By Robert Hazen, Ph.D., George Mason University
Let’s consider the various compounds of carbon and element one (hydrogen). These are called hydrocarbon compounds. And hydrocarbons are extremely important in our lives. Natural gas, methane, butane, gasoline, and many other fuels, even candle wax, are hydrocarbons. So we begin with a single carbon atom and continue with hundreds of carbon atoms that are bonded with one another to form more complex compounds.
Reasons Behind Variety in Carbon Chemistry
A key reason for the variety of carbon chemistry now lies in the fact that carbon can easily bond to itself. That’s an amazing ability. You can have two carbon atoms that share a pair of electrons with each other. That’s called a carbon-carbon single bond. And you can imagine this as two letters C with a single line drawn between them. That single line represents a pair of electrons, 2 electrons that are shared between the two atoms.
And then, if you want to, you can add hydrogens all around the outside, and you’d come up with C2H6. That’s the chemical called ethane, also a very efficient fuel. And you could always play the game this way. You could always take away a hydrogen and replace it with a group of carbons and three hydrogens, a CH3 or a methyl group that just sticks on the end.
So, methane, which is CH4, becomes ethane, C2H6. And then ethane becomes propane, which is C3H8, that’s also a very efficient fuel. By the way, if you keep on substituting carbon for hydrogen in this process—that is, every time there’s a hydrogen, take it away and put a carbon on instead—you eventually wind up with a structure of diamond.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Straight Hydrocarbons, Branched Hydrocarbons, Isomers, and Cyclic Hydrocarbon Compounds
Back to hydrocarbons, you can go from methane with one carbon, to ethane with two carbons, to propane with three, butane with four, and so forth, just by taking away hydrogen and replacing it with CH3 groups. Normal alkanes include a large series of covalently bonded material in which there’s just a long, straight chain of carbon.
But hydrocarbons can occur in branches, too. So, for example, the simplest of the branched hydrocarbons is isobutane. It’s four carbon molecules in a T-shape—three across and one down. Now, isobutane differs from normal butane, which has four carbons in a row, but the chemical formulas are exactly the same: in each case, C4H10. Normal butane and isobutane are called isomers of each other—the same composition but different structures.
A new wrinkle is provided by cyclic hydrocarbons, in which carbon atoms actually form a ring. In this case, each carbon sees two neighboring carbons, and it has two neighboring hydrogens, so every atom is happy. Now, it gets even more complicated because cyclic hydrocarbons can also have branches; they can have chains; they can have a chain linking to rings; they can have nested rings. Any imaginable combination of carbon and hydrogen in which all the atoms are happy is possible.
Learn more about phase transformations and chemical reactions.
How Carbon Bonds with Itself
Carbon can form a double bond with itself. Instead of sharing one pair of electrons, it shares two pairs of electrons—four electrons shared between two carbon atoms. Imagine two Cs connected by an equal sign. And then you have to fill up the rest of carbon’s needs with hydrogen. In this case, two hydrogens for each carbon. That leads to the compound C2H4, or ethane, which is the simplest member of the alkene series.
Now, all the variations described above—chains, branches, rings—can also feature double bonds. The formulas differ from alkanes in that for each carbon-carbon double bond, there are two fewer hydrogens. A compound of special interest is the molecule benzene, that’s C6H6, which is a ring of six carbons, and it has three double carbon-carbon bonds and three single carbon-carbon bonds shared around the ring.
Carbon can also form a triple bond with itself. In this case, sharing three pairs of electrons, a total of six electrons are shared between the two carbons, and in that case, each carbon only needs one additional hydrogen.
Properties of Hydrocarbons
The reason society values hydrocarbons so much is because of their unique and valuable properties. All of these chemicals burn. They burn with intense heat and flame. They produce carbon dioxide and hydrogen oxide— that is, water—as by-products. So they’re very clean-burning fuels for that reason.
The smallest of these molecules are typically gases or liquids, so propane and butane, for example, are fluids and, therefore, can be destroyed very easily as fuels. When you have larger straight chains, more massive molecules, then you start getting solids, and that’s typical of candle wax, where the longer 20-carbon molecule typically forms a solid. But there’s an interesting point here because as you raise the temperature, those solids become liquids, and then the liquids become gas.
Learn more about states of matter and changes of state.
How Candle Wax Behaves
If you look at the way a candle behaves, you actually see a very interesting phenomenon taking place. In the lighted candle, you have the solid portion of the candle, which is at room temperature. Near the base of the flame, at temperatures above about a hundred degrees, you have a puddle of liquid. The liquid is pulled up into the wick, and then at the hottest part of the flame, that molecule is turning into a gas, and it’s the gas that’s burning.
So we see all three states of matter here. It turns out that branching and cyclic hydrocarbons usually have lower melting and boiling temperatures because these are large, irregular molecules, and they don’t pack together nearly as well as straight chains that form the solid candle wax.
Well, hydrocarbons are the easy part of carbon chemistry. And the chemistry of carbon becomes very, very quickly complicated as additional elements come into play.
Common Questions about Hydrocarbon Compounds
Carbon can bond to itself. It also makes chemical bonding with elements such as hydrogen to form various hydrocarbon compounds.
When two hydrocarbon compounds have different structures but the same composition, they are called isomers. For example, butane and isobutane have two similar compounds but different structures.
Hydrocarbon compounds are very valuable because of their unique properties. They produce a great deal of heat and light when they burn, and the by-product that remains after burning is water. Mostly, these compounds are valuable because they are clean.
