Hydrocarbons are molecules made of only the two elements: carbon and hydrogen. One might think that with only two building blocks, hydrocarbon chemistry would be quite limited. The reality is that hydrocarbon chemistry is extensive, and plays a significant part in daily life. Hydrocarbons are the primary component of many fuels and plastics, and serve as building blocks for other chemical products as well.
Hydrocarbon chemistry really centers on the carbon atom. With four valence electrons (electrons in the outermost energy level), carbon is able to form four bonds with other atoms. This allows carbon to form chains, branches, and ring structures with ease. Hydrogen atoms have but one electron, and form exactly one bond. In hydrocarbons, hydrogen atoms occupy all the edge positions, providing the remaining bonds required to give each carbon atom four bonds. Because hydrogen and carbon atoms have a similar electronegativity, the bonds in hydrocarbons have little polarization, and hydrocarbon molecules are non-polar as well.
Hydrocarbon chains are the most familiar molecules to most people, even if they do not realize it. Octane, for example, is a familiar hydrocarbon that is present in gasoline. (Gasoline itself is mostly a mixture of hydrocarbons). Octane is a chain of eight singly-bonded carbon atoms, covered in 18 hydrogen atoms. Butane – the fuel in cigarette lighters – is a four-carbon chain (with 10 hydrogen atoms), and propane is a three-carbon chain (8 hydrogen atoms). For single-bonded hydrocarbon chains, there is a simple formula to determine how many hydrogen atoms are present. That formula is H = 2*C + 2, where H is the number of hydrogen atoms, and C is the number of carbon atoms.
Longer hydrocarbon chains exist, including familiar polymers such as polyethylene and polypropylene. Polyethylene (PE) is the simplest hydrocarbon polymer (or polyolefin). It is simply a chain of thousands of carbon atoms, with the corresponding number of hydrogen atoms. While not very exciting structurally, these long hydrocarbon chains are molded into plastic milk jugs and plastic shopping bags on a daily basis. With the familiar #2 recycling designation, they are also one of the easier plastics to recycle. (PE can also get the #4 recycling code, depending on how it is processed). Polypropylene (PP) is a similar hydrocarbon chain, except that it has an extra carbon attached to every other carbon in the chain. This small change in structure makes PP a harder plastic, suitable for making more rigid containers. PP receives the #5 recycling code.
Ring structures add a bit of excitement to hydrocarbon chemistry. While chains can flop fairly randomly, a ring has a restricted geometry. Because of carbon’s bonding angles, the most common rings consist of six carbon atoms. Rings may exist on their own, or be bonded to other rings or hydrocarbon chains. An excellent example of a ring-shaped hydrocarbon is benzene. Benzene is a flat, six carbon ring, with alternating single and double bonds between the carbon atoms. As a result, it has only six hydrogen atoms, one for each carbon atom. The benzene ring is present as a central part of many other chemical structures including dyes, drugs, and perfumes. “Fused” benzene rings (rings joined together by sharing a carbon edge make interesting planar molecules (or sections of molecules) as well. Most familiar to most people (especially older people) would be naphthalene – the chemical used to make mothballs. Naphthalene is made of just two rings, fused, for a total of ten carbons and only eight hydrogen atoms. (The 2*C + 2 rule does not apply once double bonds and rings are present. Every ring and every double bond decreases the number of hydrogen atoms by 2).
Hydrocarbons are widely used in making other chemicals. They can serve as solvents (if they are liquids) such as benzene, hexane (a six-carbon chain), or cyclohexane (a six-carbon ring with all single bonds). They can also be used as building blocks for the synthesis of larger molecules. Ethene, which has two carbon atoms connected by a double bond, is a common starting place for many syntheses.
They may only use two elements, but modern chemistry and even the modern world would not be the same without hydrocarbons.
