giving rise to a great number of isomers. The halogen atoms which are attached to the nucleus are much more firmly held than those which enter in the side-group only. Ammonia and oxidizing agents will attack and eliminate the chlorine of the sidegroup, but not affect that contained in the nucleus. This action of oxidizing agents is so distinctly different as to serve to distinguish whether the chlorine is present in the one position or the other.

SUMMARY OF HALOID SUBSTITUTION PRODUCTS.

C.HCl,

Chloro-ben- CH,Br, Bromo-ben- CHI, Iodo-benzene,

zene, b. p. 133°. CH4Cl, Dichloro

benzenes.

zene, b. p. 154°.

b. p. 185°.

CH4Bra Dibromo- Diiodo-benzenes.
benzenes.

0- 179°, m- 172°, p- 173°. | 0- 224°, m- 219°, p-219°.

CH,Clg, Trichloro-benzenes, three isomers.
CH2Cl4, Tetrachloro-benzenes, three isomers.
CHC, Pentachloro-benzene.

C.Cl, Hexachloro-benzene, m. p. 226°, b. p. 332°.

Modes of Formation.-Chlorine and bromine acting upon benzene readily yield monochloro- and monobromo-benzene. A further substitution takes place especially easily in the presence of iodine. This acts as a carrier of chlorine or bromine, and by its presence aids in the introduction of these elements.

In the homologues of benzene, if the chlorination takes place in the cold, the chlorine enters the nucleus; if at a boiling temperature, it enters the side-group.

The action of phosphorus pentachlorides upon phenols and aromatic alcohols also serves to form the halogen derivatives.

The nitro or primary amido compounds of the benzene hydrocarbons may also be converted into halogen compounds by first

converting them into the diazo compounds, and then boiling these with cuprous chloride :

CH.N=N.Cl =

Monochloro-benzene, C ̧HCl,

CHCl + Ng.

Monobromo-benzene, СHÅBг, are all colorless liquids of peMonoiodo-benzene, C ̧HI,

culiar, aromatic odor. The boiling points have been given in the table.

Benzyl Chloride, C.H. CH2Cl, is formed by the action of chlorine upon toluene at a boiling temperature. When oxidized it yields benzoic acid. This reaction, together with the fact that prolonged boiling with water or potassium carbonate solution changes it into benzyl alcohol, CH.CH2OH, shows that the chlorine is present in the side-group. Colorless liquid, boiling at 179°. It is used on a large scale for the manufacture of benzoic aldehyde (oil of bitter almonds).

Benzal Chloride, CH.CHC, is produced by the further chlorination of boiling toluene, also from benzoic aldehyde by the action of PC15.

It is used in the manufacture of benzaldehyde and benzoic acid. When heated with water or sulphuric acid, or, as is done on a large scale, with water and calcium hydrate, the benzal chloride is changed into the aldehyde, Cн ̧CHC + H2O=CH.CHO + 2HCl.

Benzo-trichloride, CH. CCl, sometimes termed Phenyl-chloroform, is used on a large scale for the manufacture of artificial benzoic acid. For this purpose it is heated with water under

pressure:

CH.CCl + 2H2O

=

CH COOH + 3HCl.

III. SULPHONIC DERIVATIVES.

The sulphonic acids of the aromatic series, like those mentioned under the methane derivatives, contain the monad group HSO, but are much more readily formed. They result from the action of strong or fuming sulphuric acid upon a great variety of aromatic compounds, and play quite an important part in the technical application of the benzene derivatives, and latterly in the manufacture of synthetic compounds of medicinal value. Thus we may have the following classes of sulphonic acid derivatives:

1. Sulphonic acids of the hydrocarbons, as benzene-sulphonic acid, CH,HSO3.

2. Sulphonic acids of the phenols, as phenol-sulphonic acid, CH(OH)HSO,.

3. Sulphonic acids of the amines, as amido-benzene-sulphonic acid, C,H,(NH,)HSOg.

4. Sulphonic acids of the diazo compounds, as diazo-benzeneSO.

sulphonic acid, CH1<N:N>

5. Sulphonic acids of the azo compounds, as azo-benzene-sulphonic acid, CH,.N: N-CH,HSO.

6. Sulphonic acids of the hydrazines, as phenyl-hydrazinesulphonic acid, CH ̧.NH-NH-HSO,.

In general, these sulphonic acids and their salts are very stable compounds, mostly soluble in water, difficultly soluble in alcohol. When fused with caustic potash or soda, they yield phenols and sulphites, C,H,.NaSO, + NaOH= C,H,.OH+Na,SO, When distilled with potassium cyanide, they yield cyanides or nitriles.

When heated with fuming hydrochloric acid or with water under pressure, they regenerate the same hydrocarbons from which they were originally formed, CH. HSO,+H2O=С ̧H ̧ + H2SO1.

