H Cl C-Cl; and hence the formula of the isomeric H H H H ethylene chloride is Cl—C—C— Cl, because these are the H H only possible formulae for the molecule C2HCl. But ethylene chloride is produced by the direct addition of chlorine to H H ethylene; hence the formula of ethylene is C-C. H H Two compounds having the molecular composition CH, 365 may exist according to the hypothesis of valency only one actually exists; but derivatives of both, e.g. chlorides, are known. It is possible that the compound the molecule of H which would have the composition H-C-C-H may be H produced; but it is not likely, because the many attempts made to form it have all resulted in the production of the H H isomeric compound C — C. H H This is an illustration of the proposition, that it is not always possible to obtain every one of the isomerides of a given composition the existence of which is indicated by the hypothesis of valency; or, in other words, that the existence of a compound of specified composition is not conditioned solely by the valencies of the atoms which form the molecule of this compound. 366 367 The hypothesis of valency leads to the conception of the molecule as a structure, the parts of which are related to each other in a definite manner. Formulae such as those given in the preceding paragraphs for ethylic alcohol, methylic ether, ethylene chloride, and ethylidene chloride, are called rational or structural formulae; they are contrasted with empirical formulae (CHO and C ̧H ̧Cl2) which express the percentage and atomic composition of molecules. Structural formulae are attempts to summarise the chief reactions of formation and decomposition of compounds in the highly symbolical language of a special hypothesis resulting from the application of the molecular and atomic theory to the chemical phenomena of isomerism. A structural formula may be found for any gasifiable compound the molecule of which is composed of atoms of known valencies; but the structural formula to be of any value must be the outcome of many experiments on the interactions of the compound to which it is given. The value of the formula consists in its suggestiveness of reactions, and in the extent to which it exhibits the analogies between the compound formulated and other compounds. The structural formulae of carbon compounds have been greatly developed. There can be no doubt that the chemistry of these compounds would not have advanced as it has done without the aid of structural formulae; indeed the remarkable predictions which have been made, and verified, regarding classes of chemical changes among carbon compounds afford satisfactory evidence that the conceptions on which structural formulae are based are accurate and well founded. It is generally possible to shew that the characteristic properties of a group of similar carbon compounds are connected with a certain arrangement of some of the atoms in the molecules of these compounds, which arrangement is common to all the members of the group, and can be expressed in a structural formula. Thus, a great many alcohols behave similarly when oxidised; the molecule of each loses 2 atoms of hydrogen thereby producing an aldehyde, and this aldehyde is then oxidised to an acid the molecule of which is composed of the same number of carbon atoms as the molecule of the alcohol. Such alcohols are called primary alcohols. The following formulae give examples of the oxidation of primary alcohols. The study of the reactions of these, and of other, primary alcohols has led to structural formulae in all of which the group H of atoms C—O— H appears. H Thus the structural formulae of the 4 alcohols in the above table are these ; CH.CH,.OH; CH ̧. CH. CH2. OH; CH. CH. CH.CH ̧.OH; and CH, CH. CH,. OH; respectively. [These formulae are shorter than, but have exactly the same meanings as, the more developed formulae; A primary alcohol is sometimes defined as an alcohol the molecule of which is composed of atoms of carbon and hydrogen in union with the atomic group CH,. OH; or it is sometimes said that the molecules of all primary alcohols contain the group of atoms CH,. OH. By these statements we understand that the study of the chemical changes undergone by those alcohols which are classed together as primary, because of their behaviour on oxidation, has led to structural formulae which represent the molecules of these alcohols as always containing at least one atom of carbon directly interacting with 