A bromine ± alkyne p complex (l max 294 nm) of 1:1 stoichiometry has been observed for the first time in the course of the bromination of 1-phenylpropyne by means of a diode-array stopped-flow technique. ); this demonstrates that the complex is actually an essential intermediate on the reaction coordinate. The bromination of a series of nine alkynes has been studied. Bromination reactions with negative apparent activation parameters lead to mixtures of E and Z vinyl dibromides, whereas reactions with positive activation energy yield the E isomers exclusively. The reason for the difference in reactivity of these alkynes compared with structurally similar alkenes most likely lies in the stability of these 1:1 charge-transfer complexes. Usually open arylvinyl cations correspond to the energetically favored product-determining intermediates; bridged bromirenium ions are formed from deactivated alkynes and react to give E isomers. The kinetic effect of alkyl groups and of p-OCH 3 , p-CN, and p-NO 2 substituents at the aryl group on the bromination of arylalkylacetylenes is discussed. Density functional calculations provide insight into the geometries, energies, and bonding of the intermediate 1:1 and 2:1 Br 2 ± acetylene complexes involved. These theoretical investigations demonstrate that the most stable trimolecular Br 2 ± Br 2 ± acetylene adduct possesses a structure very similar to a crystallographically characterized Br 2 ± Br 2 ± alkene species, which can directly yield the ionic products, Br À 3 and vinyl cation, driven by the heterolytic action of a solvent.

To help conserve the vast array of (combinations of) dyestuffs and pigments encountered in culturalheritage objects and application materials, a rapid and convenient method for dye-degradation research is required. In-solution degradation studies of dyes in a strong solvent, such as the commonly used dimethyl sulfoxide (DMSO), are potentially of interest, in addition to studies involving other solvents, such as water. The degradation of eosin and carminic acid under the influence of light was investigated in two solvents, i.e. in a mixture of DMSO and acetonitrile and in pure water. A liquid chromatography -mass spectrometry (LC-MS) method was developed for analysis of the degraded samples and identification of the individual components. The presence of DMSO generally facilitated faster degradation, which, in combination with its universal solvating properties are advantageous. However, different products were formed in the presence of DMSO. Degradation pathways for eosin and carminic acid in these solvents are proposed.

Rates and products of electrophilic bromination of ring-substituted cis- and trans-stilbenes have been investigated in acetic acid, trifluoroethanol, ethanol, methanol, and water−methanol mixtures. The mY Br relationships (linear for nucleophilic solvents only, with m = 0.8), the deviations of the two nonnucleophilic solvents from the mY Br plots (ΔAcOH and ΔTFE positive, negative, or negligible), the kinetic solvent isotope effects (k MeOH/k MeOD = 1.1−1.6), the chemoselectivity (predominant dibromide, DB, or solvent-incorporated adducts, MA), and the high dependence of the stereochemistry on the solvent and the substituents (from stereoconvergency to stereospecificity) are discussed and interpreted in terms of a mechanistic scheme, analogous to the Jencks scheme for aliphatic nucleophilic substitutions, in which preassociation, free-ion, and ion-pair pathways compete. In particular, the stereochemical outcome of these reactions is consistent with a marked change in the nucleophilic partners of the product-forming ionic intermediate arising from different ionization routes. Return, i.e. change in the rate-limiting step from ionization to product formation, is shown to be related to substituent-dependent, but not solvent-dependent, bromine bridging.