Intermolecular Forces
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Intermolecular Forces
If you are ever asked, the rest of your life, something about a compound’s boiling point, your first thought should be about this section. Imagine you have a beaker full of some compound. The zillions of molecules of this compound are swimming around as a liquid in this beaker. If this liquid is going to boil, you begin heating up the liquid until the molecules start flying away. The boiling point of the liquid is that temperature. The more the molecules stick together to each other; will you need to heat up the liquid more or less to get them to fly away? Of course, you need to heat the liquid up more to get it to boil when the molecules stick together more. If the molecules weakly stick together, the boiling point will be lower. It will not take as much heat to get them to fly away.
Here is another question. Between solid, liquid, and gas, which one has molecules that stick the most together? Of course, it is the solid. The molecules stick together very well and do not move around each other. In the liquid, the molecules stick together pretty well, but they can slide past each other. As a gas, the molecules do not stick together very well at all.The attractive forces or bonds within one molecule are called intramolecular forces. Intra means within.The attractive forces between molecules are called intermolecular forces. Inter means between.
So, the real question is, how do we know how well the molecules stick together? The answer is to look at the intermolecular forces involved. Intermolecular forces are simply the forces between molecules that hold them together. These forces are weaker than intramolecular forces or regular atom-atom bonds. There are three intermolecular forces to consider. All of them are related to dipole moments.
Dipole-Dipole Interactions
When a molecule has a dipole moment, it has a positive end and a negative end. Since opposite charges attract, the positive end of a molecule is attracted to the negative end of another molecule. This intermolecular force is called a dipole-dipole interaction.
Dipole-Dipole Interaction
Hydrogen Bonding
Hydrogen bonding is an intermolecular force that is a special, very strong case of dipole-dipole interaction. It is the strongest of the intermolecular forces. For hydrogen bonding to occur, there needs to be a very positive site and a very negative site to stick together. The very positive site involves a hydrogen atom on a very electronegative atom like nitrogen, oxygen, or fluorine. Organic compounds do not contain H-F bonds, so we usually only consider H-N or H-O bonds. Because the hydrogen is on a very electronegative atom, the N or O win the tug-of-war for the negative electrons in the bond and make the hydrogen atom a very small, concentrated, positive spot. This is a special positive location that can bond to negative spots on other molecules.
In hydrogen bonding, the negative site is a lone pair of electrons on nitrogen, oxygen, or fluorine. Since N, O, and F are the most electronegative atoms, they are strongly pulling negative electrons towards themselves. They have a very negative charge, especially with the very large, juicy lone pair of electrons on them.
These very negative spots and these very positive spots stick together strongly.
Hydrogen bonding examples
22. Rank the following compounds from the highest boiling point (1) to the lowest boiling point (3).
London Dispersion Forces
The weakest of the intermolecular forces are the London dispersion forces. We find these in non-polar compounds. ”Molecule 1” is a nonpolar molecule with the electrons evenly distributed throughout.
Molecule 1
Sometimes, when two nonpolar molecules are near each other, one of them will get a temporary dipole moment because some of the electrons will slosh more toward one end of it. This only lasts for a brief time before they will slosh back and the temporary dipole is lost. But, when the negative electrons slosh more to one side, one end of the molecule is temporarily negative and one side is temporarily positive.
Molecule 1
Molecule 2
Since we have a partially positive side in this molecule for a brief moment, what will happen to the negative electrons in Molecule 2? The negative electrons in Molecule 2 are attracted to the positive spot on Molecule 1. Molecule 2 has an induced dipole. The two molecules then stick together in a weak dipole-dipole interaction.
Molecule 1
Molecule 2
London Dispersion Interaction
A brief moment later, the electrons will slosh back, and we’ll lose this attraction. This is why London forces tend to be weak. Electrons are more likely to slosh around to make a dipole with the more electrons the nonpolar molecule has. Therefore, the London dispersion forces are greater the larger surface area a molecule has. Compare the diatomic molecules of chlorine, bromine, and iodine. These are all perfectly symmetrical nonpolar compounds. Yet, at room temperature, chlorine is a gas, bromine is a liquid, and iodine is a solid. So, the molecules must be sticking together the most for iodine and the least for chlorine. How do we explain this? I2 has many more electrons (106) than Br2 (70) and Cl2 (34). It also has a larger surface area. So, we would predict the London dispersion forces are greatest for iodine.
London forces in diatomic halides
In organic chemistry, we can make similar arguments. The following alkanes are all nonpolar. Long, straight molecules have a greater surface area than ones that have many branches coming off of it. The more branches we have, the more spherical the molecule behaves and the smaller surface area it has. So, London Dispersion forces are greatest for longer chains. Longer chains stick together better and have higher boiling points. So, more branched alkanes have lower boiling points.
London forces in branched alkanes
23. Rank the following compounds from the highest boiling point (1) to the lowest boiling point (4).
24. Rank the following compounds from the highest boiling point (1) to the lowest boiling point (4).
Answers
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