Benzene Reactions: Study with Video Lessons, Practice Problems & Examples

Benzene, an aromatic compound, undergoes substitution reactions due to its unique stability. Two key reactions are halogenation and Friedel-Crafts alkylation. In halogenation, benzene reacts with X₂ (Br₂ or Cl₂) using a catalyst like FeX₃, replacing a hydrogen atom with a halogen. Friedel-Crafts alkylation involves benzene reacting with an alkyl halide (R-X) and a catalyst (AlX₃), substituting a hydrogen with an alkyl group. Both reactions maintain benzene's aromatic stability while introducing new functional groups.

Benzene Reactions Concept 1

Now we've seen benzene a few times within this chapter. Now benzene itself is what we call an aromatic compound. And aromatic compounds have an unusually or uniquely stable condition. Benzene possesses Pi bonds, but because of its aromaticity and its unique stability, it doesn't undergo additional reactions like we would expect. Benzene undergoes substitution reactions instead, and that's unlike alkenes and alkynes which do addition, because they have pi bonds, but they're not uniquely stable. Now here we're going to cover 2 types of substitution reactions that Benzene undergoes. One being Halogenation reactions, where we're substituting in a Halogen onto the Benzene ring, and then Friedel-Craft Alkylation, where we're substituting an alkyl group onto the benzene ring. So just remember, Benzene is pretty special. It's aromatic and very stable. It's gonna do substitution reactions instead of addition reactions, because it wants to maintain this very highly stable form that it possesses.

Halogenation Reactions Concept 2

In this video, we're going to take a look at the halogenation of benzene. Here, benzene would react with either Br₂ or Cl₂. In this reaction, one of the hydrogen atoms on the benzene is substituted with an X, which stands in for either bromine or chlorine. Here we'd say that a catalyst in the form of FeX₃, where X₃ is either Br₃ or Cl₃, must be used and has to contain the same halogen as the reagent. So basically, what I mean by this is if X₂ here is Cl₂, then this has to be FeCl₃; if X₂ here is Br₂, then this has to be FeBr₃. So, if we take a look here at this benzene halogenation reaction example, we have our benzene with its six hydrogens on it. Remember, X here equals either bromine or chlorine. What happens is that one of these hydrogens gets replaced by this halogen, one of them. And so here, the H is now gone, and instead in its place we put one of the halogens. That halogen could be chlorine or bromine. This is a way that we can substitute in a halogen at the expense of losing a hydrogen on benzene.

Halogenation Reactions Example 1

Of the following, which is a correct product of benzene substitution reaction with Br2? So remember, here we have the bromination of benzene, where we're going to substitute out a hydrogen on benzene for a bromine. Option A doesn't work because we substituted in an iron, which doesn't work. Option B doesn't work because we only substitute in 1 bromine, not 2. Option C doesn't work because the hydrogen is still there. The answer here is option D because there was a hydrogen here, and we substituted it out to get the bromine there, creating bromobenzene as our answer. So here option D is the correct answer.

Name the product of the following benzene halogenation reaction.

• A. chlorobenzene
• B. 1,1-dichlorobenzene
• C. 1,2-dichlorobenzene
• D. 1,4-dichlorobenzene

Friedel Craft Alkylation Concept 3

In this video, we're going to take a look at Friedel-Crafts Alkylation. Here in this reaction, benzene reacts with an alkyl halide, and what happens is that one of the hydrogens on the benzene ring is substituted with an alkyl group. To do this, a catalyst of Aluminum _X3 must be used and has to contain the same halogen as the reagent. So basically, what we mean by this, if we take a look at this Friedel-Crafts reaction down below, is CH3X. This X could represent either bromine or chlorine. If it's bromine, then this has to be AlBr₃. If this X is chlorine, then this has to be AlCl₃. So, the halogens have to match between these two. Now, in this reaction, all that happens is that one of the hydrogens on benzene gets substituted out, and the alkyl portion of our alkyl halide comes in. The alkyl portion of our alkyl halide here would be methyl, so we end up making methylbenzene as our final answer.

Friedel Craft Alkylation Example 2

Name the major product of the following Friedel Crafts alkylation reaction. Alright. So here we have our Benzene, and we know we're still going to have Benzene at the end, so we can redraw it. And we know that essentially what happens is, one of the hydrogens on Benzene gets substituted out with the alkyl group portion of my alkyl halide. So here we have Ethyl Chloride and we have Aluminum Chloride as the catalyst. So here, we're going to attach Ethyl to my Benzene. This would be my major product form, and its name would be Ethylbenzene. So just remember with Friedel Crafts alkylation, we're just substituting out a hydrogen and bringing in an alkyl group.

