Chemistry · Ch 10 — Halogen Derivatives
Boiling point
Boiling point
Boiling points of alkyl halides are considerably higher than those of the corresponding alkanes, because of the higher polarity and higher molecular mass that the halogen brings. Within a series of alkyl halides sharing the same alkyl group, the boiling point increases with increasing atomic mass of the halogen (RI > RBr > RCl > RF), because the magnitude of the van der Waals force of attraction between molecules increases as the halogen gets larger and heavier -- for example CH3F boils at 194.6 K, CH3Cl at 248.8 K, CH3Br at 276.6 K and CH3I at 315.4 K. For a given halogen, boiling point also rises with increasing carbon number of the alkyl chain (CH3Cl 248.8 K, CH3CH2Cl 285.5 K, CH3CH2CH2Cl 320.0 K, CH3CH2CH2CH2Cl 351.5 K), because a longer chain has a larger surface area over which van der Waals forces can act. Conversely, for isomeric alkyl halides of the same molecular formula, boiling point decreases with increased branching, because a more branched, more compact molecule has less surface area and therefore weaker van der Waals attraction than its straight-chain isomer -- for example 1-bromobutane boils at 375 K, its secondary isomer 2- …
Worked out. Problem: arrange bromoform, chloromethane, dibromomethane and bromomethane in order of increasing boiling point. Solution: since all four compounds have only one carbon, their comparative boiling points are governed mainly by van der Waals forces of attraction, which depend on overall molecular size -- here, on the size and number of halogen atoms present rather than on chain length. Going from chloromethane (one small Cl) to bromomethane (one larger Br) to dibromomethane (two Br atoms) to bromoform (three Br atoms), molecular size and hence the strength of the van der Waals attraction increases steadily, so the increasing order of boiling point is: CH3Cl < CH3Br < CH2Br2 < CHBr3. Note: this worked problem is printed in the textbook at the very start of section 10.5 (immediately before the optical-isomerism discussion begins), even though its co …