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There seem to be two possible explanations of the behaviour of the hydrocarbon towards bromine and hydrobromic acid. First it will be

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noticed that dimethyldihydrobenzene contains the complex CC.C:C,

which, as is well known, exhibits an exceedingly feeble tendency to form additive products (compare Trans., 1901, 79, 379, 1287). Only one mol. of hydrogen bromide would then be added on giving the substance, CH CH

III. CMe2

CH2 CHB

CH2.

>CH or

V. CMe,

CH-CH
CH2 CHBr

>CH,

Secondly, if the above formula be correct, it contains the grouping C:CC:C, with which Thiele (Annalen, 1899, 306, 87) has dealt in his theory of partial saturation. According to this theory, when hydrogen bromide adds itself on to dimethyldihydrobenzene, only one mol. would become attached, and this at the two extreme carbon atoms of the above grouping, thus necessitating a rearrangement of the bonds with production of a substance having one of the following formulæ : CHBr CH IV. CMe, CH-CH, and in order to obtain some evidence on this point the hydrobromide was submitted to oxidation with potassium permanganate. The reaction takes place readily in the cold, and the amount of permanganate required is about one-half of that needed for the oxidation of the hydrocarbon. The products obtained were as-dimethylsuccinic and BB-dimethylglutaric acids, and the lactone of a-hydroxy-ßßdimethylglutaric acid. The formation of the two latter substances effectively disproves the supposition that the hydrobromide is represented by formula III (above), from which (if a glutaric acid resulted) only aa-dimethylglutaric acid could be formed.

Formula IV must also be rejected, for although it readily accounts for the production of the lactone by first supposing that the bromine atom is converted into a hydroxyl group, giving:

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it does not admit of the formation of ßß-dimethylglutaric acid, but only of aa-dimethylglutaric acid,

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From a compound of the formula V, ßß-dimethylglutaric acid and also the lactone may easily result:

CH-CH

CMe, CHỈ CHBr CH

CMe

CH, CO,H
CH, CO,H

As the oxidation proceeds as thus represented, the solution becomes alkaline, and the elements of hydrogen bromide may be removed from the hydrobromide, partial oxidation taking place at the double bond thus formed, giving rise to an intermediate product, which can suffer oxidation in two ways, giving either dimethylsuccinic or hydroxydimethylglutaric acids:

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and moreover it is shown in the experimental part of this paper that the lactone itself, on further oxidation, gives a-dimethylsuccinic acid. It is therefore concluded that the hydrobromide has the formula CH, CH >CH, and is 5-bromo-1 : 1-dimethyl-A3-tetrahydrobenzene, and the evidence on which this is founded seems strongly to favour the supposition that the addition of hydrogen bromide to dimethyldihydrobenzene takes place in accordance with Thiele's

CMe, CH CHBr

suggestions.

The physical properties of the hydrocarbon and its dichloro-derivatives have also been examined, with results deciding in favour of these compounds being substituted dihydrobenzenes.

This work has been done by Dr. W. H. Perkin, senr., to whom the authors desire to express their appreciative thanks for his valuable assistance.

The magnetic rotation of dimethyldihydrobenzene is 11.024, whereas if it were a tetrahydrobenzene it would be about the same as tetrahydrobenzene +2CH,, or

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Again, the refractive value of dimethyldihydrobenzene for Ha is practically identical with that calculated:

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It is also interesting to observe how very different the rotation of a dihydrobenzene is from that of tetrahydrobenzene and of benzene itself, as may be seen from the following comparison:

Dimethyldihydrobenzene. Tetrahydrobenzene +CH, x 2 = 6.393 +2.046

Benzene........

} C2H14

C&H 12

11.024

8.439

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Dimethyldihydrobenzene - CH2 x 2 = } CoHs

11.0242.046.........

The magnetic rotation of dichlorodimethyldihydrobenzene lies between that of dichlorocyclohexane and p-dichlorobenzene, allowing for difference of composition:

Dichlorocyclohexane +CH2 × 2.............

Dichlorodimethyldihydrobenzene

p-Dichlorobenzene + CH, x 2

= 10.976

= 13.377

= 15.701

On comparing the magnetic rotation of dichlorodimethyldihydrobenzene with that of the hydrocarbon, the influence of chlorine displacing hydrogen in this substance is seen thus:

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This is a very low number, but is very similar to that found for p-dichlorobenzene (2.371); the low value is most probably due to the relative position of the radicles, which in this compound as regards the halogen is 1:3, since the rotations of substances of this class are usually very similar to those of para-compounds.

EXPERIMENTAL.

