In those cases where the fresh substituting group or element was identical with har aiready occurring in the di-derivative the conseentive tri-ierivative 1:2 : 3 =

could be obtained from both the ortho

I

and meti

di-derivatives; the unsymmetrical or 1:2:4 tri-derivative

X

X

di-derivatives; and, finally, the symmetrical tri-derivative

X

can only be obtained from the meta di-derivative

In those cases, on the other hand, where the third replacing group is different from that occurring in the di-derivative, and where, therefore, six tri-derivatives are possible, these bodies would be all different. For instance, taking the nitro-dibrom benzenes :

This has been carefully worked out for the dibrom-benzenes, and it is further found that dibrom-benzene which gives only a single tri-derivative can be converted by simple means into terephthalic acid, and similarly that the dibrom-benzene giving rise to three tri-derivatives is convertible into isophthalic acid, thus fully corroborating the previous ideas that terephthalic acid was a para (1.4) compound, isophthalic a meta (1.3) compound, and leaving the ortho (1.2) as the only position for phthalic acid.

565

BENZENE AND ITS SUBSTITUTION DERIVATIVES BY CARBON FREE RADICALS.

BENZENE, CH6.

955. Benzene is obtained synthetically by passing acetylene through tubes heated to whiteness (§ 947, 1), and this probably explains its occurrence amongst the fluid products of the dry distillation of many organic bodies. Many dense hydrocarbons and homologues of benzene also yield that body on exposure to a very high temperature, which may possibly account for its presence in coal-tar oil along with many of its homologues, with phenols, aniline, pyridine bases, and solids like naphthalene, anthracene, phenanthrene, &c.

Chemically pure benzene may be prepared by carefully distilling calcic benzoate or benzoic acid with quick-lime:

CHş.CO.OH + CaO = CaCOg +CH,

and similarly by distilling the polybasic benzoic acids with excess of calcic oxide, e.g.

C6H3(CO.OH)3 + 3CaO = 3CaCO3 + C6H6;

also by passing its hydroxyl substitution derivatives over strongly heated zinc dust:

CH.OH + Zn ZnO + C6H6.

On the manufacturing scale benzene is always obtained from the coaltar oil boiling below 100°.

Coal tar is separated by distillation into three principal fractions. 1. Light oil is lighter than water, principally contains benzene and its homologues.

2. Heavy oil: sinks in water and consists principally of phenols and volatile bases and naphthaline.

3. Solid hydrocarbons.

The light oil is treated in rotation with sulphuric acid, strong potash solution, and water to remove bases and phenols, and the remaining oily portion fractionally distilled several times; the portion between 80° and 90° is then treated with about of its weight of concentrated sulphuric acid, and digested with this for some hours in order to remove hydrocarbons of the CnH2n and CH series. It is then washed with water, and the treatment repeated if strong sulphuric acid causes any blackening when warmed with it. It is again fractionally distilled and the portion under 90° submitted to a freezing mixture, when the benzene crystallises. The

2n - 2

crystals are pressed at a low temperature to remove liquid homologues and the crystallisation repeated several times.

At the ordinary temperature benzene is a mobile, colourless liquid, of peculiar odour, quickly causing headache if breathed, of sp. gr. 899 at 0°. It forms large fern-shaped crystals, which melt at 5.5°. It boils at 80-5° and has a vapour density of 2-675. It is scarcely soluble in water, to which, however, it imparts its odour, but is soluble in alcohol and ether in all proportions. On gently heating it dissolves in 506 times its bulk of sulphuric acid. Benzene is largely used as a solvent for caoutchouc and guttapercha, resins, fats, &c.; it also dissolves phosphorus, sulphur, iodine, and many organic bodies. It evaporates very rapidly even at a low temperature, and burns in air with a highly luminous, smoky flame. Benzene vapour passed through a red-hot tube yields diphenyl and hydrogen :

2CH=CH5.C6H5 + H2;

paradiphenyl-benzene, and small quantities of other products if the benzene be slightly impure. It forms an explosive compound with potassium when heated with it to 250°.

Most oxidising agents completely oxidise benzene to carbonic anhydride and water. Chromylic chloride, CrO2Cl2, however, gives in acetic solution a trichlor-quinone, CHC1302. When gently heated with sulphuric acid and manganic oxide, carbonic and formic acid are produced, and very small quantities of benzoic and phthalic acids. By heating for a long time to about 300° with concentrated fuming hydric iodide containing a small quantity of free iodine, it is converted into hexane, CH14.

ADDITION PRODUCTS OF BENZENE.

956. On exposing a mixture of benzene with bromine to sunlight, or on passing excess of chlorine gas into benzene, also in sunlight, two kinds of products are formed, addition products (§ 947, 2) and substitution products.

6

Of the former the compounds CH.Br, and C6H6C are the end products of the action of these two reagents. Intermediate bodies, C6H6Br2, C6H6Br4, &c., no doubt exist in the crude product, but have not been isolated with certainty.

The bodies CH,Bre and C&HCl, are colourless crystalline solids, insoluble in water, which decompose on heating:

C&H&Bre=3HBr + C6H3Br3.

This reaction takes place with still greater ease when they are heated with alkalies (§ 471).

Benzene hexachloride, C6H6Cl, melts at 157°, dissolves in warm alcohol, ether, and benzene, from which it crystallises in large needles. Benzene hexabromide, CH¿Bг6, cannot be melted without decomposition, and is much more insoluble than the chlorine compound.

