Carbon chemistry

The study of carbon compounds has its own name: organic chemistry. The word once meant “from living things”, because early chemists found these substances only in plants and animals and believed that only a living body could make them. In 1828 the German chemist Friedrich Wöhler showed otherwise. Starting from nothing but mineral salts, he produced urea, a substance until then known only from urine. An organic compound had been made in a flask.

The old name stayed, with a wider meaning: organic now means carbon-based, whether the compound comes from a leaf or a factory. The link with life is real, though. Proteins, fats, sugars and DNA are all built on frameworks of carbon atoms, with other elements attached. So are petrol, plastics, dyes and most medicines.

Diamond is the hardest natural material known. Graphite, the grey core of a pencil, is soft enough to rub off on paper. Yet both are pure carbon. Different forms of one element, built from the same atoms in different arrangements, are called allotropes.

In diamond, every atom is bonded to four others, which are bonded to four more, in every direction. The whole crystal is a rigid network of bonds with no weak point, so scratching it means breaking the bonds themselves. In graphite, each atom bonds strongly to only three others, forming flat layers patterned like chicken wire. Between one layer and the next the attraction is weak, so the layers slide over each other and flake off. That is what leaves a pencil line, and why powdered graphite works as a lubricant.

The simplest carbon compounds use just one other element. A molecule made of carbon and hydrogen and nothing else is a hydrocarbon. Natural gas, petrol, candle wax and tar are all hydrocarbons, or mixtures of them.

The smallest is methane: one carbon atom with a hydrogen on each of its four bonds. It is the main gas in a kitchen gas supply. Join two carbons and fill the remaining bonds with hydrogen, and you have ethane. Three carbons make propane, the gas in camping cylinders, and four make butane, the fuel in lighters. The family goes on, one carbon at a time, and its character changes steadily as the chain grows: the first four are gases, the next dozen or so are liquids, and the longest are waxy solids.

A chemical formula says which atoms a molecule contains, but with carbon that is not the whole story. Take four carbon atoms and ten hydrogen atoms. They can be joined as one straight chain of four, which is ordinary butane. Or three carbons can form the chain, with the fourth branching off the middle one. The atoms are identical in number and kind, yet the two molecules are different substances that boil at different temperatures.

Molecules with the same formula but a different arrangement of atoms are called isomers. The bigger the molecule, the more ways there are to arrange it. Four carbons give two isomers, ten carbons give seventy-five, and twenty give more than three hundred thousand. This is a large part of why carbon compounds are counted in the millions.

Crude oil is a thick, dark liquid pumped from deep underground, a mixture of thousands of different hydrocarbons. It formed from tiny sea plants and animals that sank to the seabed, were buried under mud, and were slowly changed by heat and pressure over millions of years. Straight from the ground it is of little use. Its value comes from sorting it.

The sorting is done by distillation. The oil is heated until most of it boils, and the vapour rises up a tall tower, hot at the bottom and cooler towards the top. Each hydrocarbon turns back into liquid at the height where the tower has cooled to its own boiling point, and is drawn off there. Distilling oil is an old idea: around 900 the Persian physician and chemist al-Rāzī wrote down how to distil black crude oil, again and again, until it ran as clear as water.

Why do the hydrocarbons in crude oil boil at different temperatures? Because their chains differ in length. Molecules attract their neighbours all along their surfaces, and longer chains attract more strongly, so more heat is needed to pull them apart into a gas.

That one rule sorts the whole refinery. Methane, with one carbon, boils at about minus 160 degrees Celsius. Petrol is a mixture with chains of five to twelve carbons, and most of it boils near 100. Diesel, with chains of up to about twenty, boils at around 300, and is thicker and harder to set alight. Beyond diesel come lubricating oils and road tar.

Crude oil holds more long chains than anyone wants and too few short ones. So refineries break long chains into shorter ones with heat and a catalyst, a substance that speeds a reaction without being used up. The process is called cracking.

