Clayden - Greeves Warren Wothers - Organic Chemistry

Clayden - Greeves Warren Wothers - Organic Chemistry

(Parte 1 de 17)

Organic chemistry and you

You are already a highly skilled organic chemist. As you read these words, your eyes are using an organic compound (retinal) to convert visible light into nerve impulses. When you picked up this book, your muscles were doing chemical reactions on sugars to give you the energy you needed. As you understand, gaps between your brain cells are being bridged by simple organic molecules (neurotransmitter amines) so that nerve impulses can be passed around your brain. And you did all that without consciously thinking about it. You do not yet understand these processes in your mind as well as you can carry them out in your brain and body. You are not alone there. No organic chemist, however brilliant, understands the detailed chemical working of the human mind or body very well.

We, the authors, include ourselves in this generalization, but we are going to show you in this book what enormous strides have been taken in the understanding of organic chemistry since the science came into being in the early years of the nineteenth century. Organic chemistry began as a tentative attempt to understand the chemistry of life. It has grown into the confident basis of vast multinational industries that feed, clothe, and cure millions of people without their even being aware of the role of chemistry in their lives. Chemists cooperate with physicists and mathematicians to understand how molecules behave and with biologists to understand how molecules determine life processes. The development of these ideas is already a revelation at the beginning of the twenty-first century, but is far from complete. We aim not to give you the measurements of the skeleton of a dead science but to equip you to understand the conflicting demands of an adolescent one.

Like all sciences, chemistry has a unique place in our pattern of understanding of the universe. It is the science of molecules. But organic chemistry is something more. It literally creates itself as it grows. Of course we need to study the molecules of nature both because they are interesting in their own right and because their functions are important to our lives. Organic chemistry often studies life by making new molecules that give information not available from the molecules actually present in living things.

This creation of new molecules has given us new materials such as plastics, new dyes to colour our clothes, new perfumes to wear, new drugs to cure diseases. Some people think that these activities are unnatural and their products dangerous or unwholesome. But these new molecules are built by humans from other molecules found on earth using the skills inherent in our natural brains. Birds build nests; man makes houses. Which is unnatural? To the organic chemist this is a meaningless distinction. There are toxic compounds and nutritious ones, stable compounds and reactive ones—but there is only one type of chemistry: it goes on both inside our brains and bodies and also in our flasks and reactors, born from the ideas in our minds and the skill in our hands. We are not going to set ourselves up as moral judges in any way. We believe it is right to try and understand the world about us as best we can and to use that understanding creatively. This is what we want to share with you.

Organic compounds

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. Now it is both. It is the chemistry of the compounds of carbon along with other elements such as are found in living things and elsewhere.

1What is organic chemistry?

We are going to give you structures of organic compounds in this chapter—otherwise it would be rather dull. If you do not understand the diagrams, do not worry. Explanation is on its way.

1-cis-retinal absorbs light when we see

HO NH2 serotonin human neurotransmitter

The organic compounds available to us today 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 flavouring compound from the essential oil of spearmint and cis-jasmone an example of a perfume distilled from jasmine flowers.

Even in the sixteenth century one alkaloid was 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.

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 monoxide) also gave a brown tar rich in aromatic compounds such as benzene, pyridine, phenol, aniline, and thiophene.

Phenol was used 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. A dyestuff of this kind—still available—is Bismarck Brown, which should tell you that much of this early work 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’) and propane (CH3CH2CH3, ‘calor gas’) 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 ‘fine’ chemicals. Bulk chemicals like paints and plastics are usually based on simple molecules produced in multitonne quantities while fine chemicals such as drugs, perfumes, and flavouring materials are produced in smaller quantities but much more profitably.

At the time of writing there were about 16 million organic compounds known. How many more are possible? There is no limit (except the number of atoms in the universe). Imagine you’ve just made the longest hydrocarbon ever made—you just have to add another carbon atom and you’ve made another. This process can go on with any type of compound ad infinitum.

But these millions of compounds are not just a long list of linear hydrocarbons; they embrace all kinds of molecules with amazingly varied properties. In this chapter we offer a selection.

You will be able to read towards the end of the book (Chapters 49–51) about the extraordinary chemistry that allows life to exist but this is known only from a modern cooperation between chemists and biologists.

