ChemistryChapter 9: Halogenoalkanes
9.1 Preparations of Halogenoarenes
- Halogenoalkanes (Alkyl Halides):
Explanation level
In this chapter, we begin our study of halogenoalkanes (also known as alkyl halides) and halogenoarenes (aryl halides). These are organic compounds in which one or more hydrogen atoms of an alkane or arene have been replaced by a halogen atom (fluorine, chlorine, bromine, or iodine). This section focuses on the preparation of halogenoarenes, specifically the halogenation of benzene.
Definition
- Halogenoalkanes (Alkyl Halides): Compounds with the general formula R-X, where R is an alkyl group and X is a halogen.
- Halogenoarenes (Aryl Halides): Compounds with the general formula Ar-X, where Ar is an aryl (arene) group and X is a halogen.
- Halogenation of Benzene: An electrophilic aromatic substitution reaction in which a hydrogen atom on the benzene ring is replaced by a halogen atom.
Core Concept
The core concept is that halogenoarenes are prepared by the electrophilic aromatic substitution of benzene. Benzene reacts with halogens (X2) in the presence of a Lewis acid catalyst, such as iron or iron(III) halide (FeX3). The catalyst generates the reactive electrophile (a halonium ion, X+), which attacks the electron-rich benzene ring to form a substituted benzene. This reaction is an example of the electrophilic aromatic substitution mechanism discussed in the previous chapter.
Detailed Explanation
The textbook states that halogenation of benzene occurs with halogens (X2) in the presence of Fe or Lewis acid catalyst FeX3.
General Reaction
Benzene + X_2 \xrightarrow{FeX_3} Halobenzene + HX where X = Br, Cl.
Specific Examples
1. Bromination of Benzene: C_6H_6 + Br_2 \xrightarrow{Fe \text{ or } FeBr_3} C_6H_5Br + HBr The product is bromobenzene. The catalyst can be either iron (Fe) or iron(III) bromide (FeBr3). Iron reacts with bromine to form iron(III) bromide in situ.
2. Chlorination of Benzene: C_6H_6 + Cl_2 \xrightarrow{FeCl_3} C_6H_5Cl + HCl The product is chlorobenzene. The catalyst is iron(III) chloride (FeCl3).
Mechanism Overview
The mechanism follows the general electrophilic aromatic substitution pathway:
- The halogen molecule (X2) acts as a Lewis base (donates a pair of electrons) to the Lewis acid catalyst (FeX3), which acts as a Lewis acid (accepts an electron pair).
- This generates the halonium ion (X+), which is the electrophile.
- For example, in bromination: Br_2 + FeBr_3 \rightleftharpoons Br^+ + FeBr_4^-
2. Formation of the Arenium Ion: - The electrophile (X+) attacks the benzene ring, forming a resonance-stabilized arenium ion (sigma complex). The aromatic ring is temporarily broken. Benzene + X^+ \rightarrow Arenium\ ion
3. Deprotonation and Regeneration of the Catalyst: - The FeX4- ion acts as a base, removing a proton (H+) from the arenium ion. This restores the aromatic ring and regenerates the FeX3 catalyst. Arenium\ ion + FeX_4^- \rightarrow Halobenzene + HX + FeX_3
Intermediate Level
Halogenoarenes are prepared by the electrophilic substitution of benzene. The reaction requires a halogen (Cl2 or Br2) and a Lewis acid catalyst, such as FeCl3 or FeBr3. The role of the catalyst is to polarize the halogen molecule, generating a reactive halonium ion (X+) that can attack the benzene ring. The mechanism involves the formation of an arenium ion intermediate, followed by deprotonation to restore the aromatic ring and regenerate the catalyst. This reaction is a classic example of electrophilic aromatic substitution, where the stable benzene ring is preserved. The products are aryl halides, such as bromobenzene or chlorobenzene.
Important Points
- - Preparation of Halogenoarenes: Halogenation of benzene.
- - Reagents: Halogen (X2) + Lewis acid catalyst (FeX3 or Fe).
- - Reaction Type: Electrophilic aromatic substitution.
- - Electrophile: Halonium ion (X+).
- - Product: Halobenzene (e.g., bromobenzene, chlorobenzene).
- - Example: C_6H_6 + Br_2 \xrightarrow{FeBr_3} C_6H_5Br + HBr.
- - Mechanism: Generation of electrophile → formation of arenium ion → deprotonation to restore aromatic ring.
