シクロアルカンの異性化反応　（第７報）　塩化アルミニウムによるシクロヘプタン類の異性化反応

Liquid-phase isomerization of cycloheptane, methylcycloheptane and ethylcycloheptane were investigated in the presence of anhydrous aluminum chloride as a catalyst. Cycloheptane was synthesized by a modified Wolff-Kishner reduction of cycloheptanone prepared by ring-enlarging of cyclohexanone10)-12) (Eqs. (1) and (2)), while methyl- and ethyl-cycloheptanes were prepared by the Grignard alkylation of cycloheptanone with alkylhalides (Eq. (3)). The properties and IR spectra of these 7-membered ring compounds are shown in Table 1 and Fig. 1, respectively.<br>A reaction material was placed in a small cylindrical flask at a desired temperature, and after the catalyst was added, reactions were carried out with vigorous stirring using an electromagnetic stirrer. Aliquots were withdrawn and their compositions determined by an FID gas-chromatograph equipped with a capillary separation column (squalane, 0.25mm(dia.)×90m).<br>Compositions of the isomerization products of cycloheptane at 75°C are plotted against reaction time in Figs. 2 and 3. Methylcyclohexane appeared to be the primary product from cycloheptane (Figs. 2 and 4), and underwent subsequent conversion to C₇-cyclopentanes by ca 10mol% (Fig. 3). In these reactions, product composition eventually approached the thermodynamical equilibrium of C₇-cycloalkane2). The main equilibrium component was methylcyclohexane, the most stable product, while small amounts of dimethyl- and ethyl-cyclopentanes were obtained as other components (Table 2).<br>In the case of isomerization of methylcycloheptane, at reaction temperature of 20°C, ethylcyclohexane was rapidly produced from methylcycloheptane, which was then converted to dimethylcyclohexanes (Figs. 5 and 6). Reaction paths of isomerization are represented schematically and the main route of isomerization is shown by wide arrows in Fig. 7. Carbonium ion with a carbon skeleton of methylcycloheptane changed to carbonium ion with ethylcyclohexane structure which then underwent conversion to carbonium ions with dimethylcyclohexanes structures via many carbonium ions with carbon skeletons of methylethylcyclopentanes. Finally, composition of isomerization products approached the thermodynamical equilibrium of C₈-cycloalkane1) and consisted mainly of dimethylcyclohexanes. At the same time, small amounts of ethylcyclohexane and various cyclopentanes were obtained (Table 3(a)).<br>The compositions of isomerization products of ethylcycloheptane at 15°C are plotted against reaction time in Fig. 8. Ethylcycloheptane was converted rapidly to n-propylcyclohexane, from which were produced methylethylcyclohexanes and then trimethylcyclohexanes, in this order. As shown in Fig. 9, the primary product from ethylcycloheptane was n-propylcyclohexane, the secondary products were methylethylcyclohexanes, and final were thermodynamically stable trimethylcyclohexanes. As summarized in Eq. (5), the isomerization process of ethylcycloheptane may be expressed as follows: Initially, contraction of the 7-membered to 6-membered ring proceeds, followed by slow contraction of the 6-membered to 5-membered ring and fast expansion of the 5-membered to 6-membered ring, which occur repeatedly. Ultimately, compositions of isomerization products approached the equilibrium of C₉-cycloalkane4) (Table 3(b)).