Haloalkanes and Haloarenes - From other Haloalkanes through Halogens Exchange

  • Introduction to Haloalkanes and Haloarenes
  • Role of Halogens in Organic Chemistry
  • Importance of Halogen Exchange in Haloalkanes

Definition of Haloalkanes and Haloarenes

  • Haloalkanes: Also known as alkyl halides, they are organic compounds containing a halogen atom (fluorine, chlorine, bromine, or iodine) attached to an alkyl group.
  • Haloarenes: Also known as aryl halides, they are organic compounds containing a halogen atom attached directly to an aromatic ring.

Common Examples of Haloalkanes

  • Chloromethane (CH3Cl)
  • Bromoethane (C2H5Br)
  • Iodoethane (C2H5I)
  • Chloroform (CHCl3)
  • Bromobenzene (C6H5Br)

Common Examples of Haloarenes

  • Chlorobenzene (C6H5Cl)
  • Bromobenzene (C6H5Br)
  • Fluorobenzene (C6H5F)
  • Iodobenzene (C6H5I)
  • 2,4-Dichlorobenzene (C6H4Cl2)

Methods of Preparation of Haloalkanes

  1. Alcohols to Haloalkanes:
    • Substitution of hydroxyl group (-OH) of alcohol with a halogen (X) in the presence of a strong acid catalyst.
  1. Alkenes to Haloalkanes:
    • Addition of a halogen to an alkene in the presence of a halogen carrier or under UV light.
  1. Alkanes to Haloalkanes:
    • Free radical substitution reaction of an alkane with a halogen in the presence of heat or light.

Examples: Preparation of Haloalkanes from Alcohols

  • Ethanol to Bromoethane:
    • Reaction: CH3CH2OH + HBr -> CH3CH2Br + H2O
  • Methanol to Chloromethane:
    • Reaction: CH3OH + HCl -> CH3Cl + H2O

Methods of Preparation of Haloarenes

  1. Benzene Reactions with Halogens:
    • Substitution of a hydrogen atom in the benzene ring with a halogen in the presence of a Lewis acid catalyst.
  1. Sandmeyer Reaction:
    • Conversion of an aromatic diazonium salt to a haloarene by replacement of the diazonium group with a halogen.

Examples: Preparation of Haloarenes from Benzene

  • Benzene to Chlorobenzene:
    • Reaction: C6H6 + Cl2 -> C6H5Cl + HCl
  • Benzene to Bromobenzene:
    • Reaction: C6H6 + Br2 -> C6H5Br + HBr

Properties of Haloalkanes

  • Low boiling points compared to corresponding alkanes due to weak van der Waals forces between molecules.
  • Insoluble in water due to the nonpolar nature of hydrocarbon tails.
  • Solubility depends on the polarity of the halogen atom, with smaller halogens being more soluble in water.

Properties of Haloarenes

  • High boiling points compared to corresponding haloalkanes due to the presence of an aromatic ring.
  • Insoluble in water due to the nonpolar nature of the aromatic ring.
  • Solubility depends on the polarity of the halogen atom, with smaller halogens being more soluble.

Nomenclature of Haloalkanes and Haloarenes

  • Haloalkanes: Named as alkyl halides.
    • Alkyl group is named as per the longest carbon chain, with the halogen as a substituent.
    • Numbering of the chain should give the halogen the lowest possible number.
    • Common prefixes for halogen substituents include fluoro-, chloro-, bromo-, and iodo-.
  • Haloarenes: Named as aryl halides.
    • The halogen is considered as a substituent on the aromatic ring.
    • Common prefixes for halogen substituents include fluoro-, chloro-, bromo-, and iodo-. Example:
  • 2-Chloropropane (CH3CHClCH3)
  • p-Bromoaniline (C6H4BrNH2)

Physical Properties of Haloalkanes and Haloarenes

  • Melting and boiling points increase with increasing molecular weight (compare isomers).
  • Haloalkanes have higher boiling points than haloarenes of similar molecular weight.
  • Haloarenes have lower boiling points than their corresponding alkanes due to weaker dipole-dipole interactions. Example:
  • Chloroethane (CH3CH2Cl) has a boiling point of 12.3°C.
  • Chlorobenzene (C6H5Cl) has a boiling point of 131.7°C.

