Slide 1: Haloakanes and Haloarenes - Polyhalogen Compounds

  • Polyhalogen compounds are organic compounds that contain multiple halogen atoms.
  • They can be categorized as haloalkanes or haloarenes, based on whether the halogen is bonded to a carbon atom in an alkane or an aromatic ring, respectively.

Slide 2: Haloalkanes

  • Haloalkanes, also known as alkyl halides, are organic compounds in which one or more hydrogen atoms in an alkane are replaced by halogen atoms.
  • General formula: R-X, where R represents an alkyl group and X represents a halogen (F, Cl, Br, or I).
  • Example: CH3Cl (methyl chloride)

Slide 3: Nomenclature of Haloalkanes

  • The halogen atom is considered as a substituent and is named as a prefix.
  • The alkyl group is named as a suffix.
  • The position of the halogen atom on the carbon chain is indicated by a number.
  • Example: CH3CH2Br is named as bromoethane.

Slide 4: Physical Properties of Haloalkanes

  • Haloalkanes have higher boiling points than their corresponding alkanes due to the presence of polar covalent bonds and dipole-dipole interactions.
  • They are generally insoluble in water but soluble in organic solvents.
  • As the size of the halogen atom increases, the boiling point also increases.
  • Haloalkanes exhibit optical isomerism when attached to an asymmetric carbon atom.
  1. Nucleophilic Substitution Reactions:
    • Haloalkanes can undergo nucleophilic substitution reactions where the halogen atom is replaced by a nucleophile.
    • Types of nucleophilic substitution reactions:
      • SN1 (unimolecular nucleophilic substitution)
      • SN2 (bimolecular nucleophilic substitution)
    • Example: SN1 reaction - R-X → R+ + X- (formation of carbocation followed by nucleophile attack)

Slide 6: Haloarenes

  • Haloarenes are organic compounds derived from aromatic hydrocarbons in which one or more hydrogen atoms are replaced by halogen atoms.
  • General formula: Ar-X, where Ar represents an aromatic ring and X represents a halogen (F, Cl, Br, or I).
  • Example: C6H5Cl (chlorobenzene)

Slide 7: Nomenclature of Haloarenes

  • The halogen atom is considered as a substituent and is named as a prefix.
  • The aromatic ring is named as a suffix.
  • The position of the halogen atom on the ring is indicated by a number.
  • Example: C6H5Br is named as bromobenzene.

Slide 8: Physical Properties of Haloarenes

  • Haloarenes have higher boiling points than their corresponding aromatic hydrocarbons due to the presence of polar covalent bonds.
  • They are generally insoluble in water but soluble in organic solvents.
  • As the size of the halogen atom increases, the boiling point also increases.
  • Haloarenes do not exhibit optical isomerism as the halogen is directly attached to a sp2 hybridized carbon atom.
  1. Nucleophilic Aromatic Substitution Reactions:
    • Haloarenes can undergo nucleophilic aromatic substitution reactions where the halogen atom is replaced by a nucleophile.
    • The rate of reaction depends on the nature of the halogen atom and the substituents on the aromatic ring.
    • Example: SNAr reaction - Ar-X + Nu- → Ar-Nu + X-

Slide 10: Examples of Polyhalogen Compounds

  • Dichloromethane (CH2Cl2)
  • Trichloromethane (CHCl3)
  • Tetrachloromethane (CCl4)
  • Dibromomethane (CH2Br2)
  • Tribromomethane (CHBr3)
  • Iodoform (CHI3)
  • Dichloroethane (C2H4Cl2)
  • Trichloroethane (C2H3Cl3)
  • Trichlorofluoromethane (CCl3F)

Slide 11:

Haloalkanes - Preparation Methods

  • From alcohols: Alcohols can be converted to haloalkanes by reacting with hydrogen halides (HCl, HBr, HI) or with phosphorus halides (PCl3, PCl5, PBr3, PI3).
  • From alkenes: Alkenes can undergo addition reactions with hydrogen halides to form haloalkanes.
  • From alkanes: Alkanes can be converted to haloalkanes through free radical substitution reactions using halogen gases (Cl2, Br2, I2).
  • From alkynes: Alkynes can be converted to haloalkanes through addition reactions with hydrogen halides.

Slide 12:

Haloalkanes - Uses and Applications

  • Haloalkanes are used as solvents in various industries.
  • They serve as starting materials for the synthesis of various organic compounds.
  • Some haloalkanes have medical applications as anesthetics, sedatives, or muscle relaxants.
  • They are used as refrigerants and propellants in aerosol products.
  • Haloalkanes are used in the production of polymers, plastics, and dyes.

