IRELAND 4 In-Depth Notes on Alkyl Halides and Nucleophilic Substitution Reactions

Learning Outcomes

• Ability to name alkyl halides using IUPAC nomenclature.
• Differentiate between primary, secondary, and tertiary alkyl halides, and identify the alpha-carbon.
• Understand that alkyl halides are polar compounds.

Alkyl Halides: Overview

• Alkyl Halides (or haloalkanes) are derivatives of alkanes where hydrogen atoms are replaced by halogens (fluorine, chlorine, bromine, iodine).
• Common Alkyl Groups:
  • Methane: $CH<em>4$ (alkyl group: $-CH</em>3$)
  • Ethane: $C<em>2H</em>6$ (alkyl group: $-C<em>2H</em>5$)
  • Propane: $C<em>3H</em>8$ (alkyl group: $-C<em>3H</em>7$)

Naming Alkyl Halides

1. Identify the parent chain (longest continuous carbon chain).
2. Number substituents for the lowest possible numerical designation.
3. Use prefixes (di-, tri-, tetra-) for multiple same type halogens.
4. List substituents alphabetically.
   • Example: 3-bromohexane

Types of Alkyl Halides

• Focus on the alpha-carbon:
  • Primary: Carbon bonded to one other carbon (e.g. Iodomethane)
  • Secondary: Carbon bonded to two other carbons
  • Tertiary: Carbon bonded to three other carbons
  • Note: The same classification applies for halogens (fluorine, chlorine, bromine).

Polarity of Carbon-Halide Bonds

• The bond between carbon and halogen is polar, making carbon partially positive (electrophilic) and capable of reacting with nucleophiles.
• Electronegativity values:
  • Carbon: 2.5
  • Halogens: F = 4.0, Cl = 3.1, Br = 3.0, I = 2.6

Nucleophiles Characteristics

• Nucleophiles are electron-rich and can either:
  • Carry a negative charge
  • Be neutral (possessing lone pairs)
• Form bonds by donating electrons to electrophilic atoms (e.g., hydroxide, alkoxide, water, alcohol).

Nucleophilic Substitution Reactions Summary

• Mechanism: Nucleophile + Alkyl Halide -> Product + Leaving Group
  • The nucleophile replaces the halogen in the alkyl halide.
  • Halide ions are good leaving groups due to high electronegativity, accommodating the negative charge.

Nucleophilic Substitution Mechanisms

• Two types of nucleophilic substitution reactions:
  • SN1: Unimolecular substitution, rate depends only on the substrate.
  • SN2: Bimolecular substitution, the rate depends on the concentrations of both nucleophile and substrate.

Mechanism Details and Curly Arrows

• Utilize curly arrows to illustrate electron movement during bond formation/breaking.
• Transition State: The highest energy point in the pathway from reactants to products.

Reaction Examples

• Hydroxide reacting with chloromethane:
  • Both C-Cl bond polarization and new bond formation are critical events in the reaction.
  • Square brackets denote impossible isolation of structures in transition states.

Inversion of Configuration with SN2 Reactions

• Inversion occurs at the opposite side of the leaving group.
  • Important during nomenclature to avoid ambiguity (e.g., (R) to (S)).

Summary Of SN2 Reactions

• Backside attack leads to inversion of configuration.
• Transition state formed without intermediate. Reaction rate depends on both nucleophile and substrate concentrations.
• Primary alkyl halides are more reactive than secondary; tertiary do not proceed via SN2.
• Negatively charged nucleophiles are more reactive than neutral ones.

Practice Example

• Predict products for SN2 reaction of 1-bromobutane with:
  1. NaI
  2. KOH
  3. H–C≡C–Li
  4. NH₃

Conclusion

• Keep studying the principles of alkyl halides and nucleophilic substitution to master these concepts fully.