Mechanism of the Reaction

• The mechanism of the reaction consists of several steps:
  1. Attack by the nucleophile (OH⁻) on the electrophilic carbon (carbon attached to the halogen) of the haloarene.
  2. Formation of Meisenheimer complex.
  3. Removal of the halide ion and formation of phenoxide ion.
  4. Protonation of the phenoxide ion to form the final product (phenol).

Examples

• Example 1:
  • Bromobenzene + Sodium Hydroxide → Phenol + Sodium Bromide
  • C₆H₅Br + NaOH → C₆H₅OH + NaBr
• Example 2:
  • Chlorobenzene + Sodium Hydroxide → Phenol + Sodium Chloride
  • C₆H₅Cl + NaOH → C₆H₅OH + NaCl

Factors Affecting the Reaction Rate

• The reaction rate of haloarenes with hydroxide ion depends on various factors:
  1. Nature of the haloarene: Reactivity increases with the decrease in electron density on the benzene ring due to electron-withdrawing substituents.
  2. Nature of the halogen atom: Reactivity increases in the order F < Cl < Br < I.
  3. Mechanism of nucleophilic substitution: Depending on the substituents on the benzene ring, the reaction can occur via an addition-elimination or an elimination-addition mechanism.

Reaction Conditions

• The reaction between haloarenes and hydroxide ion is usually carried out under the following conditions:
  1. Presence of a strong base: Sodium hydroxide (NaOH) is most commonly used as a strong base.
  2. Solvent: The reaction is generally carried out in an aprotic solvent such as acetone or dimethyl sulfoxide (DMSO).
  3. Elevated temperatures: The reaction is usually heated to increase the reaction rate.

Mechanism Steps: Attack by the Nucleophile

1. The nucleophile (OH⁻) attacks the electrophilic carbon (carbon attached to the halogen) of the haloarene.

2. The carbon-halogen bond breaks and the negative charge on the halogen atom is delocalized into the benzene ring.

3. A Meisenheimer complex is formed as an intermediate.

Mechanism Steps: Formation of Meisenheimer Complex

1. The negative charge on the halogen atom forms a bond with one of the carbons of the benzene ring, resulting in the formation of a Meisenheimer complex.

2. The negative charge from the oxygen atom is delocalized into the benzene ring.

Mechanism Steps: Removal of Halide Ion

1. The Meisenheimer complex undergoes rearrangement due to the repulsion between the negative charges on the oxygen and the benzene ring.

2. The rearranged complex results in the removal of the halide ion.

Mechanism Steps: Formation of Phenoxide Ion

1. The rearranged complex forms a phenoxide ion, which is stabilized by resonance.

2. The negative charge on the oxygen atom is delocalized into the benzene ring.

11. Haloakanes and Haloarenes - Reaction of Haloarenes with Hydroxide Ion

• Haloarenes are the aromatic compounds in which a halogen atom is directly attached to the benzene ring.
• In the reaction between haloarenes and hydroxide ion, the halogen atom is replaced by the hydroxyl group (-OH) on the benzene ring.
• This reaction is also known as nucleophilic aromatic substitution reaction.
• The general reaction equation can be represented as:
  • Halobenzene + Sodium Hydroxide → Phenol + Sodium Halide

12. Mechanism of the Reaction

• The mechanism of the reaction consists of several steps:
  1. Attack by the nucleophile (OH⁻) on the electrophilic carbon (carbon attached to the halogen) of the haloarene.
  2. Formation of Meisenheimer complex.
  3. Removal of the halide ion and formation of phenoxide ion.
  4. Protonation of the phenoxide ion to form the final product (phenol).

13. Examples

• Example 1:
  • Bromobenzene + Sodium Hydroxide → Phenol + Sodium Bromide
  • C₆H₅Br + NaOH → C₆H₅OH + NaBr
• Example 2:
  • Chlorobenzene + Sodium Hydroxide → Phenol + Sodium Chloride
  • C₆H₅Cl + NaOH → C₆H₅OH + NaCl

14. Factors Affecting the Reaction Rate

• The reaction rate of haloarenes with hydroxide ion depends on various factors:
  1. Nature of the haloarene: Reactivity increases with the decrease in electron density on the benzene ring due to electron-withdrawing substituents.
  2. Nature of the halogen atom: Reactivity increases in the order F < Cl < Br < I.
  3. Mechanism of nucleophilic substitution: Depending on the substituents on the benzene ring, the reaction can occur via an addition-elimination or an elimination-addition mechanism.

