Chapter 11: Reactions of Alkyl Halides

Overview

This chapter explores the chemistry of alkyl halides, focusing on their nomenclature, preparation, and the mechanisms of nucleophilic substitution and elimination reactions. These reactions are fundamental in organic synthesis and are governed by the structure of the substrate, the nature of the nucleophile or base, and the reaction conditions.

Names and Structures of Alkyl Halides

Naming Alkyl Halides

Alkyl halides are organic compounds containing a halogen atom (F, Cl, Br, I) attached to an alkyl group. Their nomenclature follows IUPAC rules to ensure clarity and consistency.

• Parent Chain Selection: The longest continuous carbon chain containing the halogen is chosen as the parent chain.

• Numbering: Number the chain from the end nearest the first substituent (halogen or alkyl group) to give the lowest possible locants.

• Multiple Substituents: If multiple substituents are present, number the chain to give the lowest set of locants to the substituents.

• Examples:

  • 5-Bromo-2,4-dimethylheptane

  • 1-Bromo-3-chloro-4-methylhexane

Functional Class Nomenclature

Types of Nomenclature

• Functional Class Nomenclature: The alkyl group is named as a substituent on the halide (e.g., pentyl chloride, cyclohexyl iodide).

• Substitutive Nomenclature (Preferred): The halogen is treated as a substituent on the alkyl chain (e.g., 2-chloropropane, bromocyclohexane).

Alcohols

Nomenclature of Alcohols

• Functional Class Nomenclature: The alcohol is named as an "alkyl alcohol" (e.g., ethyl alcohol, 1-methylpentyl alcohol).

• Substitutive Nomenclature (Preferred): The alcohol is named as an "alkanol," with the suffix "-ol" indicating the position of the hydroxyl group (e.g., hexan-2-ol, 2-methylpentan-2-ol).

• Either the "n-alkanol" or "alkan-n-ol" format is acceptable.

Substitutive Nomenclature

Numbering and Precedence

• When using substitutive nomenclature, the position of the functional group (–OH or –X) takes precedence in numbering the parent chain.

• For alcohols, the –OH group is given the lowest possible number.

• Examples:

  • 2-propylpentan-1-ol (not 4-hydroxymethylheptane)

  • 5-fluoro-2-propylpentan-1-ol (not 1-fluoro-4-propylpentan-5-ol)

Preparing Alkyl Halides from Alcohols

Conversion Reactions

Alkyl halides can be synthesized from alcohols via substitution reactions, where the hydroxyl group is replaced by a halogen.

• General Reaction: Where HX is a hydrogen halide (HCl, HBr, HI).

• Reactivity Order: Methyl < Primary < Secondary < Tertiary alcohols (tertiary alcohols react fastest).

• Methods:

  • Direct reaction with hydrogen halides (HCl, HBr, HI).

  • Alternative reagents: SOCl2, PBr3, and others for specific conversions.

Summary Table: Reactivity of Alcohols Toward Halogenation

Key Concepts

• Alkyl Halides: Organic compounds with a halogen atom bonded to an sp3 carbon.

• Alcohols: Organic compounds with a hydroxyl (–OH) group attached to an sp3 carbon.

• Substitution Reaction: A reaction in which one atom or group is replaced by another.

• Functional Class Nomenclature: Names the compound as a derivative of a parent functional group.

• Substitutive Nomenclature: Names the compound by treating the functional group as a substituent.

Example: Preparation of Alkyl Halides

• Reaction: Application: Ethanol reacts with hydrogen bromide to form bromoethane and water.

Additional info:

• These notes cover the introductory nomenclature and preparation of alkyl halides and alcohols, which are foundational for understanding their reactivity in substitution and elimination reactions discussed in later sections.