In the first step, I'm going to sulfonate. That sulfonation reaction is going to produce a sulfonic acid group. It's going to be para because like I told you guys, you get very high yields of the para product because para is favored over ortho. Now we're going to nitrate. Concentrated HNO3 is a nitration reaction. So now what we're going to do is we're going to get nitration. And where is it going to add? Well guys, it has to add here. Let's try to use a different color. It has to add here because the OH is still an ortho-para director. It's still going to direct ortho-para but it can't add para anymore because there's something there already. Then you have to add ortho. Then as you guys know, dilute acid heat is going to desulfonate and I'm going to end up with my final product that looks like this, OH and NO2. So I get my ortho substitution. I know there might be a few of you out there wondering, Johnny, if you did the nitration straight on phenol, wouldn't the major product be ortho anyway? I actually talked about how this is one of the exceptions; how you can get hydrogen bonding, so it actually favors ortho. You're exactly right and I'm glad you've been paying such close attention. But the problem is it still wouldn't be that great. It'll be like 60% if you just did it by itself. If all you did was you did concentrated HNO3, right? What you would get is a phenyl with a nitro group in about a 60% yield. But if you do this 3-step pathway, it's not much harder. You're going to get a 90% plus yield. See what I mean? Even in a situation where it would have been favored to go ortho, it's still better to use a blocking group so you can get a higher yield blocking that para position. Does that make more sense now? Awesome guys. Great example. Let's move on to the next topic.
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Blocking Groups - Sulfonic Acid - Online Tutor, Practice Problems & Exam Prep
Sulfonation is a key synthetic strategy in electrophilic aromatic substitution (EAS) that allows for ortho substitution by blocking the para position. Using concentrated sulfuric acid, sulfonation introduces a sulfonic acid group, which can later be removed through desulfonation. This method enhances yields of ortho products by preventing para addition, as the sulfonic acid acts as a meta director. Understanding this blocking group concept is crucial for optimizing synthetic pathways in aromatic chemistry.
Let's discuss the one reversible EAS reaction and how we could use it to benefit us, specifically by acting as a blocking group.
Forcing Ortho substitution
Multistep Synthesis
Video transcript
Beginning from Benzene, synthesize the following compound.
Problem Transcript
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More setsHere’s what students ask on this topic:
What is the role of sulfonic acid as a blocking group in electrophilic aromatic substitution (EAS)?
Sulfonic acid acts as a blocking group in EAS to enhance ortho substitution by preventing para addition. When sulfonation is performed using concentrated sulfuric acid, a sulfonic acid group is introduced at the para position. This group is a strong meta director, which blocks the para position and forces the incoming electrophile to add at the ortho positions. After the desired ortho substitution is achieved, the sulfonic acid group can be removed through desulfonation using heat and acid or steam, leaving the final product without the sulfonic acid group.
How does sulfonation help in achieving high yields of ortho-substituted products?
Sulfonation helps achieve high yields of ortho-substituted products by blocking the para position, which is typically more favored in EAS reactions. By introducing a sulfonic acid group at the para position using concentrated sulfuric acid, the para position is blocked, and the electrophile is directed to the ortho positions. This method ensures that the ortho substitution is favored, leading to higher yields of the desired ortho product. After the reaction, the sulfonic acid group can be removed through desulfonation, leaving the ortho-substituted product.
What is the process of desulfonation, and how is it used in synthetic pathways?
Desulfonation is the process of removing a sulfonic acid group from an aromatic ring. This can be achieved by heating the compound with dilute acid or steam. In synthetic pathways, desulfonation is used after the sulfonic acid group has served its purpose as a blocking group. Once the desired ortho substitution is achieved, the sulfonic acid group is removed, leaving the final product without the blocking group. This step is crucial for obtaining the pure ortho-substituted product without any residual sulfonic acid group.
Why is sulfonation considered a reversible reaction in electrophilic aromatic substitution?
Sulfonation is considered a reversible reaction in EAS because the sulfonic acid group introduced during sulfonation can be easily removed through desulfonation. This reversibility is achieved by heating the compound with dilute acid or steam, which breaks the bond between the aromatic ring and the sulfonic acid group. The ability to reverse the sulfonation reaction makes it a valuable tool in synthetic chemistry, allowing chemists to use the sulfonic acid group as a temporary blocking group to direct subsequent reactions.
What are the advantages of using sulfonic acid as a blocking group in aromatic chemistry?
The advantages of using sulfonic acid as a blocking group in aromatic chemistry include its ability to enhance the yield of ortho-substituted products by blocking the para position, its ease of introduction through sulfonation using concentrated sulfuric acid, and its reversibility through desulfonation. This makes sulfonic acid a versatile and effective tool for directing electrophilic aromatic substitution reactions, allowing chemists to achieve high yields of desired products with precise control over substitution patterns.