No abstract

No abstract

In trying to understand chemical reactivity, the physical organic chemist has traditionally relied on models or concepts that are qualitative and intuitive. Claude F. Bernasconi was born In Zurich, Switzerland, In 1939. He received his undergraduate degree and Ph.D. (with Heinrich Zollinger) from the Swiss Federal Institute of Technology (ETH), Following a postdoctoral year with Manfred Eigen at the Max Planck Institute for Biophysical Chemistry in Gottingen, he Joined the chemistry faculty at the University of California, Santa Cruz, in 1967, where he has been a Professor of Chemistry since 1977. His research Is focused on kinetics, organic reaction mechanisms, and structure-reactivity problems.Perhaps the best-known physical organic concept is the Hammond postulate3 along with its various extensions(1) This Account is based, in part, on a talk I presented during a symposium honoring Professor Joseph Bunnett on the occasion of his retirement from active service at the University of California at Santa Cruz. The talk was entitled "From Bunnett's Variable Transition State Theory to the Principle of Non-Perfect Synchronization". Except for the introduction, which summarizes the basic features of the principle of nonperfect synchronization (PNS), most of the material is new. Specifically, there is very little overlap with an earlier Account that was entitled "Intrinsic Barriers of Reactions and the Principle of Nonperfect Synchronization".2

No abstract

Three relatively fast kinetic processes can be detected in reactions of 2,4,6-trinitrotoluene (TNT) with lyate ions in methanol, ethanol, and in 50% dioxane-50% water. With the base in excess over TNT, formation of the 2,4,6-trinitrobenzyl anion (TNT-) is the principal process. At high base concentration a second much faster process emerges, which is difficult to identify but could be due to a Meisenheimer complex (MC) coupled to a radical-anion formation. When TNT is in excess over the base, formation of a Janovsky complex (JC) between TNTand a second molecule of TNT is observed, which is identified through its visible spectrum. Rate constants of TNTand JC formation and reversion were measured. Preliminary spectral evidence indicates that in

No abstract

A kinetic study of oligoguanylate synthesis on a polycytidylate template, poly(C), as a function of the concentration of the activated monomer, guanosine 5'-monophosphate 2-methylimidazolide, 2-MeImpG, is reported. Reactions were run with 0.005-0.045 M 2-MeImpG in the presence of 0.05 M poly(C) at 23 degrees C. The kinetic results are consistent with a reaction scheme (eq 1) that consists of a series of consecutive steps, each step representing the addition of one molecule of 2-MeImpG to the growing oligomer. This scheme allows the calculation of second-order rate constants for every step by analyzing the time-dependent growth of each oligomer. Computer simulations of the course of reaction based on the determined rate constants and eq 1 are in excellent agreement with the product distributions seen in the HPLC profiles. In accord with an earlier study (Fakhrai, H.; Inoue, T.; Orgel, L. E. Tetrahedron 1984, 40, 39), rate constants, ki, for the formation of the tetramer and longer oligomers up to the 16-mer were found to be independent of length and somewhat higher than k3 (formation of trimer), which in turn is much higher than k2 (formation of dimer). The ki (i > or = 4), k3, and k2 values are not true second-order rate constants but vary with monomer concentration. Mechanistic models for the dimerization (Scheme I) and elongation reactions (Scheme II) are proposed that are consistent with our results. These models take into account that the monomer associates with the template in a cooperative manner. Our kinetic analysis allowed the determination of rate constants for the elementary processes of covalent bond formation between two monomers (dimerization) and between an oligomer and a monomer (elongation) on the template. A major conclusion from our study is that bond formation between two monomer units or between a primer and a monomer is assisted by the presence of additional next-neighbor monomer units. This is consistent with recent findings with hairpin oligonucleotides (Wu, T.; Orgel, L. E. J. Am. Chem. Soc. 1992, 114, 317). Our study is the first of its kind that shows the feasibility of a thorough kinetic analysis of a template-directed oligomerization and provides a detailed mechanistic model of these reactions.

No abstract