550 CHEMISTRY [ORGANIC. Name of Compound. Rational Formula. CO {NH 2 I NH 2 Urea. H Trimethyl-methane. C Graphic Formula. H O H I II I N C N i i H ^ H i I i H C C C H CH H H H-C-H H Isomerism. It has been mentioned that organic sub stances may have the same percentage composition and molecular formula, and yet exhibit totally distinct chemical and physical characters. To this phenomenon the general term isomerism is applied, and the compounds are termed isomerides, or are said to be isomeric. The differences between isomeric bodies are well explained on the view that such bodies possess different constitutions, or that their elements are grouped in different ways. Constitutional formulas are thus absolutely indispensable for the repre sentation of isomerides. As will be seen subsequently, there can be several kinds of isomerism, but we shall here restrict the term to two classes of cases. (1.) Isomerism proper or Physical Isomei ism. In these cases the substances are identical in their composition, vapour-density, and chemical behaviour, but exhibit dif ferent physical properties. Thus there are several hydro carbons known as terpenes, having the formula C 10 H 10 , which exist in the oils of turpentine, lemon, bergamot, orange, &c., and which exhibit the same behaviour under the influence of chemical reagents, differing only in their odour and action upon polarized light. Again, the empiri cal formula C 4 H G G includes several organic acids (tartaric acid being one of the number) which are physical isomerides differing from one another only in their crystalline form and action upon polarized light. (2.) Chemical Isomerism. In these cases the compounds are identical in composition and in their molecular formula;, but differ in physical properties and chemical behaviour in certain reactions. For instance, by taking one atom of hydrogen from the hydrocarbon propane (C 3 H 8 ) we obtain the radicle propyl (C 3 H 7 ), and if we suppose one atom of hydrogen in marsh gas or methane (CH 4 ) to be replaced by propyl, we get the hydrocarbon tetrane or diethyl H Methane. Now propyl can be written in two different ways, according as the elements are grouped differently; thus (CH 3 CH 2 CH 2 ) , or (CHCH 3 C H 3 ) . The differences are still more strikingly shown by the use of graphic formulas H H H H H H H C C C- H H H Propyl. H C-C C H I I I H H Tseudopropyl. Distinguishing these radicles from each other by the names propyl and pseudopropyl, it is clear that we can have a tetrane containing propyl and another containing pseudopropyl CHCH 3 CH 3 H Tetrane or propyl-methane. Pseudopropyl methane. These formulas can, of course, be written in a more con densed form ; thus CH 3 .C 3 H 7 CH 3 .CH(CH 3 ) 2 3 O i Tetrane or propyl-methane. A a/ i > Methyl-pseudopropyl. or (C a H 5 ) 3 Tetrane or diethyl. Ti CH(CH 3 ) 3 . methyl-methane. The the graphic formulae help to show still more clearly that elements can be grouped only in two different ways H H H H H C H H C C C H H C H ll H H C H H C H H C H H H Tetrane or d ethyl. Trimethyl-methane. The fact thus shown possible by formulation is borne out experimentally. Two tetranes actually exist, one, which from its mode of formation can be shown to be propyl-methane or diethyl, having a boiling point of 1 C., the other, which can be shown to be pseudopropyl-methane, boiling at - 15 C. The experimental confirmation does not, however, end here. The same isomerism can be shown to exist among all the derivatives of these two propyls. Thus we have NORMAL SERIES. ISOMERIC SERIES. Name of Compound. Formula. Boiling point. Specific gravity Name of Compound. Formula. Boiling point. Specific gravity. Hexane or dipropyl C 3 H 7 .C 3 H 7 70 669 at 16 Di- pseudopropyl ( CH(CH 3 ) 2 j CH(CH 3 ) 2 58 67 at 17 Propyl chloride or chloropropane C 3 H 7 C1 46-5 915 at Pseudopropyl ) chloride " CH(CH 3 ) 2 C1 39 874 at 10 Propyl bromide or ) bromopropaue C 3 H 7 Br 71 1-35 at 16 Pseudopvopyl ) bromide ) CH(CH 3 ) 2 Br 61 1-32 at 13 iouopropane C 3 H 7 I 102 1-76 at 16 Pseudopropyl iodide CH(CH 3 ) 2 I 89-5 1-70 at 15 Propyl alcohol or ) ethyl carbinol C 3 H 7 .OH 97-4 806 at 15 Pseudopropyl al- ) cohol or dimethyl > carbinol ) CH(CH 3 ) 2 .OH 82-85 786 at 16 Propylamine or ) amidopropane C 3 H 7 .NH 2 49-5 728 at Pseudopropylamine CH(CH 3 ) 2 .NH S 32 69 at 18 Butyric acid C ; ,H 7 .COOH 162-5 982 at Pseudobutyric acid CH(CH 3 ) 2 .COOH 154
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