Burning is a reaction with the oxygen in air. When a hydrocarbon burns with plenty of air, each kind of atom in it pairs up with oxygen. The hydrogen becomes water, which leaves as steam. The carbon becomes carbon dioxide, a gas with two oxygen atoms on every carbon. A candle, a gas hob and a jet engine all turn out the same two products, and the energy released on the way is the reason fuels are burned at all.

When air is short, the burning is incomplete. Some carbon is left as soot, the black of a smoky flame, and some leaves with only one oxygen atom, as carbon monoxide. This gas has no colour and no smell, and it is poisonous: it clings to haemoglobin, the blood’s oxygen carrier, far more tightly than oxygen does. A blocked flue or a faulty heater can kill this way.

Coal, oil and natural gas are called fossil fuels because they formed from living things buried long ago. The carbon in them was taken from the air by plants and sea life over millions of years and then locked away underground. Burning returns it to the air as carbon dioxide within a couple of centuries.

That matters because of what carbon dioxide does to heat. Sunlight warms the ground, and the ground gives the warmth back as invisible heat rays. Carbon dioxide in the air absorbs some of them and sends part of the heat back down. This is the greenhouse effect, and without it the Earth would be frozen. But more carbon dioxide means more heat held back. Before factories and engines, the air held about 280 parts of carbon dioxide per million. It now holds more than 420, and the world has warmed by over one degree Celsius.

Swap one hydrogen on a hydrocarbon for an oxygen atom carrying a hydrogen of its own, and the molecule joins a new family, the alcohols. That small oxygen-hydrogen group changes its character. Unlike hydrocarbons, small alcohols mix freely with water, and they dissolve many things that water cannot.

The best known is ethanol, with two carbons. It is the alcohol in hand sanitiser, where at about 70 per cent strength it kills most germs. It is used as a solvent and blended into petrol as a fuel, and it is the intoxicating part of wine and beer.

The word itself is Arabic. Al-kuḥl was kohl, the dark eye cosmetic, a very fine powder of ground mineral. European chemists borrowed the word for any fine powder, then for any purified essence, and in the 1500s began applying it to the spirit they distilled from wine.

Ethanol can be made in a factory, but most of it is made by a living thing. Yeast is a single-celled fungus that feeds on sugar. With no air to breathe, it splits each sugar molecule into two products, ethanol and carbon dioxide, and lives on the energy released. This is fermentation.

Bakers use the same process for a different reason. Yeast mixed into dough ferments the sugars in the flour, and as it gives off carbon dioxide the gas is trapped as bubbles, which make the dough rise. Nearly all of the little ethanol made evaporates in the oven.

Left open to the air, a fermented liquid changes again. Bacteria use oxygen to turn its ethanol into ethanoic acid, the sharp, sour acid of vinegar. The name says as much: it comes from the French vin aigre, sour wine.

Carbon’s chains can grow very long indeed. A polymer is a giant molecule made by joining thousands of small molecules end to end, like beads on a string. The small molecule that is repeated is called the monomer. The names are Greek: mono means one, poly means many, and meros means part.

Nature makes polymers constantly. The cellulose in wood and cotton is a chain of sugar units, and proteins and DNA are polymers too. Chemists learned to make their own in the twentieth century, and the results are plastics. Polythene, the commonest of them, is nothing but ethene, a two-carbon gas made by cracking, joined into chains many thousands of units long. It makes bags and bottles. Nearly all plastics start as hydrocarbons from oil or gas, cracked into small molecules and then linked up.

Plastics are useful because they do not rot. That is also the problem with them. Wood, paper and food scraps are biodegradable: bacteria and fungi can digest them and return their carbon to the soil and the air. The long chains of most plastics are something almost no microbe can take apart. A plastic bottle in the ground does not decay. It only breaks, over centuries, into smaller and smaller fragments.

One answer is to use the material again. Many plastics can be melted down and remoulded into new objects, that is, recycled. In practice little has been. Of all the plastic waste produced up to 2015, about 9 per cent was recycled, 12 per cent was burned, and the remaining 79 per cent went to landfill or ended up in the environment.