Organic chemistry started as the chemistry of life, when that was thought to be different from the chemistry in the laboratory. Then it became the chemistry of carbon compounds, especially those found in coal. But now it is both. It is the chemistry of the compounds formed by carbon and other elements such as are found in living things, in the products of living things, and wherever else carbon is found. The most abundant organic compounds are those present in living things and those formed over millions of years from dead things. In earlier times, the organic compounds known from nature were those in the ‘essential oils’ that could be distilled from plants and the alkaloids that could be extracted from crushed plants with acid. Menthol is a famous example of a f l avouring compound from the essential oil of spearmint and cis-jasmone an example of a perfume distilled from jasmine flowers. Natural products have long been used to cure diseases, and in the sixteenth century one became famous—quinine was extracted from the bark of the South American cinchona tree and used to treat fevers, especially malaria. The Jesuits who did this work (the remedy was known as ‘Jesuit’s bark’) did not of course know what the structure of quinine was, but now we do. More than that, the molecular structure of quinine has inspired the design of modern drug molecules which treat malaria much more effectively than quinine itself. The main reservoir of chemicals available to the nineteenth century chemists was coal. Distillation of coal to give gas for lighting and heating (mainly hydrogen and carbon mon oxide) also gave a brown tar rich in aromatic compounds such as benzene, pyridine, phenol, aniline, and thiophene.

Phenol was used in the nineteenth century by Lister as an antiseptic in surgery, and aniline became the basis for the dyestuffs industry. It was this that really started the search for new organic compounds made by chemists rather than by nature. In 1856, while trying to make quinine from aniline, an 18-year-old British chemist, William Perkin, managed to produce a mauve residue, mauveine, which revolutionized the dyeing of cloth and gave birth to the synthetic dyestuffs industry. A related dyestuff of this kind—still available—is Bismarck Brown: much of the early work on dyes was done in Germany.

In the twentieth century oil overtook coal as the main source of bulk organic compounds so that simple hydrocarbons like methane (CH4, ‘natural gas’), propane, and butane (CH3CH2CH3 and CH3CH2CH2CH3, ‘Calor gas’ or LPG) became available for fuel. At the same time chemists began the search for new molecules from new sources such as fungi, corals, and bacteria, and two organic chemical industries developed in parallel—’bulk’ and ‘fi ne’ chemicals. Bulk chemicals like paints and plastics are usually based on simple molecules produced in multitone quantities while fi ne chemicals such as drugs, perfumes, and fl avour ing materials are produced in smaller quantities but much more profitably. At the time of writing there were over 16 million organic compounds known. How many more might there be? Even counting only moderately sized molecules, containing fewer than about 30 carbon atoms (about the size of the mauveine structure above), it has been calculated that something in the region of 1,000,000,000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000 (1063) stable compounds are possible. There aren’t enough carbon atoms in the universe to make them all. Among the 16 million that have been made, there are all kinds of molecules with amazingly varied properties. What do they look like? They may be crystalline solids, oils, waxes, plastics, elastics, mobile or volatile liquids, or gases. Familiar ones include sugar, a cheap natural com pound isolated from plants as hard white crystals when pure, and petrol, a mixture of colour less, volatile, flammable hydrocarbons. Isooctane is a typical example and gives its name to the octane rating of petrol.

The compounds need not lack colour. Indeed, we can soon dream up a rainbow of organic compounds covering the whole spectrum, not to mention black and brown. In this table we have avoided dyestuffs and have chosen compounds as varied in structure as possible.