Benzene-monosulphonic Acid, CH. HSO,, is obtained by heating benzene with concentrated sulphuric acid. The excess of sulphuric acid is removed by addition of barium or lead carbonate, as both the barium and lead salts of the sulphonic acid are soluble. It forms small, crystalline leaflets, deliquescing in the air, and easily soluble in water. Because of the fact stated above, that when fused with caustic alkalies it yields phenol, it is of importance in the manufacture of synthetic carbolic acid. Benzene-disulphonic Acids, CH1(HSO3)2.—Three isomers are The para compound when fused with caustic alkali yields the diatomic phenol, resorcin. (See Resorcin.)

obtainable.

Toluene-sulphonic Acids, C.H(CH)HSO,, exist in three isomeric modifications.

Xylene-sulphonic Acids, CH(CH),HSO,.-The different isomers of this substance are utilized to separate the isomeric xylenes. (See p. 688.)

The higher homologues of benzene, except hexamethyl-benzene, are all capable of yielding sulphonic acids.

The sulphonic acids of the several classes of benzene derivatives will be mentioned, when of sufficient importance, under the different groups of compounds.

IV. NITRO DERIVATIVES.

By the action of strong nitric acid upon the aromatic hydrocarbons, derivatives in which one or more hydrogen atoms are replaced by the monad group, NO,, are formed. These are mostly soluble in the strong nitrating acid, and are thrown out of solution on dilution with water. Thus, from benzene we have CH2+ NO2.OH=CH.NO2 + H2O, and from toluene CH.CH2+ NO2OH=CH(CH ̧)NO2+ HOH. When more than one such replacement takes place, we distinguish by the names mononitro-benzene, dinitro-benzene, trinitro-benzene, etc.

2

In all these cases the NO, group attaches itself to the nucleus, and not to the side-group. Whether one or more such replacements shall take place is dependent upon the strength of the acid and the length of time of its action.

The nitro compounds are mostly pale yellowish liquids, distilling undecomposed in a current of steam, or, in the case of the higher derivatives, yellowish needles or prisms. They are heavier than water, and insoluble therein, but generally soluble in alcohol, ether, and glacial acetic acid. All the nitro compounds are reduced in acid solution by the action of nascent hydrogen, forming the corresponding amido compounds. This may be effected by the action of tin and hydrochloric acid, stannous chloride and hydrochloric acid, or iron and acetic acid. Thus, CH. NO2+H ̧⇒C ̧H ̧.NH2+2H2O and C ̧H ̧(NO2)2+ H12 =CH(NH2)2 + 4H2O.

12

The nitro compounds are of very great technical importance as necessary steps in the manufacture of aniline and similar amido compounds.

Nitro-benzene, CH. NO2, is formed by adding benzene gradually to well-cooled fuming nitric acid as long as it seems to dissolve. After some standing the nitro-benzene is separated by the addition of water, washed with water and dilute caustic soda, and purified by distilling in a current of steam. It forms a light yellowish liquid smelling strongly of bitter almonds, boiling at 205°, and crystallizing in needles at 3°. It is manufactured on a large scale as a step in the manufacture of aniline, and pharmaceutically, under the name of “mirbane oil," as a basis of toilet preparations, soaps, etc., because of its characteristic odor. It is poisonous.

Dinitro-benzenes, CH(NO), are formed when benzene is added to a mixture of equal volumes of concentrated nitric and

sulphuric acids and the mixture heated to boiling. All three isomers are solid and crystallizable.

Nitro-toluenes, CH(CH)NO.—When toluene is acted upon by strong nitric acid, the ortho- and paranitro-toluenes are formed, the meta compound being absent, although obtainable by indirect means. By boiling with fuming nitric acid the nitrotoluenes are changed into dinitro-toluenes.

Commercial benzene, being always a mixture of benzene and toluene, yields on nitration a mixture of nitro-benzene with the several nitro-toluenes above mentioned.

The presence of a halogen atom replacing an atom of hydrogen in the benzene nucleus does not interfere with the nitration, so that compound derivatives are easily formed. Thus, chloro-benzene is readily nitrated, and yields three isomeric chloronitrobenzenes.

A derivative of this character is trinitrochloro-benzene, CH2 (NO2)3. Cl, which acts like an acid chloride, and corresponds to CH2(NO),OH, the so-called "picric acid" (see Trinitro-phenol, or Picric Acid).

V. AMIDO DERIVATIVES of the BenzenE HYDROCARBONS. In studying these compounds it is necessary to remember that they can be looked at from two points of view: first, as benzene in which one or more atoms of hydrogen of the nucleus are replaced by the group NH,, or as ammonia in which one or more hydrogen atoms are replaced by the radical CН, or similar aromatic hydrocarbon radical. Thus, CH,NH2 is called amidobenzene when looked at from the first point of view, or phenylamine when looked at from the second point of view, and considered as analogous to methylamine, CH,. NH2. Moreover, just as we had secondary and tertiary amines like dimethylamine and trimethylamine, so we have diphenylamine and triphenylamine. We have, also, the classes of monamines and diamines according as one or two molecules of ammonia are taken as the basis of replacement. The amido group NH, can also replace hydrogen of the side-group in the homologues of benzene.

In the accompanying tables we have indicated first the primary amines, which, containing the group NH2, may also be called amido compounds, and then compared with these the secondary and tertiary amines, containing NH and N respectively, combined with aromatic radicals.