Файл: Л.И. Иваницкая Сборник текстов для студентов II курса химических специальностей (английский язык).pdf

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Experiments with Medicines

Most human diseases are caused by microorganisms mat penetrate the tissue and disturb me normal chemical processes in them. One of ways of controlling these microbes is to "poison" them by bringing into contact with some chemical compounds.

At the beginning of this century j/ara-aminobenzene sulphonamide (sulphanilamide) was obtained as an intermediate in the production of dyes. It was noticed mat this compound kills the microorganisms responsible for some diseases. After numerous tests, sulphanilamide was used as a medicine and called streptocide. If a tablet of streptocide is heated on a metal plate, a violet melt is formed that smells of ammonia. This test identifies the amino group in the substance. Now, boil a tablet of streptocide in a test tube in a small amount of concentrated nitric acid, dilute the solution with water, and add barium chloride solution. As a result, a white precipitate is produced, which proves the presence of a sulpho group - SО2-.

Why can streptocide kill microorganisms? It turns out that the normal function of a bacterium requires para-aminobenzoic acid, which is also called Hi vitamin, para-aminobenzoic acid is a component of folic acid, from which the bacteria produce enzymes. Vitamin Hi is a growth factor without which bacteria can neither grow nor reproduce. A molecule of streptocide is similar in size and chemistry to a molecule of para-aminobenzoic acid. Bacteria utilize the sulphanilamide to form "folic" acid and then their enzymes, but then the forgery comes to light for the enzymes do not function. Thus, the normal bacterium's metabolism is disrupted and the bacterium dies.

Streptocide is the simplest sulphanilamide. Numerous other sulphanilamide derivatives are now known and they are all used as medicines. They include sulphadi-mezine (sulphadimedine, sulphamethazine), sulphathiazole (norsulphazole), sulphagua-nidine (sulgin), and ethazole (sul- pha-ethidole). For example, phthalyl sulphathiazole is like a molecule of streptocide, but the hydrogen atoms in the amino groups are replaced by the residues of complex organic compounds. These substituents prevent the medicine from penetrating the intestine walls into the blood stream and thus being distributed throughout the organism. Therefore, phthalyl sulphathiazole is effective in controlling disease-carrying microorganisms in the stomach and intestine (Fig. 32).

Heat a tablet of sulphathiazole in a test-tube: a dark brown melt is formed which smells of hydrogen sulphide. The latter is due to a sulphur atom in the five-mem-bered ring contained in the sulphathiazole molecule.

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Now, place sodium hydroxide solution into each of the three test tubes, and add sulphathiazole to the first test tube, ethazole to the second, and sulphadimedine to the third test tube. If copper sulphate solution is then added to each test tube, precipitates of different colours will be formed: a dirty violet precipitate appears in the first test tube, a green compound (which turns dark after a while) precipitates in the second, and a yellow-green compound, which later becomes brown, precipitates in the third test tube.

The derivatives of para-aminobenzoic acid are widely used in medicine. If the hydrogen atom of its carboxyl group is replaced by an ethyl radical, anesthesin (benzocaine) is obtained, while novocaine (procaine) is

Fig. 32. Structural formulas of certain medicines

formed when the hydrogen atom is replaced by a more complex radical, viz. –CH2CH2N(C2H2)2. Both compounds are used as local anaesthetics and analgesics. Salicylic acid somewhat resembles para-ammobeuzoic acid. It is a derivative of benzene, in which two neighbouring hydrogen atoms are replaced by a carboxyl and a hydroxyl group. Salicylic acid is an important medicine; it is used as a bactericide and to treat rheumatism. The sodium salt of salicylic acid is used as an analgesic, antipyretic, antiphlogistic, and antirheumatic agent. The methyl ester of this acid (in which –COOCH3 group

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substitutes for -COOH group) is used in ointments as antiphlogistic and antirheumatic agent. Everyone is familiar with acetylsalicylic acid, in which me -Oil group is replaced by the -OCOCH3 group (it is called aspirin). The phenyl ester of acetylsalicylic acid, i.e. phenyl salicylate (where - COOH group is replaced by -COOC6H5), which is also called salol, and is a good antiseptic and disinfectant. The sodium salt of para-aminosalicylic acid "(this compound contains two amino groups in the benzene ring) is antitubercular agent.

All phenols are turned violet by ferric chloride solution. This can be verified by reacting an alcohol solution of salol with a drop of ferric chloride. When this test is performed with acetylsalicylic acid, no violet colouration is observed. The reason is that the acid has no free hydroxyl group, and the hydrogen is replaced by the acetyl group. However, we can easily obtain salicylic and acetic acids from acetylsalicylic acid by boiling a tablet of the latter in a sodium hydroxide solution in a test tube.

After cooling, the solution is acidified with dilute sulphuric acid. The precipitated salicylic acid can be identified by the violet colouration which appears when a drop of ferric chloride solution is added to the precipitate.

Phenacetin is an effective medicine for fever. A molecule of this compound includes benzene ring with attached -NHCOCH3 and -OC2H5 groups. Place a small piece of phenacetin tablet into a test tube containing dilute nitric acid. First, the solution turns yellow, arid then a yellow precipitate forms. Now, boil a piece of phenacetin tablet in a small amount of dilute hydrochloric acid in a test tube, and after cooling the solution add a drop of potassium dichromate solution. The violet colour which first appears, changes to red.

The derivatives of pyrazolone (ketopyrazoline) comprise another class of antipyretics and analgesics. They include antipyrine (analgesin), amidopyrine (pyramidon), and analgin (novaminsulphon, dipyrone) and have similar formulas. Antipyrine gives red colouration with ferric chloride solution, while with acidified sodium nitrite solution it produces a green nitroderivative. In contrast to antipyrine, amidopyrine reacts with ferric chloride to give a blue solution, from which a brown substance precipitates after some time. If this solution is acidified with hydrochloric acid, a violet colouration is observed. Amidopyrine solution also goes the same colour when treated by silver nitrate solution.

Derivatives of barbituric acid are soporifics. The barbiturates include veronal (barbital), phenobarbital (luminal), bar-bamyi (sodium amobarbital),

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nembutal (sodium pentobarbital), and others. Add sodium hydroxide and copper sulphate solutions to a solution of barbital. The solution first turns blue, and then a red precipitate forms. A molecule of barbamyi contains sodium, which can be identified by dipping a wire into the substance and placing the wire into a flame. The flame colours bright yellow.

Finally, a few words about antibiotics. These are produced by certain microorganisms and can kill the causative agents (pathogens) of many diseases. For example, penicillin does not permit pathogenic bacteria to assimilate amino acids fi-om food, and therefore, bacteria cannot construct cell membranes. Terramycin (oxytetracycline) disturbs phosphorylation in a bacterium, i.e. the process by which it releases energy. In the presence of tetracycline or streptomycin (agrimycin, agri-strep), bacteria cannot synthesize individual proteins. Certain antibiotics undergo specific chemical reactions. Add a drop of sodium hydroxide to a few drops of streptomycin and boil the mixture for a few seconds. Then, acidify the liquid with hydrochloric acid and add ferric chloride: a red-violet colouration appears.

When an alcohol solution of ferric chloride is added to a solution of terramycin, the solution turns brown. If several crystals of terramycin are dissolved in a drop of concentrated sulphuric acid, a red colouration appears. If a tablet of levomycetin (chloramphenicol) is dissolved in a sodium hydroxide solution with heating, the solution first turns yellow and then red.

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