1.1 Intermolecular Forces

Intermolecular forces are attractive interactions between the molecules. These forces are responsible for keeping molecules in a liquid in close proximity with neighboring molecules.

Figure 1.2: Relative strengths of some attractive intermolecular forces.

Dispersion is the weakest intermolecular force and is the dominant intermolecular force in non-polar molecules. It is important to note that molecules can exhibit multiple intermolecular forces. Dispersion exists in all substances.

Dipole-dipole interactions exist between molecules with dipoles. Hydrogen bonding is a special type of dipole-dipole interaction that exists in molecules that contain a hydrogen bound to a highly electronegative atom (N, O, or F). Let us analyze the boiling points of series of hydrides for the following main group elements to highlight the enhanced nature of hydrogen bonding.

Figure 1.3: Hydrides of the following main group elements

Stronger intermolecular forces require more energy to break these interactions and generally lead to higher boiling points. These hydrides will be organized by Group (15, 16, and 17, respectively) moving down the periodic table. The boiling points are plotted by the period of the main group element.

Figure 1.4: Boiling points of some hydrides of main group elements

The data indicates that hydrogen bonding tend to be stronger than other dipole-dipole interactions.

The boiling point of some simple substances are given below and categorized by the dominant intermolecular force they exhibit. Note that ammonium chloride (NH₄Cl), included for comparison, exhibits ionic intramolecular forces.

Figure 1.5: Boiling points of some molecules

It is clear that the types of intermolecular forces span a wide range of boiling points and, therefore, interaction energies. The classes of intermolecular forces overlap in terms of strength; however, a general trend is still seen. Dispersion tends to be the weakest intermolecular force while hydrogen-bonding, a special type of dipole-dipole interaction, tends to be strong.

Dispersion interactions are stronger in molecules with larger masses.

Name	Formula	m.m. (g mol^–1)	Boiling Pt. (°C)
Astatine	At₂	420	610
Iodine	I₂	254	457
Bromine	Br₂	160	332
Chlorine	Cl₂	71	238
Fluorine	F₂	38	85

Dispersion also tends to be stronger in molecules with larger surface area.

Name	Formula	m.m. (g mol^–1)	Boiling Pt. (°C)
n–pentane	C₅H₁₂	72.15	36.0
isopentane	C₅H₁₂	72.15	27.0
neopentane	C₅H₁₂	72.15	9.5

Pentane

Isopentane

Neopentane

Name	Formula	m.m. (g mol^–1)	Boiling Pt. (°C)
n–hexane	C₆H₁₄	86.18	69.0
isohexane	C₆H₁₄	86.18	60.0
neohexane	C₆H₁₄	86.18	49.8

Hexane

Isohexane

2-Methylpentane

Neohexane

2,2-Dimethylbutane

Explain the following boiling points (below) using intermolecular forces.

Name	Formula	Dominant IMF	Dipole	BP
Sodium chloride	NaCl	Ion-Ion	9.00	1465.00
Ethylene glycol	(CH₂OH)₂	Hydrogen bonding	2.75	197.60
Pentanol	C₅H₁₁OH	Hydrogen bonding	1.70	138.00
Benzene	C₆H₆	Dispersion	0.00	80.08
Methanol	CH₃OH	Hydrogen bonding	1.69	64.70
Pentane	C₅H₁₂	Dispersion	0.00	36.06
Neopentane	C₅H₁₂	Dispersion	0.00	9.50
Ethane	C₂H₆	Dispersion	0.00	-88.60
Methane	CH₄	Dispersion	0.00	-161.50

Ethylene glycol

Pentanol

Benzene

Methanol

Pentane

Neopentane

Ethane

Methane

CH₄

Practice

Determine the dominant intermolecular force for the following molecules and rank the boiling points from smallest to largest.

water (H₂O)
butane (C₄H₁₀)
formaldehyde (H₂CO)
methane (CH₄)
methanol (CH₃OH)

Solution

Dominant IMFs:

water – hydrogen bonding
butane (C₄H₁₀) – dispersion
formaldehyde (H₂CO) – dipole-dipole
methane (CH₄) – dispersion
methanol (CH₃OH) – hydrogen bonding

Boiling points:

methane < butane < formaldehyde < methanol < water

The intermolecular forces between particles give rise to the properties of different liquids.

Hydrogen-bonding in DNA