Acceleration of synthetic organic reactions using supercritical water: Noncatalytic Beckmann and pinacol rearrangements

A high-pressure and high-temperature FTIR was used to study noncatalytic Be ckmann and pinacol rearrangements using supercritical water (scH(2)O). Sign ificant acceleration of Beckmann and pinacol. rearrangements can be achieve d by using scH(2)O, especially near the critical point,, even in the absenc e of any acid catalysts. It has been demonstrated that scH(2)O acts effecti vely in the place of-conventional acid catalysts for both the rearrangement s. The rate of pinacol rearrangement using scH(2)O is significantly larger by a factor of 28200 than that in 0.871 M HCl solution at 46.7 MPa under di stillation conditions, The activation energy for me former at 25 MPa was fo und to be markedly reduced to about one-third of that for the latter. The a ccelerated rates of reaction may be attributed to a great increase in the l ocal proton concentration around the organic reactants. In addition, the na ture of scH(2)O can be adjustable to weak acidity in the near-critical regi on, and then it does not catalyze the pinacol rearrangement, but opens a ne w reaction pathway from pinacol through a completely dehydrated product and then to a Diels-Alder adduct between the dehydrated products.