CHEM2100 CH 10 FS2024

Chapter 10: Organohalides

1. Uses of Alkyl Halides

  • Solvents: Commonly used as degreasers and in dry cleaning fluids.

  • Anesthetics:

    • Halothane (CF3CHClBr) is a prominent example.

    • Prior use of chloroform (CHCl3) as an anesthetic.

  • Refrigerants:

    • Chlorofluorocarbons (CFCs) like Freon 12 (CF2Cl2) have been replaced by Freon 22 (CF2CHCl) due to less ozone layer depletion.

  • Pesticides:

    • Examples include DDT, and bromomethane as fumigants.

  • Reagents for synthesis: Used in the creation of other chemical compounds.

2. Physical Properties of Haloalkanes

  • Electronegativity Order: F > Cl > Br > I.

  • Bond Lengths: Order of C-X bond lengths: C-F < C-Cl < C-Br < C-I.

  • Bond Strengths (kcal/mol):

    • C-F: 110

    • C-Cl: 85

    • C-Br: 70

    • C-I: 57.

  • Boiling Points:

    • Methane (CH4): Bp = -161.7 °C.

    • Iodomethane (Methyl iodide, CH3I): Bp = 42.4 °C; reacts readily as a strong alkylating agent.

3. Names and Structures of Alkyl Halides

  • Common Terminology:

    • Methylene halides: CH2X2.

    • Haloforms: CHX3.

    • Carbon tetrahalides: CX4.

  • Examples:

    • Methylene chloride (CH2Cl2).

    • Chloroform (CHCl3).

    • Carbon tetrachloride (CCl4).

4. Preparing Alkyl Halides from Alkanes: Radical Halogenation

  • Mechanism Overview:

    • Initiation: Formation of radicals through homolytic cleavage.

    • Propagation: Radical reacts with stable compounds to generate more radicals.

    • Termination: Reactions that eliminate free radicals.

5. Radical Chain Reaction Process

  • Chlorination of Methane:

    • Initiation Step: Chlorine molecule splits into atoms, generating free radicals.

    • Propagation: Chlorine radical abstracts a hydrogen atom, forming HCl and a methyl free radical.

    • Termination Steps: Include combination of free radicals to halt the reaction.

6. Reactivity and Selectivity in Chlorination

  • Relative Reactivity:

    • Fluorine reacts explosively, chlorine at a moderate rate, bromine requires heat, and iodine has negligible reactivity.

  • Monochlorination: Given different hydrogen types, actual product distribution alters from theoretical expectations due to varying hydrogen reactivity.

7. Stability of Carbon Radicals

  • Radical Stability:

    • Tertiary > Secondary > Primary based on hyperconjugation effects.

8. Other Preparation Methods for Alkyl Halides

  • From alcohols using reagents like

    • SOCl2: produces chlorides.

    • PBr3: produces bromides.

    • HF-pyridine: produces fluorides.

9. Reactions of Alkyl Halides

  • Grignard Reagents: Used extensively in synthetic chemistry, forming organomagnesium derivatives.

10. Oxidation and Reduction in Organic Chemistry

  • Focused on qualitative understanding of organic oxidative and reductive processes.

11. Summary of Key Concepts

  • Understanding the naming, properties, and reactions of alkyl halides is crucial for synthesis in organic chemistry.