Alkyl Halides: Structure and Classification

Definition and Types of Halides

Alkyl halides are organic compounds in which a halogen atom is bonded to a carbon atom. The nature of the carbon atom and its hybridization determines the classification:

• Alkyl halide: Halogen bonded to an sp3 hybridized carbon of an alkyl group.

• Vinyl halide: Halogen bonded to an sp2 hybridized carbon of an alkene.

• Aryl halide: Halogen bonded to an sp2 hybridized carbon of an aromatic ring.

Example:

• Alkyl halide: CH3CH2Br

• Vinyl halide: CH2=CHCl

• Aryl halide: C6H5I

Nomenclature

Alkyl halides are named using IUPAC and common names:

• IUPAC: Haloalkane (e.g., 1-chloropropane)

• Common name: Alkyl halide (e.g., ethyl chloride)

• Examples of common solvents:

  • CH2Cl2: methylene chloride

  • CHCl3: chloroform

  • CCl4: carbon tetrachloride

Primary, Secondary, and Tertiary Halides

• Primary (1°) halide: Halogen attached to a carbon bonded to one other carbon.

• Secondary (2°) halide: Halogen attached to a carbon bonded to two other carbons.

• Tertiary (3°) halide: Halogen attached to a carbon bonded to three other carbons.

Geminal and Vicinal Dihalides

• Geminal dihalide: Two halogens bonded to the same carbon.

• Vicinal dihalide: Two halogens bonded to adjacent carbons.

Example:

• Geminal dibromide: Br2C

• Vicinal dichloride: ClCH2CH2Cl

Common Uses of Alkyl Halides

Solvents

• Carbon tetrachloride (CCl4): Used for dry cleaning.

• Methylene chloride (CH2Cl2) and chloroform (CHCl3): Used for cleaning and degreasing.

• Methylene chloride: Previously used in decaffeinated coffee production.

Reagents

• Alkyl halides serve as starting materials for synthesizing more complex organic molecules.

Bond Polarity and Reactivity of Alkyl Halides

Bond Polarity

Alkyl halides are highly reactive due to the polarity of the carbon-halogen bond, which arises from differences in electronegativity:

• Carbon has a partial positive charge (electrophilic), while the halogen has a partial negative charge.

• Nucleophiles can attack the electrophilic carbon.

Bond Polarity Table:

Preparation of Alkyl Halides

Free-Radical Halogenation

• Not an effective method for selective synthesis due to product mixtures.

• Useful when all hydrogens are equivalent or halogenation is highly selective.

Example:

• Cyclohexane + Cl2 (hv) → chlorocyclohexane

• Isobutane + Br2 (hv) → t-butyl bromide

Allylic Halogenation

Allylic halogenation is selective because the allylic radical is resonance-stabilized.

• Allylic positions are more reactive due to resonance stabilization.

• Common reagent: N-bromosuccinimide (NBS) generates a low concentration of Br2 for selective bromination.

Example:

• Cyclohexene + Br2 (hv) → 3-bromocyclohexene

Reactions of Alkyl Halides

Substitution and Elimination

Alkyl halides are easily converted to other functional groups due to the halide being a good leaving group.

• Substitution: Another atom replaces the halide ion.

• Elimination: The halide ion leaves with another atom or ion, forming a double bond.

General Equations:

• Nucleophilic substitution:

• Elimination:

Example:

• Substitution: CH3Br + OH- → CH3OH + Br-

• Elimination: CH3CH2Br + OH- → CH2=CH2 + H2O + Br-

Summary Table: Types of Alkyl Halides

Key Concepts

• Alkyl halides are important intermediates in organic synthesis.

• Bond polarity and electrophilicity of carbon make alkyl halides reactive toward nucleophiles.

• Substitution and elimination reactions are fundamental transformations for alkyl halides.

• Preparation methods include free-radical halogenation and selective allylic halogenation.

Additional info: Later sections of the chapter (not shown in these slides) typically cover detailed mechanisms (SN1, SN2, E1, E2), factors affecting reactivity, and stereochemistry.