In each pair of compounds, which compound has the higher boiling point? Explain your reasoning.
(a) octane or 2,2,3-trimethylpentane
(b) nonane or 2-methylheptane
(c) 2,2,5-trimethylhexane or nonane
The boiling points increase smoothly with increasing numbers of carbon atoms and increasing molecular weights. Larger molecules have larger surface areas, resulting in increased intermolecular Van der Waals attractions. These increased attractions must be overcome for vaporization and boiling to occur. Thus, a larger molecule, with greater surface area and greater Van der Waals attractions, boils at a higher temperature.
- Both octane and 2,2,3-trimethylpentane have same molecular formula
Octane has larger surface area than 2,2, 3-trimethylpentane. Thus increases Van der Waal interactions. Hence, octane has the higher boiling point than 2,2, 3-trimethylpentane.
b) The dispersion force increases with increasing molar mass because molecules or atoms of higher molar mass generally have more electrons dispersed over a greater volume.
Molar mass of 2-methylnonane (142.17 /mol) is heavier than heptane (100.13 /mol).
Thus, increase electron cloud volume of 2-methylnonane, the greater dispersed forces result in increasing boiling points. Hence, 2-methylnonane have higher boiling point than heptane.
c) Both nonane and 2,2,5-trimethylhexane have same molecular formula
Nonane has larger surface area than 2,2,5-trimethylhexane. Thus, increases Van Der Wall
Interactions, Hence, nonane has the higher boiling point than 2,2,5-trimethylhexane.