Chemists have catalogued many millions of compounds, and the great majority contain carbon. One element, out of more than a hundred, makes up the backbone of fuels, plastics, medicines, dyes and every living thing. The reason lies in its bonding. Carbon forms four bonds and bonds strongly to itself, so it can build long chains, branches and rings. Silicon, just below carbon in the table, can also form four bonds, but its bonds to itself are weaker and it bonds so firmly to oxygen that most of it ends up locked in rock.

The end of a life force

Two centuries ago, chemists divided substances into two kinds. Inorganic substances came from rocks, air and water. Organic substances came from living things, such as sugar, oils and urea, a compound found in urine. Many chemists held a view called vitalism: organic compounds required a special life force and could not be made from lifeless materials.

In 1828 the German chemist Friedrich Wöhler heated ammonium cyanate, a compound regarded as inorganic, and found he had made urea. Wöhler's 1828 synthesis of urea showed that a compound from living bodies could be made from non-living starting materials. The story is often told as the moment vitalism died. Historians tell it more carefully. Wöhler's starting chemicals had themselves been prepared from animal products, and other chemists did not change their minds overnight. Vitalism faded gradually over the following decades, rather than falling to a single experiment, as more and more organic compounds were made in the laboratory. Today organic chemistry is the chemistry of carbon compounds, whatever their source.

Building blocks

The simplest organic compounds are hydrocarbons, made only of carbon and hydrogen. Methane, the main part of natural gas, has one carbon. The octane in gasoline has eight in a chain. Hydrocarbons are nonpolar and burn well, which is why so many fuels belong to this family.

Add other atoms and the molecule's behavior changes. A functional group is a small cluster of atoms, such as the oxygen and hydrogen of an alcohol, that gives a molecule its characteristic behavior. Ethane is a gas. Attach that oxygen and hydrogen group to it and you get ethanol, the alcohol in beer and wine, a liquid that mixes with water. Organic chemists learn the families of functional groups the way language learners learn verb endings.

Same atoms, different molecules

In organic chemistry, a formula alone is not enough. Isomers are compounds with the same atoms in different arrangements, and they can have very different properties. Ethanol and dimethyl ether have exactly the same atoms. One is a drinkable liquid; the other is a gas used as a propellant. In 1865 August Kekulé proposed that the six carbons of benzene form a ring, a key step in learning to think about molecules as structures in space. He later told a story of dreaming of a snake biting its tail, though some historians doubt the tale.

Space matters in another way. Some molecules are chiral: they come in two mirror-image forms, like left and right hands, that cannot be superimposed. Louis Pasteur discovered this in 1848, sorting tiny crystals of a salt of tartaric acid by hand under a microscope into left-handed and right-handed sets. Mirror-image molecules often behave identically in a test tube but differently in a living body, because the body's receptors and enzymes are themselves chiral, so each fits one hand better than the other. One form of the molecule carvone smells of spearmint; its mirror image smells of caraway seeds.

Molecules of life

Living things are built mainly from four families of carbon compounds: carbohydrates, lipids, proteins and nucleic acids. Most are polymers. A polymer is a long molecule built by linking many small repeating units, such as the sugar units of starch or the amino acids of a protein. Here the point is chemical: life uses the same bonds and the same rules as a test tube, on a vast and intricate scale.

Worked example: thalidomide and the mirror

In 1957 a West German company began selling thalidomide as a sedative. It was widely considered safe and was soon given to pregnant women for morning sickness in dozens of countries. Within a few years doctors linked it to severe birth defects, especially shortened or missing limbs. More than 10,000 children were affected worldwide, and many died in infancy.

Thalidomide is chiral. Research has linked one mirror-image form to its calming effect and the other to the harm, and people often say that selling only the "good" hand would have prevented the tragedy. It would not have. In the body, each form slowly converts into the other, so a pure dose soon becomes a mixture. Chemistry alone could not have fixed the drug.

What did make a difference was testing. In the United States, Frances Kelsey, a pharmacologist at the Food and Drug Administration, refused to approve thalidomide in 1960 and 1961 because the company's safety evidence was thin. In 1962 Congress passed laws requiring drug makers to prove that new drugs work and to test them more carefully. The case shows how a molecule's shape, a regulator's caution and the rights of patients not to be treated as unwitting test subjects all connect.