Chemical Reactions of Haloalkanes

  • Nucleophilic Substitution Reactions: Substitution of the halogen atom by a nucleophile.
  • SN1 and SN2 mechanisms.
  • SN1 reactions proceed via a carbocation intermediate and are favored with tertiary haloalkanes.
  • SN2 reactions occur in a single step and are favored with primary and secondary haloalkanes. Example:
  • SN1 Reaction: CH3Br + H2O -> CH3OH + HBr
  • SN2 Reaction: CH3Cl + OH- -> CH3OH + Cl-

Chemical Reactions of Haloalkanes

  • Elimination Reactions: Removal of a halogen atom and a proton to form a double bond.
  • E1 and E2 mechanisms.
  • E1 reactions proceed via a carbocation intermediate and are favored with tertiary haloalkanes.
  • E2 reactions occur in a single step and are favored with primary and secondary haloalkanes. Example:
  • E1 Reaction: CH3CH2Br -> CH2=CH2 + HBr
  • E2 Reaction: CH3CH2Br + OH- -> CH2=CH2 + H2O + Br-

Chemical Reactions of Haloarenes

  • Nucleophilic Aromatic Substitution (SNAr) Reactions: Substitution of a halogen atom in an aromatic ring by a nucleophile.
  • Common nucleophiles include amines, alkoxides, and thiols.
  • Reaction rate depends on the nature of the halogen, presence of electron-withdrawing groups, and ring substituents. Example:
  • SNAr Reaction: C6H5Br + NH3 -> C6H5NH2 + HBr

Reactions of Haloalkanes and Haloarenes with Metals

  • Reaction with Sodium and Potassium:
    • In the presence of dry ether, alkyl halides react with sodium or potassium to form alkanes.
    • The halogen is replaced by a metal atom. Example:
  • 2-Bromopentane + Na -> Pentane + NaBr
  • Reaction with Magnesium:
    • The reaction of alkyl halides with magnesium in the presence of dry ether forms Grignard reagents.
    • Grignard reagents are used in the synthesis of various organic compounds. Example:
  • 2-Bromopropane + Mg -> (CH3)2CHMgBr

Preparation of Haloalkanes from Alkenes

  • Addition of Halogens:
    • Alkenes react with halogens (Cl2, Br2) or halogen gases (HBr, HCl) in the presence of a halogen carrier or under UV light to form haloalkanes. Example:
  • Addition of Chlorine to Ethene:
    • Reaction: CH2=CH2 + Cl2 -> CH2ClCH2Cl

Preparation of Haloalkanes from Alkanes

  • Free Radical Substitution Reaction:
    • Alkanes react with halogens (Cl2, Br2) in the presence of heat or light to form corresponding haloalkanes.
    • A chain reaction is involved, with initiation, propagation, and termination steps. Example:
  • Chlorination of Ethane:
    • Reaction: CH3CH3 + Cl2 -> CH3CH2Cl + HCl

Preparation of Haloarenes from Benzene

  • Nitration and Halogenation Reactions:
    • Benzene undergoes electrophilic aromatic substitution reactions with nitric acid (HNO3) or halogens (Cl2, Br2) in the presence of a catalyst to form haloarenes. Example:
  • Bromination of Benzene:
    • Reaction: C6H6 + Br2 -> C6H5Br + HBr