3:5-Dichloro-1 : 1-dimethyl-A: -dihydrobenzene,

(CH2)2C

CH CCI-
➤CH.
CH2∙CCI

CH, CCI

One hundred and fifty grams (1 mol.) of dimethyldihydroresorcin and 450 grams of dry chloroform were placed in a flask attached to a reflux condenser and 450 grams (2 mols.) of phosphorus pentachloride added. A somewhat vigorous reaction sets in and the dimethyldihydroresorcin,

which is not completely soluble in the amount of chloroform used, readily goes into solution, After adding about one-fourth of the phosphorus pentachloride in small quantities, the remainder may be added in one lot, and the whole gently heated on a water-bath until complete solution is effected. The heating is continued more strongly for 1 to 2 hours, when the evolution of hydrogen chloride is practically complete; the chloroform is then evaporated, and the clear yellow liquid product poured into ice-cold water and extracted with ether. The ethereal solution, after shaking with sodium hydroxide solution until alkaline, is washed with water, carefully dried over calcium chloride, the ether evaporated off, and the residue distilled in a vacuum.

The following fractions were collected under 23 mm. pressure : 91-920-133 grams; 92-98° = 35 grams; 98-115°*-5 grams, On redistilling the fraction 92-98°, a considerable quantity was found to boil between 91° and 92°, and on finally redistilling this fraction 155 grams (82 per cent. of theory) were obtained of constant boiling point. On analysis, the following numbers were obtained:

0.1580 gave 0.3120 CO, and 0.0788 H2O. C=53·86; H=5·54. 0.2026 required 0-3915 AgNO.† Cl=40·34.

CH1Cl2 requires C-54.23; H-5-65; Cl-40·11 per cent.

10

3:5-Dichloro-1 : 1-dimethyl-A2:4-dihydrobenzene is a colourless, mobile, highly refractive liquid boiling at 92° under 23 mm. pressure, and having a sp. gr. 1·1394 at 15°/15°. It is volatile with steam, but not without slight decomposition. It has a sharp, although not very pronounced, aromatic odour, resinifies slowly on exposure to air, and cannot be kept for very long, even in well-stoppered bottles, as it slowly decomposes, turning yellow and evolving hydrogen chloride. On heating with 20 per cent. sulphuric acid, it is slowly reconverted into dimethyldihydroresorcin.

Action of Bromine.-Two grams of dichlorodimethyldihydrobenzene were dissolved in dry chloroform and a solution of bromine in chloroform slowly added in the dark. The colour of the bromine remained permanent when 1.8 grams had been added, which corresponds with the formation of a dibromide, but this compound could not be isolated in a pure state owing to the ease with which it undergoes decomposition with evolution of hydrogen bromide.

Action of Halogen Acids.-Dichlorodimethyldihydrobenzene was mixed with excess of a solution of hydrogen bromide in glacial acetic acid, when, on shaking, the former gradually went into solution, and on

* Evidence has been obtained of the presence of other substances in this higher fraction, and the results will be communicated shortly.

†The volumetric method for the determination of halogens as recommended by Walker has been used in all cases mentioned in this investigation.

standing crystals separated which were found to melt at 168° with decomposition and evolution of gas. This substance was proved to be identical with the hydrobromide of dimethyldihydroresorcin previously described (Crossley, Trans., 1899, 75, 776). If, however, the reaction be interrupted, as soon as the dichlorodimethyldihydrobenzene has gone into solution, by pouring the whole into ice-cold water, dimethyldihydroresorcin separates out in theoretical amount.

Dichlorodimethyldihydrobenzene is also acted on by solution in glacial acetic acid saturated with hydrogen chloride, and on allowing the solution to stand crystals formed, which were separated, washed by decantation with cold acetone, dried on bibulous paper, then in a vacuum for a short time, and the chlorine determined:

0.1180 required 0·1113 AgNO. Cl=19.69.

CH12O2, HCl requires Cl 20·11 per cent.

=

Dimethyldihydroresorcin hydrochloride crystallises in large, rhombic plates which, when heated in a capillary tube, soften at 105°, partially melt and give off gas at 126°, and melt to a clear yellow liquid at 131°. It is a very unstable substance, which could only be obtained crystalline in the above-mentioned manner. On solution in other solvents, hydrogen chloride is given off, and dimethyldihydroresorcin crystallises out.

A specimen of the hydrochloride made by dissolving dimethyldihydroresorcin in glacial acetic acid saturated with hydrogen chloride showed all the above properties.

Dichlorodimethyldihydrobenzene is also acted on by hydrogen chloride in alcoholic solution. Two grams of the dichloride were dissolved in 15 grams of absolute alcohol, and after saturating with hydrogen chloride allowed to stand for 44 hours. The whole was then poured into water, extracted with ether, &c., when 2 grams (calc. 2.2 grams) of an oil were obtained, distilling at about 250° and solidifying on cooling. The solid was boiled with light petroleum (b. p. 40-60°), filtered from a very small amount of solid (dimethyldihydroresorcin), when large crystals separated which melted at 58-59°, and proved to be the monoethyl ether of dimethyldihydroresorcin (compare Crossley, Trans., 1899, 75, 775).

Densities, Magnetic Rotation, and Refractive Values of Dichlorodimethyldihydrobenzene.

The following physical data were determined by Dr. W. H. Perkin,

sen.:

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Densities: d 4°/4° 1.1493; d 10°/10° 1.1435; d 15°/15° 1·1394; d 20°/20° 1.1357; d 25°/25° = 1.1318.

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