gas through the bromine contained in the long limb of the bent glass tube in fig. 22.

As soon as, by either of these methods, the bromine is nearly saturated with ethylene, shown by its being nearly decolorised, the product is shaken with dilute solution of alkali till colourless, the under layer separated from the aqueous solution, and, after mixing with concentrated sulphuric acid to fix the water, distilled on a sand bath, when pure ethylene dibromide passes over.

It is a mobile, colourless, sweet-smelling liquid of sp. gr. 2.163; it solidifies at 0° to colourless crystals, melting at +9°. The boiling point is 129°. By alcoholic solution of potassic hydrate it is converted into monobrom ethylene, CH,:CHBr (boiling point 23°), from which a series of brominated ethanes and ethylenes can be prepared, similar to the chlorine derivatives (§ 473).

Ethylene dibromide can be regenerated from monobrom ethylene, by heating in closed vessels to 100°, with a solution of hydrobromic acid saturated at +6°:

Monobrom ethylene unites more slowly, and apparently in quite a different manner, with dilute hydrobromic acid. If the acid saturated at +6° be diluted with one-third of its volume of water, and then employed, ethylidene dibromide is the main product :

CH2I

Ethylene unites

with iodine when exposed to light or on gentle heating, especially if the iodine employed be somewhat damp. The compound is solid, crystallising in colourless silky needles or prisms, which melt at 75°, and on stronger heating decompose into ethylene and free iodine. This change also occurs at ordinary temperatures, though very slowly.

Ethylene diiodide can only be sublimed or preserved colourless in vessels filled with ethylene gas.

Alcoholic potassic bydrate decomposes ethylene diiodide very readily. Only a little moniodo-ethylene, CH,:CHI, is obtained, as a colourless oil, boiling at 55°, the greater part being converted into acetylene, C2H2:

Ethylene chloro-iodide, C2H,ClI = CH2Cl.CH2I, can be prepared by the action of ethylene on iodine chloride, and by decomposition of ethylene diiodide with a little mercuric chloride :

2C2HI2+ HgCl2 = HgI2 + 2C2H ̧CU.

=

It is a heavy liquid of sweet odour, boiling at 137°-138°. Ethylene brom-iodide, C,H,BrI CH,Br.CH,I, is formed when hydriodic acid (saturated at +4°) is heated at 100° with brominated ethylene, and when iodine bromide is saturated with olefiant gas. It is a colourless mass, melting at 28°, which boils at 163° and has sp. gr. 2.7 at + 1°.

Halogen Compounds of Propene.

477. The divalent alcohol radical C3H6 can possess two different constitutions, namely:

CH3

CH2

478. Propylene dichloride, C3H6Cl2 = CH3.CHCI.CH,Cl, is obtained directly from propylene and chlorine gas, and by the action of chlorine gas upon isopropylic chloride in sunlight :

in the latter case obtained together with the metameric methyl chloracetol (§ 445). It is a colourless liquid, boiling at 96°-97°, and has sp. gr. 1.1656 at 14°. By alcoholic potassic hydrate it is converted into monochlor propylene (boiling at 23°):

the same which is formed from acetone by the action of phosphoric

chloride. The chlor propylene, CH3.CH: CHC1, isomeric with this, has not yet been observed.

Propylene dibromide, C3H6Br2 = CH3.CH Br.CH,Br, is formed by the absorption of propylene gas by bromine, and by heating isopropylic bromide with liquid bromine. It is an oil boiling at 142°, having the sp. gr. 1·974. It is converted into monobrom propylene, CH3.CBr.CH, (boiling point 57°-58°), by action of alcoholic potassic hydrate, from which, by heating with hydrobromic acid saturated at +6°, propylene dibromide is regenerated, whilst by employment of more dilute acid, methyl brom-acetol (§ 445), CH3.CBr2.CH3, is

formed.

Propylene diiodide, C3H6I2 = CH3.CHI.CH2I, obtained by direct union of iodine and propylene at a temperature of 50°-60°, is a colourless oil of powerful odour, decomposing on distillation.

Propylene chloro-bromide, CH3.CHВr.CH2Cl, is obtained by heating monobrom propylene with concentrated hydrochloric acid. It boils at 1120-113°, and is reconverted into monobrom propylene by alcoholic potassic hydrate:

CH..CHBr.CH,C1 + KOH = KCl + H,O + CHg.CBr:CH,.

The isomeric propylene bromo-chloride, CH3.CHC1.CH2Br, is ob- · tained by continued boiling of propylene dibromide with mercuric bromide :

2CH3.CHBг.CH,Br + HgCl2 = HgBr2 + 2CH3.CHCl.CH2Br. It boils constantly between 119° and 120°, and yields monochlor propylene on boiling with alcoholic potassic hydrate :

CH3.CHCI.CH2Br + KOH = KBr + OH2 + CH3.CCI: CH2. On heating propylene haloid compounds with water to 210°-220° in sealed glass tubes, there is obtained, together with halogen hydroacid, not, as might have been expected, propylene oxide, formed by the direct replacement of the halogen by an atom of oxygen, but the compounds isomeric with this, acetone and propionic aldehyde. Without doubt in these decompositions it is not both hydrogen atoms of the water molecule that are employed to form the halogen hydroacid, but only one of them, together with a hydrogen atom of the propylene; so that the process proceeds with intermediate phases in the following way:

CH2.CHCI.CH2Cl + HOH = HCl + CH3.CC1:CH2 + HOH
= 2HCl + CH3.CO.CH3,

and CH3.CHCI. CH2.Cl + HOH = HCl + CH3.CH: CHC1
+ HOH = 2HCl + CH3.CH2.CHO.

