Supercritical water alkylation

The anti-Markovnikov alkylation of phenol (PhOH) with /-butyl alcohol (/-BuOH) can be achieved in supercritical water at 673 K in the absence of an added catalyst. 

Alkylation of phenols using primary, secondary and tertiary alcohols was achieved using supercritical water (at the near-critical region, 250-350 °C). This process eliminates the need for environmentally hazardous organic solvents and acid catalysts. Both ortho-and para-alkylphenols were formed in these reactions, their ratio being dependent on the... 

Carbon dioxide is not the only nonflammable, biocompatible, and widely available solvent. Let us not forget about water. In particular, supercritical water oxidation and related reactive processes have shown a tremendous capacity for reducing toxic chemicals to innocuous constituents. Akiya and Savage have authored a recent review. Of particular interest is the section on hydrolysis in SCF water. Numerous references are tabulated according to chemical family. With regard to the more toxic compounds, Klein et al. have been especially active for many years and Tester et al. have focused on alkyl halides. ... 

Chapter 6 presents estimations of thermochemical properties of intermediates, transition states and products important to destruction of the aromatic ring in the phenyl radical + O2 reaction system. We have employed both DFT and high-level ab initio methods to analyze the substituent effects on a number of chemical reactions and processes involving alkyl and peroxyl radicals. Partially based on the results obtained in the vinyl system, high-pressure-limit kinetic parameters are obtained using canonical Transition State Theory. An elementary reaction mechanism is constructed to model experimental data obtained in a combustor at 1 atm, and in high-pressure turbine systems (5-20 atm), as well as in supercritical water [31]. 

Later studies showed that the mechanism of reactions, in particular ionic versus free-radical, could vary. Townsend [15] has studied the reaction of a series of coal model compounds (alkyl-aryl hydrocarbons and ethers) in supercritical water. For the hydrocarbons a free-radical pyrolysis route does not take advantage of the medium. However, for the ethers enhanced rates of reaction through a hydrolysis route occurs. As a result of different possible pathways, decomposition products of some organics in supercritical water have been shown by several workers to vary with solvent strength. In the absence of water, Pr(H20) = 0, pyrolysis is dominant and yields a variety of products including polycondensates. The main products of decomposition of neat methoxy... 

Other authors investigated the synthesis of Cu NPs in different SCF media. When, in particular, Ziegler et at. employed supercritical water and alkyl thiols [255], the latter species were found to behave both as capping and reducing agent, assisting the conversion of Cu precursor and intermediate species into capped Cu NPs (see Figure 1.24). 

The tuning of solubility with a relatively small jump or fall in pressure can possibly bestow many benefits with respect to rates, yields, and selectivity. Reaction parameters can be changed over a wide range. Replacement of solvents with high boiling points by supercritical (SC) fluids offers distinct advantages with respect to removal of the solvent. SC fluids like CO2 are cheap and environmentally friendly the critical temperature of CO2 is 31 C and the critical pressure 73.8 atm (Poliakoff and Howdle, 1995). Eckert and Chandler (1998) have given many examples of the use of SC fluids. Alkylation of phenol with tcrt-butanol in near critical water at 275 °C allows 2- erf-butyl phenol to be formed (a major product when the reaction is kinetically controlled 4-rert-butyl phenol is the major product, when the reaction is... 

In work by Hanrath and Korgel, H-terminated Ge nanowires were exposed to hexene in a supercritical fluid reactor at 220 °C, and the resulting hexyl-terminated nanowires appear resistant to oxidation in either air or water. The image reveals an abrupt interface for the alkyl-terminated Ge nanowire (Figure 5.12(b)) compared to the nanowires removed from the reactor without termination (Figure 5.12(a)) [103]. 

Water can be used as the solvent in the presence of added surfactants. Reactions in ionic liquids and supercritical fluids are also feasible. A variety of reagents promote cychzation, which can be achieved at room temperature. Examples of compounds that promote and accelerate the reaction include A-methyhnorphohne A-oxide, trimethylamine A-oxide, phosphine oxides, dimethylsulfoxide, alkyl methyl snlfides, molecular sieves, and lithium perchlorate. A comparison of a few promoters is seen in Scheme 246. Promoters... 

One special application of SEE involves its joint use with subcritical water to extract Dacthal and its acid metabolites from soil the herbicide is converted to monoacid and diacid derivatives that are more readily soluble in water than the parent compound. Thus, following extraction of Dacthal with supercritical CO, at 400 bar at 150°C for 15 min, its monoacid and diacid metabolites were extracted from soil in 10 min, using subcritical water conditions (viz. 200 bar and 50°C). The metabolites were trapped in situ on a strong anion-exchange disc placed over the exit frit in the extraction cell and subsequently combined with Dacthal by placing the disc into the GC autosampler vial containing the SEE extract to allow their simultaneous elution from the disc and deriva-tization to their ethyl esters. Only a single sample was analysed also, because the disc-catalysed alkylation reaction did not transesterify Dacthal, its speciation was maintained [180]. 

---