Showing papers on "Steric effects published in 1968"

Molecular conformation of poly(S‐lactic acid)

Abstract: The predominant role of van der Waal interactions in determining the helical conformations of a simple synthetic linear polymer, as well as helical polypeptides, was pointed out in systematic studies by Liquori et al.1,2. In the case of homopolypeptides the conformational analysis carried out on the basis of a simple semiempirical function describing the van der Waal pairwise interactions between the non directly bonded atoms lead to the conclusion that only five helices are allowed (Rα, β, γ, δ, Lα).2,3 In view of the close similarities with poly-L-alanine, we have investigated by x-ray and conformational analysis the molecular conformation of poly(S-lactic acid) which has recently been described by Kleine and Kleine4 and Schuls and Schwaab5 and studied in solution by Goodman and D'Alagni.6 In fact, this polymer may be related to the polypeptide by the interchange of a peptide bond with ester bond along the main chain. This operation is expected to involve only a relatively small change in the steric interaction within the possible helical conformation, but obviously rules out any possibility of hydrogen bonding.

Thermal decomposition of aliphatic nitro-compounds

Abstract: Literature data on the thermal decomposition reactions of aliphatic nitro-compounds are surveyed and described systematically. Information concerning the influence of molecular structure on the rates and mechanisms of the gas-phase reactions is presented and the structures of the transition states in the C-N dissociation and HNO2 elimination processes are examined. The characteristic features of the decomposition of nitro-compounds in the condensed phase are desctibed. Data are presented on the C-N bond dissociation energy and the influence on the latter of electronic, steric, and conformational effects. The bibliography includes 86 references.

Redistribution Equilibria of Organometallic Compounds

Abstract: Publisher Summary This chapter reviews work on redistribution equilibria and describes the principles underlying the mathematical treatment of such equilibria as well as the general implications of these equilibria with respect to general chemistry. The overall process merely is a rearrangement of covalent bonds, with the total number of σ-bonds of any type remaining constant. However, other effects may also be quite significant-effects such as hybridization changes or nonbonding interactions, including steric factors and dipole interactions. When monofunctional and difunctional substituents are sorted on a polyfunctional moiety, a large number of molecular structures are possible ranging from the simple structures in which all of the sites of the polyfunctional moiety are filled by mono-functional substituents through molecules of increasing complexity to the most complicated structures in which the poly-functional moieties are held together by difunctional or bridging substituents. Equilibrium constants for the exchange of the constituent atoms between building units truly apply only to the ring-free portion of the molecules and the presence of simple cyclic molecules can be shown to cause a continuous increase in the apparent value of such equilibrium constants as the amount of bridging units is increased with respect to the mono-functional substituents.

Steric effects in the basic hydrolysis of poly(ethylene terephthalate)

Abstract: A comparative study of the reactivity of hydroxide and various alkoxide anions was made by determining percentage weight loss of poly(ethylene terephthalate) fiber. Regardless of the basicity of the anions, the order of reactivity was found to be hydroxide < tert-butoxide < sec-propoxide < methoxide < ethoxide. This observed order follows the nucleophilicity of the bases, and the relatively lower reactivity of sec-propoxide and tert-butoxide is assumed to be due to steric retardation during the equilibrium reactions.

Kinetic Studies on Substitution Reactions of Carbonylmetal Complexes

Abstract: Kinetic studies of substitution reactions of carbonylmetals and of carbonylmetal derivatives in which other ligands (n or π donors) besides CO are bound to the metal atom have recently attracted great interest, and the results of these investigations throw a new light on theoretical and preparative problems. The substitution mechanism is determined mainly by steric and electronic factors.

Structural effects in reactions of organophosphorus compounds. I. Reactions of phosphorus oxychloride with hindered phenols

Abstract: 2,6-Dialkyl-phenols exhibit little steric hindrance on reaction with phosphoryl chloride unless the alkyl substituent is bulky (i.e. t-butyl). Where bulky substituents are present reaction with phosphoryl chloride occurs only in the presence of Friedel–Crafts type catalysts after either a dealkylation or a rearrangement of the ortho-placed alkyl groups, followed by O-phosphorylation, has occurred.

Reaction Mechanisms, Structure, and Properties of Methylol Compounds in Cross-Linking Cotton1

Abstract: Kinetic and thermodynamic studies on the formation and dissociation of N-methylol compounds, as well as a study of their equilibrium states in relation to concentration, temperature, and catalysts, on the cross-linking of N-methylol and N-alkoxymethyl compounds with nucleophilic reactants, such as the hydroxy groups of cellulose, on the stability of the N-alkoxymethyl compounds in alkaline and acid media, and on the processes involved in chlorine retention, have led to an explanation of the mechanisms underlying these reactions, and to an understanding of the influence of such factors as electron density at the nitrogen atom (as determined by the methylolation rate in general or by the carbonyl wave number. in the case of ureas and acid amides), basic centers, possible steric hindrance and anchimeric groups on the application properties of these substances. Regular relationships have been established between their chemical consti tution and properties, which make possible a more reliable prediction of the...

