Chapter 11: Reactions of Alkyl Halides

Introduction

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 foundational in organic synthesis and are essential for understanding the behavior of organic molecules under various conditions.

Naming and Structures of Alkyl Halides

Systematic Nomenclature

• Alkyl halides are named by identifying the longest carbon chain as the parent hydrocarbon.

• Number the chain to give the substituent (halogen or other group) the lowest possible number.

• If multiple substituents are present, assign numbers to give the lowest set of locants.

• Halogen substituents are indicated by prefixes (e.g., bromo-, chloro-, fluoro-).

• Examples:

  • 5-Bromo-2,4-dimethylheptane

  • 1-Bromo-3-chloro-4-methylhexane

Functional Class Nomenclature

• Two main types:

  1. Functional class nomenclature: The alkyl group is named as a substituent on the halide (e.g., pentyl chloride, cyclohexyl iodide).

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

Nomenclature of Alcohols

Alcohol Naming Conventions

• Similar to alkyl halides, alcohols can be named by functional class or substitutive nomenclature.

• Functional class nomenclature: The alcohol is named as an "alkyl alcohol" (e.g., ethyl alcohol).

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

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

Substitutive Nomenclature Rules

• Number the chain so that the OH or halogen substituent gets the lowest possible number.

• For alcohols, the OH group takes precedence over other substituents and alkyl groups in numbering.

• 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

Substitution Reactions

• Conversion of alcohols to alkyl halides involves replacing the OH group with a halogen (X).

• General reaction:

$\text{R-OH} + \text{HX} \rightarrow \text{R-X} + \text{H}_2\text{O}$

• Common reagents: HCl, HBr, SOCl2, PBr3, HF.

• Reactivity order for alcohols: methyl < primary < secondary < tertiary (tertiary alcohols react fastest).

Mechanisms of Nucleophilic Substitution Reactions

SN2 Reaction (Bimolecular Nucleophilic Substitution)

The SN2 reaction is a concerted process where the nucleophile attacks the substrate from the opposite side of the leaving group, resulting in inversion of stereochemistry.

• Rate law:

$\text{Rate} = k[\text{RX}][\text{Nu}^-]$

• Occurs in a single step with a transition state.

• Favored by primary substrates and strong nucleophiles.

• Results in inversion of configuration at the reaction center.

• Steric hindrance decreases the rate: tertiary < secondary < primary.

SN1 Reaction (Unimolecular Nucleophilic Substitution)

The SN1 reaction proceeds via a two-step mechanism involving formation of a carbocation intermediate.

• Rate law:

$\text{Rate} = k[\text{RX}]$

• Favored by tertiary substrates and weak nucleophiles.

• Results in racemization due to planar carbocation intermediate.

• Carbocation stability is crucial: tertiary > secondary > primary.

Summary Table: Reactivity Trends

Key Terms and Concepts

• Nucleophile: A species that donates an electron pair to form a new covalent bond.

• Leaving group: An atom or group that departs with an electron pair in substitution or elimination reactions. Good leaving groups stabilize the negative charge (e.g., Br-, I-).

• Stereochemistry: SN2 reactions invert configuration; SN1 reactions produce racemic mixtures.

Examples and Applications

• SN2 Example: Reaction of 1-bromopropane with hydroxide ion yields 1-propanol and bromide ion.

• SN1 Example: Reaction of tert-butyl bromide with water yields tert-butyl alcohol and bromide ion.

Additional info:

• These notes are based on textbook-style lecture slides and cover the essential nomenclature and mechanistic details for alkyl halide reactions in organic chemistry.