Structure and Reactivity of Alkyl Halides

The 3D model of methyl chloride shows its Molecular Electrostatic Potential (MEP), highlighting the polar C-Cl bond. Red areas indicate higher, and blue areas lower, electron density. This polarity is due to chlorine's greater electronegativity, giving it a partial negative charge (δ-) and carbon a partial positive charge (δ+). This positive carbon makes alkyl halides susceptible to nucleophilic substitution. Nucleophiles (electron-rich species) are attracted to this positive center and can displace the halide. Understanding this polarity (visualized by the MEP) is crucial for understanding alkyl halide reactivity and properties. The following sections will explore electronegativity, bond polarity, and their influence on alkyl halide properties and reactions, including nucleophilic attack.

 

Element Electronegativity (Pauling Scale) ΔEN with Carbon Comments
Carbon (C) 2.55 - Forms polar bonds with halogens; acts as the less electronegative atom in C–X bonds.
Fluorine (F) 3.98 1.43 Most electronegative element; creates very polar C–F bonds with strong dipole moments.
Chlorine (Cl) 3.16 0.61 High electronegativity; C–Cl bonds are quite polar, but less so than C–F bonds.
Bromine (Br) 2.96 0.41 Moderate electronegativity; C–Br bonds are polar but weaker in polarity compared to C–Cl.
Iodine (I) 2.66 0.11 Least electronegative halogen; C–I bonds are the least polar among common alkyl halides.

Explanation

  • Electronegativity Difference: The difference in electronegativity between carbon (2.55) and each halogen determines the polarity of the C–X bond.
  • Polar C–X Bonds: The most polar bond occurs between carbon and fluorine due to the significant difference in electronegativity, while the C–I bond is the least polar because the difference is smallest.
  • Trend in Polarity: As electronegativity decreases from fluorine to iodine, the C–X bond becomes less polar, affecting properties like dipole moment and intermolecular forces.

The polarity of an alkyl halide depends on the electronegativity difference between the carbon atom and the halogen atom. Here’s how polarity changes across different alkyl halides:

  1. C–F (Fluoroalkanes): The C–F bond is the most polar among alkyl halides because fluorine is highly electronegative. The significant electronegativity difference (about 1.5) results in a large dipole moment, making fluoroalkanes highly polar.

  2. C–Cl (Chloroalkanes): The C–Cl bond is less polar than the C–F bond because chlorine is less electronegative than fluorine, with an electronegativity difference of about 0.5. Chloroalkanes still have a considerable dipole moment, but it is weaker than that of fluoroalkanes.

  3. C–Br (Bromoalkanes): The C–Br bond is even less polar, with an electronegativity difference of about 0.3. The dipole moment is smaller, making bromoalkanes moderately polar.

  4. C–I (Iodoalkanes): The C–I bond has the lowest polarity among the common alkyl halides, with an electronegativity difference of only about 0.1. As a result, the dipole moment is small, and iodoalkanes are the least polar alkyl halides.

 

Bond Bond Length (pm) Bond Strength (kJ/mol) Electronegativity Difference Polarity
C–F ~135 ~485 1.5 Very polar
C–Cl ~177 ~338 0.5 Moderately polar
C–Br ~194 ~285 0.3 Less polar
C–I ~214 ~238 0.1 Least polar

This table illustrates how bond length increases and bond strength decreases as you move from fluorine to iodine, while the electronegativity difference decreases, affecting the polarity of the bond.

 

Alkyl Halide Molecular Formula Boiling Point (°C) Comments
Methyl fluoride CH₃F -78 Very low boiling point; small, non-polarizable molecule
Methyl chloride CH₃Cl -24 Low boiling point; small molecule, moderate dipole
Methyl bromide CH₃Br 4 Higher than CH₃Cl due to increased polarizability
Methyl iodide CH₃I 42 Significant increase from CH₃Br due to large, polarizable iodine
Ethyl chloride C₂H₅Cl 12 Slightly higher than methyl chloride; increased mass and surface area
Ethyl bromide C₂H₅Br 38 Higher boiling point; larger molecule with more dispersion forces
Ethyl iodide C₂H₅I 72 Much higher due to increased polarizability of iodine
n-Propyl chloride C₃H₇Cl 47 Higher boiling point than ethyl chloride; increased carbon chain length
n-Propyl bromide C₃H₇Br 71 Higher boiling point due to greater polarizability of bromine
n-Propyl iodide C₃H₇I 102 Significantly higher boiling point due to iodine's large, polarizable electron cloud
  1. Increase with Halogen Size: For a given alkyl group, the boiling point increases as the halogen changes from fluorine to iodine. This is due to the increasing polarizability and stronger London dispersion forces associated with larger halogen atoms.
  2. Increase with Alkyl Chain Length: As the alkyl group increases in size (e.g., from methyl to propyl), the boiling point rises. This is because larger alkyl groups have more surface area, leading to stronger dispersion forces.
  3. Combined Effects: The highest boiling points are observed for alkyl iodides with larger alkyl groups, where both the polarizability of iodine and the increased dispersion forces from a longer carbon chain contribute to the overall increase.

This table and explanation should provide a clear understanding of how molecular structure influences the boiling points of alkyl halides. Let me know if you need more details or any additional comparisons!