479. Trimethene dibromide, CH2Br.CH2.CH2Br, is formed by heating allylic bromide with hydrobromic acid (saturated at 0°) for a short time to 100° in sealed tubes. Some propylene dibromide is always formed also. The process can be expressed by the equation: CH,Br

CH2

CH, + (a−b) CHBr

CH2Br
Allylic bromide.

CH,Br
Trimethene bromide.

CH,Br
Propylene bromide.

The isomeric products are separated by fractional distillation.

Scarcely anything but trimethene dibromide is obtained when hydrobromic acid is passsed into allyl alcohol at 0° :

Trimethene dibromide is a colourless liquid insoluble in water, boiling at 160°-163° (about 20° higher than propylene dibromide) and having sp. gr. 2.0177 at 0°. It is reconverted into allylic bromide by alcoholic potassic hydrate:

Trimethene dichloride is obtained, by heating trimethene dibromide for a long time at 160° with mercuric chloride, as a colourless oil boiling at 117°, having at 15° sp. gr. 1.201, and being converted into allylic chloride by alcoholic potassic hydrate:

480. Butene Dihaloids.-As yet only few haloid compounds of the three butylenes (§ 476) are known.

Normal butylene dibromide, or methyl-allyl dibromide:

CH..CH,.CHBr.CH,Br,

boils at 156-159°, but has probably not yet been obtained quite pure. The bromine compound of the butylene obtained by action of zinc diethyl on monobrom ethylene boils at 166°.

Pseudo-butylene dibromide, CH3.CHBr.CHBr.CH3, obtained from butylene and bromine, boils at 159°.

Two halogen compounds of isobutylene are known. The dichloride, (CH3)2:CCI.CH2Cl, boils at 130°, the dibromide at 149°. Similarly to the propylene dihaloids, which yield propionic aldehyde on boiling with water, isobutylene dibromide gives isobutyric aldehyde :

(CH3): CBr.CH2Br + H2O = 2HBr + (CH3)2: CH.CHO. 481. Normal amylene dibromide, or ethyl-allyl dibromide : CH3.CH2.CH2.CH Br.CH2Br,

boils at about 175°.

Isoamylene dichloride, (CH3)2.CC1.CHC1.CH3, is a colourless distillable oil. Isoamylene dibromide, (CH3)2.CBr.CHBr.CH 3, boils with slight decomposition between 170° and 175°, and yields with alcoholic potassic hydrate monobrom amylene, (CH3)2:CH.CBr: CH3, whose boiling point is 112°.

482. The dibromides of the higher members of the series, such as hexylene, octylene, and diamylene, are oily liquids which decompose on distillation. Some of these bodies will be mentioned later.

The Diacid Alcohols or Glycols, CnH2n(OH)2.

483. The methods used for preparing the glycols or dihydrates of the divalent alcohol radicals vary with the nature of these bodies. In most cases the dihaloid compounds are employed, and are first converted into the acetates.

A good yield is obtained by employing the diiodides or dibromides of the olefines, and heating them with argentic acetate in presence of glacial acetic acid :

CnH2nI2+2AgO.C,H,O= 2AgI + CnH2n(O.C2H2O)2.

The diacetate is first purified by distillation, and then saponified by addition of just the requisite quantity of alkali:

CnH2n(O.C2H3O)2 + 2KOH = 2KO.C2H ̧O + СnH2n(OH)2.

Another, if also analogous, way consists in heating the dibromides or dichlorides of the olefines with alcohol and potassic acetate to 100°, until the reaction is complete. There is then formed potassic and ethylic acetates together with the olefine hydrate acetate :

CnH2nBr2+ HOC2H5 + 2KO.C2H30 = 2KBr + C2H2O.C2H2O + CnH2n(OH)(O.C2H2O),

which is also purified by rectification and then decomposed by the requisite quantity of strong basic hydrate, and the glycol separated by fractional distillation.

The double tertiary glycols or pinacones are prepared from the ketones by action of nascent hydrogen.

484. The glycols are mostly syrupy liquids, the lower members mixing in all proportions with water, but, like the alcohols, CnH2n+1.OH, becoming less soluble with rise in molecular weight. They are soluble in alcohol without exception, mostly also in ether. So far as the members of this series have yet been obtained, they distil unaltered and generally taste sweet.

By action of hydrochloric acid gas they are converted into monohydrate chlorides :

CnH2n(OH)2 + HCl = C2H2n {O

2n OH

which by action of phosphoric chloride are converted into the dichlorides:

CnH2n OH+PU1, = POC13 + HCl + CnH2nCl2.

он

Hydrobromic, and especially hydriodic, acids convert the glycols with greater readiness and completely into the respective dihaloids.