Summary of Benzene Reactions Concept 4

Here we have a summary of our benzene reactions. We have halogenation, in it we have the reagent of X2, which could represent Br2 or Cl2. We use our catalyst to get things going. In essence what happens is that we substitute out one of the hydrogens on benzene with one of these halogens here. If this is Br2, then this has to be FeBr3. If this is Cl2, then this has to be FeCl3. At the end, the H is substituted out, the halogen comes in.

Next, we have Friedel-Crafts alkylation. Here we use as our reagent an alkyl halide. X again could be Cl or Br, but now we're using the catalyst of aluminum, and it could be either aluminum chloride or aluminum bromide. Here, we substitute out our hydrogen and bring in our alkyl group. Here if the X is Cl, then this is aluminum chloride. If the X here is bromine, then this is aluminum bromide. So remember, both of these reactions are quite similar, they use similar types of catalysts, and they're just substituting out an H. This one substitutes in a halogen. This one here an alkyl group.

Provide a complete reaction of benzene Friedel Crafts alkylation with 2-bromo-2-methylpropane and name the product.

• A.
• B.
• C.
• D.

Fill in the missing reagent for the provided reaction.

• A.
• B.
• C.
• D.

Here’s what students ask on this topic:

What is the halogenation reaction of benzene?

The halogenation reaction of benzene involves substituting one of the hydrogen atoms on the benzene ring with a halogen (either bromine or chlorine). The general reaction is:

$C_6{H}_6+X_2\to C_6{H}_5{X}_1+H_1{X}_1$

Here, X can be either Br or Cl. A catalyst such as FeX₃ (FeBr₃ for bromine or FeCl₃ for chlorine) is required to facilitate the reaction. The catalyst helps in the formation of the electrophile, which then substitutes the hydrogen atom on the benzene ring.

What is Friedel-Crafts alkylation of benzene?

Friedel-Crafts alkylation is a reaction where an alkyl group is introduced to the benzene ring. Benzene reacts with an alkyl halide (R-X) in the presence of a catalyst, typically AlX₃ (AlCl₃ for chlorine or AlBr₃ for bromine). The general reaction is:

$C_6{H}_6+R_X\to C_6{H}_5{R}_1+H_1{X}_1$

In this reaction, the alkyl group (R) from the alkyl halide replaces one of the hydrogen atoms on the benzene ring, forming an alkylbenzene. The catalyst helps in generating the electrophile, which then attacks the benzene ring.

Why does benzene undergo substitution reactions instead of addition reactions?

Benzene undergoes substitution reactions instead of addition reactions due to its aromatic stability. Benzene is an aromatic compound, meaning it has a conjugated π-electron system that is highly stable. Addition reactions would disrupt this stable π-electron system, leading to a loss of aromaticity. Substitution reactions, on the other hand, allow benzene to retain its aromatic stability by replacing a hydrogen atom with another substituent without disturbing the conjugated π-electron system. This unique stability is why benzene prefers substitution over addition reactions.

What catalysts are used in the halogenation and Friedel-Crafts alkylation of benzene?

In the halogenation of benzene, the catalysts used are iron halides such as FeCl₃ for chlorination and FeBr₃ for bromination. These catalysts help in the formation of the electrophile, which then substitutes a hydrogen atom on the benzene ring with a halogen.

In Friedel-Crafts alkylation, the catalysts used are aluminum halides such as AlCl₃ for reactions involving alkyl chlorides and AlBr₃ for reactions involving alkyl bromides. These catalysts facilitate the generation of the electrophile, allowing the alkyl group to replace a hydrogen atom on the benzene ring.

What is the role of the catalyst in benzene halogenation?

The catalyst in benzene halogenation, such as FeCl₃ or FeBr₃, plays a crucial role in generating the electrophile needed for the reaction. The catalyst reacts with the halogen (Cl₂ or Br₂) to form a complex that makes the halogen more electrophilic. This electrophilic halogen can then attack the benzene ring, substituting one of the hydrogen atoms with a halogen atom. Without the catalyst, the halogen molecule would not be sufficiently reactive to initiate the substitution reaction with benzene.