Slide 13:

Nucleophilic Substitution Reactions - SN1 Mechanism (Unimolecular)

  • SN1 mechanism involves a two-step process:
    1. Formation of a carbocation intermediate due to heterolysis (breaking of the C-X bond).
    2. The nucleophile attacks the carbocation to complete the substitution reaction.
  • SN1 reactions proceed through a planar transition state.
  • Rate of reaction depends on the concentration of the haloalkane only.
  • Example: R-X → R+ + X-, R+ + Nu- → R-Nu + X-

Slide 14:

Nucleophilic Substitution Reactions - SN2 Mechanism (Bimolecular)

  • SN2 mechanism involves a direct attack of the nucleophile on the carbon atom while the leaving group departs.
  • SN2 reactions proceed through an inversion of stereochemistry.
  • Rate of reaction depends on the concentration of both the haloalkane and the nucleophile.
  • Steric hindrance can impact the reaction rate.
  • Example: R-X + Nu- → R-Nu + X-

Slide 15:

Nucleophilic Aromatic Substitution Reactions - SNAr Mechanism

  • SNAr mechanism involves the substitution of a halogen atom in a haloarene with a nucleophile.
  • The mechanism proceeds through a cyclic intermediate called a Meisenheimer complex.
  • Electrophilic aromatic substitution is followed by a nucleophilic attack and elimination of the leaving group.
  • Rate of reaction depends on the nature of the substituents on the aromatic ring.
  • Example: Ar-X + Nu- → Ar-Nu + X-

Slide 16:

Haloarenes - Preparation Methods

  • From diazonium salts: Aromatic diazonium salts react with metal halides or sodium halides to form haloarenes.
  • From aromatic hydrocarbons: Aromatic hydrocarbons can react with halogens in the presence of a Lewis acid catalyst to form haloarenes.
  • From phenols: Phenols can be converted to haloarenes by treating them with halogen acids (HCl, HBr) or phosphorus halides.

Slide 17:

Haloarenes - Uses and Applications

  • Haloarenes are used as starting materials for the synthesis of various pharmaceuticals and agrochemicals.
  • Some haloarenes are used as flame retardants in plastics and textiles.
  • They find applications as intermediates in organic synthesis.
  • Haloarenes can be used as refrigerants and coolants in various systems.
  • They are used as solvents and reagents in the laboratory.

Slide 18:

Reactivity of Haloalkanes vs. Haloarenes

  • Haloarenes are comparatively less reactive than haloalkanes due to the resonance stabilization provided by the aromatic ring.
  • Haloarenes undergo nucleophilic aromatic substitution reactions, while haloalkanes undergo nucleophilic substitution reactions.
  • Haloalkanes can undergo elimination reactions (E1 and E2), while haloarenes do not show significant elimination reactions.

Slide 19:

Environmental Impact of Polyhalogen Compounds

  • Polyhalogen compounds, especially halogenated hydrocarbons, have been identified as environmental pollutants.
  • Some haloalkanes and haloarenes are toxic and can bioaccumulate in organisms, leading to harmful effects.
  • Certain haloalkanes, such as chlorofluorocarbons (CFCs), have been implicated in the depletion of the ozone layer.
  • There are regulations and restrictions on the use and disposal of polyhalogen compounds to minimize their impact on the environment.

Slide 20:

Summary

  • Polyhalogen compounds are organic compounds that contain multiple halogen atoms.
  • Haloalkanes and haloarenes are two main categories of polyhalogen compounds.
  • Haloalkanes are alkyl halides derived from alkanes, while haloarenes are aromatic compounds with halogen substituents.
  • Haloalkanes undergo nucleophilic substitution reactions (SN1 and SN2), while haloarenes undergo nucleophilic aromatic substitution reactions (SNAr).
  • Both haloalkanes and haloarenes have various applications and environmental implications. This is the continuation of the previous slides:

Slide 21:

  • Haloalkanes and haloarenes are important classes of organic compounds with unique properties and reactivity.
  • The presence of halogen atoms in these compounds significantly influences their physical and chemical properties.
  • Understanding the behavior of polyhalogen compounds is crucial for various applications in medicine, industry, and environmental studies.

Slide 22:

Haloalkanes - Reactivity:

  • The reactivity of haloalkanes is influenced by the nature and position of the halogen atom.
  • Haloalkanes can undergo substitution and elimination reactions.
  • Nucleophilic substitution reactions occur via SN1 or SN2 mechanisms.
  • Elimination reactions (E1 and E2) involve the removal of a halogen atom and a neighboring hydrogen atom to form a double bond.