Colour is not the only characteristic by which we recognize compounds. All too often it is their odour that lets us know they are around. There are some quite foul organic compounds too; the infamous stench of the skunk is a mixture of two thiols—sulfur compounds containing SH groups. But perhaps the worst smell ever recorded was that which caused the evacuation of the German city of Freiburg in 1889. Attempts to make thioacetone by the cracking of Tri thioacetone gave rise to ‘an offensive smell which spread rapidly over a great area of the town causing fainting, vomiting, and a panic evacuation...the laboratory work was abandoned’. It was perhaps foolhardy for workers at an Esso research station to repeat the experiment of cracking Tri thioacetone south of Oxford in 1967. Let them take up the story. ‘Recently we found ourselves with an odour problem beyond our worst expectations. During early experi ments, a stopper jumped from a bottle of residues, and, although replaced at once, resulted in an immediate complaint of nausea and sickness from colleagues working in a building two hundred yards away. Two of our chemists who had done no more than investigate the cracking of minute amounts of Tri thioacetone found themselves the object of hostile stares in a restaurant and suffered the humiliation of having a waitress spray the area around them with a deodorant. The odours defi ed the expected effects of dilution since workers in the laboratory did not find the odours intolerable ... and genuinely denied responsibility since they were working in closed systems. To convince them otherwise, they were dispersed with other observers around the laboratory, at distances up to a quarter of a mile, and one drop of either acetone gem-dithiol or the mother liquors from crude Tri thioacetone crystallizations were placed on a watch glass in a fume cupboard. The odour was detected downwind in seconds.’ There are two candidates for this dreadful smell—propane dithiol (called acetone gem dithiol above) or 4-methyl-4-sulfanylpentan-2-one. It is unlikely that anyone else will be brave enough to resolve the controversy. But nasty smells have their uses. The natural gas piped into homes contains small amounts of deliberately added sulfur compounds such as tert-butyl thiol (CH3)3CSH. When we say small, we mean very small—humans can detect one part in 50,000,000,000 parts of natural gas. Other compounds have delightful odors. To redeem the honour of sulfur compounds we must cite the truffle, which pigs can smell through a metre of soil and whose taste and smell is so delightful that truffle es cost more than their weight in gold. Damascenes are responsible for the smell of roses. If you smell one drop you will be disappointed, as it smells rather like turpentine or camphor, but next morning you, and the clothes you were wearing, will smell powerfully of roses. Many smells develop on dilution. Humans are not the only creatures with a sense of smell. We can find mates using all our senses, but insects cannot do this. They are small in a crowded world and they find those of the opposite sex of their own species by smell. Most insects produce volatile compounds that can be picked up by a potential mate in incredibly weak concentrations. Only 1.5 mg of sericornin, the sex pheromone of the cigarette beetle, could be isolated from 65,000 female beetles—so there isn’t much in each beetle. Nevertheless, the slightest whiff of it causes the males to gather and attempt frenzied copulation. The sex pheromone of the beetle Popilia japonica, also given off by the females, has been made by chemists. As little as 5 μg (micro grams, note!) was more effective than four virgin females in attracting the males.

The pheromone of the gypsy moth, disparlure, was identifi ed from a few μg isolated from the moths: as little as 2 × 10−12 g is active as a lure for the males in fi eld tests. The three phero mones we have mentioned are available commercially for the specific trapping of these destructive insect pests.

Don’t suppose that the females always do all the work; both male and female olive flies produce pheromones that attract the other sex. The remarkable thing is that one mirror image of the molecule attracts the males while the other attracts the females! Mirror image isomers of a molecule called frontalin are also emitted by male elephants; female elephants can tell the age and appeal of a potential mate from the amount of each isomer he produces.

What about taste? Take the grapefruit. The main flavour comes from another sulfur com pound and human beings can detect 2 × 10⁻⁵ parts per billion of this compound. This is an almost unimaginably small amount equal to 10⁻⁴ mg per tonne or a drop, not in a bucket, but in a fairly large lake. Why evolution should have left us so extraordinarily sensitive to grape fruit, we leave you to imagine. For a nasty taste, we should mention ‘bittering agents’, put into dangerous household sub stances like toilet cleaner to stop children drinking them by accident. Notice that this com plex organic compound is actually a salt—it has positively charged nitrogen and negatively charged oxygen atoms—and this makes it soluble in water.

Other organic compounds have strange effects on humans. Various ‘drugs’ such as alcohol and cocaine are taken in various ways to make people temporarily happy. They have their dangers. Too much alcohol leads to a lot of misery and any cocaine at all may make you a slave for life.

Again, let’s not forget other creatures. Cats seem to be able to go to sleep anywhere, at any time. This surprisingly simple compound, isolated from the cerebrospinal fluid of cats, appears to be part of their sleep-control mechanism. It makes them, or rats, or humans fall asleep immediately.

This compound and disparlure (above) are both derivatives of fatty acids. Fatty acids in the diet are a popular preoccupation, and the good and bad qualities of saturates, monounsatu rates, and polyunsaturates are continually in the news: one of the many dietary molecules reckoned to have demonstrable anticancer activity is CLA (conjugated linoleic acid), which is found in dairy products and also, most abundantly, you may be interested to know, in kanga room eat. Resveratrole is another dietary component with beneficial effects: it may be responsible for the apparent ability of red wine to prevent heart disease. It is a quite different sort of organic compound, with two benzene rings. For a third edible molecule, how about vitamin C? This is an essential factor in your diet— that is why it is called a vitamin—and in the diet of other primates, guinea-pigs, and fruit bats (other mammals possess the biochemical machinery to make it themselves). The disease scurvy, a degeneration of soft tissues from which sailors on the long voyages of past centuries suffered, results from a lack of vitamin C. It also is a universal antioxidant, scavenging for rogue reactive radicals and protecting damage to DNA. Some people think an extra-large intake may even protect against the common cold.