Catalysis of Organic..

Angueira and White 277Table 1. 27 Al-NMR chemical shifts in [n-BuMIM] + [Al n Cl 3n+1 ] - ; n = 1, 3/2, & 2n Peak 1 Peak 2 Peak 31 103.43/2 102 .2 98.22 102.2 98.2Assignment [AlCl 4 ] - [ Al 2 Cl 7 ] - [AlCl l 4 ] -3 -AlCPredicted CS’s 72.4 69.8; 69.7 62.7; 60.0Apparently, the mole fraction of the species [Al 2 Cl 7 ] was governed by thestructure of the cation where the concentration of this anion was highest when thechain length of R was shortest (ethyl) and it was smallest when the chain length of Rwas longest (dodecyl). The rate constants could be correlated by a linear function ofthe mole fraction of the [Al 2 Cl 7 ] - , Figure 6. In the proposed mechanism, Scheme 1,the (RMIM) + structure determines the equilibrium distribution, K, between species I& II. HCl reacts with I to form a super acidic species (III), which combines withCO to give the formyl cation (IV). This formyl cation reacts with substrate to formtolualdehyde that is stabilized as an adduct with AlCl 3 in species V. The aldehydeproduct can be recovered upon addition of water to the system. Without wateraddition, the aldehyde adduct cannot be used again either 1) for lack of sufficientacidity, or 2) there is no other aluminum species to stabilize the aldehyde product.These data can be explained by the reaction mechanism, Scheme 1.-Table 2. Effect of cation structure upon themole fraction [Al 2 Cl 7 ] - and the observedtoluene carbonylation rate constant.Cation x[Al 2 Cl 7 ] - k, kmin -1+EMIM 0.812 4.67+BMIM 0.702 3.92+HMIM 0.599 2.98k[Al]o[H + ][CO], kmin -154.543.532.521.5y = 7.7336x - 1.5923OMIM + 0.512 2.34DoMIM + 0.377 1.3510.500 0.2 0.4 0.6 0.8 1x, mole fraction of 102.2 peakFigure 6. Correlation of observedrate constants with x[Al 2 Cl 7 ] -