Organ Chem Ch 10 Organohalides and Alkyl Halides

Organohalides and Alkyl Halides

• Definition: Organohalides are organic compounds containing one or more halogen atoms.

• Examples: Found in solvents, inhaled anesthetics, and fumigants.

• Structure: Alkyl halides have a halogen atom bonded to a saturated, sp³-hybridized carbon atom.

Naming and Structures of Alkyl Halides

• Haloalkanes: Alkenes with a halogen substituent.

• Naming Process:

  • Step 1: Identify the longest carbon chain as the parent (considering double/triple bonds).

  • Step 2: Number the parent chain starting from the end nearest the first substituent.

  • Step 3: If both ends give the same numbering, start from the end closer to the substituent with alphabetical preference.

  • Halogens listed along with alkyl groups in alphabetical order.

• Example: CH₃I is named iodomethane (methyl iodide).

Comparison of Halomethanes

• Trends:

  • Size of halogens increases down the periodic table.

  • C–X bond lengths increase and C–X bond strengths decrease.

Polarity of C–X Bonds

• Halomethanes exhibit significant dipole moments:

  • Carbon (δ+) exhibits slight positive charge.

  • Halogens (δ−) exhibit slight negative charge.

• Reactivity: C–X bond makes carbon behave as an electrophile in polar reactions.

Radical Halogenation of Alkanes

• Process: Alkanes react with Cl₂ or Br₂ in the presence of light via radical chain-reaction.

• Requirements: Initiation, propagation, and termination.

• Reactivity: Tertiary radicals are more stable than secondary and primary.

Allylic Bromination

• Reactant: Alkenes treated with N-bromosuccinimide (NBS) to produce alkyl halides.

• Mechanism: Hydrogen is substituted with bromine at allylic position, characterized by radical formation.

Stability of The Allyl Radical

• Hybridization: Allyl radicals adopt sp² hybridization.

• Delocalization: Unpaired electrons are stabilized through resonance, resulting in increased stability.

• Reactivity: Allyl radicals can yield a mixture of products when brominated.

Preparing Alkyl Halides from Alcohols

• Method: Treat alcohols with HCl, HBr, or HI for alkyl halide formation.

• Optimal Conditions: Use thionyl chloride or phosphorus tribromide for primary and secondary alcohols.

Reactions of Alkyl Halides: Grignard Reagents

• Formation: Reaction of RX (alkyl halide) with magnesium in ether/THF forms Grignard reagents (RMgX).

• Properties: Carbon in Grignard reagents is nucleophilic and basophilic.

Organometallic Coupling Reactions

• Gilman Reagents: Alkyl lithium compounds formed from lithium and alkyl halides.

• Reactions: Gilman reagents can undergo coupling reactions with organohalides to form carbon–carbon bonds.

Suzuki–Miyaura Reaction

• Reaction: Coupling of an aromatic or vinyl substituted boronic acid with its corresponding organohalide under palladium catalysis.

Oxidation and Reduction in Organic Chemistry

• Oxidation: Decreases electron density, forming C–O, C–N, C–X bonds.

• Reduction: Increases electron density, forming C–H or C–C bonds.

• Example of Oxidation: Chlorination of methane is an oxidation reaction.

Summary of Key Concepts

• Alkyl halides are crucial in organic chemistry and have well-studied reactions.

• Prepared from alkenes or alcohols through various methods including radical reactions and Grignard formations.

• Understanding the properties and reactivity of alkyl halides is essential for mastering organic reactions.