You can read about polymers and plastics in Chapter 52 and about fine chemicals throughout the book.

OH menthol cis-jasmone N

MeO HO quinine benzene N pyridine OHphenol

NH2 aniline S thiophene

Bismarck Brown Y n = an enormous number length of molecule is n + 3 carbon atoms n = an enormous number length of molecule is n + 2 carbon atoms

What do they looklike? They may be crystalline solids, oils, waxes, plastics, elastics, mobile or volatile liquids, or gases. Familiar ones include white crystalline sugar, a cheap natural compound isolated from plants as hard white crystals when pure, and petrol, a mixture of colourless, 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 smell of the skunk is a mixture of two thiols—sulfur compounds containing SH groups.

Organic compounds3

Colour Description Compound Structure red dark red hexagonal plates 3¢ -methoxybenzocycloheptatriene- 2¢ -one orangeamber needlesdichloro dicyano quinone (DDQ) yellowtoxic yellow explosive gasdiazomethane greengreen prisms with a9-nitroso julolidine steel-blue lustre bluedeep blue liquid with aazulene peppery smell purpledeep blue gas condensingnitroso trifluoromethane to a purple solid

O MeO

CH2 N N

CNCl Cl u m skunk spray contains:

volatile inflammable liquid white crystalline solid

CH3 C C

CH CH3

CH3 CH3 CH3 sucrose – ordinary sugar isolated from sugar cane or sugar beet isooctane (2,3,5-trimethylpentane) a major constiuent of petrol

But perhaps the worst aroma was that which caused the evacuation of the city of Freiburg in 1889.

Attempts to make thioacetone by the cracking of trithioacetone gave rise to ‘an offensive smell which spread rapidly over a great area of the town causing fainting, vomiting and a panic evacuationºthe laboratory work was abandoned’.

effects of dilution since workers in the laboratory did not find the odours intolerableand genu-

It was perhaps foolhardy for workers at an Esso research station to repeat the experiment of cracking trithioacetone south of Oxford in 1967. Let them take up the story. ‘Recentlyºwe found ourselves with an odour problem beyond our worst expectations. During early experiments, 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 trithioacetoneºfound themselves the object of hostile stares in a restaurant and suffered the humiliation of having a waitress spray the area around them with a deodorantº. The odours defied the expected inely 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 trithioacetone crystallisations 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. Nasty smells have their uses. The natural gas piped to our homes contains small amounts of delib- erately added sulfur compounds such as tert-butyl thiol (CH3)3CSH. When we say small, we mean verysmall—humans can detect one part in 50000000000 parts of natural gas.

Other compounds have delightful odours. 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 truffles cost more than their weight in gold. Damascenones 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. Just like the compounds from trithioacetone, this smell develops on dilution.

Humans are not the only creatures with a sense of smell. We can find mates using our eyes alone (though smell does play a part) but insects cannot do this. They are small in a crowded world and they find others of their own species and the opposite sex 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 serricornin, the sex pheromone of the cigarette beetle, could be isolated from 65000 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 Japanese beetle, also given off by the females, has been made by chemists. As little as 5 µg (micrograms, note!) was more effective than four virgin females in attracting the males.

The pheromone of the gypsy moth, disparlure, was identified from a few µg isolated from the moths and only 10 µg of synthetic material. As little as 2 ·10–12g is active as a lure for the males in field tests. The three pheromones we have mentioned are available commercially for the specific trapping of these destructive insect pests.

thioacetone trithioacetone; Freiburg was evacuated because of a smell from the distillation this compound propane dithiol

4-methyl-4- sulfanylpentan- 2-one two candidates for the worst smell in the world no-one wants to find the winner!

CH3 S CH3 damascenone - the smell of roses the divine smell of the black truffle comes from this compound

H serricornin the sex pheromone of the cigarette beetle Lasioderma serricorne japonilure the sex pheromone of the Japanese beetle Popilia japonica

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!

What about taste? Take the grapefruit. The main flavour comes from another sulfur compound and human beings can detect 2 ·10–5parts per billion of this compound. This is an almost unimaginably small amount equal to 10–4mg per tonne or a drop, not in a bucket, but in a good-sized lake. Why evolution should have left us abnormally sensitive to grapefruit, we leave you to imagine.

(Parte 1 de 17)

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