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10.S: Organohalides (Summary)

Last updated

Jan 15, 2023
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- 10.9: Oxidation and Reduction in Organic Chemistry
- 11: Reactions of Alkyl Halides- Nucleophilic Substitutions and Eliminations

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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

Reactions Alkyl Halides

Concepts & Vocabulary

Skills to Master

Summary of Reactions

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