15. Reaction Conditions

• The reaction between haloarenes and hydroxide ion is usually carried out under the following conditions:
  1. Presence of a strong base: Sodium hydroxide (NaOH) is most commonly used as a strong base.
  2. Solvent: The reaction is generally carried out in an aprotic solvent such as acetone or dimethyl sulfoxide (DMSO).
  3. Elevated temperatures: The reaction is usually heated to increase the reaction rate.

16. Mechanism Steps: Attack by the Nucleophile

1. The nucleophile (OH⁻) attacks the electrophilic carbon (carbon attached to the halogen) of the haloarene.

2. The carbon-halogen bond breaks and the negative charge on the halogen atom is delocalized into the benzene ring.

3. A Meisenheimer complex is formed as an intermediate.

17. Mechanism Steps: Formation of Meisenheimer Complex

1. The negative charge on the halogen atom forms a bond with one of the carbons of the benzene ring, resulting in the formation of a Meisenheimer complex.

2. The negative charge from the oxygen atom is delocalized into the benzene ring.

18. Mechanism Steps: Removal of Halide Ion

1. The Meisenheimer complex undergoes rearrangement due to the repulsion between the negative charges on the oxygen and the benzene ring.

2. The rearranged complex results in the removal of the halide ion.

19. Mechanism Steps: Formation of Phenoxide Ion

1. The rearranged complex forms a phenoxide ion, which is stabilized by resonance.

2. The negative charge on the oxygen atom is delocalized into the benzene ring.

20. Summary

• The reaction between haloarenes and hydroxide ion involves the replacement of the halogen atom with a hydroxyl group on the benzene ring.
• The reaction can occur via a mechanism involving nucleophilic attack, formation of a Meisenheimer complex, removal of the halide ion, and formation of the phenoxide ion.
• Factors such as the nature of the haloarene, nature of the halogen atom, and the mechanism of nucleophilic substitution influence the reaction rate.
• The reaction is usually carried out with the presence of a strong base and elevated temperatures. Sure! Here are the slides 21 to 30 on the topic “Haloakanes and Haloarenes - Reaction of Haloarenes with Hydroxide Ion”:

Slide 21:

• Factors that affect the rate of reaction:
  • Nature of the substituents on the benzene ring
  • Electron density on the benzene ring
  • Steric hindrance around the carbon-halogen bond
  • Nature of the halogen atom attached to the benzene ring
  • Mechanism of nucleophilic substitution

Slide 22:

• Mechanism Steps: Protonation of Phenoxide Ion
  1. The phenoxide ion reacts with an acid (H⁺).
  2. Protonation of the oxygen atom occurs.
  3. The final product, phenol, is formed.

Slide 23:

• Example 3:
  • Fluorobenzene + Sodium Hydroxide → Phenol + Sodium Fluoride
  • C₆H₅F + NaOH → C₆H₅OH + NaF
• Example 4:
  • Iodobenzene + Sodium Hydroxide → Phenol + Sodium Iodide
  • C₆H₅I + NaOH → C₆H₅OH + NaI

Slide 24:

• Comparison with Alkyl Halides:
  • Alkyl halides undergo nucleophilic substitution more readily than haloarenes due to the higher reactivity of alkyl halides.
  • Haloarenes require harsh reaction conditions and longer reaction times compared to alkyl halides.

Slide 25:

• Uses of Phenols:
  • Antiseptics and disinfectants
  • Pharmaceuticals
  • Dyes and pigments
  • Polymers and plastics
  • Flavorings and fragrances

Slide 26:

• Applications of Nucleophilic Aromatic Substitution:
  • Synthesis of pharmaceuticals and agrochemicals
  • Preparation of organic intermediates
  • Modification of natural products
  • Introduction of functional groups
  • Design and synthesis of new materials

Slide 27:

• Limitations of Nucleophilic Aromatic Substitution:
  1. Low reaction rates and yields
  2. Regioselectivity issues in some cases
  3. Lack of availability and high cost of reactants in some instances

Slide 28:

• Summary:
  • Haloarenes can undergo nucleophilic substitution with hydroxide ion to form phenols.
  • The reaction rate depends on the nature of the substituents on the benzene ring, electron density, steric hindrance, nature of the halogen atom, and mechanism of nucleophilic substitution.
  • Protonation of the phenoxide ion leads to the formation of the final product, phenol.

Slide 29:

• Summary (contd.):
  • Examples of haloarenes reacting with hydroxide ion include bromobenzene, chlorobenzene, fluorobenzene, and iodobenzene.
  • The reaction of haloarenes with hydroxide ion is a useful method for the synthesis of phenols.
  • Phenols have various applications in industries such as medicine, cosmetics, and plastics.

Slide 30:

• Thank you for your attention!
• Feel free to ask any questions you may have.