Electronic steric effect

The magnetic moments rise only slightly at elevated temperatures (see Table 5), which led the authors to conclude that some population of the higher sT2(Oh) state is possible. No clear distinction can be made as to which of the influencing factors, viz. electronic effects, steric hindrance, and crystal solvent effects, plays the dominant role here, because all of these are operative to some extent. Data from the UV-vis spectra of the nickel(II) complexes indicate that the ligands have field strengths in the iron(II) crossover region. 

Besides electronic effects, steric effects have also been investigated. Contrary to expectations, the presence of reasonably hindered substituents at the ortho position, favours the reaction, preorganising the formation of the spirocyclic intermediate. In the presence of a single ortho substitution, the rate of reaction follows the trend t-butyl > z-propyl > methyl [21]. The... 

A study of several substituted alkenes in methanol developed some generalizations pertaining to the capture of bromonium ions by methanol For both E- and Z-disubstituted alkenes, the addition of both methanol and Br was completely anti stereospecific. The reactions were also completely regioselective, in accordance with Markovnikov s rule, for disubstituted alkenes, but not for mono substituted alkenes. The lack of high regioselectivity of the addition to monosubstituted alkenes can be interpreted as competitive addition of solvent at both the mono- and unsubstituted carbons of the bromonium ion. This competition reflects conflicting steric and electronic effects. Steric factors promote addition of the nucleophile at the unsubstituted position, whereas electronic factors have the opposite effect. 

Apart from energetic and electronic effects, steric (geometric) effects also play an important role in chemisorption and heterogeneous eatalysis [32]. The porosity and the surface of solids must therefore also be taken into account. A steric factor means that a molecule has to be adsorbed on the catalyst in such a manner that it fits properly on die surface atoms. Only then can it be readily activated. 

In addition to electronic effects, steric factors also influence the reactivity of isocyanates. Thus, o-tolyl isocyanate reacts more slowly than does phenyl isocyanate, and a secondary hydroxyl group adds more slowly than a primary one. 

Auto-association of A-4-thiazoline-2-thione and 4-alkyl derivatives has been deduced from infrared spectra of diluted solutions in carbon tetrachloride (58. 77). Results are interpretated (77) in terms of an equilibrium between monomer and cyclic dimer. The association constants are strongly dependent on the electronic and steric effects of the alkyl substituents in the 4- and 5-positions, respectively. This behavior is well shown if one compares the results for the unsubstituted compound (K - 1200 M" ,). 4-methyl-A-4-thiazoline-2-thione K = 2200 M ). and 5-methyl-4-r-butyl-A-4-thiazoline-2-thione K=120 M ) (58). 

Deviation includes, in fact, the summation of steric and electronic effects, and basicity is somewhat a useful predictor for properties of complex dyes (solvent sensitivity, isomeric forms of trinuclear dyes) and gives also semiquantitative data for color structure relation (atomic)... 

The quatemization of the nitrogen atom of the thiazole ring (the Menschutkin s reaction) by alkyl halide or methyl tosylate can be used to measure the reactivity of this atom and thus to evaluate steric and electronic effects of ring substituents. 

The greater stability of more highly substituted double bonds is an exam pie of an electronic effect The decreased stability that results from van der Waals strain between cis substituents is an example of a steric effect... 

Table 17 3 compares the equilibrium constants for hydration of some simple aldehydes and ketones The position of equilibrium depends on what groups are attached to C=0 and how they affect its steric and electronic environment Both effects con tribute but the electronic effect controls A hydr more than the steric effect... 

Electronic and steric effects operate m the same direction Both cause the equilib rium constants for hydration of aldehydes to be greater than those of ketones... 

Effects of Structure on Rate Electronic and steric effects influence the rate of hydra tion m the same way that they affect equilibrium Indeed the rate and equilibrium data of Table 17 3 parallel each other almost exactly... 

Steric and electronic effects influence the rate of nucleophilic addition to a proton ated carbonyl group m much the same way as they do for the case of a neutral one and protonated aldehydes react faster than protonated ketones... 

The BDE theory does not explain all observed experimental results. Addition reactions are not adequately handled at all, mosdy owing to steric and electronic effects in the transition state. Thus it is important to consider both the reactivities of the radical and the intended coreactant or environment in any attempt to predict the course of a radical reaction (18). AppHcation of frontier molecular orbital theory may be more appropriate to explain certain reactions (19). 

The importance of both electronic and steric effects is clear in cycloadditions as in cross-oxidations. One example is a heterocycHc modification leading to the thermodynamically less stable natural form of juglone derivatives such as ventiloquinones JT [124917-64-2] (84) and I [124917-65-3] (85) (83). The yields are 97% (84) from 6-chloro-2,3-dimethoxy-l,4-ben2oquinone [30839-34-0] and 100% (85) upon hydrolysis. 

Hydrolysis reactions involving tetrahedral intermediates are subject to steric and electronic effects. Electron-withdrawing substituents faciUtate, but electron-donating and bulky substituents retard basic hydrolysis. Steric effects in acid-cataly2ed hydrolysis are similar to those in base-cataly2ed hydrolysis, but electronic effects are much less important in acid-cataly2ed reactions. Higher temperatures also accelerate the reaction. 

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