A series of four methyl halides—CH3F, CH3Cl, CH3Br, and CH3I—exhibits interesting trends in bond polarity and molecular dipole moment. Relevant data are provided below.
Pauling electronegativities: F = 4.0, Cl = 3.0, Br = 2.8, I = 2.5, C = 2.5
Average bond lengths: C–F = 1.39 Å, C–Cl = 1.78 Å, C–Br = 1.94 Å, C–I = 2.14 Å
Use this information to analyze the effect of changing the halogen on the distribution of partial charges.
(a) For each C–X bond (X = F, Cl, Br, I), designate which atom carries δ− and which carries δ+. State the principle you applied.
(b) Rank the four C–X bonds in order of expected bond dipole moment magnitude based solely on electronegativity differences. Show your reasoning.
(c) Incorporate the bond-length data to refine your ranking from part (b). Identify any deviation from the trend predicted by electronegativity alone and account for it in two or more well-structured sentences.
(d) Using your refined analysis, predict which methyl halide has the largest overall molecular dipole moment. Justify your prediction by considering both vector addition of bond dipoles in a tetrahedral geometry and the quantitative comparisons made earlier.