lech201

## Haloalkanes and Haloarenes: Introduction - Replacement of hydrogen atom(s) in hydrocarbons by halogen atom(s) leads to: - **Alkyl halides** (haloalkanes): Halogens attached to sp3 hybridised carbon in an alkyl group. - **Aryl halides** (haloarenes): Halogens attached to sp2 hybridised carbon in an aryl group. - Presence of halogenated compounds in nature with significant industrial and clinical uses: - Solvents for non-polar compounds. - Starting materials for organic synthesis. - Examples: - Chloramphenicol (antibiotic for typhoid fever). - Thyroxine (iodine hormone regulated for goiter). - Chloroquine (malaria treatment). - Halothane (anaesthetic). - Fluorinated compounds as potential blood substitutes. ## Learning Objectives - Name haloalkanes and haloarenes using IUPAC nomenclature. - Describe preparation and reactions of haloalkanes and haloarenes. - Correlate structure with reaction types. - Apply stereochemistry to reaction mechanisms. - Analyze the applications and environmental impact of organo-metallic compounds. ## Classification of Haloalkanes and Haloarenes - **Classification by Number of Halogens**: - **Monohalogen**, **Dihalogen**, **Polyhalogen** (tri-, tetra-, etc.). #### Alkyl Halides (Haloalkanes) - Formula: CnH2n+1X (X = halogen) - Classified as: - **Primary (1°)**: Halogen attached to primary carbon. - **Secondary (2°)**: Halogen attached to secondary carbon. - **Tertiary (3°)**: Halogen attached to tertiary carbon. #### Other Halide Categories - **Allylic Halides**: Halogen on sp3 carbon adjacent to C=C bond. - **Benzylic Halides**: Halogen on sp3 carbon next to an aromatic ring. - **Vinylic Halides**: Halogen on sp2 carbon of a C=C bond. - **Aryl Halides**: Directly bonded to sp2 carbon of an aromatic ring. ## Nomenclature of Haloalkanes - **Common Naming**: Alkyl group + halide. - **IUPAC Naming**: Halosubstituted hydrocarbons. - Dihalide naming: - Common name: o-, m-, p- prefixes. - IUPAC name: use numerals 1,2; 1,3; 1,4. #### Dihaloalkanes - Classified as: - **Geminal Halides**: Both halogens on the same carbon. - **Vicinal Halides**: Halogens on adjacent carbons. ### Examples of Common Names and Structures: | Structure | Common Name | IUPAC Name | |------------------------- |---------------------|-------------------------------------| | CH3CH2CH(Cl)CH3 | sec-Butyl chloride | 2-Chlorobutane | | CH2=CHCl | Vinyl chloride | Chloroethene | | CHBr3 | Bromoform | Tribromomethane | ## Carbon-Halogen Bond Characteristics - The carbon-halogen bond in alkyl halides is polarized: - Carbon (partial positive charge) <--- Halogen (partial negative charge). - Bond lengths vary by halogen size: C—F < C—Cl < C—Br < C—I. ## Preparation Methods for Haloalkanes #### 1. From Alcohols - Methods: - Alcohol + HCl or concentrated HBr (with ZnCl2 for primary and secondary). - Thionyl chloride preferred for yielding alkyl halides with gaseous byproducts. - Order of reactivity: 3° > 2° > 1°. #### 2. From Alkanes - **Free Radical Halogenation**: Chlorination or bromination. - **Addition of Hydrogen Halides**: Reactions with alkenes follow Markovnikov's rule. - **Finkelstein Reaction**: Rxn of R-Cl with NaI (producing R-I). - **Swarts Reaction**: Production of R-F using metallic fluoride. ### Nucleophilic Substitution Reactions - Substitutions categorized as: - **SN1 Mechanism**: Unimolecular; rate depends only on alkyl halide. - **SN2 Mechanism**: Bimolecular; rate depends on concentration of both alkyl halide and nucleophile. - Order of reactivity: R—I > R—Br > R—Cl >> R—F (stronger bonds are harder to break). ## Physical Properties of Haloalkanes - **Boiling Points**: Higher than hydrocarbons of similar molecular weight due to stronger intermolecular forces (dipole-dipole forces). ### Solubility Characteristics - Haloalkanes: Poorly soluble in water; soluble in organic solvents. ## Environmental Impact of Polyhalogen Compounds - Persistent due to resistance to breakdown. - Examples of polyhalogen compounds with industrial applications: - Methylene chloride: Solvent with CNS effects. - Chloroform: Historical anaesthetic with health risks. - Carbon tetrachloride: Industrial cleaning agent; contributes to ozone depletion. - DDT: Effective pesticide but environmental and health issues due to biomagnification. ## Summary - Haloalkanes and haloarenes are pivotal in organic chemistry for synthesis and industrial applications. - Chiral molecules play a crucial role in determining reaction mechanisms. - Knowledge of both physical properties and reactivity patterns is essential in understanding these halogenated compounds.