CHM210 Class 11/03

Introduction to Organic Halides

  • Overview of chapters dedicated to organic halides, including Chapter 11, which is considered a separate entity.

Definition of Organohalides

  • Organohalides are organic compounds that contain halogen atoms (Group 17 elements: fluorine, chlorine, bromine, iodine).

Physical Properties of Organohalides

  • Familiar chemistry with organohalides will be explored.

  • Reactions involving organohalides have been previously introduced in earlier chapters.

  • Exclusion of fluorine in responsibilities: Flurocarbons are commonly discussed in the popular press but fluorine behaves differently than other halogens.

    • Utility of Organohalides: Used in fragrances, solvents, and pharmaceuticals, albeit with concerns regarding their environmental impact.

    • Recognition that many organohalides are naturally occurring.

Naming Organohalides

  • Halogens are named as substituents:

    • Chlorine as chloro group

    • Bromine as bromo group

  • Rules for naming form consistent patterns established in earlier chapters.

Physical Properties and Trends

  • Important trends in the physical properties and parameters of organic halides:

    • As you descend the periodic table from fluorine to iodine, the carbon-halogen bond becomes longer and weaker due to increased electron count.

      • This is logical because:

      • Fewer electrons lead to stronger, shorter bonds. Hence, carbon-fluorine bonds are strong and short.

      • Conversely, iodine's larger size and increased electron count lead to longer and weaker bonds.

    • Carbon-hydrogen bonds also display similar trends of increased bond length and decreased strength as you go down the periodic table.

  • Understanding bond lengths, strengths, and dipole moments are crucial but do not require memorization of specific values.

Dipole Moments in Organohalides

  • Dipole moments are influenced by charge separation between carbon and halogen in organic compounds.

    • Methyl chloride as an example shows significant dipole moments related to charge separation and bond lengths.

    • Understanding dipole moments is important for later chapters, particularly Chapter 11, as applying this knowledge is pivotal in evaluating chemical preferences.

Reactions and Synthesis of Alkyl Halides

Overview of Alkyl Halide Formation

  • Alkyl halides can be synthesized using various methods, as previously discussed:

    • Reaction of alkenes with hydrohalic acids (HX, where X = Cl, Br, I).

    • Formation of vicinal dihalides by adding halogens across carbon-carbon bonds.

  • This synthesis may not have been previously covered in detail, indicating a step forward in the complexity of organic reactions.

Radical Mechanisms in Organic reactions

  • Details on the radical reactions where electrons move singly rather than in pairs are critical:

    • Conventional reactions typically operate under a homolytic bond cleavage mechanism.

    • A clear distinction between homolytic vs. heterolytic mechanisms is made where most organic reactions utilize electron pairs, while homolytic reactions rely on single electron movements represented by fish hook arrows.

Reaction of Methane with Chlorine

  • Introduction of free radical chain reactions illustrated through the chlorination of methane:

    • Mechanism involves initiation, propagation, and termination steps:

      • Initiation: Ultraviolet light causes homolytic cleavage of the Cl-Cl bond, forming chlorine radicals.

      • Propagation: Chlorine radical reacts with methane (CH₄) to abstract a hydrogen atom, forming a methyl radical (CH₃•) and hydrochloric acid (HCl).

      • Termination: The radicals collide and ultimately slow down the reaction, terminating the chain by depleting the available radicals, affecting yields.

    • This radical mechanism can lead to complexities such as polyhalogenation, where multiple chlorines replace hydrogens.

    • Control issues arise, especially in achieving desired product mix with potential for significant byproducts when scaling up reactions.

Factors Affecting Reactivity in Free Radical Reactions

  • Insight into the reactivity of different hydrogen atoms:

    • Tertiary hydrogen atoms are more reactive to chlorine compared to primary and secondary hydrogen atoms due to stability differences in the resulting radicals.

    • Empirical data suggests that tertiary C-H bonds are five times more reactive than primary C-H bonds, aligning with stability trends discussed previously in carbocation stability.

  • The reactivity order established reflects the stability of free radicals— tertiary > secondary > primary radical stability.

Bromination Reactions

  • Mechanism for bromine is similar to chlorine but shows greater selectivity toward secondary hydrogens over primary ones.

    • Bromine radicals show preference for stronger secondary C-H bonds, resulting in notable differences in product distribution.

    • A direct comparison of numbers and observed data reveals significant deviations in expected vs. actual reaction products.

Summary and Implications

  • Recognizing the distinct mechanisms and factors influencing organohalide reactions is critical for advanced organic synthesis.

  • Importance and challenges associated with controlling free radical reactions, particularly in industrial contexts and synthetic organic chemistry.

  • Comparative analysis between chlorination and bromination provides insight into reactivity and selectivity of the halogen atoms in organic reactions, which are necessary for developing strategies in chemical synthesis.