560 CHEMISTRY [ORGANIC. thus furnishing a good illustration of physical isomerism (see p. 550). They are generally clear, highly refractive liquids, possessing characteristic odours, and easily poly merized by the action of heat, H 2 SO 4 , BF 3 , &c. Several polymerides exist ready formed in oils of copaiba, cubebs, &c. The terpenes are characterized by the extreme readi ness with which they undergo isomeric changes under the influence of reagents. They act as unsaturated compounds combining with at most four monad atoms. Thus, they combine with HC1, forming such compounds as C 10 H 17 C1, C 10 H 1S C1 2 , and some terpenes can be made to combine with water, forming such bodies as C 10 H 20 O 2 , C 10 H 18 O, and C 20 H 34 O, which, in certain respects, resemble alcohols. The best known terpene is turpentine oil, which is obtained from the resinous exudation of certain French and Ameri can species of Pinus and Abies. Camphor (C 10 H 16 O) is an oxidized compound closely allied to the terpenes contained in the leaves of Laurus Camphora. The relationship of the terpenes to the group of aromatic hydrocarbons is shown by the production of cymene (C 10 H 14 ), a member of the C n H 2n _ 6 or benzene series, from turpentine oil and camphor, by the action of certain re agents. Fifth Series, C n H 2n _ 6 . Benzene or Aromatic Series. The general characters of these hydrocarbons, as com pared with the preceding series, have been previously con sidered (p. 552). Occurrence. Small quantities of these hydrocarbons are found in petroleum, but the chief source is the tar obtained in the destructive distillation of coal for the manufacture of coal-gas. General Method of Synthetic Formation. By heating a mixture of a mono-iodated paraffin and a brominated ben zene hydrocarbon with sodium : (C n H 2n+1 I) m + C n H 2n _ (6+m) Br m + Na 2m = CJV^ + (Nal) m + (NaBr) w . Formulation and Isomerism of Benzene and its Deriva tives. Like the paraffins, the C,JH 2n _ 6 hydrocarbons form a parent series giving rise to a vast number of derivatives, and just as the higher paraffins can be regarded as formed from methane by the substitution of C n H 2n+1 radicles for H, so the homologues of benzene can be considered as derived from this latter hydrocarbon by similar substitu tions. Thus Benzene. C 6 H 5 .CH 3 = C 7 Methyl benzene. Ethyl benzene. j.CgHf = C 9 H 12 Propyl benzene, The synthesis of benzene from ethine has already been alluded to (p. 552). In accordance with this mode of for mation, the structural formula of benzene may be written H H
/
C C / 3 H C 1 4C H . ./ c=c / H H This formula, as will be seen on inspection, is in accord ance with the fact that benzene can act as a hexatomic radicle, combining directly (though with difficulty) with 6 halogen atoms. Owing to the complex structure of the ben zene molecule, an immense number of isomers are capable of existing. A few of the most important cases may be here elucidated. Let us, in the first instance, suppose one atom of H to be replaced by another element or radicle. Supposing (as there is at present no reason for doubting) that all the H and C atoms in the molecule have the same property, it is obvious that there can be no isomer of a mono-substi tution derivative, and thus we know only one methyl ben zene, chlorobenzene, nitrobenzene, &c. Passing on to the poly-substitution derivatives, we have several possible causes of isomerism. Thus, the H atom maybe replaced by one radicle, or more than one H atom may be replaced by an equivalent number of radicles, furnishing the same total number of C and H atoms. For instance H H I I C C H (/ C H
/
H II C C H C
C H
c=c CH 3 CH 3 Hf TIT 26 Dimethyl-benzone Is metameric with Ethyl-benzene. Similarly diethyl-benzena is isomeric with tetramethyl- benzene, &c., &c. Another cause of isomerism is to be found in the C,,H 2n+1 or other radicles replacing the H in the benzene molecule, since these radicals themselves admit of isomeric modifications, as has already been seen when the paraffin series were discussed. Thus, we may have C 6 H 5 .C 3 H 7 and C 6 TT 6 .CH(CH 3 ) 2 . Propyl-benzene Tseudo-propyl-benzene. With regard to substitution derivatives of the homologues of benzene, the H atoms may be replaced either in the benzene molecule or in the radicle ; thus C 6 H 4 C1 . CH 3 C 6 H 5 . CH 2 C1 ilethyl-chlorobenzene or 1 , ,*, w ui, fChloromethyl-benzene monochlorotolucne j 13 or benzyl chloride. Another cause of isomerism depends on the relative posi tions of the replaced H atoms of the benzene molecule. Thus, there can be only one of each mono-substitution derivative, because it is immaterial which of the six H atoms is replaced. Di-derivatives containing two atoms of the same element or radicle are capable of three isomeric modifications, according as the replaced atoms are those numbered in the graphic formula 1 2 or 1 3 or 1 4. All other arrangements will be found on trial to be reducible to these (X representing the substituted element or radicle) Benzene. Ortho-di-derivative. Mcta-di-derivative. Para-di-derlvatlve. (LHHHHHH C.XXHHHH C B XHXHHH C,XHHXHH 23156 123156 1 3166 123156 In like manner there can be three tri- or tetra-derivatives, and only one penta- or hexa-derivative, when the H atoms are replaced by the same element or radicle. When the H atoms are replaced by three or more different elements or radicles the possible number of isomerides is greater than when replaced by the same number of similar elements or radicles, because the arrangements of the latter with regard to one another are then capable of variation. Thus, there can be only three di-derivatives containing two different elements er radicles XY, because the reversal of their order does not affect the relative posi tions of X to Y. But when three H atoms are replaced by XXY elements or radicles we may have, for every iso- meride depending on the relative position of XXY to one another and to the remaining II atoms, corresponding iso merides depending on the order XYX. The same reason ing applies to the higher derivatives.
The following is a list of the hydrocarbons of this series :