-substituted, poly-isopropyl benzene. All I wanted was a three carbon chain and look what a disaster this is. This is why alkylation is so limited and why we're usually going to go with an acylation over an alkylation because alkylation just gives us a mess of rearranged products and possible poly-substituted products. Now there is one way to overcome this which would be for this specific reaction what we could do is we could increase the equivalence of benzene. Now I noticed here I put one equivalent each of both of these. If I really want to use alkylation I could increase the equivalence.

Let's say I made it to one hundred equivalence of benzene. Now that would promote mono-substitution. That would promote mono-substitution because now I have excess benzene that most likely isn't going to find that many carbocations to react with, most likely it only reacts with one but still that would still give me an isopropyl benzene so even in that case I would still have isopropyl benzene because it's going to rearrange. So it's like acylation give us exactly what we want but unfortunately there's that stupid ketone. If only there was a way to get rid of that ketone then we would have a three carbon chain exactly the way we want it. Wait, we do have a way to get rid of that ketone and that is the glorious advantage of Clemmenson reduction. So it turns out that acylation products can be converted to alkylbenzenes using a reagent called the zinc amalgam in a reaction called the Clemmenson reduction. Now the mechanism for this reaction is still unknown to this day. No one really knows what the mechanism is but you do need to recognise and memorise the reagents. The reagents are a zinc, mercury amalgam over a strong acid H C L and what that's going to give us is it's actually going to add hydrogens to where the carbonyl was was and it's just going to give us an alkyl product meaning that guys if we want to get a three carbon chain instead of using an alkylation I could just use an acylation and then do a Clemmenson reduction and get rid of that carbonyl and we're done so let me show you guys the true, the best way to prepare n-propylbenzene. Now if you guys don't remember if you see that little N in the front that just means it's a straight chain so we're trying to get N straight chain three carbon benzene. What we could do is we could acylate first that's going to give me a mono-substituted product that looks like this it's a ketone that's not what we want but wait we've got Clemmenson reduction. What does Clemmensen reduction do? It zaps that carbonyl completely so I'm going to take myself out of the screen what I wind up getting is just n-propyl benzene which is what I wanted. So this is why we're going to really emphasize that acylation is better than alkylation and any time you want to add an R group try adding it with an acylation first and then doing a Clemmenson because it's probably going to be more efficient. So that's it for this video, let's move on to the next.