Polarity Part 2: Intermolecular Interactions & Physical Properties

Introduction

In the last post, we discussed the origin of non-covalent (intermolecular) interactions between molecules, and outlined the main interactions that are encountered at undergraduate. The next step is start looking at the effects of the these interactions on physical properties (or, simplistically, the question “Why do we care that these interactions occur?”). Just by inspecting the type and number of intermolecular interactions, you will start to gain an idea of which molecules will have higher or lowing boiling points, and which will be more or less soluble in various solvents.

I’m not going to pretend that this is anything other than a very simple introduction to this complex area but it should be suitable for first year undergraduates. Like all simple introductions, there is enough information to give you an appreciation of the bigger picture, and hopefully, an appreciation of how simple chemical principles can have real world consequences. This is not a physical chemists look at this subject. There are no phase diagrams nor even the mention of vapor pressure (which is probably an oversight).

Boiling points

The boiling point of a molecule is influenced by non-covalent interactions. The more, and the stronger, the interactions between molecules of a substance, the higher its boiling point will be. For example, acetone has a higher boiling point than butane even though their size and mass are very similar.

The effect of dipole-dipole interactions on boiling point (bp)

Acetone has strong dipole-dipole (permanent dipole-permanent dipole) interactions between molecules while the only interactions between molecules of butane are induced dipole-induced dipole interactions or dispersion interactions. These are much weaker and butane has a lower boiling point.

Hydrogen bonding has the biggest effect on boiling points as you can see when we compare the properties of the four compounds below, water, methanol, hydrogen sulfide and methane:

The effect of hydrogen bonding on boiling point

Water has two hydrogen bond donors (O–H bonds) and two hydrogen bond acceptors (lone pairs of electrons on oxygen). It is perfectly set-up to make four hydrogen bonds per molecule and it has a ridiculously high boiling point. Methanol has one hydrogen bond donor less, and it has a bulky (when compared to hydrogen) methyl group that hinders how close two molecules can approach. This causes the boiling point to drop even though the mass of the molecule has increased. Hydrogen sulfide replaces the oxygen of water for sulfur, the element directly below oxygen on the periodic table. You might predict that it should behave the same, but the low boiling point shows that elements of the third row and below are poor hydrogen bond donors/acceptors due to lower electronegativity and more diffuse lone pairs. Methane cannot form any hydrogen bonds and has a boiling point of just –162 °C.

Induced dipole-induced dipole interactions or dispersion forces occur between all molecules, and this makes them very important. If you look at the isomers of a simple hydrocarbon, such as pentane, you can see the effect of these small dispersion forces. Each isomer has a different shape and this influences the strength of the dispersion forces by effecting how readily the electrons can be re-distributed and how many attractive dispersion interactions are possible. This determines how polarisable the molecule is. The bigger the surface area (long, linear chains of carbon atoms) the greater the polarisability and the bigger the induced dipole-induced dipole interaction. Compare this to branched molecules that have a more spherical shape and thus a smaller surface area and are less polarisable. Inspecting the boiling points of the isomers of pentane shows this effect off nicely. Pentane itself, with a straight chain of five carbon atoms, has the largest surface area and the highest boiling point. 2,2-Dimethylpropane is almost a sphere, redistribution of the electrons leads to a smaller molecular dipole, and so it has less dispersion forces and a lower boiling point.

The influence shape of a molecule on dispersion forces and boiling point

This is one of the many reasons why chemists like myself love the shape of molecules.

Shape also explains the difference between the oils and fats found in our foods. Both classes of molecules are triesters made from glycerol (propan-1,2,3-triol) and long greasy carboxylic acids. The long hydrocarbon chains in fats are predominantly saturated, meaning they have fewer double bonds. The chains are largely straight (actually, they are the standard alkane zig-zag but I hope you know what I mean!), and pack together well. This means there are numerous, strong dispersion interactions between molecules leading to high melting points. Fats are normally solids.

The difference between oils and fats can partially be explained by dispersion interactions

Oils are polyunsaturated, which means they contain multiple cis-alkenes. This puts a kink in the chain and forces them apart. The molecules can’t get close to each other and there are less opportunities for attractive dispersion interactions. As the attraction between molecules is diminishes so the oils tend to have lower melting points and are liquids.

Solubility and miscibility

The ability of one substance to dissolve in another, the solubility of a solid in a liquid, or the ability of two liquids to mix, the miscibility, is determined by non-covalent interactions. The rough rule of thumb is that polar compounds mix with other polar compounds and non-polar compounds mix with non-polar compounds. This is summarised as “like likes like” or “like dissolves like”. What this means is that polar molecules form strong interactions with other polar substances. In a pure solid or liquid these interactions are between identical molecules. Only another polar substance can disrupt or break these interactions and allow mixing. As a result, non-polar molecules do not mix with polar molecules.

A cartoon showing why polar molecules mix with polar molecules

The most common polar liquid or solvent, especially if you have an interest in biological systems, is water. Compounds that form strong and/or multiple non-covalent interactions with water will dissolve in it. We have all seen salt dissolve in water, this occurs due to multiple ion-dipole interactions. Similarly, sugar (and here we are referring to table sugar or sucrose, a disaccharide made from glucose and fructose), contains eight alcohol groups, along with three acetal oxygen atoms and can form multiple hydrogen bonds. It is soluble in water (and hence philistines can spoil a good cup of tea). Molecules that dissolve or mix with water are hydrophilic or water ‘loving.’

Sodium chloride dissolves in water due to strong non-covalent interactions

Acetone is miscible in water due to strong non-covalent interactions

Many other organic molecules are non-polar, having large areas of non-polar C–C and C–H bonds. These do not interact with water and will not mix (or more accurately, there are very weak interactions … we’ve already told you that all molecules can interact due to dispersion forces). Such molecules are known as hydrophobic molecules.

Hexane is not miscible in water as there are little non-covalent interactions

Teaching large classes has revealed that many students would prefer things to be ‘black or white’, right or wrong, hydrophilic or hydrophobic. It isn’t, and we should stop thinking in absolutes. Many compounds are not simply polar or non-polar but contain areas with each characteristic. Even worse (especially when it comes to exams), polarity should be considered as a scale with some molecules being more polar than others, while some are less polar. If you just consider the solubility of a handful of carboxylic acids in water, you will see that as the alkyl chain gets longer so the overall polarity of the compound becomes less. Acetic acid dissolves in water without issue but by the time you reach pentanoic acid the molecule is only partially miscible in water. The carboxylic acid functionality is still polar, it can still form hydrogen bonds with water but now the non-polar alkyl chain is starting to exert a greater influence on the properties of the molecule. Add just three more methylene groups and octanoic acid is effectively immiscible in water or is hydrophobic.

The miscibility of simple organic acids in water

Conclusions

This summary gives a brief introduction to polarity in molecules and the concept that electrons are not evenly distributed between atoms or throughout a molecule. The polarity of individual bonds add together to give the polarity, or not, of the molecule. The distribution of electrons within a molecule then controls the interactions between molecules through what are known as non-covalent interactions.  They can influence the physical and, as we shall see, the reactivity of molecules.

Download this WORKSHEET for some practice questions.