AR

27 09 2022 alkyl halides introduction and synthesis

Alkyl Halides Overview

  • Alkyl halides: Compounds formed when an alkane's hydrogen is replaced by a halogen.

  • Classification:

    • Primary: Halogen attached to primary carbon.

    • Secondary: Halogen attached to secondary carbon.

    • Tertiary: Halogen attached to tertiary carbon.

Specialized Types of Alkyl Halides

  • Vinyl Halides: Halogen bonded to a vinylic (Sp2) carbon.

  • Allylic Halides: Halogen bonded to a carbon next to a vinylic carbon (sp3).

  • Aryl Halides: Halogen attached directly to a benzene ring.

  • Aralkyl Halides: Halogen attached to benzene ring, separated by sp3 carbons.

IUPAC Nomenclature of Alkyl Halides

  • Two naming systems:

    • Functional Class Nomenclature: Alkyl group and halide are separate words.

    • Substitutive Nomenclature: Halogens treated as substituents on the alkane chain.

  • Chain Numbering: Number the chain to give lower locant to substituted carbon.

Preparation of Alkyl Halides

  • From alcohols reacting with hydrogen halides:

    • Reactivity of hydrogen halides: HI > HBr > HCl > HF.

    • Tertiary alcohols convert rapidly to alkyl chlorides; primary and secondary require a more reactive halide (HBr) and higher temperatures.

    • Reactions are substitution: Halogen replaces hydroxyl group.

Mechanistic Pathways

  • Tertiary Alcohols: Follow S1 mechanism due to carbocation stability.

  • Primary Alcohols: Follow SN2 mechanism, avoiding carbocation formation.

  • Alcohol reactions with Thionyl Chloride: Gives alkyl chlorides and produces gases for easy isolation.

Radical Halogenation

  • Mechanism for chlorination and bromination involves:

    • Initiation: Homolytic bond cleavage, forming radicals.

    • Propagation: Radicals abstract hydrogen atoms, generating new radicals.

    • Termination: Combination of radicals reduces overall radical concentration.

  • The reaction yields a mixture of halogenated products at high temperatures or light exposure.

Selectivity in Radical Halogenation

  • Chlorination and Bromination yield different product distributions.

  • Bromination is more selective for more substituted radicals.

  • Hydrocarbon selection affects chlorination outcomes; 2-bromobutane is favored in bromination vs chlorination.

Reactivity–Selectivity Principle

  • Bromine reacts more selectively than chlorine due to differences in reactivity:

    • Radical formation rates when abstracting hydrogen vary by radical type (Br > Cl).

  • Activation energy for hydrogen abstraction by bromine is significantly higher than by chlorine, influencing reaction outcomes.