Chapter 5: Chemistry of Alkyl Halides

Chemistry of Alkyl Halides

Introduction to Alkyl Halides (Haloalkanes)

• Definition: Alkyl Halides (also known as Haloalkanes) are organic compounds characterized by at least one carbon-halogen bond (C-X), where X is any halogen:

  • F (Fluorine)

  • Cl (Chlorine)

  • Br (Bromine)

  • I (Iodine)

• They replace hydrogen atoms in alkanes with halogen atoms.

• Halogens are bonded to sp³ hybridized carbon atoms.

Properties and Uses of Alkyl Halides

• Functional Properties: Alkyl halides can contain multiple C-X bonds.

• Applications:

  • Fire-resistant solvents

  • Refrigerants

  • Pharmaceuticals and precursors

Examples

• Halothane:

  • Structure: F-C-C-H with halogens on carbon atoms.

  • Use: An inhaled anesthetic.

• Carbon Compounds: Examples include:

  • Dichlorodifluoromethane

  • Bromomethane (fumigant)

Classification of Alkyl Halides

• Alkyl halides can be classified based on the carbon atom to which the halogen is attached:

  • Primary (1º): Carbon with one alkyl group.

  • Example: CH₃-CH₂-Br

  • Secondary (2º): Carbon with two alkyl groups.

  • Example: CH₃-CH-CH₃ (where CH is the central carbon bonded to Br)

  • Tertiary (3º): Carbon with three alkyl groups.

  • Example: CH₃-C-CH₃ (where C is bonded to Br)

Types of Halides

• Vinyl Halides: Halogen is bonded to a C-C double bond.

• Aryl Halides: Halogen is bonded to a benzene ring.

• Allylic Halides: Halogen is bonded to a carbon adjacent to a C-C double bond.

• Benzylic Halides: Halogen is bonded to a carbon adjacent to a benzene ring.

  • Examples:

    • Propyl chloride (1-chloropropane)

    • Allyl chloride

    • Benzyl chloride

Naming Alkyl Halides (Haloalkanes)

• Familiar naming conventions apply; can refer to compounds as "Alkyl Halides."

• Specific Naming Examples:

  • 1-Iodo-ethane or ethyl iodide (for CH₃-CH₂-I).

  • 2-Chloro-butane or sec-butyl chloride (for CH₃-CH-CH₂-CH₃ with Cl on the second carbon).

  • 2-Bromo-propane or isopropyl bromide.

Physical Properties of Alkyl Halides

• Polarity: Alkyl halides are weak polar molecules exhibiting dipole-dipole interactions due to the polar C-X bond.

• Intermolecular Forces: They cannot undergo hydrogen bonding due to predominantly C-C and C-H bonds.

Boiling Point Trends

• Boiling Point Order: The boiling point increases with size and weight of the alkyl halides.

  • More substituents may lead to lower boiling points due to reduced intermolecular forces.

• Bond Characteristics:

  • C-X bond strength and length increase down the periodic table with halogens.

  • Polarization of the C-X bond results in partial charges on the carbon (δ⁺) and halogen (δ⁻).

Table 10.1: Comparison of Halomethanes

• 
  

• Bond Strength and Length Data:
  

  Halomethane	Bond Length (pm)	Bond Strength (kJ/mol)	Dipole Moment (D)
  CH₃F	139	452	1.85
  CH₃Cl	178	351	1.87
  CH₂Br	193	293	1.81
  CH₃I	214	234	1.62
  Synthesis of Alkyl Halides

  • Halogenation of Alkanes: Achieved through the exposure of alkanes to halogens in the presence of heat or light:

    • Reaction Formula:
      R-H + X_2
      ightarrow R-X + H-X

    • Halogen Variability:

    • F₂: Explosive

    • I₂: Endothermic, not economical

  Halogenation Thermodynamics

  • Reaction Energy Changes:

    • ext{For Methane (CH₄) and Chlorine (Cl₂):}
      ext{Enthalpy change (ΔH) for CH₄ + Cl₂ → CH₂Cl + HCl} = -25 ext{ kcal/mol}

    • Other reactions include Bromination and Fluorination with respective ΔH values.

