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LXXXIII.-Substituted Dihydrobenzenes.

A2:4 Dimethyldihydrobenzene.

Part I.

By ARTHUR WILLIAM CROSSLEY and HENRY RONDEL LE SUEUR. WHEN dimethyldihydroresorcin is treated with phosphorus pentachloride, two oxygen and two hydrogen atoms are removed apparently as hydroxyl groups, and these are replaced by chlorine atoms, giving -COrise to dichlorodimethyldihydrobenzene: CMe, CH, C(OH) CH2


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The action of phosphorus haloids on substituted dihydroresorcins is at present being thoroughly investigated, and a detailed discussion of the course of the reaction will therefore be left over for future consideration. It need only be pointed out here that there are two possible formula for dichlorodimethyldihydrobenzene as represented above (I and II).

The position of the two chlorine atoms in this substance can be readily proved, for on oxidation with dilute nitric acid it yields, among other substances, 3: 5-dichlorobenzoic acid,

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and on boiling with dilute sulphuric acid, dimethyldihydroresorcin is regenerated. Presumably this is a case of simple hydrolysis, whereby the dienolic form of dimethyldihydroresorcin is momentarily produced,

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and which at once passes over into the more stable and usual ketoenolic


form, CMe, CH, C(OH)

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Ber., 1894, 27, 3020).

>CH (compare Klages and Knoevenagel,

A similar result follows from the action of halogen acids, for if dichlorodimethyldihydrobenzene be dissolved in glacial acetic acid saturated with hydrogen bromide, dimethyldihydroresorcin or its hydrobromide is obtained, and in glacial acetic acid, saturated with hydrogen chloride, dimethyldihydroresorcin or its hydrochloride is formed, according to the length of time that the substances are allowed to remain in contact.

If, on the other hand, the dichloride be treated with hydrogen 3 I VOL. LXXXI.

chloride in absolute alcoholic solution, the dihydroresorcin, or its -CH2 -CO>CH, is obtained. Presumably this is also a time reaction, the dichloride being first hydrolysed with production of dimethyldihydroresorcin, which is then esterified by the alcohol and hydrogen chloride.

monoethyl ether, CMe, CHC(OEt)

After many experiments, it has also been found possible to decide on the position of the double bonds in dichlorodimethyldihydrobenzene, or, in other words, to establish the fact that this substance is homogeneous and not a mixture of two substances of the formulæ I and II (page 821). When oxidised with nitric acid, with chromic acid mixture, or with potassium permanganate, the two main products are as-dimethylsuccinic and dimethylmalonic acids, a fact which would at first sight seem to prove definitely that the substance is a mixture, dimethylsuccinic acid resulting from the oxidation of a compound having formula I:





and dimethylmalonic acid from a substance having formula II:

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The most satisfactory experiments are those in which potassium permanganate is used, and the authors therefore base their conclusions mainly on them. Dichlorodimethyldihydrobenzene is attacked only very slowly by potassium permanganate in the cold, and at first the oxidations were carried out by boiling the solution, when the results arrived at were those just indicated. If, however, the oxidation is allowed to take place at the ordinary atmospheric temperature, dimethylsuccinic acid is the sole oxidation product, and although careful search was made for dimethylmalonic acid, not a trace could be isolated.

This is strong evidence in favour of the supposition that the dichlorodimethyldihydrobenzene is homogeneous and has the formula

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The formation of dimethylmalonic acid when heat was employed during the oxidation is therefore probably due to the further oxidation of dimethylsuccinic acid at the moment of its production.

Moreover, if dimethylmalonic acid resulted from the oxidation of a substance of formula II, then malonic (or acetic) acid should also have been obtained, but not a trace could be detected.

Lastly, there can be no doubt that the usual form in which dihydroresorcins exist is the ketoenolic form, for example,



and if further proof of this were needed, it is to be found in the action of phosphorus trichloride on this substance, which gives rise to a com


pound of the formula CMe, CH, CCP >CH (details of this reaction will be published in a subsequent communication). Presumably, phosphorus pentachloride would act in a similar manner, replacing the hydroxyl group by chlorine, in which case the position of one of the double bonds is assured. The ketone group would then be replaced by two chlorine atoms, and hydrogen chloride eliminated as follows (compare Klages and Knoevenagel, Ber., 1894, 27, 3020):

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It is therefore concluded that the above is the correct formula of dichlorodimethyldihydrobenzene, a conclusion which is further supported by a consideration of the constitution of dimethyldihydrobenzene itself (see below).

When dichlorodimethyldihydrobenzene is treated with sodium in moist ethereal solution, the chlorine atoms are replaced by hydrogen. CH=CHatoms, giving rise to dimethyldihydrobenzene, CM, CH, CH CH, a substance which shows a strong resemblance to the natural terpenes in its odour and general chemical behaviour. It is very readily attacked by potassium permanganate in the cold, and only one single oxidation product could be obtained, namely, as-dimethylsuccinic acid:



CMe, CH, CO_H•


This seems to prove definitely the correctness of the proposed formula for dimethyldihydrobenzene and also that of its dichloroderivative from which it is obtained to the extent of more than 70 per cent. of the calculated quantity.

Towards bromine or hydrobromic acid, dimethyldihydrobenzene behaves as if it contained only one double linking, for it absorbs one molecular proportion of each. In view of the fact that Knoevenagel (Annalen, 1896, 289, 131) described a series of hydrocarbons which he believed to be dihydrobenzenes, but which were shown at a later date (Annalen, 1897, 297, 113) to be tetrahydrobenzenes, every care has been taken to prove that the hydrocarbon described in the present communication is really a dihydro- and not a tetrahydro-benzene.

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