Slide 1: Haloalkanes and Haloarenes - Preparation of Haloalkanes

  • Haloalkanes are the organic compounds containing halogen atoms in their structure.
  • They are commonly used as intermediates in the synthesis of various organic compounds.
  • Several methods are employed for the preparation of haloalkanes.
  • Let’s discuss some important methods for their preparation.

Slide 2: Addition of Halogen to Alkenes

  • One of the most common methods for the preparation of haloalkanes is the addition of halogen to alkenes.
  • In this reaction, the double bond of the alkene is broken and replaced by a halogen atom.
  • The reaction is usually carried out using halogens such as chlorine or bromine.
  • Example:
    • Ethene + Bromine -> 1,2-Dibromoethane

Slide 3: Addition of Halogen Acid to Alkenes

  • Another method for the preparation of haloalkanes is the addition of halogen acid to alkenes.
  • The reaction involves the addition of hydrogen halide (H-X) to an alkene, where X represents a halogen atom.
  • This method is also known as hydrohalogenation reaction.
  • Example:
    • Propene + HCl -> 2-Chloropropane

Slide 4: Addition of Halogen to Alkanes

  • Haloalkanes can also be prepared by the addition of halogens to alkanes.
  • This reaction requires the presence of a strong oxidizing agent.
  • Example:
    • Methane + Chlorine -> Chloromethane + Hydrogen Chloride

Slide 5: Addition of Alcohol to Hydrogen Halide

  • Haloalkanes can be prepared by the addition of alcohol to hydrogen halide.
  • The reaction involves the substitution of the hydroxyl group (OH) of the alcohol with a halogen atom.
  • The reaction is known as the nucleophilic substitution reaction.
  • Example:
    • Ethanol + HBr -> Bromoethane + Water

Slide 6: Free Radical Halogenation

  • Free radical halogenation is another method for the preparation of haloalkanes.
  • The reaction involves the substitution of a hydrogen atom in an alkane with a halogen atom.
  • The reaction requires the presence of UV light or heat to initiate the free radical reactions.
  • Example:
    • Methane + Chlorine -> Chloromethane + Hydrogen Chloride

Slide 7: Preparation of Haloalkanes using Alcohols

  • Haloalkanes can also be prepared by the substitution of a hydroxyl group (OH) in alcohols with a halogen atom.
  • The reaction involves the reaction of alcohol with a halogen acid.
  • Example:
    • Ethanol + HCl -> Chloroethane + Water

Slide 8: Preparation of Haloalkanes using Alkenes

  • Haloarenes can also be prepared by the substitution of a hydrogen atom in benzene with a halogen atom.
  • The reaction involves the reaction of benzene with a halogen acid.
  • Example:
    • Benzene + HCl -> Chlorobenzene + Hydrogen

Slide 9: Preparation of Haloalkanes using Alkanes

  • Haloarenes can also be prepared by the halogenation of benzene using a strong oxidizing agent.
  • The reaction requires the presence of a Lewis acid catalyst.
  • Example:
    • Benzene + Chlorine + AlCl3 -> Chlorobenzene + HCl

Slide 10: Preparation of Haloalkanes using Aromatic Compounds

  • Haloarenes can also be prepared by the halogenation of aromatic compounds using halogens or halogenating agents.
  • The reaction requires the presence of a Lewis acid catalyst.
  • Example:
    • Toluene + Br2 + AlCl3 -> Bromotoluene + HBr

Slide 11: Preparation of Haloalkanes using Grignard Reagents

  • Haloalkanes can be prepared using Grignard reagents.
  • Grignard reagents are organometallic compounds containing a carbon-metal bond.
  • The reaction involves the addition of a Grignard reagent to a halogenated compound or carbonyl compound.
  • Example:
    • RMgX + CH3Br -> R-CH3 + MgXBr

Slide 12: Preparation of Haloalkanes using Aldehydes or Ketones

  • Haloalkanes can be prepared by reacting aldehydes or ketones with a halogenating agent.
  • The reaction involves the substitution of a hydrogen atom in the aldehyde or ketone with a halogen atom.
  • Example:
    • RCHO + PBr3 -> R-CH2Br + HBr

Slide 13: Preparation of Haloalkanes using Alkyl Halides

  • Alkyl halides can be converted into haloalkanes by a reaction known as dehalogenation.
  • The reaction involves the removal of the halogen atom from the alkyl halide using reducing agents.
  • Example:
    • R-Br + Zn -> R-H + ZnBr

Slide 14: Preparation of Haloalkanes using Allyl Halides

  • Allyl halides can be prepared by the halogenation of allylic compounds.
  • The reaction involves the substitution of a hydrogen atom in the allylic compound with a halogen atom.
  • Example:
    • CH2=CH-CH3 + Cl2 -> CH2Cl-CH=CH2 + HCl

Slide 15: Preparation of Haloalkanes using Oxidation of Alcohols

  • Haloalkanes can be prepared by the oxidation of primary alcohols or aldehydes.
  • The reaction involves the substitution of the hydroxyl group of the alcohol or aldehyde with a halogen atom.
  • Example:
    • R-CH2-CH2-OH + PCl5 -> R-CH2-CH2-Cl + POCl3 + HCl

Slide 16: Preparation of Haloalkanes using the Appel Reaction

  • The Appel reaction is a method for preparing haloalkanes from alcohols or phenols.
  • The reaction involves the reaction of the alcohol or phenol with a mixture of an alkyl halide and a phosphorus compound.
  • Example:
    • R-OH + PPh3 + CCl4 -> R-Cl + PPh3O + CCl3Ph

Slide 17: Preparation of Haloalkanes using Diazomethane

  • Haloalkanes can be prepared by the reaction of diazomethane with various compounds.
  • The reaction involves the substitution of a hydrogen atom in the compound with a methyl group.
  • Example:
    • R-H + CH2N2 -> R-CH3 + N2