  Mechanism of Halogenation

  Initiation Step

  • Requirement: Light or heat to dissociate Cl₂ into two reactive chlorine radicals (Cl•).

  Propagation Steps

  1. Chlorine radical abstracts hydrogen from alkane, forming:

     • CH₄ + Cl•
       ightarrow CH₃• + HCl

  2. The resulting alkyl radical reacts with another chlorine molecule:

     • CH₃• + Cl₂
       ightarrow CH₃Cl + Cl•

  • Regeneration: Chlorine radical is reused in the cycle.

  Selectivity in Chlorination

  • Selectivity Limitation: Chlorination may produce mixtures instead of pure chlorinated products, especially in larger alkanes.

  • Regioselectivity: All halogens react similarly but bromination offers more selectivity:

    • Example: 1-chlorobutane versus 2-chlorobutane formation ratios.

  Nucleophilic Substitution Reaction

  • Basic Equation: R-X + :Nu ightarrow R-Nu + X:

    • Where R is the alkyl group, Nu is the nucleophile, and X is the leaving group.

  Reactivity of Alkyl Halides

  • Leaving Groups: Stability of the leaving group plays a key role:

    • Larger halogen atoms are generally better leaving groups due to their ability to stabilize negative charge.

    • Ranking of Leaving Groups (best to worst): I⁻ > Br⁻ > Cl⁻ > F⁻.

  • Important to Avoid: OH⁻, MeO⁻, NH₂⁻, as they are poor leaving groups.

  Mechanisms of Substitution Reactions for Alkyl Halides

  SN² Mechanism (2nd Order)

  • Characteristics:

    • A single concerted step reaction where nucleophile attacks the electrophile opposite to the leaving group.

    • The rate of the reaction depends on the concentration of the substrate and the nucleophile:
      ext{Rate} = k[ ext{Substrate}][ ext{Nucleophile}]

    • SN² is favored with primary halides.

  SN1 Mechanism (1st Order)

  • Overview:

    • Includes a rate-determining step where a carbocation is formed after the leaving group departs.

    • The nucleophile attacks the carbocation in a rapid second step:

    • Rate-dependent only on substrate concentration:
      ext{Rate} = k[ ext{Substrate}]$$

  Substrate Influence on SN1/SN2 Mechanisms

  • SN2 Mechanism: Favored for primary substrates due to minimal steric hindrance.

  • SN1 Mechanism: Occurs with stable secondary and tertiary halides, ensuring the formation of stable carbocations.

  Carbocation Stability

  • Stability Ranking:

    • 3º > 2º > 1º > Methyl.

    • Stabilization attributed to hyperconjugation and inductive effects from adjacent alkyl groups.

  Nucleophilicity Influencers

  1. Charge: Conjugate bases are better nucleophiles (e.g., HO⁻ > H₂O).

  2. Electronegativity: Nucleophilicity decreases with increasing electronegativity.

  3. Solvent Dependence: Polar protic solvents hinder nucleophilicity due to solvation effects.

  4. Sterics: Bulkiness inhibits approach to the reaction site, reducing nucleophilicity.

  Common Nucleophiles Order of Reactivity

  Nucleophile	Effectiveness
  Good	Br⁻, I⁻
  Moderate	HO⁻, CH₃CO⁻
  Poor	H₂O, NH₃

  SN1 vs SN2 Stereochemistry

  • SN2 Reaction: Results in inversion of configuration, while SN1 forms a racemic mixture due to the planar nature of the carbocation intermediate.

  Conclusion

  • Understanding the chemistry of alkyl halides is essential for predicting their behavior in nucleophilic substitution reactions, geared toward different synthetic pathways in organic chemistry. Their reactivity, stability, and the influence of mechanisms provide a comprehensive basis for studying substitution processes in complex organic synthesis.