Remember
- - Halogenation of benzene = electrophilic aromatic substitution.
- - Catalyst is required (Fe or FeX3).
- - Electrophile is X+.
- - Product is a halobenzene.
- - Example: Benzene + Br2 (FeBr3) → bromobenzene.
- - Example: Benzene + Cl2 (FeCl3) → chlorobenzene.
- - The aromatic ring is preserved.
Common Mistakes
- - Forgetting the catalyst. The reaction does not proceed without a Lewis acid catalyst like FeBr3.
- - Thinking the product is an addition product. The reaction is a substitution, not an addition. The aromatic ring is preserved.
- - Using the wrong halogen. The reaction works for chlorine and bromine, but fluorine is too reactive and iodine is too slow.
- - Confusing the name of the product. The product is a halogenoarene (e.g., bromobenzene), not an alkyl halide.
- - Not recognizing the reaction as electrophilic aromatic substitution. It follows the same mechanism as other substitutions on benzene (e.g., nitration).
Quick Revision
Halogenoarenes are prepared by the halogenation of benzene, an electrophilic aromatic substitution reaction. Benzene reacts with a halogen (Cl2 or Br2) in the presence of a Lewis acid catalyst (FeCl3 or FeBr3) to form a halobenzene (e.g., chlorobenzene or bromobenzene). The catalyst generates a reactive halonium ion (X+), which attacks the benzene ring to form an arenium ion intermediate. Deprotonation restores the aromatic ring and regenerates the catalyst. This reaction is an example of electrophilic aromatic substitution and preserves the stable aromatic system.
60-Second Revision
Halogenoarenes are prepared by the halogenation of benzene, an electrophilic aromatic substitution. Benzene reacts with a halogen (Cl2 or Br2) in the presence of a Lewis acid catalyst (FeX3) to form a halobenzene. The catalyst generates the electrophile (X+), which attacks the ring to form an arenium ion. Deprotonation restores the aromatic ring. Example: benzene + Br2 (FeBr3) → bromobenzene.
Frequently Asked Questions (FAQs)
Q1. What is a halogenoarene? Answer: A halogenoarene (or aryl halide) is an organic compound with the general formula Ar-X, where Ar is an aryl (arene) group and X is a halogen atom. An example is bromobenzene (C6H5Br).
Q2. How is a halogenoarene prepared from benzene? Answer: A halogenoarene is prepared by the halogenation of benzene, which is an electrophilic aromatic substitution reaction. Benzene is reacted with a halogen (Cl2 or Br2) in the presence of a Lewis acid catalyst such as FeCl3 or FeBr3.
Q3. What is the role of the Lewis acid catalyst in the halogenation of benzene? Answer: The Lewis acid catalyst (e.g., FeBr3) reacts with the halogen molecule (e.g., Br2) to generate a reactive electrophile, the halonium ion (Br+). This strong electrophile is needed to attack the stable benzene ring.
- Q4. Write the chemical equation for the bromination of benzene.
- Answer: The equation is:
- C_6H_6 + Br_2 \xrightarrow{FeBr_3} C_6H_5Br + HBr
- The product is bromobenzene and hydrogen bromide.
- Q5. Write the chemical equation for the chlorination of benzene.
- Answer: The equation is:
- C_6H_6 + Cl_2 \xrightarrow{FeCl_3} C_6H_5Cl + HCl
- The product is chlorobenzene and hydrogen chloride.
Q6. What is the electrophile in the halogenation of benzene? Answer: The electrophile is the halonium ion (X+), such as Br+ or Cl+. It is generated by the reaction of the halogen with the Lewis acid catalyst.
Q7. Is the halogenation of benzene an addition or substitution reaction? Answer: It is a substitution reaction. The halogen atom replaces a hydrogen atom on the benzene ring, and the aromatic ring is preserved. It is specifically an electrophilic aromatic substitution.
Q8. Why is a catalyst required for the halogenation of benzene? Answer: The catalyst is required to generate a strong electrophile (X+). Without the catalyst, the halogen molecule (X2) is not reactive enough to attack the stable benzene ring.
- Q9. What are the products of the halogenation of benzene?
- Answer: The products are a halogenoarene (e.g., bromobenzene or chlorobenzene) and a hydrogen halide (e.g., HBr or HCl).
Multiple Choice Questions (MCQs)
Q1. Which catalyst is commonly used in the halogenation of benzene with Cl2 or Br2? A. H2SO4 B. FeCl3 C. AlCl3 D. NaOH Correct Answer: B Explanation: The textbook states that halogenation of benzene occurs with halogens in the presence of Fe or Lewis acid catalyst FeX3. For chlorination, FeCl3 is used.
