Page:EB1911 - Volume 22.djvu/46

acid obtained by heating the anhydride of the trans-acid, forms prisms which melt at 192° C. When heated with hydrochloric acid it passes into the trans-variety. The racemic trans-acid is produced by the reduction of the dihydrobromide of ∆⁴-tetrahydrophthalic acid or ∆^2·6 dihydrophthalic acid. It is split into its active components by means of its quinine salt (A. Werner and H. E. Conrad, Ber., 1899, 32, p. 3046). Hexahydroisophthalic acids (cyclo-hexane-1·3-dicarboxylic acids) are obtained by the action of methylene iodide on disodio-pentane tetracarboxylic ester (W. H. Perkin, Journ. Chem. Soc., 1891, 59, p. 798); by the action of trimethylene bromide on disodio-propane tetracarboxylic ester; and by the reduction of isophthalic acid with sodium amalgam, the tetrahydro acids first formed being converted into hydrobromides and further reduced (A. v. Baeyer and V. Villiger, Ann., 1893, 276, p. 255). The cis- and trans- forms can be separated by means of their sodium salts. The trans-acid is a racemic compound, which on heating with acetyl chloride gives the anhydride of the cis-acid.

Hexahydroterephthalic acids (cyclo-hexane-1·4-dicarboxylic acids). These acids are obtained by the reduction of the hydrobromides of the di- and tetra-hydroterephthalic acids or by the action of ethylene dibromide on disodio-butane tetracarboxylic acid. An important derivative is succino-succinic acid, C₆H₆O₂(CO₂H)₂, or cyclo-hexanedione-2-5-dicarboxylic acid-1·4, which is obtained as its ester by the action of sodium or sodium ethylate on succinic ester (H. Fehling, Ann., 1844, 49, p. 192; F. Hermann, Ann., 1882, 211, p. 306). It crystallizes in needles or prisms, and dissolves in alcohol to form a bright blue fluorescent liquid, which on the addition of ferric chloride becomes cherry red. The acid on heating loses CO₂ and gives cyclo-hexanedione-1·4.

Tetrahydrobenzoic acid (cyclo-hexene-1-carboxylic acid-1), C₆H₉·CO₂H. Three structural isomers are possible. The ∆¹ acid results on boiling the ∆² acid with alkalis, or on eliminating hydrobromic acid from 1-brom-cyclo-hexane-carboxylic acid-1. The ∆² acid is formed on the reduction of benzoic acid with sodium amalgam. The ³ acid is obtained by eliminating the elements of water from 4-oxy-cyclo-hexane-1-carboxylic acid (W. H. Perkin, jun., Journ. Chem. Soc., 1904, 85, p. 431) Shikimic acid (3·4·6-trioxy-∆¹-tetrahydrobenzoic acid) is found in the fruit of Illicium religiosum. On fusion with alkalis it yields para-oxybenzoic acid, and nascent hydrogen reduces it to hydroshikimic acid. Sedanolic acid, C₁₂H₂₀O₃, which is found along with sedanonic acid, C₁₂H₁₈O₃, in the higher boiling fractions of celery oil, is an ortho-A oxyamyl-∆⁵-tetrahydrobenzoic acid, sedanonic acid being orthovaleryl-∆¹-tetrahydrobenzoic acid (G. Ciamician and P. Silber, Ber., 1897, 30, pp. 492, 501, 1419 seq.). Sedanolic acid readily decomposes into water and its lactone sedanolid, C₁₂H₂O₂, the odorous constituent of celery oil.

Tetrahydrophthalic acids (cyclo-hexene dicarboxylic acids), C₆H₈(CO₂H)₂. Of the ortho-series four acids are known. The ∆¹ acid is obtained as its anhydride by heating the ∆² acid to 220° C., or by distilling hydropyromellitic acid. Alkaline potassium permanganate oxidizes it to adipic acid. The ∆² acid is formed along with the ∆⁴ acid by reducing phthalic acid with sodium amalgam in hot solutions. The ∆⁴ acid exists in cis- and trans- forms. The trans-variety is produced by reducing phthalic acid, and the cis-acid by reducing ∆^2·4 dihydrophthalic acid.

In the meta-series, four acids are also known. The ∆² acid is formed along with the ∆⁴ (cis) acid by reducing isophthalic acid. The trans ∆⁴ acid is formed by heating the cis-acid with hydrochloric acid under pressure. The ∆³ acid is formed when the anhydride of tetrahydro rimesic acid is distilled (W. H. Perkin, junr., Journ. Chem. Soc., 1905, 87, p. 293).

In the para-series, three acids are known. The ∆¹ acid is formed by the direct reduction of terephthalic acid; by boiling the ∆² acid with caustic soda; and by the reduction (in the heat) of ∆^1·4 dihydroterephthalic acid. The ∆^1·3 acid exists in cis- and trans- forms; these are produced simultaneously in the reduction of ∆^1·3 or ∆^1·5 dihydroterephthalic acids by sodium amalgam.

