The essence of last week's column was that simple hydrocarbons are made from a carbon chain festooned with hydrogen atoms in a fashion that fully satisfies and saturates any valence bonding position left on each carbon atom.
Think of a hydrocarbon as a long chain of carbon atoms holding hands with the carbon atoms on each side and with hydrogen atoms occupying the remaining positions. Essentially, this is what a simple hydrocarbon chain looks like. We can now make chains so long that they extend for hundreds of thousands of atoms in length.
Indeed, this is polyethylene. It is formed by taking two carbon ethylene units and plugging them together until you have a chain of ridiculous length. It is a bit like the plastic bead necklaces that little kids play with.
And for the same reason that simple hydrocarbons shift from a gas to a liquid to a wax or tar as the chains get longer, really long hydrocarbon chains become solids. The length of the chain makes it almost impossible for the atoms to move.
However, if all that one could do with carbon atoms was to build a very long straight chain, organic chemistry would be both easy and boring. Far from it. Organic chemistry has layers of complexity that arise because it is also possible to build in branches and to add other elements into the mix leading to the 8 million or so known organic compounds.
Branching is fairly straightforward. With one, two, or three carbons, there really isn't an option to build in a branch. But starting with four carbons, there is a choice.
If you have a chain of three carbons, you can add the next one on the end to give you "normal butane". But you can also add the next one to the middle carbon where it branches off. This gives you something that looks like a "Y". This, according to the rules of organic chemistry, is called "methyl propane".
The "methyl" arises from the CH3- unit called a methyl radical by chemists. The propane is the longest continuous chain possible in the molecule.
When you get to five carbons, it is possible to add to both the end and the middle of methyl propane giving methyl butane and dimethyl propane. The more carbons, the more branches are possible and the greater the number of possible hydrocarbons or constitutional isomers.
By the time you get to eight carbons, there are 18 constitutional isomers. At 20 carbons, there are 366,319 possible constitutional isomers. With 30 carbons, there are over 4 billion possible constitutional isomers. And that is just for hydrocarbons where every carbon is fully saturated.
Remove two hydrogen atoms from a hydrocarbon chain and two carbon atoms are left with unsatisfied valences. As a consequence, they form double bonds - both cis and trans.
Double bonds add rigidity. You can spin different portions of a molecule around single bonds. It is jiving while only holding one hand. Spinning your partner is easy.
But put a double bond into play and suddenly it is not so easy to spin. It is much more akin to trying to do the jive when you are holding both of your partner's hands in yours. Spins take a lot more effort and lead to funny contortions.
In molecules, conversions between cis and trans require major amounts of energy. Further, these double bonds lead to unsaturation and multiply the possible arrangements of atoms exponentially. The number of unique compounds that can be built from carbon and hydrogen alone is astronomical.
The presence of double bonds also changes the fluidity of the organic molecules. Where a long straight chain saturated polymer might be solid like a wax, adding a double bond here and there can result in something that is much more malleable with a butter-like consistency.
This is how they make margarine but it doesn't mean that margarine is a plastic.
In any case, the complexity of the hydrocarbon soup means that simply saying that a liquid is crude oil is actually saying a mouthful. Crude oil typically contains hundreds if not thousands of different chemical compounds.
The same is true for bitumen which is really a collective noun. It describes a particular type of material which is composed of many thousands of chemical compounds. It is not easy to characterize and because of the nature of the compounds involved, it is both heavy and almost solid in its natural state. It is only through the addition of lighter hydrocarbons that it can be made to flow.
As to the organic compounds present, we haven't even touched the full range of possibilities because they can also contain oxygen, nitrogen, sulphur, and other members of the periodic table.
