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- 0 — H appears. H Thus the structural formulae of the 4 alcohols in the above table are these ; CH, CH,.OH; CH ̧. CH. CH,. OH; CH ̧.CH,.CH.CH.OH; and CH ̧.CH.CH,. OH; respectively. 2 [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 of oxygen also directly interacts with an atom of hydrogen. The examination of the aldehydes obtained by oxidising the primary alcohols has shewn that the best structural formulae which can be assigned to these aldehydes always H 'contain the group' C[CHO]; and the structural formulae given to the acids obtained by oxidising these aldehydes always 'contain the group' C by 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+H,O. alcohol aldehyde (2) CH,.CHO+O=CH.CO.OH. acetic acid. 368 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. 369 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 CH2. 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. 6 6 The hydrocarbon ethane, CH, interacts with chlorine to form chlorethane and hydrogen chloride; thus C.H, + Cl2 = C2H ̧Cl + HCl : chlorethane and caustic potash interact to produce ethylic alcohol and potassium chloride; thus CHCI + KOH = C2H2O + KCl : when ethylic alcohol is oxidised ethylic aldehyde is formed, and when this is oxidised acetic acid is produced; thus C.HO+O=C,H,O+ H2O, and CHO+0=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 ; (1) CH. CH2+ Cl2 = CH ̧. CH ̧Cl + HCl. 2 (2) CH. CH,Cl + KOH = CH2. CH ̧ . OH + KCI. CH,.CH.OH+O=CH,.CHỎ + H,O. 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. 2 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, C.H,. Br, C,H,. CH. OH, CH. CN, CH. NH, CH. CHO. The group of atoms CH, is therefore an example of a compound radicle. 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 CHO2. The acid is monobasic; from this we conclude that one of the 4 hydrogen atoms is related to the rest of the molecule differently from the other 3 hydrogen atoms; we therefore adopt the formula C,H,O,. H. Phosphorus pentachloride interacts with acetic acid; the interaction consists in the removal of one oxygen and one hydrogen atom from the molecule CHO, and the putting of one chlorine atom in their place; this interaction is thus expressed in an equation, C2H2O2+PC1=CH ̧OC1+POCl2+HCl. From this we conclude that one of the oxygen atoms in the molecule C,H2O, directly interacts with an atom of hydrogen, and that the relation of this oxygen atom to the rest of the molecule is different from that of the other oxygen atom to the rest of the molecule; we therefore adopt the formula CH2O. OH, and we express the interaction with phosphorus pentachloride thus, 4 2 3 3 C2H2O. OH + PC11 = C2H2O. Cl + POCI ̧ + HCl. 3 If this interpretation of the mechanism of these changes is correct, then the group of atoms C2H2O is a compound radicle; this group remains unchanged when acetic acid interacts with phosphorus pentachloride, and it is common to the 2 molecules CHO. OH and CH2O. Cl. If acetic acid has the formula CHO.OH the formula of sodium acetate is C,H,O. ONa : when this salt is mixed with solid caustic soda and the mixture is heated, sodium carbonate (Na,CO,) and methane (CH) are produced. Therefore in this change the atomic group CHO is decomposed; one of the carbon atoms and the oxygen atom are removed, and the remaining CH, combines with an atom of hydrogen to form CH. The change is most simply represented thus, CH,. CO. ONa + NaOH = Na,CO2+CH ̧H. Because of this reaction we adopt for acetic acid the structural formula CH,. CO. OH; and we say that the molecule of this acid is composed of the compound radicle CH, combined with the other compound radicles CO and OH. The Had we stopped this investigation after examining the interaction between PCl, and CHO̟2, we should have given 4 to acetic acid the structural formula C,H,O. OH, and we should have said that the molecule of this acid is composed of the compound radicles C,HO and OH. Further investigation however obliges us to modify this conclusion, inasmuch as it shews that the atomic group C,H2O is itself composed of the simpler groups CH, and CO. The formula CH. CO. OH expresses all that is expressed by the formula CH ̧O. OH, and it also suggests the interaction in which methane is produced from sodium acetate. 3 3 It appears then that a compound may have more than 371 one structural formula; that formula is the best which tells most about the characteristic reactions of the compound. In Chap. XI. pars. 210 and 211 we glanced at the reactions of compounds of ammonia, NH,. We found that these reactions were analogous to those of the alkali potash, KOH; to bring out these analogies we wrote the formulae of the compounds produced by the interaction of an aqueous solution of ammonia with acids as compounds of the hypothetical compound radicle ammonium, NH. The interpretation of these reactions given by the molecular and atomic theory is that in the molecule of an ammonium compound e.g. NHCl, NH.NO,, (NH)SO, (NH)CO, &c. we have always direct interaction between an atom of nitrogen and 4 atoms of hydrogen; in other words, we have the compound radicle or atomic group, NH. As we speak of the valency of an atom in this or that molecule, meaning thereby the number of other atoms with which the specified atom directly interacts in the molecule, so we speak of the valency of an atomic group or compound radicle in a molecule. The atomic groups CH,, CH, CH, &c. are monovalent; the group CH2OH is also monovalent; the group CO is divalent; and so on. In par. 357 it was mentioned that an atom which combines 372 with one divalent atom to form a molecule is often regarded as thereby proved to be itself divalent. Thus the atom of oxygen is divalent because of the existence of the gaseous molecules OC1, and OH,; one atom of carbon combines with one atom of oxygen to form the gaseous molecule CO; therefore, it has been urged, the atom of carbon is divalent in the molecule CO. Again, one atom of carbon combines with 2 atoms of oxygen to form the gaseous molecule CO,; therefore, it is said, the atom of carbon is tetravalent in M. E. C. 17 |
