The boiling point of CH₃CH₂Br (bromoethane) is 38.4 °C whereas the boiling point of CH₃CH₂I (iodoethane) is 71.5 °C. Which of the following best explains the difference between the boiling points of the two alkyl halides?

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11ecba2d_124f_ff74_a3bf_338eed2f5a84_MD0008_00 The boiling points of compounds are significantly influenced by intermolecular forces, which cause attractions between molecules. Structures with weaker intermolecular forces have lower boiling points because less energy is required to overcome these attractions between molecules during the phase change from a liquid to a gas. London dispersion forces are intermolecular attractions between instantaneous dipoles induced by momentary distortions in the distribution of the electron "cloud" around atoms. As such, London dispersion forces are greater in larger molecules, which have a larger electron cloud and thus are more polarizable. London dispersion forces are among the weakest intermolecular forces (as an individual interaction). However, the combined effect of numerous dispersion force interactions often make London dispersion forces a very strong net influence on physical properties, especially in larger molecules. Comparing bromoethane and iodoethane, both molecules have an equivalent, nonpolar CH₃CH₂ chain, but the iodine substituent of iodoethane is larger and more polarizable than the bromine substituent of bromoethane. As such, iodoethane experiences greater London dispersion forces and has a higher boiling point than bromoethane. (Choice A) C-Br bonds are stronger (ie, require more energy to break) than C-I bonds, but this is irrelevant with regard to the boiling point. Boiling occurs when enough energy is gained to overcome noncovalent intermolecular forces. C-Br and C-I are covalent bonds, which are not broken during boiling. (Choice B) Bromine atoms are smaller and less polarizable than iodine atoms. Because iodine atoms polarize more easily, they contribute greater London forces than bromine atoms when present as substituents in molecules. (Choice C) Although the C-Br bond is slightly more polar than the C-I bond, the dipole is negligible, and the lower boiling point of bromoethane shows that any effect from C-Br dipole-dipole interactions does not compete with the more significant London forces in iodoethane. Educational objective: Structures with stronger intermolecular forces have higher boiling points because more energy is required to overcome the attractions between molecules. Larger molecules tend to have higher boiling points because the increased size causes greater London dispersion forces.

The Boiling Point of CH3CH2Br (Bromoethane) Is 38

The Boiling Point of CH₃CH₂Br (Bromoethane) Is 38