Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/561

Rh ORGANIC.] CHEMISTRY 549 It was shown by analysis that 197 5 parts of platinum represented 614 5 parts of the double salt, so that Making use of the percentage numbers given by ultimate analysis, we have the following obvious proportions for finding the weights of the respective elements contained in this molecular weight : 100 100 100 101 101 101 71-29 14-85 13-86 Whence C = 72 0, H = 14 9, atoms are : = 13 9, and the numbers of the By the atomic theory these numbers must be integral, so that the numbers of atoms are 6, 15, and 1; and the molecular formula is C 6 H 15 N. When a base does not readily form double platinum salts, the molecular formula is deduced from the analysis of an anhydrous normal salt. In the case of compounds which are neither acid nor basic, and which do not admit of vaporization, the molecular formula can only be indirectly arrived at by considering the chemical transformations of the compound and its relationship to known substances. A molecular formula obtained by this means implies that, could the substance be vaporized, its molecular volume would correspond to the molecular volume of hydrogen. Rational, Constitutional, or Structural Formulae. The molecular formula of an organic compound simply expresses the fact that the molecule of such compound contains so many atoms of each of its constituent elements, and in the earlier stages of the science chemists were contented with such representation of their analytical results. As the science developed, however, it soon became evident that substances might have the same percentage composition, or even the same molecular formula, and yet exhibit under the influence of the same reagents totally distinct characters. These facts, which will be more fully discussed in a sub sequent part of this article, led to the necessity of devising some method by which organic formulae could be made to represent the behaviour of the respective compounds under the influence of decomposing agents in other words, the manner in which the compound was capable of splitting up or of being resolved, and, as a necessary result, the converse fact of representing the manner in which the elements of a compound were grouped together. These rational, consti tutional, or structural formulae must be regarded solely from a chemical point of view ; they are symbolic repre sentations of chemical facts, and in no way represent the physical grouping of the atoms in space. They may be most conveniently defined as artificial epitomes of the re actions of compounds, indicating that when decomposed the compounds separate into such and such groups, and that when it is possible to bring these groups or radicles together, the compound can in most cases be built up or synthesized. Let us now, by way of illustration, proceed to consider the method of arriving at the constitutional formula of some typical compound. The molecular formula of acetic acid, as previously shown by its ultimate analysis and the determination of silver in its silver salt, is C 2 H 4 2 - Being a monobasic acid, one of its hydrogen atoms is replaceable by metals. This fact is expressed, as in the case of inorganic acids, by the formula H.C 2 H 3 O 2. But this formula does not express the whole of the decompositions possible to the acid ; the residue C 2 H 3 O 2 being capable of further subdivision, the formula may be further developed. Thus, acetic acid may be formed by the action of acetyl chloride upon water, according to the reaction C 2 H 3 O.C1 + OH 2 = C 2 H 3 2 .H + HC1. Thus the radicle acetyl C 2 H 3 is shown to enter into the composition of acetic acid, and the formula therefore be- H.O.C 2 H 3 0. In confirmation of this formula several reactions might be mentioned in which the acetyl group is left unchanged, while the hydroxyl, HO, is withdrawn and replaced by other elements or radicles. For example H.O.C 2 H 3 O + PCL = C 2 H 3 O.C1 + HC1 + POC1 3 Acecacid. *&quot; Acety, chloride. Hydr^loric ngjhn^ 5(H.O.C 2 H 3 O) + P 2 &amp;gt;S 5 = 5(H.S.C 2 H 3 O) + P 2 5 Phosphorus Thiaretic acid Phosphoms pentasulphide. &quot;UacettC acid. pentoxide. Na.O.C 2 H 3 O Sodium acetate. C 2 H 3 O.C1 = C 2 H 3 O.C 2 H 3 O.O Acetyl chloride. Acetic oxide. NaCl. chloride Next with respect to acetyl itself. When acetic acid is electrolyzed, hydrogen is evolved at the positive pole and carbon dioxide and ethane (C 2 H G ) at the negative. Now, ethane can be shown to be identical with di-methyl (CH 3 ) 2 , so that the radicle methyl is thus shown to exist in acetic acid a fact which receives confirmation from several re actions, two of which may be now considered. When potassium cyanide acts upon methyl iodide, a substance known as acetonitrile (CH 3 .CN) is produced CH 3 I + KCN = CH 3 .CX + KI. Methyl iodide. Potassium cyanide. Acetonitrile. Potassium iodide. By heating acetonitrile with water or caustic potash solu tion, acetic acid and ammonia are formed, thus CH 3 .CX + 20H 2 = CH 3 .CO.O. H + NH 3. Acetonitrile. Water. Acetic acid. Ammonia. When barium acetate is submitted to dry distillation, it decomposes in the manner shown by the following equation : Ba.O 2 .(GH 3 .CO) 2 = CO.(CH 3 ) 2 + BaC0 3. Barium acetate. Acetone. J$%EL Thus the most developed formula of acetic acid is CH 3 .CO.O.H, or, as it is more conveniently expressed (CH 3 fCOOH. The bracket signifies that the two carbon atoms are directly united. Graphic Formulae. Graphic formulae having already been explained (see p. 473), it is here only necessary to illustrate their application to organic compounds. The following are typical examples : Name of Compound. Rational Formula Acetic acid. J CH, CO( COOH Triethylamine. N C 2 H Graphic Formula. H H C H 0=C H H H C H H C H H H i H H i i H C C N C C H II II H H H H