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
Sections
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
Benzyne
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

The birch reduction is a dissolving metal reduction, except reacting with benzenes instead of alkynes. The product of an unsubstituted benzene is a simple isolated cyclohexadiene.

Concept #1: Birch Reduction Mechanism

Transcript

Hey guys! In this video, we're going to talk about a specific type reduction reaction that can happen with benzene. That's called the birch reduction.
Let's just take a look at the general reaction for a second. What a birch reduction does is it combines elemental sodium within an amine and alcohol to turn a benzene into what we call an isolated diene. Specifically, if this were to happen with an unsubstituted benzene like we have here, our product would be an isolated cyclohexadiene, two double bonds that are far apart from each other in a 1,4 position on a cyclohexane.
If you take a closer look at these reagents, they might look familiar because these are very similar to the reagents that we use on a dissolving metal reduction. This is a reaction from Orgo 1 that we learned a long time ago that worked with alkynes. It was a radical mediated mechanism. It turns out that this mechanism is really the same exact mechanism, except it’s going to work with benzene instead of with an alkyne. Let's get right into it.
The mechanism for this reduction is going to proceed through elemental sodium which means it’s going to possess just one electron. When that one electron donates to any of the carbons, we're going to have to break a bond. But this is going to be a mechanism where we have a combination up half-headed arrows and normal arrows just like the dissolving metal reduction, how there were some arrows that moved one radical and some arrows that moved a lone pair. When we make that bond, we have to break this bond in order to make room for the radical.
In order to keep these charges as far away from each other as possible or these intermediates as far away from each other as possible, this double bond is going to ionize into a lone pair on to the very bottom. Basically the furthest position possible from the radical, we're going to get an anion. Let’s go ahead and draw the product of this first step. What we're now going to get is a single radical at the top, double bonds on both sides and now a lone pair at the bottom which is going to be a carbanion.
This intermediate is called a radical anion which makes sense because that's what it is. It's a radical and it’s an anion. This is where our ethanol comes in. Our ethanol is going to serve as a propagating agent. Just so you know, ethanol isn’t the only alcohol you can use. Some text use tert-butanol. It doesn't matter. It’s the source of hydrogen. That's the biggest deal.
EtO-H my anion is going to grab the H and give a negative charge. What I’m going to get is a molecule that looks like this. I got my two double bonds. I still have my radical. But now I have two H’s at the bottom because I have one originally and now I just added a second which is the one that came from the ethanol.
At this point, I react with another equivalent of my elemental sodium. That elemental sodium is going to donate electrons to that same location. Now I'm going to get a lone pair anion. This is just a carbanion intermediate. This reaction just repeats itself. That's one thing about maybe dissolving metal reduction if your recall. It was the same thing twice.
Here, we would react again with another equivalent of ethanol. We would wind up getting our isolated diene because now I’ve got two H’s in the bottom. I've got two H’s on the top. I’ve got my isolated diene which is this molecule here.
For this reason and the fact that it reacts twice, sometimes you might see professors actually write ethanol times two or alcohol times two. It doesn't matter. It’s just going to have enough equivalents to make the reaction go to completion.
That's really it. That's the mechanism for Birch reduction. Now what we're going to do is we’re going to talk about specific regiochemistry that you have to consider with a Birch reduction.

Mechanism:

Concept #2: Regiospecific products

Transcript

Since this reaction always passes through an anion intermediate we can actually use activating groups and deactivating groups to direct the site of the isolated diene. How does that work? Well let's just take a look at the anion or the carbanion intermediates. This would be the point where we have the two double bonds, we have the two H's and we have a lone pair negative at the top. Let me ask you a question, if I add an electron withdrawing group to that anion, what do you think it does for stability? Do you think it makes that anion more stable or less stable? So hold that thought, now what happens if I add an electron donating group to that anion? So whatever I add to that is going to give more electrons to the negative, what does that do for the stability?

So the answer was that the first one is going to make it more stable because it pulls electrons away. An electron donating group is actually going to make it less stable because it's going to push more electrons into the anion so it turns out that these different groups are going to direct where the double bonds go. So as you guys can see withdrawing groups are going to what I say isolate themselves from the diene and I specifically chose that word for a reason because withdrawing and isolate kind of mean the same thing, if you're withdrawing from the crowd that means you're isolating yourself so a withdrawing group is going to be isolated from the double bond, it's going to be away from the double bond and why is that? It's not just because we memorised it, it's because you know that it's going to stabilise the negative charge. So it's going to want to be where the negative charge was whereas donating groups are going to attach themselves directly to the diene like in this situation.

Why? Because I have electrons going into the ring and I don't want it to be here because it was there it would make my anion less stable, so I'm trying to put it in a place where it's not going to affect the stability where it's going to be fine. So electron donating groups attached to the ring and withdrawing groups isolate from the ring. If you don't remember the mechanism you can at least remember the way that I'm telling you which is that withdrawing isolates, so you can think of just your isolating yourself from the crowd you're withdrawing or donating attaches which is basically the opposite. Awesome guys, so really that's it for this topic. Let's move on to the next.

Substituents affect the course of the mechanism, yielding regiospecific products. 

Practice: Predict the major product from the Birch Reduction

Practice: Predict the major product from the Birch Reduction