­Crude oil is the term for "unprocessed" oil, the stuff that comes out of the ground. It is also known as petroleum. Crude oil is a fossil fuel, meaning that it was made natural­ly from decaying plants and animals living in ancient seas millions of years ago -- most places you can find crude oil were once sea beds. Crude oils vary in color, from clear to tar-black, and in viscosity, from water to almost solid.

Crude oils are such a useful starting point for so many different substances because they contain hydrocarbons. Hydrocarbons are molecules that contain hydrogen and carbon and come in various lengths and structures, from straight chains to branching chains to rings.

There are two things that make hydrocarbons exciting to chemists:

·         Hydrocarbons contain a lot of energy. Many of the things derived from crude oil like gasoline, diesel fuel, paraffin wax and so on take advantage of this energy.

·         Hydrocarbons can take on many different forms. The smallest hydrocarbon is methane (CH₄), which is a gas that is a lighter than air. Longer chains with 5 or more carbons are liquids. Very long chains are solids like wax or tar. By chemically cross-linking hydrocarbon chains you can get everything from synthetic rubber to nylon to the plastic in Tupperware. Hydrocarbon chains are very versatile!

The major classes of hydrocarbons in crude oils include:

·         Paraffins

     general formula: C_nH_2n+2 (n is a whole number, usually from 1 to 20)

m    straight- or branched-chain molecules

     can be gasses or liquids at room temperature depending upon the molecule

    examples: methane, ethane, propane, butane, isobutane, pentane, hexane

·         Aromatics

    general formula: C₆H₅ - Y (Y is a longer, straight molecule that connects to the benzene ring)

    ringed structures with one or more rings

    rings contain six carbon atoms, with alternating double and single bonds between the carbons

    typically liquids

     examples: benzene, napthalene

·         Napthenes or Cycloalkanes

    general formula: C_nH_2n (n is a whole number usually from 1 to 20)

     ringed structures with one or more rings

    rings contain only single bonds between the carbon atoms

     typically liquids at room temperature

    examples: cyclohexane, methyl cyclopentane

·         Other hydrocarbons

    Alkenes

·         general formula: C_nH_2n (n is a whole number, usually from 1 to 20)

·         linear or branched chain molecules containing one carbon-carbon double-bond

·         can be liquid or gas

·         examples: ethylene, butene, isobutene

    Dienes and Alkynes

·         general formula: C_nH_2n-2 (n is a whole number, usually from 1 to 20)

·         linear or branched chain molecules containing two carbon-carbon double-bonds

·         can be liquid or gas

·         examples: acetylene, butadienes