Blocking Groups - Sulfonic Acid: Study with Video Lessons, Practice Problems & Examples

Multistep Synthesis

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.

Beginning from Benzene, synthesize the following compound.

Hey guys, let's take a look at the following practice question. So here it says, Beginning from Benzene, synthesize the following compound. So here we look. It says we're making this compound here which is 1,2-disubstituted. And here I'm saying this is the only isomer which means I cannot make any side products. Now, here we have an alkyl group. Alkyl groups are ortho-para directors. And here, since it's an ortho-para director, it would want the chlorine to go here, here or here. Now, it would have preferred the chlorine to go to the para position. Now, chlorine itself is also an ortho-para director. Now, chlorine is an ortho-para director so it would prefer it to go here, here, or here. But it would have wanted it to go para. So something's weird about this problem. More stable para product you have a 1,2-disubstituted benzene, then a blocking group could have been used. So when I say blocking group, I mean that the para position was blocked somehow by a group so that the next incoming group had no choice but to go ortho, thus giving us this 1,2-disubstituted product. Then later the blocking group was taken off. Now, the blocking group that we're talking about here is your SO3H group, your sulfonic acid group which we add through sulfonation. We know we're going to have to do sulfonation at some point here and then later take it off. Here we have to decide who goes on first. Should the alkyl group go on first or should the chlorine go on first? Alkyl halides, one limitation they have is that when meta directors are directly attached to benzene, they don't work. This is true for alkylation and acylation. Now, chlorine is not a meta group but it is weakly deactivating. So it's better to be safe than sorry. Here, we're going to say, let's add the alkyl group first and then add the chlorine later. Because even though it's not a meta director, it's still weakly deactivating. So, there's a chance that it cou

Here’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.