petroleum
/ puh-TROH-lee-um /
Petroleum, or crude oil, is the dark, sticky liquid pumped from underground that is the main raw material of the organic chemical industry. It is the buried, heated, pressure-cooked remains of ancient marine plankton — millions of years of slow geological change turned biological matter into a soup of hydrocarbons. Together with natural gas (mostly methane) it supplies the carbon for our fuels, plastics, medicines, and dyes.
Crude oil is not one compound but a mixture of thousands, overwhelmingly alkanes and cycloalkanes from one carbon up to fifty or more, plus some aromatics. To make it useful, refineries separate it by fractional distillation: the oil is heated and its components boil off at different temperatures, the lightest gases at the top of a tall column and the heaviest tars at the bottom. This sorts the jumble into fractions — refinery gas, gasoline, kerosene, diesel, lubricating oil, and bitumen — roughly by chain length.
Petroleum matters because it is the cheap, abundant feedstock of carbon skeletons that organic chemists then cut, branch, and decorate into everything else. Processes like cracking (breaking big alkanes into smaller, more valuable ones and alkenes) and reforming turn low-value fractions into fuels and building blocks. It is also finite and its combustion releases carbon dioxide, which is why so much green chemistry aims to replace it with renewable carbon sources.
In a refinery's distillation column, gasoline (about C5 to C10, boiling 40 to 200 degrees Celsius) condenses high up, while heavy bitumen (above C40) stays at the hot bottom.
Fractional distillation sorts the mixture by boiling point, which tracks chain length and intermolecular force.
Distillation separates an existing mixture by boiling point; it does not make new molecules. Cracking and reforming, which actually rearrange the carbon, are separate chemical steps.