Slide 23:

Haloalkanes - Examples of Substitution Reactions:

  1. Hydrolysis:
    • Haloalkanes can undergo hydrolysis reactions in the presence of water or hydroxide ions.
    • The reaction can proceed via SN1 or SN2 mechanisms.
    • Example: CH3CH2Cl + H2O → CH3CH2OH + HCl.
  1. Nucleophilic Substitution with Ammonia:
    • Haloalkanes can react with ammonia (NH3) to form a primary amine.
    • This reaction occurs via an SN2 mechanism.
    • Example: CH3CH2Br + NH3 → CH3CH2NH2 + HBr.

Slide 24:

Haloalkanes - Examples of Elimination Reactions:

  1. Dehydrohalogenation:
    • Haloalkanes can undergo dehydrohalogenation reactions to form alkenes.
    • The reaction can proceed via an E1 or E2 mechanism.
    • Example: CH3CH2Br → CH2=CH2 + HBr.
  1. Dehydrogenation:
    • Certain haloalkanes can undergo dehydrogenation reactions to form alkynes.
    • Example: CH3CH2Br → CH≡CH + HBr.

Slide 25:

Haloarenes - Reactivity:

  • The reactivity of haloarenes is influenced by the nature and position of the halogen atom, as well as the substituents on the aromatic ring.
  • Haloarenes generally undergo nucleophilic aromatic substitution (SNAr) reactions.
  • The rate of reaction depends on the electron-withdrawing or electron-donating nature of the substituents.

Slide 26:

Haloarenes - Examples of Substitution Reactions:

  1. Reduction:
    • Haloarenes can undergo reduction reactions to form corresponding arylamines (anilines).
    • The reducing agents used are usually strong reducing agents like tin and hydrochloric acid (Sn/HCl).
    • Example: C6H5Br + 6[H] → C6H5NH2 + HBr.
  1. Coupling:
    • Haloarenes can undergo coupling reactions with aryl compounds to form biaryl compounds.
    • The reaction is commonly known as the Suzuki coupling reaction.
    • Example: C6H5Br + C6H5B(OH)2 → C6H5-C6H5 + HBr.

Slide 27:

Haloarenes - Examples of Nucleophilic Aromatic Substitution:

  1. Substitution with Nitrogen Nucleophiles:
    • Haloarenes can react with nitrogen nucleophiles, such as amines, to form arylamines.
    • The reaction occurs via an SNAr mechanism.
    • Example: C6H5Cl + NH3 → C6H5NH2 + HCl.
  1. Substitution with Oxygen Nucleophiles:
    • Haloarenes can react with oxygen nucleophiles, such as phenols, to form aryl ethers.
    • The reaction occurs via an SNAr mechanism.
    • Example: C6H5Br + C6H5OH → C6H5OC6H5 + HBr.

Slide 28:

Comparing Reactivity of Haloalkanes and Haloarenes:

  • Haloalkanes are generally more reactive than haloarenes due to the greater ease of breaking carbon-halogen bonds in haloalkanes.
  • The resonance stabilization provided by the aromatic ring in haloarenes makes them less reactive towards substitution and elimination reactions.
  • Haloalkanes readily undergo both substitution and elimination reactions, while haloarenes primarily undergo nucleophilic aromatic substitution reactions.

Slide 29:

Chemical Tests and Identification of Haloalkanes and Haloarenes:

  • The presence of halogen atoms in organic compounds can be confirmed using specific tests:
  1. Silver Nitrate Test:
    • AgNO3 can react with haloalkanes or haloarenes to form insoluble silver halides (AgX).
    • AgBr and AgCl form white precipitates, while AgI forms a yellow precipitate.
  1. Sodium Fusion Test:
    • When a haloalkane or haloarene is fused with sodium metal, sodium halides are formed.
    • The liberated halides can be identified using specific tests.

Slide 30:

Summary:

  • Polyhalogen compounds, including haloalkanes and haloarenes, play significant roles in various fields like medicine, industry, and environmental studies.
  • Haloalkanes undergo nucleophilic substitution and elimination reactions, whereas haloarenes mainly undergo nucleophilic aromatic substitution reactions.
  • Understanding the reactivity and properties of polyhalogen compounds is crucial for their applications and environmental impact assessment.
  • Specific tests are available to identify the presence of halogens in organic compounds.