Sandmeyer Reaction

  • Conversion of Aromatic Amines to Haloarenes:
    • Aromatic amines react with nitrous acid (HNO2) to form aromatic diazonium salts.
    • Aromatic diazonium salts can be further reacted with a copper(I) halide to form haloarenes. Example:
  • Conversion of Aniline to Bromobenzene:
    • Reaction: C6H5NH2 + HNO2 -> C6H5N2+Cl- + H2O -C6H5N2+Cl- + CuBr -> C6H5Br + CuCl + N2

Reactions of Haloalkanes with Bases

  • Haloalkanes can undergo elimination reactions when treated with strong bases.
  • The base abstracts a proton from a neighboring carbon to form an alkene.
  • Two main types of elimination reactions: E1 and E2. Example:
  • E2 Reaction: CH3CH2Br + OH- -> CH2=CH2 + H2O + Br-

E1 Mechanism for Elimination Reactions

  • E1 stands for unimolecular elimination reactions.
  • These reactions occur in two steps.
  • First, the haloalkane undergoes ionization to form a carbocation.
  • Then, a base or a molecule with a nucleophilic center abstracts a proton from the carbon adjacent to the carbocation, leading to the formation of an alkene.

E1 Mechanism for Elimination Reactions (contd.)

  • E1 reactions are favored by tertiary haloalkanes, as they form more stable carbocations.
  • The rate of reaction depends on the concentration of the haloalkane but is independent of the concentration of the base. Example:
  • E1 Reaction: CH3CH2Br -> CH2=CH2 + HBr

E2 Mechanism for Elimination Reactions

  • E2 stands for bimolecular elimination reactions.
  • These reactions occur in a single step.
  • The base and the haloalkane react simultaneously to form an alkene and the halide ion.
  • E2 reactions are favored by primary and secondary haloalkanes.

E2 Mechanism for Elimination Reactions (contd.)

  • The rate of an E2 reaction depends on the concentrations of both the haloalkane and the base.
  • Strong bases like hydroxide ion (OH-) or alkoxides are commonly used for E2 reactions. Example:
  • E2 Reaction: CH3CH2Br + OH- -> CH2=CH2 + H2O + Br-

Synthetic Applications of Haloalkanes

  • Haloalkanes serve as useful starting materials for the synthesis of various organic compounds.
  • They can be used to form alkyl or aryl groups in organic synthesis.
  • Grignard reagents, formed by the reaction of haloalkanes with magnesium, are extensively used in organic synthesis. Example:
  • Use of Haloalkanes in the synthesis of alcohols, amines, and carboxylic acids.

Introduction to Haloarenes Substitution reactions

  • Haloarenes can undergo nucleophilic substitution reactions.
  • The halogen atom in the haloarene is replaced by a nucleophile.
  • The substitution reactions take place on the aromatic ring. Example:
  • SNAr Reaction: C6H5Br + NH3 -> C6H5NH2 + HBr

Nucleophilic Aromatic Substitution (SNAr) Reactions

  • SNAr reactions occur due to the formation of sigma complexes.
  • The nucleophile attacks the sigma complex, leading to substitution of the halogen.
  • The rate of SNAr reactions depends on the nature of the halogen, the presence of electron-withdrawing groups on the ring, and the presence of substituents.

Nucleophilic Aromatic Substitution (SNAr) Reactions (contd.)

  • Aromatic diazonium salts can also undergo SNAr reactions to form haloarenes.
  • Diazonium salts can be prepared by the reaction of aromatic amines with nitrous acid. Example:
  • Conversion of Aniline to Bromobenzene:
    • Reaction: C6H5NH2 + HNO2 -> C6H5N2+Cl- + H2O
    • C6H5N2+Cl- + CuBr -> C6H5Br + CuCl + N2

Importance of Haloalkanes and Haloarenes in Everyday Life

  • Haloalkanes and haloarenes have various practical applications.
  • Haloalkanes, such as Freon, have been used as refrigerants and propellants.
  • Haloarenes, like chlorobenzene, are used as solvents and as starting materials in the synthesis of pharmaceuticals and dyes. Example:
  • Use of Haloalkanes in the production of plastics and insecticides.