- Q2. In the reaction of benzene with Cl2 in the presence of a catalyst, the product is:
- A. Chloromethane
- B. Chlorobenzene
- C. Dichloromethane
- D. Benzyl chloride
- Correct Answer: B
- Explanation: The reaction is the chlorination of benzene, which produces chlorobenzene.
- Q3. What is the major product of the reaction between benzene and Br2 with FeBr3 as a catalyst?
- A. Benzene bromide
- B. Bromobenzene
- C. Dibromobenzene
- D. Bromomethane
- Correct Answer: B
- Explanation: The reaction is the bromination of benzene, producing bromobenzene.
- Q4. What is the role of FeBr3 in the bromination of benzene?
- A. It acts as a base.
- B. It acts as a Lewis acid catalyst.
- C. It acts as a nucleophile.
- D. It acts as a solvent.
- Correct Answer: B
- Explanation: FeBr3 is a Lewis acid catalyst that generates the electrophile (Br+).
- Q5. The electrophile in the bromination of benzene is:
- A. Bromide ion
- B. Bromonium ion
- C. Aluminium bromide
- D. Bromine molecule
- Correct Answer: B
- Explanation: The electrophile is the bromonium ion (Br+), generated by the reaction of bromine with the catalyst.
- Q6. The reaction of benzene with a halogen in the presence of FeX3 is an example of:
- A. Nucleophilic substitution
- B. Electrophilic aromatic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: B
- Explanation: The reaction is an electrophilic aromatic substitution, where an electrophile replaces a hydrogen atom on the benzene ring.
- Q7. Which of the following is the correct product for the chlorination of benzene?
- A. C6H6Cl2
- B. C6H5Cl
- C. C6H4Cl2
- D. C6H5Cl2
- Correct Answer: B
- Explanation: The product is chlorobenzene (C6H5Cl).
- Q8. What is the by-product of the bromination of benzene?
- A. H2
- B. HBr
- C. Br2
- D. H2O
- Correct Answer: B
- Explanation: The by-product is hydrogen bromide (HBr).
- Q9. What is the by-product of the chlorination of benzene?
- A. HCl
- B. Cl2
- C. H2
- D. H2O
- Correct Answer: A
- Explanation: The by-product is hydrogen chloride (HCl).
- Q10. Which of the following is a halogenoarene?
- A. Chloroethane
- B. Bromobenzene
- C. Chloromethane
- D. Bromoethane
- Correct Answer: B
- Explanation: Bromobenzene is a halogenoarene (aryl halide). The others are halogenoalkanes (alkyl halides).
Multiple Choice Questions (MCQs)
- Q1. Which catalyst is commonly used in the halogenation of benzene with Cl2 or Br2?
- A. H2SO4
- B. FeCl3
- C. AlCl3
- D. NaOH
- Correct Answer: B
- Explanation: FeCl3 is commonly used as a catalyst.
- Q2. In the reaction of benzene with Cl2 in the presence of a catalyst, the product is:
- A. Chloromethane
- B. Chlorobenzene
- C. Dichloromethane
- D. Benzyl chloride
- Correct Answer: B
- Explanation: The product is chlorobenzene.
- Q3. What is the major product of the reaction between benzene and Br2 with FeBr3 as a catalyst?
- A. Benzene bromide
- B. Bromobenzene
- C. Dibromobenzene
- D. Bromomethane
- Correct Answer: B
- Explanation: The product is bromobenzene.
- Q4. What is the role of FeBr3 in the bromination of benzene?
- A. It acts as a base.
- B. It acts as a Lewis acid catalyst.
- C. It acts as a nucleophile.
- D. It acts as a solvent.
- Correct Answer: B
- Explanation: FeBr3 is a Lewis acid catalyst.
- Q5. The electrophile in the bromination of benzene is:
- A. Bromide ion
- B. Bromonium ion
- C. Aluminium bromide
- D. Bromine molecule
- Correct Answer: B
- Explanation: The electrophile is the bromonium ion.
- Q6. The reaction of benzene with a halogen in the presence of FeX3 is an example of:
- A. Nucleophilic substitution
- B. Electrophilic aromatic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: B
- Explanation: It is an electrophilic aromatic substitution.
- Q7. Which of the following is the correct product for the chlorination of benzene?