By the action of a cold aqueous solution of chlorous acid on benzene for some days, a

Benzene trichlor trihydrate, C6H6C13(OH)3, is produced. It is extracted from the mixture by shaking with ether, and forms colourless plates melting at 10°, nearly insoluble in water, but dissolving in

most other solvents. On warming with dilute sodic hydrate solution it passes into

Phenose, benzene hexahydrate, CH(OH), an amorphous hygroscopic body, which gives secondary hexyl iodide, C6H13I, on heating to 120° with hydric iodide.

MONO-SUBSTITUTION DERIVATIVES OF BENZENE OR
PHENYL COMPOUNDS.

957. Haloid Derivatives.-Chlorine and bromine displace hydrogen in benzene at the ordinary temperature, especially in the presence of iodine. The first products, C6H5Cl and C6H5Br, readily pass into the higher haloid bodies, from which they require to be separated by distillation. Iodo-benzene can only be obtained by the simultaneous action of iodine and iodic acid on benzene :

5C6H6 +212 + HIO3 = 5C6Н¿I + 3H2O.

The reaction requires a temperature of about 200°. All the monohaloid compounds may be easily obtained by the action of haloid phosphorus compounds on phenol :

C6H5OH + PC15 = POC13 + HCl + C¿H¿Cl.

The reaction :

is

C&H5.OH + HCl = H2O + C6H5Cl,

very difficult to obtain and requires a temperature of at least 200°. Monochlor-benzene, phenyl chloride, CH,Cl, is a colourless liquid; it boils at 132°, sp. gr. 1·128 at 0°.

Monobrom-benzene, phenyl bromide, CH,Br, boils at 154°, sp.

gr. 1.517.

1.69.

Moniod-benzene, phenyl iodide, CH,I, boils at 185°, sp. gr.

Monofluor-benzene, phenyl fluoride, CH,Fl, obtained by distilling fluobenzoic acid with lime, melts at 40° and boils a little above 180°.

Oxygen Derivatives.

958. Hydroxy-benzene, phenyl hydrate, phenol or carbolic acid, CH5.OH, occurs in heavy coal-tar oil, in castor oil (castoreum), and in the urine of herbivorous animals.

It is obtained from heavy tar oil by shaking with strong soda solution, in which it dissolves to forni the compound C6H5.ONa. The aqueous layer is separated from the neutral oils, and the phenol precipitated by neutralising with hydric chloride. The crude product, containing cresol and other phenols, is then dried by means of calcic chloride and distilled. The fraction obtained between 180° and 190° crystallises on cooling, the crystals being separated by pressure and filtration from the portions remaining in a liquid state.

Pure phenol forms large colourless prisms, which melt at 40°, have a peculiar smoky odour and burning taste, dissolve in 15 parts of water at the ordinary temperature and in all proportions in alcohol and ether. It boils at 183° and has sp. gr. 1·08 at 0°. It acts very powerfully on the skin, completely destroying it, and acts as an ener

getic poison to plants and animals, germs, &c., and for this reason is largely employed as a disinfectant and antiseptic to prevent decomposition and putrefaction in organic substances. The efficacy of the meat-smoking process depends essentially on the presence of small quantities of phenol in the smoke.

An aqueous phenol solution gives a beautiful violet reaction with ferric chloride and a white precipitate of tribrom-phenol, CH,Br3.OH, even in very dilute solutions when mixed with bromine water.

959. Phenylates.-These bodies are the metallic derivatives of phenol; they correspond to the alkylates, and are derived from phenol by the action of basic oxides and hydrates. They are decomposed even by carbonic acid.

The alkaline phenylates are obtained by simply dissolving phenol in the alkaline hydrate solution. They are white crystalline bodies, soluble in alcohol and ether. Sodium phenylate, С¿Í ̧.ОNa, is more soluble than the potassic compound. Baric and calcic phenylates are also crystalline soluble compounds.

960. Phenyl ether, oxydibenzene, diphenyl oxide, CH5.O.C6H5, is obtained by mixing diazo-benzene sulphate (§ 977) and phenol, and together with other products by the dry distillation of cupric benzoate. It crystallises in long colourless needles, melting at 28° and boiling at 246°, and is soluble in alcohol and ether, but not in

water.

By heating potassic phenylate with alkyl iodides, the alkyl phenyl ethers are formed:

CH5.O.K+IC2H2+1 = KI + C6H5.O.CnH2n+1

They are all colourless liquids, insoluble in water, and which may be distilled. On heating with strong hydric iodide to 130° they are reconverted into alkyl iodide and phenol.

Methyl-phenyl-ether, anisol, CH3.O.C6H5, is prepared by distilling anisic acid with baric hydrate :

Ethyl-phenyl-ether, C6H5.O.C2H5, boils at 172°. Isoamyl-phenylether, CH11.0.C6H5, boils at 225°.

Ethylene-diphenyl-ether, CH,(O.C6H5)2, obtained from ethylene dibromide and sodium phenylate, melts at 95°.

961. Ethereal Salts of Phenol.-Acids do not yield salts with phenol directly, but salts may be obtained from the anhydrides and acid chlorides.

6 5/2

Phenyl phosphates, C6H5.H2PO, and (CH3)2HPO4, are prepared from phosphoric anhydride and phenol. (CH),PO, is obtained by the action of phosphoric chloride on phenol. Acetylic chloride acting on phenol gives phenylic acetate, CH,.O.CO.CH3, a colourless liquid boiling at 193°. Carbonic oxychloride gives a phenylic carbonate, (CHO),CO, forming shining colourless needles, melting at 78°.

Sulphur Derivatives.

962. Hydro-thiobenzene, phenyl mercaptan, CH5.SH, prepared