Studies of Organic Catalytic Reactions. VI. The Function of Pyridine and Copper in the Rosenmund-von Braun Reaction

Abstract: The Rosenmund-von Braun reaction with aryl halide, with its Ar-X+CuCN-Base→Ar-CN+CuX-Base, has been studied in nitrobenzene in place of an organic base at 160–180°C. The free nitriles formed during the reaction were determined exactly by gas chromatography. In the use of bases with suitable coordination forces to CuCN, the reactivity of the base for the substitution was found to be in the order: prim.>sec.>tert. A steric hindrance of the base was observed, and the reactivity of the pyridine homologue followed the order: pyridine>quinoline>α-picoline>2, 6-lutidine. Ortho-substituted groups in the halobenzene seem to manifest no steric hindrance on the reaction. From the results of the kinetic study, it can be said that CuCN participates in the reaction essentially in the form of a Base(1 mol)-CuCN complex. The overall reaction is second-order, dc⁄dt=k[Complex][Ar-X]. The rate-determining step may be in the halogen exchange stage in the intervening complex, Base-CuCN-Ar-X, where the plane including CN-Cu-X ...

A CNDO study of steric effects in biphenyl

Abstract: The conformation of biphenyl is studied by the CNDO method. A fixed length of the twisted bond allows no minimum for the total energy between the values 0° and 90° of the angle of twist.

Stable Phenoxy-radicals

Abstract: The present review is devoted to an examination and analysis of published data on reactions leading to the formation of phenoxy-radicals, and the structures and properties of these radicals. In the description of the methods of preparing phenoxy-radicals, we examine the interaction of sterically hindered phenols with organic peroxides and inorganic oxidising agents, the mechanism of the thermal decomposition of cyclohexadienones, the autoxidation of sterically hindered phenols, and also the action of γ-radiation and ultraviolet radiation on phenols. In the section devoted to the properties of phenoxy-radicals, we discuss the influence of steric hindrance of the reactive centre, and also the degree of delocalisation of the unpaired electron, on the stability of the phenoxy-radicals. The available published data on the chemical transformations of substituted phenoxy-radicals are reviewed. Their recombination, disproportionation, isomerisation, and reaction with oxygen and halogens are examined. The influence of the steric and polar properties of substituents on the kinetics and mechanism of the transfer of hydrogen from organic compounds to stable phenoxy-radicals is analysed. The bibliography contains 180 references.

Cyclohexane compounds. VII. Nucleophilic scission of the stereoisomeric 3-methoxycyclohexene oxides

Abstract: The mode of scission of cis- and trans-3-methoxycyclohexene oxides by hydrogen chloride, hydrogen bromide, ammonia, acetic acid, and methanol (under both acidic and alkaline conditions) has been examined using vapor-phase chromatography to detect and isolate minor isomers Approximately 10% of the product from opening of the trans oxide is formed by attack at position-2 with each of these nucleophiles, whereas opening of the cis oxide proceeds exclusively by attack at position-1 The results are interpreted as reflecting very similar transition states for all of these reactions, with differences in product distributions for the two oxides being governed by the inductive effect of the methoxyl group and by steric factors in the corresponding transition states

E.S.R. spectra of free radicals induced in nucleic acid purine and pyrimidine bases and selected analogues by exposure to hydrogen atoms.

Abstract: SummaryPurine and pyrimidine bases, selected to permit determination of effects of structure, were exposed to hydrogen or deuterium atoms within an E.S.R. cavity. Spectra were recorded at intervals during the exposure. Conclusions about pyrimidines are: atoms add to all pyrimidines tested; addition is to the 5–6 double bond of 1,3-dideuterouracil; methyl groups on carbons 5 or 6 exert a dominant directive effect on addition; dihydropyrimidinyl radicals appear to have a near planar conformation at room temperature, but 1,3-dimethyldihydropyrimidinyl radicals are non-planar; steric blocking of H atom addition appears to be unimportant; and oxo and amino groups on carbon 4 have slightly different effects on the electronic structure of the radicals. Reaction of H atoms with 5,6-dihydropyrimidines gives the same radicals by abstraction as are formed by addition to the corresponding pyrimidine. All purines tested give radicals with nearly identical electronic structures.