2 atoms of hydrogen and one of oxygen, which atom oxygen also directly interacts with an atom of hydrogen. The examination of the aldehydes obtained by oxidising of 368 369 the primary alcohols has shewn that the best structural formulae which can be assigned to these aldehydes always 'contain the group' C [CHO]; and the structural H formulae given to the acids obtained by oxidising these aldehydes always 'contain the group' C< 0 - H [CO. OH]. For instance, the oxidation of ethylic alcohol to aldehyde and then to acetic acid, is represented thus in structural formulae ;— (1) CH.CH.OH+O=CH.CHO+HẠO. alcohol aldehyde (2) CH.CHO+O=CH.CO. OH. acetic acid. Now suppose a new alcohol is discovered; analyses, and determinations of the spec. gravity of the gaseous alcohol, enable an empirical formula to be given to it. The behaviour of the alcohol on oxidation is now examined; it is found to lose 2 atoms of hydrogen per molecule and to form an aldehyde, one molecule of which then combines with an atom of oxygen and forms an acid. The new alcohol therefore belongs to the class of primary alcohols. The structural formula of the alcohol will therefore, very probably, 'contain the group' CH,.OH. The reactions of the alcohol are studied, and if possible a structural formula is found for it which represents the molecule as containing the atomic group CH,. OH. This formula tells a great deal about the alcohol; for instance it suggests the formulae, and therefore many of the reactions, of the aldehyde and acid which are produced by oxidising the alcohol. In applying structural formulae in this way it is always to be remembered that a compound may be produced which exhibits most of the class-marks of a certain group but which nevertheless does not belong to that group. Thus an alcohol might be formed which oxidised to an aldehyde, and then to an acid containing the same number of carbon atoms per molecule as the alcohol, but which was not a true primary alcohol, and could not be justly represented by a molecular formula containing the group CH,. OH characteristic of the primary alcohols. We have spoken of the molecules of certain classes of compounds as all containing the same group of atoms. This conception of a group of atoms forming part of a molecule, and exerting a definite influence on the properties of the molecule, is of much importance. 5 : 6 The hydrocarbon ethane, CH, interacts with chlorine to form chlorethane and hydrogen chloride; thus C.H. + Cl, = C2H ̧Cl + HCl chlorethane and caustic potash interact to produce ethylic alcohol and potassium chloride; thus C2H ̧CI + KOH = C2H ̧O + KCl: when ethylic alcohol is oxidised ethylic aldehyde is formed, and when this is oxidised acetic acid is produced; thus C ̧H2O+0=C2H ̧0+ H ̧O, and C,H,O + O = C,H,O We already know the structural formulae of most of the carbon compounds taking part in these changes; let us write the equations expressing the changes in structural formulae ;— 2 4 (1) CH. CH, + Cl2 = CH,. CH,Cl + HCl. 2 2 (2) CH. CH CI + KOH CH, CH,. OH + KCl. 3 CH,.CH.OH+O=CH,.CHO + HẠO. (4) CH.CHO+O=CH, . CỎ.OH. All the molecules of these carbon compounds are represented as containing the atomic group CH,. If the formulae are correct, then the group of atoms CH, has remained intact during this series of changes. If the sodium salt of acetic acid is prepared, mixed with solid caustic soda, and heated, we get methane (CH) and sodium carbonate formed; this change is represented in structural formulae thus CH ̧. CO. ONa + NaOH = CH ̧ . H + Na ̧CO ̧. Here again the atomic group CH, has remained undecomposed. A group of atoms which forms a part of several molecules, 370 and which remains undecomposed through a series of reactions undergone by these molecules, is called a compound radicle. The following compounds have been obtained, the passage from one to the other has been effected, and the structural formula given to each is the outcome of quantitative experiments on the methods of preparation and the interactions of each compound :-C,H,. Cl, CH,. Br, C,H,. CH ̧. OH, CH.. CN, CH. NH, CH,. C,H,O,. The group of atoms CH, is therefore an example of a compound radicle. 5 2 5 5 The study of the reactions of acetic acid affords a good example of the meaning of the term compound radicle, and of the use of structural formulae. The empirical formula of acetic acid is C,H,O,. The acid is monobasic; from this we conclude that one of the 4 hydrogen atoms is related to the |
