Text 3. Butlerov’s theory of the chemical structure of organic compounds. Part I

A certain number of organic substances were known to man and used by him in ancient times (acetic acid, ethyl alcohol, oils, starch, dyes, and some others). But the rapid advance of organic chemistry as a science began only in the XIX century, when the requirements of developing production caused many new organic substances to be discovered and investigated.

In the course of these discoveries there emerged facts that could not be interpreted in terms of the chemical theories current at the time.

In the fifties of the XIX century the valence of the chemical elements had become a scientific concept. The German chemist August Kekule established the quadrivalence of carbon. But in many organic compounds the valence of elements appeared to be invalid. Indeed, try to determine the valence of carbon, say, in ethane C₂H₆ or in propane C₃H₈ by the method familiar to you from inorganic chemistry. You will obtain a strange result: the carbon in ethane will appear to be trivalent (6/2), while in propane it will even seem to have a fractional valence (8/3).

Other problems arose too: why does the composition of the molecules in the hydrocarbon series increase every time precisely by one carbon atom and two hydrogen atoms; what is the reason for the diversity of organic compounds; why is it carbon and not some other element that forms so many compounds. Other facts also defied explanation, such as the existence of substances of absolutely identical composition and, hence, having the same molecular formula, but exhibiting markedly different properties.

All these and many other facts were explained by the theory advanced in the sixties of the XIX century by the great Russian chemist Alexander Butlerov.

The analysis of a vast amount of scientific data led Butlerov to the conclusion that the explanation of the baffling facts that had accumulated should be sought in the structure of molecules, made up of atoms. Contrary to the views of other scientists, who attached no significance to the structure of molecules and even regarded it as something that science could not probe, Butlerov maintained that chemical structure could fully be established and that only an understanding of the inner structure of molecules could resolve the deadlock in science and arm it with a theory that would enable it to make further successful progress.

Butlerov’s teachings about the structure of molecules came to be known as the theory of chemical structure.

The main points of the theory of chemical structure may be formulated as follows:

1. The arrangement of atoms in molecules is not a random one; they are linked to one another in a definite order, according to their valence.

2. The properties of substances depend not only on which atoms and in what amounts make up the molecules, but on the order in which these atoms are connected.

Tasks on the text

1. Memorize the following words and word combinations.

acetic acid [q`sJtIk xsId] – уксусная кислота ethyl alcohol [`eTIl `xlkqhql] – этиловый спирт starch [stRtS] – крахмал to cause [kO:z] – послужить причиной, поводом (для чего-л.) in terms of – в терминах, в переводе на, говоря языком; исходя из requirement [rI`kwaIqmqnt] – требование, необходимое условие valence [`veIlqns] (valance, valency) – валентность invalid – недействительный, необоснованный to obtain – получать hydrocarbon [,haIdrqu`kRb(q)n] – углеводород

defy [dI`faI] explanation – не поддаются объяснению hence – следовательно, поэтому to accumulate [q`kju:mju,leIt] – накапливать, собирать to attach to – прикреплять, привязывать; придавать значение to probe [prqub] – прощупывать, исследовать to maintain [meIn`teIn] – поддерживать, сохранять inner structure [`strAktSq] – внутренняя структура resolve [rI`zPlv] the deadlock – выйти из тупика arrangement [q`reIGment] – расположение random – случайный, произвольный