Friedel Crafts Alkylation of Benzene Reaction Mechanism - Tons of Examples!
TLDRThis video script delves into the Fréchet-Crafts alkylation reaction, focusing on the process of substituting a hydrogen atom in benzene with an alkyl group using an alkyl chloride and aluminum chloride as a catalyst. It outlines the basic reaction, discusses limitations such as polyalkylation, carbocation rearrangements, and the reaction's ineffectiveness with strongly deactivated rings. The script also explores alternative methods for alkylation, including using alcohols and hydrofluoric acid, and presents a strategy for synthesizing propyl benzene via the Fréchet-Crafts acylation followed by reduction.
Takeaways
- 🔬 The Fréchet-Crafts alkylation reaction involves the substitution of a hydrogen atom on a benzene ring with an alkyl group using an alkyl halide and a Lewis acid catalyst like aluminum chloride.
- 🚫 Limitations of the Fréchet-Crafts alkylation include the possibility of polyalkylation, where more than one alkyl group can be added to the benzene ring, leading to multiple products.
- 🔄 Carbocation rearrangements can occur during the reaction, leading to the formation of more stable carbocations and thus affecting the final product, as seen with propyl chloride leading to isopropyl benzene instead of propyl benzene.
- 💥 The reaction does not work well with strongly deactivated rings, such as those with nitro groups, due to the reduced nucleophilicity of the benzene ring.
- 🌐 Steric hindrance plays a role in determining the position of alkylation on the benzene ring; bulky groups tend to alkylate at the para position rather than ortho due to less steric repulsion.
- 📚 Alkyl groups are ortho/para directors, but the steric bulk can influence the actual position of the incoming alkyl group on the benzene ring.
- 🔌 The mechanism of the Fréchet-Crafts alkylation involves the formation of a carbocation intermediate, followed by electrophilic attack of the benzene ring and subsequent deprotonation to reform the aromatic system.
- ❌ Formation of unstable primary carbocations is avoided in the reaction; instead, more stable carbocations are formed through hydride or alkyl shifts.
- 🌐 Alternative methods to the Fréchet-Crafts alkylation, such as using alcohols with boron trifluoride or hydrofluoric acid with alkanes, can be employed to alkylate benzene rings.
- 🛠️ The Clemmensen or Wolff-Kishner reduction can be used to convert a ketone, formed from the Fréchet-Crafts acylation reaction, into an alkane, allowing the synthesis of specific alkyl benzenes like propyl benzene.
- 🔑 Understanding the limitations and mechanisms of the Fréchet-Crafts alkylation is crucial for predicting and controlling the products of aromatic substitution reactions.
Q & A
What is the general reaction involved in the Friedel-Crafts alkylation?
-The general reaction in the Friedel-Crafts alkylation involves the substitution of a hydrogen atom on a benzene ring with an alkyl group (R group) using an alkyl chloride and aluminum chloride as a catalyst.
What is the major product when benzene reacts with tert-butyl chloride in the presence of AlCl3?
-The major product of this reaction is tert-butyl benzene, which is the mono-alkylated product formed by the substitution of a hydrogen atom on the benzene ring with a tert-butyl group.
Can you explain the mechanism of the Friedel-Crafts alkylation reaction using tert-butyl chloride as an example?
-The mechanism begins with the reaction of tert-butyl chloride with AlCl3, forming an intermediate where the chlorine atom is attached to the aluminum atom. The chlorine then leaves, creating a tertiary carbocation intermediate. The benzene ring then attacks this carbocation, breaking the pi bond and forming a new carbon-carbon bond with the tert-butyl group. Finally, a base (AlCl4-) removes a hydrogen to regenerate the benzene ring, resulting in tert-butyl benzene.
Why does polyalkylation occur in the Friedel-Crafts alkylation reaction?
-Polyalkylation occurs because once an alkyl group is added to the benzene ring, it becomes more reactive than the original benzene due to the weakly activating nature of the alkyl group, making it more susceptible to further alkylation.
What is the limitation of using ethyl chloride in the Friedel-Crafts alkylation reaction?
-While ethyl benzene can be formed as the mono-alkylated product, the reaction can continue to form polyalkylated products due to the increased reactivity of the benzene ring after the first alkylation. Additionally, ethyl chloride can lead to the formation of ortho and para substituted products due to its ortho-para directing nature.
Why is it difficult to obtain propyl benzene as the major product when using propyl chloride in the Friedel-Crafts alkylation?
-The formation of a primary carbocation intermediate, which is unstable, is unfavorable. Instead, a hydride shift occurs to form a more stable secondary carbocation, leading to the major product being isopropyl benzene rather than propyl benzene.
What is the role of AlCl4- in the Friedel-Crafts alkylation mechanism?
-AlCl4- acts as a base to abstract a proton from the intermediate formed after the benzene ring attacks the carbocation, which helps to regenerate the aromatic benzene ring.
Why does the Friedel-Crafts alkylation reaction fail with strongly deactivated rings like nitrobenzene?
-The nitro group is a strong deactivator and makes the benzene ring less nucleophilic, preventing the Friedel-Crafts alkylation reaction from occurring effectively.
Can the Friedel-Crafts alkylation be performed without using an alkyl chloride? If so, how?
