Chapter 10: Organohalides

Learning Objectives of Chapter 10

• Names and Structures of Alkyl Halides
• Preparing Alkyl Halides from Alkanes: Radical halogenation
• Preparing Alkyl Halides from Alkenes: Allylic bromination
• Stability of the Allyl Radical: Resonance revisited
• Preparing Alkyl Halides from Alcohols
• Reactions of Alkyl Halides: Grignard reagents and organometallic coupling reactions
• Oxidation and Reduction in Organic Chemistry

Organohalides and Alkyl Halides

• Definition: An organohalide is an organic compound with one or more halogen atoms (F, Cl, Br, I).
• Presence in Nature: Commonly found in nature and used as solvents, inhaled anesthetics, and fumigants.
• Alkyl Halides: Comprised of a halogen bonded to a saturated (sp³-hybridized) carbon atom.

Naming Alkyl Halides

• Haloalkanes: Alkenes with halogen substitutions.
• Systematic Naming Steps:
  • Step 1: Identify the longest carbon chain as the parent.
  • Step 2: Number the carbon atoms starting from the end closest to the first substituent.
  • Step 3: Name halogens in alphabetical order when multiple halogens are present.
• Example: $CH_3I = $Iodomethane$ (methyl iodide).

Comparison of Halomethanes

• As you move down the periodic table:
  • Size of halogens increases.
  • C–X bond lengths increase.
  • C–X bond strengths decrease.

Polarity of C–X Bonds

• Dipole Moment: Halomethanes have a significant dipole moment.
• Charge Distribution: Slight positive charge ($ ext{δ}^+$) on carbon and slightly negative ($ ext{δ}^-$) on halogens.
• Electrophilic Behavior: The C–X carbon is an electrophile in polar reactions.

Preparing Alkyl Halides from Alkanes

• Radical Halogenation: Alkanes react with $Cl2$ or $Br2$ in the presence of light to form alkyl halides through a radical chain reaction.
• Mechanistic Requirements:
  • Initiation, propagation, termination.
• Reactivity Order: Reactivity varies with hydrogen types; tertiary radicals are more stable than primary or secondary.

Preparing Alkyl Halides from Alkenes: Allylic Bromination

• Reaction with NBS: Alkenes react with N-bromosuccinimide (NBS) under light, leading to hydrogen substitution at the allylic position, generating alkyl halides.
• Stability of Allyl Radicals:
  • Adopts sp² hybridization.
  • Exhibits resonance with two structures, resulting in increased stability.

Preparing Alkyl Halides from Alcohols

• Methods: Treat alcohols with HCl, HBr, or HI for simple conversion.
• Optimal Methods: Use thionyl chloride or phosphorus tribromide for better conversion rates in primary and secondary alcohols.

Reactions of Alkyl Halides: Grignard Reagents

• Formation: Alkyl halides (RX) react with Mg in ether/THF to form Grignard reagents (RMgX).
• Nucleophilic Behavior: The carbon in Grignard reagents acts as both nucleophilic and basophilic, effectively forming carbanions.

Organometallic Coupling Reactions

• Preparation of Organometallic Compounds: Similar to the formation of Grignard reagents; involves reactions with lithium metal.
• Gilman Reagents: Reaction between lithium and alkyl halides forms alkyllithium compounds.
• Coupling Reactions: Gilman reagents couple with organochlorides, bromides, and iodides.

Oxidation and Reduction in Organic Chemistry

• Oxidation Definition: Decreases electron density on carbon, forms C–O, C–N, C–X or breaks C–H bonds.
• Reduction Definition: Increases electron density on carbon, forms C–H (or C–C) or breaks C–O, C–N, C–X bonds.

Summary of Chapter 10

• Alkyl halides play an essential role in organic chemistry, featuring well-studied reactions.
• They can be formed from alkenes through NBS reactions, and Grignard reagents are produced from the reaction of alkyl halides with magnesium.
• Gilman reagents arise from reactions with lithium, leading to significant applications in organometallic chemistry.