Chemistry of Alkyl Halides Study Notes

Chapters 5: Chemistry of Alkyl Halides

1. Alkyl Halides (Haloalkanes)

  • Definition: Alkyl halides, also known as haloalkanes, are organic compounds characterized by the presence of at least one carbon-halogen bond (C-X).
    • Halogens Include:
    • X = F (Fluorine)
    • Cl (Chlorine)
    • Br (Bromine)
    • I (Iodine)
  • Hybrids: Alkyl halides contain a halogen atom bonded to an sp³ hybridized carbon atom.
    • Can have multiple C-X bonds.

2. Properties and Applications

  • Uses of Alkyl Halides:
    • Fire-resistant solvents
    • Refrigerants
    • Pharmaceuticals and precursors
  • Examples:
    • Halothane (C₂HBrF₃) – an inhaled anesthetic
      Structure: F-C-C-H
    • Dichlorodifluoromethane (CCl₂F₂) – a refrigerant
      Structure: Cl-C-F
    • Bromomethane (CH₃Br) – a fumigant
      Structure: H-C-Br

3. Classification of Alkyl Halides

  • Alkyl halides can be classified into three categories based on the number of carbon atoms bonded to the carbon with the halogen atom:
    • Primary (1º): Halogen bound to a carbon that has one other carbon atom. Examples:
      • CH₃-CH₂-Br
      • CH₃-CH-CH₂-Cl (Propyl bromides)
    • Secondary (2º): Halogen bound to a carbon that has two other carbon atoms. Examples:
      • CH₃-CH-CH₃ (Isobromopropane)
      • CH₃-CH-CH₂-CH₃ (2-chlorobutane)
    • Tertiary (3º): Halogen bound to a carbon that has three other carbon atoms. Examples:
      • CH₃-C-CH₃ (Bromocyclopentane)

4. Types of Halides

  • Vinyl Halides: Halogen attached to a C-C double bond.
    Example: Vinyl chloride (C₂H₃Cl)
  • Aryl Halides: Halogen attached to a benzene ring.
    Example: Chlorobenzene (C₆H₅Cℎ)
  • Allylic Halides: Halogen attached to the carbon adjacent to a C-C double bond.
    Example: Allyl bromide (C₃H₅Br)
  • Benzylic Halides: Halogen attached to the carbon adjacent to a benzene ring.
    Example: Benzyl chloride (C₇H₇Cl)

5. Naming Alkyl Halides

  • Alkyl halides can be named using the following conventions:
    • Simple examples:
    1. 1-iodo-ethane = ethyl iodide
    2. 2-chloro-butane = sec-butyl chloride
    3. 2-bromo-propane = isopropyl bromide
    • In general, use the halogen as a substituent and name the molecule by identifying the longest carbon chain.

6. Physical Properties

  • Polarity: Alkyl halides are weak polar molecules exhibiting dipole-dipole interactions due to their polar C-X bonds.
  • Hydrogen Bonding: Although polar, they cannot engage in hydrogen bonding because they do not contain N-H or O-H bonds.
  • Boiling Points: The boiling point order can be described as follows:
    • Larger sizes & weights increase boiling points
    • As the number of substituents in the structure increases, the boiling point decreases.

7. Structure of Alkyl Halides

  • Bond Length and Strength: The C-X bond variations include:
    • Bond Length: Increases down the periodic table (Fluorine is shortest and Iodine is longest).
    • Bond Strength: Decreases down the periodic table, with fluorine having the strongest bond and iodine the weakest.
    • Charge: The bond is polarized, with the carbon atom bearing a partial positive charge and the halogen a partial negative charge.

Table 10.1: Bond Characteristics of Halomethanes

| 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 |

Note: Energy levels indicate relative reactivity and dipoles towards electrophilic reactions.

8. Synthesis of Alkyl Halides

  • Halogenation of Alkanes: This process involves the reaction of alkanes with halogens in the presence of heat or light:
    • General Reaction: R-H + X_2
      ightarrow R-X + H-X
    • This reaction can be explosive with Fluorine and is endothermic with Iodine.

9. Mechanism of Halogenation

9.1 Initiation

  • Step: Heat or light leads to the homolytic cleavage of the halogen molecule, producing highly reactive radical chlorine atoms (Cl•).

9.2 Propagation

  • Step 1: Chlorine radical abstracts a hydrogen atom from methane leading to the formation of a methyl radical (CH₃•).
  • Step 2: The methyl radical then abstracts a chlorine atom from a chlorine molecule, regenerating the chlorine radical and producing methyl chloride (CH₃Cl).

9.3 Summary of the Reaction Mechanism:

  • HV + CH₄ + Cl_2
    ightarrow CH₃Cl + HCl

10. Selectivity in Halogenation

  • Limitation: The chlorination of alkanes shows a lack of selectivity, yielding multiple products rather than the desired structure, particularly for larger alkanes.
  • Regioselectivity: The nature of regioselectivity reveals that reactivity is geometric in relation to how many carbons are adjacent to the reaction site. This leads to products that vary in composition.
    • Example: Chlorination of butane gives both 1-chlorobutane and 2-chlorobutane in different ratios.

11. Regioselectivity Of Bromination and Chlorination

  • Bromination exhibits more selectivity due to the stabilization of carbocation intermediates.
    • In reactions, bromination favors substitution at tertiary carbons while chlorination is less so.

12. Nucleophilic Substitution Reactions

  • General Reaction:
    • R-X + :Nu
      ightarrow R-Nu + X^-
    • Yields alkyl types and/or nucleophiles depending on the conditions.

13. Factors Affecting Nucleophilicity

  1. Charge: Conjugate bases (strong nucleophiles) have more electrons available. Examples: HO: > H₂O, H₃N > NH₃.
  2. Electronegativity: Nucleophilicity increases with decreasing electronegativity, as less electronegative atoms hold onto electrons weakly.
  3. Solvent Interactions: Polar protic solvents hinder nucleophilicity relative to polar aprotic solvents.
  4. Steric Hindrance: The bulkiness of the nucleophile affects its ability to approach and react with the electrophile.

14. Mechanisms of Nucleophilic Substitution

  • SN₂ Mechanism: Features a single step where the nucleophile attacks from the opposite side of the leaving group, creating inversion of configuration. This is a concerted reaction.
  • SN1 Mechanism: Consists of two steps and results in a carbocation intermediate before the nucleophile attacks. Carbocation stability is critical to the mechanism, with tertiary carbocations being preferred.

15. Comparison of SN1 and SN2 Mechanisms

  • SN2: Occurs with methyl and primary halides, results in stereochemical inversions and is concerted.
  • SN1: Occurs with secondary and tertiary halides due to carbocation reactivity, with racemic mixtures produced under chiral environments.

16. Other Characteristics and Rearrangements

  • Rearrangements: Carbocation stabilization via rearrangement can occur in SN1 mechanisms, favoring more stable carbocations (e.g., tertiary over secondary).
  • Stereochemical Analysis: SN1 results in racemic products where the carbocation has no stereochemical preference before recombination. Conversely, SN2 results in a unique product changing configurations.

17. Conclusion

  • Understanding alkyl halides requires a grasp of their reactions, classifications, physical properties, and how molecular structure affects reactivity. The synthetic applications, while useful, need to consider regioselectivity and stereochemistry to yield products effectively.