Question

the molecular formulas and boiling points of three alcohols are shown below:
name molecular formula boiling point (°c)
methanol ch₃oh 64.6
ethanol ch₃ch₂oh 78.3
1-propanol ch₃ch₂ch₂oh 97.2
which of the following intermolecular forces best accounts for the trend in boiling points seen above? hint: what is the same about the formulas? what is different about the formulas?
○ hydrogen bonding
○ ion-dipole interactions
○ dipole-dipole interactions
○ london dispersion forces

Explanation:

Step1: Analyze molecular structures

All three alcohols (methanol, ethanol, 1 - propanol) have an -OH group, so they can form hydrogen bonds. But we also look at the hydrocarbon chain length. Methanol has 1 C, ethanol 2 C, 1 - propanol 3 C.

Step2: Relate to intermolecular forces

Ion - dipole interactions require ions, which these molecules don't have. Dipole - dipole interactions are present, but the main trend here is due to the increase in the size of the non - polar hydrocarbon part. London dispersion forces depend on the size of the molecule (number of electrons). As the carbon chain length increases, the number of electrons increases, so London dispersion forces increase. Hydrogen bonding is present in all, but the trend in boiling points (increasing with carbon chain length) is mainly due to the increase in London dispersion forces (since the hydrogen bonding ability is similar in terms of the -OH group, and the difference in boiling points is due to the growing hydrocarbon chain which affects London dispersion forces). Ion - dipole is out. Dipole - dipole is part of it, but the main contributor to the trend (as chain length increases, boiling point increases) is London dispersion forces because the magnitude of London dispersion forces increases with the size of the molecule (more electrons, larger electron cloud, stronger temporary dipoles).

Answer:

London dispersion forces (the option: London dispersion forces)