Bruce Rickborn

Acid-catalyzed Rearrangements of Epoxides

The parallels between acid-catalyzed reactions of epoxides and the pinacol rearrangement are noted in Chapter 3.2 of this volume. Epoxides react more or less readily with acids, but they are at least moderately stable to all but the strongest of bases. It is difficult to find a persuasive instance of epoxide opening, even under basic conditions, that does not involve some probable form of electrophilic assistance, and a case can be made for the view that electrophile bonding/coordination to oxygen is required in order to cleave an epoxide. This electrophilic assistance may be as mild as hydrogen bonding by water in the opening of epoxides with aqueous amines, or as strong as the interactions that occur with concentrated H2SO4 or powerful Lewis acids

The Pinacol Rearrangement

The acid-catalyzed reaction of vicinal diols, which results in dehydration and the migration of an alkyl or aryl group, or a hydrogen atom, to form an aldehyde or ketone is known as the Pinacol rearrangement. 1 The name is derived from the material used in the earliest recorded example. Although the structures of both the starting material and product were unknown when the experiment was published in 1860, Fittig 2 found that treatment of pinacol ( 1 ) with sulfuric acid gave pinacolone ( 2 ); equation 1).

Substituted tetraphenylbutadienes as fast scintillator solutes

Abstract Sixteen previously unknown derivatives of the fast scintillator solute, 1,1,4,4-tetraphenyl-1,3-butadiene (TPB) have been synthesized by substitution with bromine atoms, alkyl groups, and phenyl groups, both individually and in pairs. Their fluorescence quantum yields and decay times have been measured, as well as their absorption and emission spectra and solubilities in pseudocumene. These parameters are all affected by the type, number, and position of the substituents. Quantum yields range from about 70% to less than 2% and decay times from 2.2 ns to about 100 ps. As expected, bromination reduces both quantum yield and fluorescence decay time by heavy-atom quenching. Surprisingly, asymmetrically substituted alkyl groups, which were added to increase solubility, also reduce the quantum yield and the decay time, and the effect is even larger than with bromine atoms. Symmetrical substitution with four propyl groups restored the quantum yield to approximately the value for TPB. Added phenyl groups increased the quantum yield and the decay time. In addition, aromatic solvents were found to quench the fluorescence relative to aliphatic solvents, and viscosity also plays a role. Finally, as expected, the asymmetrically substituted compounds are more soluble than the symmetrically substituted ones, and in the case of alkyl substitution, the longer the chain the greater the solubility.