There are five possible dihydrobenzoic acids. One was obtained in the form of its amide by the reduction of benzamide in alkaline solution with sodium amalgam (A. Hutchinson, Ber., 1891, 24, p. 177). The ∆^1·3 acid is obtained on oxidizing dihydrobenzaldehyde with silver oxide or by the reduction of meta-trimethylaminobenzoic acid (R. Willstätter, Ber., 1904, 37, p. 1859).

Of the dihydrophthalic acids, five are known in the ortho-series, two of which are stereo-isomers of the cis- and trans-type, and a similar number are known in the para-series. The ∆^1·4 acid is obtained as its anhydride by heating ∆^2·4 dihydrophthalic anhydride with acetic anhydride. When boiled, with caustic soda it isomerizes to a mixture of the ∆^2·4 and ∆^2·4 dihydrophthalic acids. The ∆² acid is obtained by boiling the dihydrobromide of the ∆² acid with alcoholic potash or by continued boiling of the ∆² acid with caustic soda.

The ∆^2·6 acid is formed when phthalic acid is reduced in the cold by sodium amalgam or by heating the ∆^2·4 and ∆^3·5 acids with caustic soda. The trans-modification of ∆^3·5 acid is produced when phthalic acid is reduced by sodium amalgam in the presence of acetic acid. When heated for some time with acetic anhydride it changes to the cis-form. The trans-acid has been resolved by means of its strychnine salts into two optically active isomerides, both of which readily pass to ∆^2·6 dihydrophthalic acid (A. Neville, Journ. Chem. Soc. 1906. 89. p. 1744).

Of the dihydroterephthalic acids, the ∆^1·3 acid is obtained by heating the dibromide of the ∆² tetrahydro acid with alcoholic potash. It cannot be prepared by a direct reduction of terephthalic acid. On warming with caustic soda it is converted into the ∆^1·4 acid. The ∆^1·4 acid is also obtained by the direct reduction of terephthalic acid. It is the most stable of the dihydro acids. The ∆^1·5 acid is obtained by boiling the cis- and trans-∆^2·5 acids with water, which are obtained on reducing terephthalic acid with sodium amalgam in faintly alkaline solution. The relationships existing between the various hydrophthalic acids may be shown as follows:—

Cyclo-heptane (suberane), C₇H₁₄, obtained by the reduction of suberyl iodide, is a liquid which boils at 117° C. On treatment with bromine in the presence of aluminium bromide it gives chiefly pentabromtoluene. When heated with hydriodic acid to 230° it gives methylhexamethylene. On oxidation with nitric acid (sp. gr. 1·4) it yields pimelic acid. Disuberyl, C₇H₁₃·C₇H₁₃, a thick oily liquid, boiling at 290–291° C., is obtained by the reduction of suberyl bromide.

Cyclo-heptene, C₇H₁₂, is obtained by the action of alcoholic potash on suberyl iodide; and from cyclo-heptane carboxylic acid, 'the amide of which by the action of sodium hypobromite is converted into cyclo-heptanamine, which, in its turn, is destructively methylated (R. Willstätter, Ber., 1901, 34, 131). Cyclo-heptadiene 1·3, C₇H₁₀, is obtained from cyclo-heptene (Willstätter, loc. cit.). It is identical with the hydrotropilidine, which results by the destructive methylation of tropane.

Euterpene (trimethyl-1·4·4-cyclo-heptadiene 1·5), C₁₀H₁₆, is prepared from dihydroeucarveol. By the action of hydrobromic acid (in glacial acetic acid solution) and reduction of the resulting product it yields 1·2-dimethyl-4-ethyl benzene (A. v. Baeyer, Ber., 1897, 30, p. 2075). Cyclo-heptatriene (tropilidine), C₇H₈, is formed on distilling tropine with baryta; and from cyclo-heptadiene by forming its addition product with bromine and heating this with quinoline to 150–160° C. (R. Willstätter, loc. cit.). Chromic acid oxidizes it to benzoic acid and benzaldehyde. With bromine it forms a dibromide, which then heated to 110° C. decomposes into hydrobromic acid and benzyl bromide.

Cyclo-heptanol, C₁H₁₃OH, is formed by the reduction of suberone, and by the action of silver nitrite on the hydrochloride of cyclo- hexanamine (N. Demjanow, Centralblatt, 1904, i. p. 1214).

Cyclo-heptanone (suberone), C₇H₁₂O, is formed on the distillation of suberic acid with lime, and from α-brom-cyclo-heptane carboxylic acid by treatment with baryta and subsequent distillation over lead peroxide (R. Willstätter, Ber. 1898, 31, p. 2507) It is a colourless liquid having a peppermint odour, and boiling at 178·5–170·5° C. Nitric acid oxidizes it to 𝑛-pimelic acid.