Comparing Compounds Based on Boiling Point

Instructor: Sarah Pierce

Sarah has taught high school chemistry and biology, as well as college level chemistry(general, organic, analytical, biochemistry), and has a doctorate in chemistry.

This lesson will discuss how the boiling point of a compound changes based on its molecular structure. We'll review intermolecular forces and how they influence the boiling point of molecules with different structures.

A Watched Pot Never Boils

Have you ever heard the phrase ''a watched pot never boils''? It basically means that if you are waiting for water to boil, it seems to take a very long time if you are watching it.

But some things take even longer than water to boil, and some faster. Why? It turns out different molecules have different boiling points. The boiling point is the moment that the liquid starts to turn into a gas.

You can even look at the structure of the molecule and predict the boiling point if you know a little bit about the intermolecular forces of the molecule.

Intermolecular Force Review

Before we get too far, let's quickly review intermolecular forces or the attractive forces that hold molecules together. Remember, there are several types of intermolecular forces:

  • London dispersion forces (aka induced dipole-induced dipole forces) - weak attraction that all molecules have due to a disruption in the electrons around an atom
  • dipole-dipole attractions - the attraction between the opposite charges of polar molecules
  • hydrogen bonding - the attraction between a hydrogen atom that is bound to a very electronegative atom (generally a F, O, or N) and another molecule with a F, O, or N atom

The stronger the intermolecular force that holds the molecules together, the higher the boiling point. In general, hydrogen bonding is stronger than dipole-dipole attractions, which are stronger than London dispersion forces.

How Functional Groups Affect Boiling Point

When a hydrocarbon has different atoms that are attached to it, it results in different intermolecular forces, which affects the boiling point. These different groups of atoms are generically referred to as functional groups.

Ether

Propane and ethyl methyl ether look very similar, but have very different boiling points. They both have three carbon atoms, but notice that ethyl methyl ether has an oxygen atom. Ethyl methyl ether has both London dispersion forces and dipole-dipole interactions because of the polar oxygen bond. Propane only has London dispersion forces, so it has a lower boiling point than ethyl methyl ether.


Ethyl methyl ether has a higher boiling point than propane because it has stronger intermolecular forces
propane


Alcohol

Let's compare 1-propanol to propane and ethyl methyl ether. 1-Propanol has London dispersion forces, dipole-dipole interactions and hydrogen bonding, so which do you believe will have the highest boiling point? If you guessed 1-propanol, you are right!


1-propanol has a higher boiling point than either propane or ethyl methyl ether because of hydrogen bonding
1propanol


Amines

Now, compare propane and ethyl methyl ether to propylamine. Propylamine can form hydrogen bonds while propane and ethyl methyl ether cannot, so it has a higher boiling point.


Propylamine has a higher boiling point than ethyl methyl ether
propylamine


But wait, what about comparing propylamine to 1-propanol? Which has the higher boiling point? While both propylamine and 1-propanol have the same types of intermolecular forces (London dispersion forces, dipole-dipole attractions, and hydrogen bonding), 1-propanol has a higher boiling point. Why? Well, it turns out that oxygen is more electronegative than nitrogen, so it forms stronger hydrogen bonds.

How Shape Affects Boiling Point

The shape of the molecule or how it's arranged in space can also affect the boiling point. Let's look at a few examples.

Alkanes - Straight Chain Versus Branched

Alkanes just have carbon and hydrogen atoms with no functional groups, so the only intermolecular force that influences the boiling point is London dispersion forces. The more the molecules can touch each other, the more London dispersion forces there are, and the higher the boiling point.

For example, pentane has a boiling point of 36.1 C, while 2,2-dimethylpropane has a boiling point of 9.5 C.


A linear alkane has a higher boiling point than a branched alkane
pentanebp


This is similar to spaghetti and meatballs. Spaghetti noodles are like pentane, long and very sticky. They are hard to separate. Meatballs, on the other hand, are like 2,2-dimethyl propane. They don't touch as much as the spaghetti noodles, so they are easier to separate.

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