Benzene Reactions - Online Tutor, Practice Problems & Exam Prep

Benzene undergoes electrophilic aromatic substitution (EAS) reactions, requiring strong electrophiles and often a Lewis acid catalyst. Key reactions include halogenation (using FeX3), nitration (HNO3 and H2SO4), and sulfonation (H2SO4 and SO3). Friedel-Crafts reactions, such as alkylation (using alkyl halides and AlCl3) and acylation (using acyl chlorides), add R groups to benzene. Carbocations, strong electrophiles, can also react with benzene, enhancing organic synthesis potential. Understanding these mechanisms is crucial for manipulating aromatic compounds in organic chemistry.

EAS Reactions Overview

In this video, I'm going to introduce all of the different reactions that can occur to benzene through an EAS mechanism. As previously discussed, benzene is going to require very strong electrophiles in order to react through EAS. Some of these reagents are going to require catalysts in order to become strong enough for the benzene to react with them. Let's just take the first one into account, EAS halogenation. Halogenation would be a reaction with a double bond and a diatomic halogen, so something like X₂. But actually X₂ is not a reactive enough electrophile to react with a benzene. Guess what? If you just put X₂ in a mixture of benzene, you're not going anywhere. You're not going to make a substituted benzene. How do we get the X₂ to react? We need a catalyst, guys. This is going to be a Lewis acid catalyst, FeX₃. This is going to work for bromine. I'm going to put here where X is equal to specifically bromine or chlorine. You always want to make sure that when you're performing one of these EAS halogenations, that you're using the same halogen in your catalyst as you are in the actual electrophile. That's going to become important later when we look into the mechanism. As you can see, the product is now going to be a monohalogenated benzene, which is great. That's what we wanted. Now, it's important to distinguish this mechanism from another reaction we learned in the past just called halogenation. It's important that when you discuss this mechanism, you call it EAS halogenation, because specifically, this is the one that requires a Lewis acid catalyst.

Let's move on to the next one. It turns out that iodination is a little bit tricky. Iodine does not react with the Lewis acid catalyst of iron and 3 halogens to perform this EAS reaction. We're going to have to use nitric acid instead, HNO₃. That's one little peculiar thing that you have to memorize that iodination, in order to put an iodine on a benzene, we're going to need to use nitric acid, not our iron Lewis acid catalyst.

Let's move on. The next one we want to discuss is nitration. A nitro group is not a group that we've discussed a whole lot at this point of the course. I do want to just help you guys understand what a nitro group is by drawing it. It would be something like this. We have a nitrogen with a double bond O and a single bond O, and there's going to be formal charges. The single-bonded O has a negative charge. The nitrogen since it has four bonds has a positive. But notice that this has no net charge because they cancel out. That's why when you draw NO₂, you draw it neutral without charges even though those charges do exist. Now there are two different ways to add a nitro group to a benzene. But they're really the same thing. It's just that different websites, different textbooks like to use different combinations. The most common would be using nitric acid, HNO₃ and sulfuric acid. A combination of sulfuric acid and nitric acid is going to react together to make the active electrophile, as we will see later when we go into the mechanism. But just keep in mind that you could also generate this electrophile just through using concentrated HNO₃. In the first example, you have one acid reacting with another. When you use concentrated HNO₃, it just reacts by itself. One HNO₃ molecule reacts with another one and makes that active electrophile. Regardless of how you draw it, it's really going to produce the same product. That would be EAS Nitration. Also, one thing to keep in mind is that nitric acid can also be written differently. It can be written as HONO₂. This is just a little technicality that actually could make you guys stumble on an exam because the same compound as nitric acid, HNO₃, is just written differently. It's basically written on the connectivity in a condensed structure instead of just based on the molecular formula.

Let's go on to sulfonation, which is actually going to be the kind of the most special reaction we're going to talk about on this page because sulfonation is the only one with these two arrows. Let's discuss what that is. First of all, the product of sulfonation is it's going to make what we call a sulfonic acid group. That's what we have at the top. By the way, I forgot to write what the product of nitration would be. It would be a nitro group. You got nitro, and we've got sulfonic acid. These are literally functional groups that we just learned in this course. Sulfonic acid. The way that we do a sulfonation is to use sulfuric acid, so H₂SO₄ and O₃, sulfur trioxide. It turns out that when you heat up H₂

Here’s what students ask on this topic:

What is electrophilic aromatic substitution (EAS) in benzene reactions?

Electrophilic aromatic substitution (EAS) is a reaction mechanism where an electrophile replaces a hydrogen atom on a benzene ring. Benzene's aromaticity makes it less reactive, so strong electrophiles and often a Lewis acid catalyst are required. Common EAS reactions include halogenation, nitration, sulfonation, and Friedel-Crafts alkylation and acylation. These reactions are crucial for modifying benzene rings in organic synthesis, allowing the introduction of various functional groups.

How does halogenation of benzene occur?

Halogenation of benzene involves the substitution of a hydrogen atom with a halogen (X₂, where X is typically Cl or Br). Benzene itself is not reactive enough to undergo halogenation without a catalyst. A Lewis acid catalyst like FeX₃ is used to generate a more reactive electrophile. For example, in bromination, Br₂ reacts with FeBr₃ to form Br⁺, which then substitutes a hydrogen atom on the benzene ring, forming bromobenzene.

What reagents are used in the nitration of benzene?

Nitration of benzene involves the introduction of a nitro group (NO₂) to the benzene ring. The reagents used are concentrated nitric acid (HNO₃) and sulfuric acid (H₂SO₄). Sulfuric acid acts as a catalyst, protonating the nitric acid to form the nitronium ion (NO₂⁺), which is the active electrophile. This nitronium ion then reacts with benzene to form nitrobenzene.

What is the difference between Friedel-Crafts alkylation and acylation?

Friedel-Crafts alkylation and acylation are both methods to introduce substituents onto a benzene ring. In alkylation, an alkyl halide (R-X) reacts with benzene in the presence of a Lewis acid catalyst like AlCl₃, adding an alkyl group (R) to the ring. In acylation, an acyl chloride (RCOCl) reacts with benzene, also using AlCl₃, to add an acyl group (RCO) to the ring. Alkylation forms alkylbenzenes, while acylation forms ketones (aryl ketones).

How does sulfonation of benzene occur and how can it be reversed?

Sulfonation of benzene involves the introduction of a sulfonic acid group (SO₃H) to the benzene ring. This is achieved using sulfuric acid (H₂SO₄) and sulfur trioxide (SO₃). The reaction can be represented as benzene + SO₃ → benzene-SO₃H. To reverse this reaction, known as desulfonation, dilute sulfuric acid (H₂SO₄) or steam is used, which removes the sulfonic acid group, regenerating benzene.