- A. C6H6Cl2
- B. C6H5Cl
- C. C6H4Cl2
- D. C6H5Cl2
- Correct Answer: B
- Explanation: The product is chlorobenzene.
- Q8. What is the by-product of the bromination of benzene?
- A. H2
- B. HBr
- C. Br2
- D. H2O
- Correct Answer: B
- Explanation: The by-product is HBr.
- Q9. What is the by-product of the chlorination of benzene?
- A. HCl
- B. Cl2
- C. H2
- D. H2O
- Correct Answer: A
- Explanation: The by-product is HCl.
- Q10. Which of the following is a halogenoarene?
- A. Chloroethane
- B. Bromobenzene
- C. Chloromethane
- D. Bromoethane
- Correct Answer: B
- Explanation: Bromobenzene is a halogenoarene.
Multiple Choice Questions (MCQs)
- Q1. What is the general formula of a halogenoarene?
- A. R-X
- B. Ar-X
- C. R-OH
- D. Ar-OH
- Correct Answer: B
- Explanation: Halogenoarenes have the general formula Ar-X.
- Q2. Which of the following is an example of a halogenoarene?
- A. Chloroethane
- B. Bromobenzene
- C. Chloromethane
- D. Bromoethane
- Correct Answer: B
- Explanation: Bromobenzene is a halogenoarene.
- Q3. What is the role of the halogen in the halogenation of benzene?
- A. It acts as a nucleophile.
- B. It acts as an electrophile.
- C. It acts as a catalyst.
- D. It acts as a solvent.
- Correct Answer: B
- Explanation: The halogen is converted into an electrophile (X+) by the catalyst.
- Q4. What type of reaction is the halogenation of benzene?
- A. Nucleophilic substitution
- B. Electrophilic aromatic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: B
- Explanation: It is an electrophilic aromatic substitution.
- Q5. What is the catalyst used in the bromination of benzene?
- A. FeBr3
- B. AlCl3
- C. H2SO4
- D. NaOH
- Correct Answer: A
- Explanation: FeBr3 is used as a catalyst.
- Q6. What is the product of the reaction between benzene and Br2 in the presence of FeBr3?
- A. Bromobenzene
- B. Benzene bromide
- C. Dibromobenzene
- D. Bromomethane
- Correct Answer: A
- Explanation: The product is bromobenzene.
- Q7. What is the by-product of the chlorination of benzene?
- A. HCl
- B. Cl2
- C. H2
- D. H2O
- Correct Answer: A
- Explanation: The by-product is HCl.
- Q8. Which of the following is NOT a halogenoarene?
- A. Bromobenzene
- B. Chlorobenzene
- C. Chloroethane
- D. Iodobenzene
- Correct Answer: C
- Explanation: Chloroethane is a halogenoalkane, not a halogenoarene.
- Q9. Why is a catalyst needed for the halogenation of benzene?
- A. To generate a strong electrophile.
- B. To increase the temperature.
- C. To act as a solvent.
- D. To neutralize the acid.
- Correct Answer: A
- Explanation: The catalyst generates the strong electrophile (X+) needed to attack the benzene ring.
- Q10. What is the electrophile in the chlorination of benzene?
- A. Cl-
- B. Cl+
- C. Cl2
- D. HCl
- Correct Answer: B
- Explanation: The electrophile is the chloronium ion (Cl+).
Key Points from the Textbook
- The reaction is an example of electrophilic aromatic substitution.
- The catalyst is essential because it generates the strong electrophile needed to attack the stable benzene ring.
- The product is a halogenoarene (e.g., bromobenzene, chlorobenzene).
- The reaction is also called halogenation of benzene.
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Review the key facts, formulas and checks before you move on.9.1 Preparations of Halogenoarenes
Sticky Notes · Quick Revision
Key Point 1
- Halogenation of benzene: Electrophilic aromatic substitution.
Key Point 2
- Reagents: X2 + FeX3 (or Fe).
Key Point 3
- Electrophile: X+.
Key Point 4
- Product: Halobenzene (e.g., bromobenzene).
Key Point 5
- Example: C_6H_6 + Br_2 \xrightarrow{FeBr_3} C_6H_5Br + HBr.
Key Point 6
- Example: C_6H_6 + Cl_2 \xrightarrow{FeCl_3} C_6H_5Cl + HCl.
Key Point 7
- Mechanism: Arenium ion intermediate.
Key Point 8
- Aromatic ring is preserved.
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