Improving on nature

Ultimately, we reach the very complex or­ganic chemicals produced by the synthetic chemist. A synthetic chemist combines chemi­cal elements and compounds to duplicate nat­urally occurring substances. He also produces compounds that do not occur naturally, in­cluding many drugs and pesticides. Many of these compounds have provided extraordinar­ily complex challenges to the synthetic chem­ist. Yet many of the compounds found natu­rally in plants and animals are produced with greater efficiency in nature than in the labora­tory. It is thus more economical to extract them from a natural source, rather than to make them in the laboratory.

On the other hand, by studying the struc­tures of natural substances, it has been possi­ble in many instances to improve on them and obtain products even better suited to our needs. Synthetic chemists can produce varia­tions of naturally-occurring penicillin that are

suitable for different kinds of therapeutic treat­ment. Similarly, a naturally-occurring insecti­cide in pyrethrum flowers has been modified to give a range of synthetic insecticides. These have a greater variety of pesticidal uses than the natural insecticide.

Oil and natural gasare

obtained by drilling. They are the chief starting materi­als for a wide range of im­portant organic chemicals. These include plastics, pes­ticides, dyes, synthetic tex­tiles, and some pharmaceu­ticals.

The foxglove plantDigi­talis purpurea was for many years the principal source of the drug digitoxin. It was used to treat heart condi­tions such as cardiac failure. Today, this drug has gener­ally been replaced by dig-oxin, obtained from an­other species of foxglove, Digitalis lanata. Many other drugs and useful organic chemicals are still obtained from plants or animals. In some cases, these sub­stances cannot be synthe­sized economically. In other cases, chemists have not yet succeeded in synthesizing them at all.

Natural gasis a major source of alkanes. It used to be burned off as a waste product at oil drilling instal­lations. Increasingly, how­ever, the gas associated with oil deposits is not wasted. It may be piped ashore from offshore fields, transported as a liquid in re­frigerated ships, or con­verted to methanol for transport over longer dis­tances.

Saturated

Aliphatic

Hydrocarbons

The saturated aliphatic hydrocarbons form a series of organic molecules that contain only hydrogen and carbon, hence the term hydro­carbon. Each carbon atom is linked to four other atoms—the maximum number possible— and is therefore called saturated (completely full). In every member of the series, the mole­cules are arranged in the shape of straight or branched chains with open ends (aliphatic), rather than in closed rings. Formerly known as paraffins, the members of this series are now called alkanes.

The alkanes

The first member of the alkane series and the simplest organic compound is methane, the chief component of natural gas. A molecule of methane consists of a single carbon atom linked to four atoms of hydrogen. Each of its chemical bonds involves two atoms—the car­bon atom and one of the hydrogen atoms. Each bond consists of a pair of electrons, one

provided by each atom. The removal of one hydrogen atom from methane forms a methyl group. Because of the ability of carbon atoms to form chains, two methyl groups can join to form a compound that consists of two carbon atoms and six hydrogen atoms. This substance—called ethane—is the second mem­ber of the alkane series.

Just as ethane can be derived from meth­ane, so can propane be derived from ethane. If one of the hydrogen atoms from the ethane molecule is replaced by a methyl group, a chain of three carbon atoms with eight hydro­gen atoms is formed. This is propane. Its mid­dle carbon atom, being linked to two other carbon atoms, has room for only two hydro­gen atoms.

Isomerism

By repeating the process of replacing a hydro­gen atom with a methyl group, the alkane se­ries can be further extended to butane (with four carbon atoms), pentane (with five carbon atoms), hexane (with six carbon atoms), and even larger molecules.

Once an alkane molecule has four or more carbon atoms, it need no longer have the shape of a simple unbranched chain. Instead of adding another methyl group to the end of propane's three-carbon chain, for example, the methyl group can be attached to the middle carbon atom. This forms a branched chain. The central atom is linked to three other carbon atoms and only one hydrogen atom. Each of the other three carbons has three hydrogens attached.

This branched-chain molecule is called iso-butane, or 2-methylpropane (indicating that the additional methyl group is attached to the second carbon atom). The formulas of isobu-tane and ordinary straight-chain butane (called /7-butane, where n stands for normal) are the same. Both forms have the same numbers of carbon atoms (4) and hydrogen atoms (10). But their structures are different.

This phenomenon, in which molecules have the same chemical composition but a dif­ferent structural arrangement of atoms, is called structural isomerism. The different forms are called isomers. The physical proper­ties of structural isomers can vary widely. For example, /7-butane boils at a temperature of 31° F. ( — 0.5° C), whereas isobutane boils at 10.9°F. (-11.7° C).

Date: 2015-12-11; view: 1008