Slide 18: Preparation of Haloalkanes using Sandmeyer Reaction

  • The Sandmeyer reaction is a method for preparing aryl halides.
  • The reaction involves the substitution of a diazonium salt with a halide.
  • Example:
    • C6H5-N2+Cl- + HCl -> C6H5-Cl + N2 +H+

Slide 19: Preparation of Haloalkanes using Nucleophilic Substitution

  • Haloalkanes can be prepared by nucleophilic substitution reactions.
  • The reaction involves the substitution of a halogen atom in a compound with a nucleophile.
  • Example:
    • R-Br + NaOH -> R-OH + NaBr

Slide 20: Preparation of Haloalkanes using Nucleophilic Addition Elimination

  • Haloalkanes can also be prepared by nucleophilic addition elimination reactions.
  • The reaction involves the addition of a nucleophile to a compound, followed by the elimination of a leaving group.
  • Example:
    • R-Cl + KOH -> R-OH + KCl

Slide 21:

Preparation of Haloalkanes using Nucleophilic Substitution (SN1 Reaction)

  • Nucleophilic substitution (SN1) reaction involves the substitution of a halogen atom in a compound by a nucleophile.
    • The reaction proceeds through a two-step mechanism: ionization of the leaving group, followed by nucleophilic attack.
  • In SN1 reaction, the rate-determining step is the formation of a carbocation intermediate.
  • Example:
    • R-Cl + AgNO3 -> R-OH + AgCl

Slide 22:

Preparation of Haloalkanes using Nucleophilic Substitution (SN2 Reaction)

  • Nucleophilic substitution (SN2) reaction also involves the substitution of a halogen atom in a compound by a nucleophile.
    • The reaction proceeds through a one-step mechanism: nucleophilic attack and simultaneous departure of the leaving group.
  • In SN2 reaction, the reaction rate depends on both the concentration of the nucleophile and the substrate.
  • Example:
    • R-Cl + NaOH -> R-OH + NaCl

Slide 23:

Preparation of Haloalkanes using Nucleophilic Substitution (SNi Reaction)

  • Nucleophilic substitution with internal return (SNi) reaction involves the substitution of a halogen atom in a compound by a nucleophile.
    • The reaction proceeds through a complex mechanism: ionization of the leaving group followed by nucleophilic attack and internal return.
  • SNi reactions typically occur in cyclic compounds or compounds with a neighboring group that can stabilize the intermediate.
  • Example:
    • R-Cl + HOCH2CH2Br -> R-OH + ClCH2CH2Br

Slide 24:

Preparation of Haloalkanes using Nucleophilic Substitution (SNAr Reaction)

  • Nucleophilic aromatic substitution (SNAr) reaction involves the substitution of a halogen atom in an aromatic compound by a nucleophile.
    • The reaction proceeds via the attack of the nucleophile at the position ortho or para to the leaving group.
  • SNAr reactions are typically seen in compounds with activated aromatic rings due to the presence of electron-withdrawing groups.
  • Example:
    • C6H5-Cl + NaOH -> C6H5-OH + NaCl

Slide 25:

Preparation of Haloalkanes using Appel Reaction

  • The Appel reaction is a method for preparing haloalkanes from alcohols or phenols.
  • The reaction involves the reaction of the alcohol or phenol with a mixture of an alkyl halide and a phosphorus compound.
  • The reaction proceeds via the formation of an alkyl phosphonium salt, which is then converted into the haloalkane.
  • Example:
    • R-OH + PPh3 + CCl4 -> R-Cl + PPh3O + CCl3Ph

Slide 26:

Preparation of Haloalkanes using Diazomethane

  • Haloalkanes can be prepared by the reaction of diazomethane with various compounds.
  • The reaction involves the substitution of a hydrogen atom in the compound with a methyl group from diazomethane.
  • Diazomethane is highly reactive and should be handled with caution due to its explosive nature.
  • Example:
    • R-H + CH2N2 -> R-CH3 + N2

Slide 27:

Preparation of Haloalkanes using Sandmeyer Reaction

  • The Sandmeyer reaction is a method for preparing aryl halides.
  • The reaction involves the substitution of a diazonium salt with a halide.
  • The reaction proceeds via the formation of a reactive aryl cation intermediate, which then reacts with the halide.
  • Example:
    • C6H5-N2+Cl- + HCl -> C6H5-Cl + N2 +H+

Slide 28:

Preparation of Haloalkanes using Nucleophilic Addition-Elimination Reaction

  • Haloalkanes can also be prepared by nucleophilic addition-elimination reactions.
  • The reaction involves the addition of a nucleophile to a compound, followed by the elimination of a leaving group.
  • The reaction can occur in both primary and secondary haloalkanes.
  • Example:
    • R-Cl + NH3 -> R-NH2 + HCl

Slide 29:

Preparation of Haloalkanes using Oxidation of Alcohols

  • Haloalkanes can be prepared by the oxidation of primary alcohols or aldehydes.
  • The reaction involves the substitution of the hydroxyl group of the alcohol or aldehyde with a halogen atom.
  • The oxidation can be carried out using different halogenating agents such as PCl5 or PBr3.
  • Example:
    • R-CH2-CH2-OH + PCl5 -> R-CH2-CH2-Cl + POCl3 + HCl

Slide 30:

Summary

  • Haloalkanes can be prepared using various methods, such as addition reactions, substitution reactions, oxidation, or using specific reagents.
  • The choice of method depends on the starting material and desired haloalkane.
  • It is important to follow proper safety precautions and handle reactive reagents with care.
  • Understanding the preparation methods is essential in organic synthesis and studying the reactivity of different functional groups.