Assignment_ Stereoelectronic Effects in Organic Reactions in Organic Chemistry.

Assignment : Stereoelectronic Effects in Organic Reactions in Organic Chemistry

Abstract

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 flavoring 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

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 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

‘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 flavoring 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

1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,0

00 (10^63) 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 compound isolated from plants as hard white crystals when pure, and petrol, a

mixture of colorless, volatile, flammable hydrocarbons. Isooctane is a typical example and

gives its name to the octane rating of petrol.

The compounds need not lack color. 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.

Color is not the only characteristic by which we recognize compounds. All too often it is

their odor 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 trithioacetone 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 trithioacetone south of Oxford in 1967.

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 honor of sulfur compounds,

we must cite the truffle, which pigs can smell through a meter of soil and whose taste and

smell are 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. 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.

The sex pheromone of the beetle Popilia japonica, 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: as little as 2 × 10^−12 g 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. 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