-Yes, the Friedel-Crafts alkylation can be performed using other reagents like 2-methylpropane with hydrofluoric acid or tert-butanol with boron trifluoride. These methods involve the formation of a carbocation intermediate that reacts with the benzene ring to form the alkylated product.
How can propyl benzene be synthesized from benzene if the Friedel-Crafts alkylation with propyl chloride yields isopropyl benzene as the major product?
-Propyl benzene can be synthesized by first using the Friedel-Crafts acylation reaction to introduce a keto group, followed by a reduction step, such as Clemmensen reduction, to convert the ketone to an alkane.
Outlines
🔬 Friedel-Crafts Alkylation of Benzene
This paragraph introduces the Friedel-Crafts alkylation reaction involving benzene, where an alkyl group is substituted for a hydrogen atom on the benzene ring using an alkyl chloride and aluminum chloride as a catalyst. The paragraph outlines the basic mechanism, including the formation of a carbocation intermediate and the subsequent reaction with the benzene ring. It also poses a question about the major product and mechanism when benzene reacts with tert-butyl chloride, highlighting the limitations of the reaction, such as polyalkylation due to the increased reactivity of the benzene ring after the initial alkylation.
📚 Ethyl Benzene Formation and Polyalkylation
The second paragraph delves into the reaction of benzene with ethyl chloride, using aluminum chloride as a catalyst, to form ethyl benzene. It discusses the potential for polyalkylation and the formation of ortho and para substituted products due to the ethyl group's ortho-para directing nature. The mechanism for ethyl benzene formation is explained, emphasizing the avoidance of unstable primary carbocations. The paragraph also touches on the increased reactivity of ethyl benzene compared to benzene and the resulting complex mixture of products that can arise from further alkylation reactions.
🔍 Carbocation Rearrangement in Propyl Chloride Reaction
This section examines the reaction of benzene with propyl chloride, where instead of forming propyl benzene, isopropyl benzene is the major product due to a carbocation rearrangement. The mechanism is detailed, explaining the formation of an unstable primary carbocation and its rearrangement to a more stable secondary carbocation. The reaction of the benzene ring with this secondary carbocation leads to the formation of isopropyl benzene. The paragraph also summarizes the limitations of the Friedel-Crafts alkylation, including polyalkylation, carbocation rearrangements, and the reaction's ineffectiveness with strongly deactivated rings.
🌐 Alternative Alkylation Methods
The fourth paragraph explores alternative methods for alkylating a benzene ring without using an alkyl chloride. It describes the use of 2-methylpropane with hydrofluoric acid, leading to the formation of a tertiary carbocation and ultimately butyl benzene. The mechanism involves the reaction of the alkene with HF, the formation of a carbocation, and the subsequent attack by the benzene ring. The paragraph also mentions the use of alcohols, such as tert-butanol with boron trifluoride, as another method for benzene alkylation, outlining the steps involved in this process.
🛠 Synthesis of Propyl Benzene via Friedel-Crafts Acylation
The final paragraph addresses the challenge of synthesizing propyl benzene from benzene, which cannot be achieved through the Friedel-Crafts alkylation due to the formation of isopropyl benzene as the major product. The solution presented involves using the Friedel-Crafts acylation reaction with an acid chloride and aluminum chloride to form a ketone, followed by reduction using the Clemmensen or Wolff-Kishner reduction to obtain propyl benzene. This method circumvents the limitations of the alkylation reaction and provides a direct route to the desired product.
Mindmap
Keywords
💡Friedel-Crafts Alkylation Reaction
💡Alkyl Halide
💡Catalyst
💡Carbocation
💡Electrophile
💡Polyalkylation
💡Carbocation Rearrangement
💡Deactivating Group
💡Alcohol
💡Clemmensen Reduction
Highlights
Introduction to the Friedel-Crafts alkylation reaction of benzene.
Use of alkyl chloride and aluminum chloride as catalyst in the reaction.
Net effect of replacing a hydrogen atom with an alkyl group in the reaction.
Discussion on the limitations of the Friedel-Crafts alkylation reaction.
Basic example of reacting benzene with tert-butyl chloride and the expected major product.
Proposed mechanism for the mono-alkylation product formation.
Formation of a tertiary carbocation intermediate in the reaction.
Benzene ring's reaction with the electrophile, the tertiary carbocation.
Use of AlCl4- as a base to remove hydrogen and regenerate the benzene ring.
The increased reactivity of tert-butyl benzene compared to benzene leading to polyalkylation.
Ortho and para directing effects of alkyl groups in the reaction.
Reaction of benzene with ethyl chloride and the expected mono-alkylated product.
Mechanism for the formation of ethyl benzene and the avoidance of unstable primary carbocations.
Potential for polyalkylation with ethyl benzene and the formation of di-substituted products.
The use of propyl chloride in the reaction and the formation of isopropyl benzene instead of propyl benzene.
Explanation of carbocation rearrangements leading to the major product, isopropyl benzene.
Limitations of the Friedel-Crafts alkylation with strongly deactivated rings like nitrobenzene.
Alternative methods for alkylating benzene without using an alkyl chloride, such as with 2-methylpropane and hydrofluoric acid.
Mechanism for the formation of tert-butyl benzene using an alcohol and boron trifluoride.
Strategies for producing propyl benzene from benzene using the Friedel-Crafts acylation reaction and reduction methods.
Transcripts
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