More Hydrocarbons

I mentioned methane is the simplest hydrocarbon. It is reasonable to wonder how many hydrocarbons there are, and the answer is there's no absolute limit. Still, there is a sequence of hydrocarbons that are really helpful to know about. They not only exist as compounds in their own right, but their molecules also form the basis of lots of more complex molecules. As you can see from the table below, this is a series of similar molecules made of repeating units:

   H
   |
H--C--H
   |
   H

This series is called the alkanes. It goes on forever. After decane comes undecane, dodecane, etc., all using regular Greek number prefixes. Notice what I've done with the skeletal formulas for ethane onward. I've removed the hydrogens from the drawing; they're strictly implied now. I've also removed the carbons and replaced them with corners. Each line represents a bond between two carbon atoms, unless some other atom is specified. This is a very common way to represent the structures of complex molecules, and it makes the structure very easy to understand.

Alkanes have a way that they can stick together, though only weakly. As the electrons orbit the nuclei, there will be times when one electron is kind of away from another molecule that has an electron that's kind of toward the nucleus of the first one. The result is a weak attractive force. This is called a van der Waals force. It's actually the same force that enables geckos to climb walls. Yet it's the weakest of all intermolecular forces. Hats off to you, geckos, for managing to use the weakest intermolecular force to support your entire weight.

Most intermolecular forces are proportional to the inverse square of the distance, just like light from a light bulb, and for the same reason. But van der Waals forces fall off with the sixth power of the distance! The reason is intuitive once you figure it out. The atoms in alkanes have what's called polarizability, where the presence of a nearby electron can slightly repel the alkane's electrons, causing the external electron to "see" a little bit of the positive charge of the atoms' nuclei. Well, that polarizing effect is also going to fall off with the inverse square of the distance — on both sides. So, power of 2 for the polarization of molecule A, times power of 2 for the polarization of molecule B, times power of 2 for the strength of the resulting attraction... makes 6th power!

The octane in the table, by the way, isn't the same octane that makes cars peppy. I know, I know, they have to make it more complicated for us, but it's not the fault of chemistry, it's just petroleum executives being imprecise with their language. The good stuff in gasoline is actually iso-octane. Iso just means made up of the same parts in a different arrangement. Two different molecules made of the same atoms are isomers. But "octane" rolls off the tongue more easily than "iso-octane" with its awkward vowels next to each other. The opposite of iso- is n- (for "normal"), so the octane in the table above is also known as n-octane. There are many many kinds of iso-octane, but the familiar one is actually this:

 CH3     CH3
 |       |
HC--CH2--C--CH3
 |       |
 CH3     CH3

There's also another irregular hydrocarbon that deserves mention, neopentane:

     CH3
     |
H3C--C--CH3
     |
     CH3

Here are the models of ethane and propane: