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10.S: Organohalides (Summary)

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    Concepts & Vocabulary

    10.1 Introduction to Organohalides

    • Alkyl halides (and allyl and benzyl halides) are more reactive than vinyl and aryl halides.

    10.2 Names and Properties of Alkyl Halides

    • Reactivity of alkyl halides is often related to the substitution of the carbon atom the halogen is attached to.
    • Alkyl halides are categorized by the number of bonds to other alkyl groups (primary, secondary, and tertiary).
    • Carbon-halogen bonds are polarized with partial positive charges on carbon and partial negative charges on the halogen.
    • Fluorine is the most electronegative of the halogens while iodine is the least electronegative.
    • Iodine is the largest of the halogens yielding the longest/weakest bonds to carbon of the halogens.
    • Since haloalkanes have dipole-dipole interactions, they have greater intermolecular forces than similar sized alkanes and therefore higher boiling points.
    • Alkyl halides are either slightly soluble or insoluble in water, but are soluble in organic solvents.

    10.3 Preparing Alkyl Halides from Alkanes - Radical Halogenation

    • Halogenation of alkanes is exothermic, so it is energetically favorable.
    • Radical chain mechanisms consist of three steps: initiation, propagation and termination.
    • Hydrogens on more substituted carbon atoms are more reactive to radical halogenation.

    10.4 Preparing Alkyl Halides from Alkenes - Allylic Bromination

    • More substituted radicals and radicals with resonance structures are more stable than other radicals.
    • Radical substitution can be carried out at the allylic or benzylic carbon by reacting with NBS.

    10.5 Stability of the Allyl Radical - Resonance Revisited

    • Allyl cations, anions and radicals have resonance structures. To draw these resonance structures non-bonded and pi-bond electrons can be moved.
    • Resonance hybrids are used to show the combination of all resonance structures for a molecule or ion.

    10.6 Preparing Alkyl Halides form Alcohols

    • Alcohols can be reacted with hydrohalogen acids or a mixture of halogen salts and a stronger acid (to form hydrohalogen acids in situ).
    • Alcohols will also react with thionyl chloride or with phosphorus halides to from haloalkanes.

    10.7 Reactions of Alkyl Halides - Grignard Reactions

    • Organometallic reagents can be formed from alkyl halides and reactive metals (such as lithium and magnesium).
    • Alkyl magnesium halide compounds are callled Grignard reagents.
    • Grignard reagents react as bases where the alkyl group gets protonated and the metal complexes to the conjugate base of the reacting acid.

    10.8 Organometallic Coupling Reactions

    • Lithium dialkyl copper compounds are called Gilman reagents.
    • Gilman reagents have different reactivity from the other organometallics (lithium and Grignard reagents).
    • Organometallics can be reacted with alkyl halides to join to alkyl groups (coupling reactions).

    10.9 Oxidation and Reduction in Organic Chemistry

    • Gaining bonds to hydrogen for organic molecules is reduction.
    • Losing bonds to hydrogen for organic molecules is oxidation.

    Skills to Master

    • Skill 10.1 Differentiate between types of halides (alkyl, allyl, aryl, benzyl, and vinyl).
    • Skill 10.2 Differentiate between substitution of alkyl halides (primary, secondary, and tertiary).
    • Skill 10.3 Identify relative reactivity of carbon-hydrogen bonds to radical halogenation.
    • Skill 10.4 Draw resonance structures for radical compounds.
    • Skill 10.5 Draw mechanisms for radical halogenation of alkanes (initiation, propagation and termination).
    • Skill 10.6 Calculate the enthalpy change of a reaction using bond dissociation energies of reactants and products.
    • Skill 10.7 Determine products for allylic bromination reactions.
    • Skill 10.8 Draw resonance structures for allylic and other similar compounds and ions.
    • Skill 10.9 Draw products of reactions of alcohols to form alkyl halides.
    • Skill 10.10 Write equations to form Grignard reagents from alkyl halides.
    • Skill 10.11 Draw reaction products for Grignard reagents acting as bases.
    • Skill 10.12 Write equations for the formation of Gilman reagents.
    • Skill 10.13 Draw reaction products of organometallic coupling reactions.
    • Skill 10.14 Explain oxidation and reduction in organic molecules.

    Summary of Reactions

    Preparation of Alkyl Halides

    Prep Alkyl Halides.png

    Reactions Alkyl Halides

    Reations Alkyl Halides.png

    10.S: Organohalides (Summary) is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Dietmar Kennepohl & Layne Morsch.