Dan Fǎrcaşiu

The liquid-phase reaction of hexane on acid mordenite

Abstract The batch reaction of hexane (H) on acid Mordenite at 160°C does not occur by consecutive carbocationic isomerization (H→2MP,3MP→2,3DMB→2,2DMB). Instead, a mixture of all the isomers in constant ratios was obtained at all conversions between 2% and 13%, as expected for quenching or desorption of an equilibrium mixture. The ratios of the monobranched isomers to the dibranched isomers did not correspond, however, to their equilibrium at the reaction temperature, or to the equilibrium of the corresponding carbocations. The existence of mechanistic differences between the reaction under these conditions and the experiments conducted either with nonoxidizing superacids in solution (the source of the accepted mechanistic model) or with a Pt-containing catalyst in the presence of hydrogen, in the gas phase, is indicated by these data.

Toward a quantitative evaluation of Lewis acid strength. A 13CNMR study of the interaction of boron trifluoride with diethyl ether

Abstract The rate of exchange between boron trifluoride diethyl etherate and free diethyl ether can be conveniently measured by 13 C dynamic NMR spectroscopy (DNMR) between 25°C and −50°C in dichloromethane solution. The same rates and activation parameters are found by the line shape analysis of either the methyl or the methylene group. The rates do not vary with the ratio of free ether to complex between 0.61 and 2.4, indicating a rate-determining unimolecular decomposition of the latter. The literature claim of a bimolecular mechanism for the ether exchange is thus found to be incorrect. An enthalpy of activation (ΔH‡) of 9.7 kcal/mol was calculated for this decomposition. This value is a good measure of the strength of interaction of BF3 with diethyl ether, because the recombination to form the complex (2) should have a very low energy barrier, and it is close to the ΔH° value of 10.9 kcal/mol determined by Brown and Adams for the decomposition of 2 to gaseous diethyl ether and BF3. The 13 C DNMR method can be applied in principle to the interaction of ether with other Lewis acids, thus providing a method for comparison of Lewis acid strengths.

Metal ion catalysis in chemisorption anddehydrogenation of alkanes on aluminium hydroxide clusters, revealed bytheoretical calculations

Electron-correlated DFT calculations with a large basis set show that propane adds to coordinatively unsaturated aluminium, as in the clusters (HO)3Al(OH2)x (x = 0, 1), by aluminium insertion into a C–H bond, followed by hydrogen migration to an oxygen atom and predict correctly experimental observations; the alternative pathway involving alkyl–oxygen interaction has a much higher energy barrier and does not predict correctly the experimental results.

The effect of cycloalkanes on the reaction of hexane with trifluoromethanesulfonic acid

The chain reaction of hexane catalyzed by trifluoromethanesulfonic acid (TFMSA) under mild conditions (disproportionation and cracking) was suppressed by small amounts (1–5%) of cycloalkanes, namely methylcyclopentane (MCP), cyclohexane (CH) and cyclopentane (CP). The reaction then proceeded in the isomerization mode. The addition of ferric ions to the acid overcomes the stabilizing effect of cycloalkanes. The formation of substituted allyl cations during the induction period for cracking was also inhibited by very small amounts of MCP. Addition of MCP or 3-methylhexane had the same effect on reduction of the induction period and (minor) on the increase in rate of the reaction stabilized against cracking by CP addition. Incorporation of deuterium into the products from hexane and MCP-1-d shows that MCP acts as a hydride relay in the reaction. Hydride transfer catalysis by the reaction product is disproved for any catalyst. Both the reaction of hexane with MCP-1-d and of uniformly labeled hexane-U-d4.3 with MCP showed H/D scrambling which is extensive among all products (2- and 3-methylpentane, cracking products and CH) and the acid catalyst, smaller in MCP, and is virtually zero in hexane. Thus, the main features of the reaction are the initiation by oxidation and the formation and hydronation of alkenes.

Carbodications. 5.1 Ring opening of the cyclopropanecarbonyl cation in superacid

The cyclopropanecarbonyl cation (11) was prepared from cyclopropanecarbonyl chloride in 1∶1 HF–SbF5, 1∶1 FSO3H–SbF5, and 4∶1 FSO3H–SbF5. Ring opening occurred in the strongest superacids 1∶1 HF–SbF5 and (much slower) 1∶1 FSO3H–SbF5, but not in 4∶1 FSO3H–SbF5. The crotyl (2) and methacryloyl (14) cations were formed in 1∶1 FSO3H–SbF5, but very little or no 14 accompanied 2 in 1∶1 HF–SbF5. Thus, 2 is formed by acid catalysis only, whereas formation of 14 involves base catalysis supplementing the acid catalysis in superacids. Dehydrochlorination of the 4-chlorobutanoyl cation in HF–SbF5 and H/D exchange at C3 of 2 (involving attack by the acid at C3 of 3-butenoyl cation) in 1∶1 DF–SbF5, both reported before, cannot involve intramolecular assistance with the formation of ring-hydronated 11 as intermediate. Instead, a 1,4 acyl alkyl dication in a tight ion pair is indicated by the results. Reaction in 1∶1 FSO3H–SbF5 under CO pressure followed by methanol quenching gave the methyl esters of glutaric (major) and methylsuccinic acid (minor); at least the latter should be formed by an SN2-like attack by CO. The reaction of 11 in deuterated superacids 1∶1 DF–SbF5 and 1∶1 FSO3D–SbF5 was much slower than the reaction in the corresponding protio-acids. At the same time, H/D exchange in the ring of unreacted 11 was observed. The extent of exchange could be assessed for the reaction in 1∶1 FSO3H–SbF5, where conversion to 2 was small. The deuteration of the ring in this medium is similar in rate to the ring cleavage. Together with the observed rate reduction in the deuterated acids, this result suggests that H/D exchange in 11 and its ring opening do not occur on the same reaction pathway.