Steric effects structure

Keywords Bond Bond valence Bond valence theory Bonding geometry Bonding strength Lewis acids and base strengths Lone pairs Steric effects Structure prediction... 

The solubility of a compound is thus affected by many factors the state of the solute, the relative aromatic and aliphatic degree of the molecules, the size and shape of the molecules, the polarity of the molecule, steric effects, and the ability of some groups to participate in hydrogen bonding. In order to predict solubility accurately, all these factors correlated with solubility should be represented numerically by descriptors derived from the structure of the molecule or from experimental observations. 

If, on the other hand, the encounter pair were an oriented structure, positional selectivity could be retained for a different reason and in a different quantitative sense. Thus, a monosubstituted benzene derivative in which the substituent was sufficiently powerfully activating would react with the electrophile to give three different encounter pairs two of these would more readily proceed to the substitution products than to the starting materials, whilst the third might more readily break up than go to products. In the limit the first two would be giving substitution at the encounter rate and, in the absence of steric effects, products in the statistical ratio whilst the third would not. If we consider particular cases, there is nothing in the rather inadequate data available to discourage the view that, for example, in the cases of toluene or phenol, which in sulphuric acid are nitrated at or near the encounter rate, the... 

Reactions such as catalytic hydrogenation that take place at the less hindered side of a reactant are common m organic chemistry and are examples of steric effects on reactivity Previously we saw steric effects on structure and stability m the case of CIS and trans stereoisomers and m the preference for equatorial substituents on cyclo hexane rings... 

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... 

Write structural formulas for maleic anhydride (M ) and stilbene (M2). Neither of these monomers homopolymerize to any significant extent, presumably owing to steric effects. These monomers form a copolymer,... 

The substituent effects in aromatic electrophilic substitution are dominated by resonance effects. In other systems, stereoelectronic effects or steric effects might be more important. Whatever the nature of the substituent effects, the Hammond postulate insists diat structural discussion of transition states in terms of reactants, intermediates, or products is valid only when their structures and energies are similar. 

We will discuss shortly the most important structure-reactivity features of the E2, El, and Elcb mechanisms. The variable transition state theoiy allows discussion of reactions proceeding through transition states of intermediate character in terms of the limiting mechanistic types. The most important structural features to be considered in such a discussion are (1) the nature of the leaving group, (2) the nature of the base, (3) electronic and steric effects of substituents in the reactant molecule, and (4) solvent effects. 

Deviations from this generalization may have several sources, including charge repulsion, steric effects, statistical factors, intramolecular hydrogen bonding, and other structural effects that alter electron density at the reaction site. Hague - ° P has discussed these effects. 

Hydrolysis of an enamine yields a carbonyl compound and a secondary amine. Only a few rate constants are mentioned in the literature. The rate of hydrolysis of l-(jS-styryl)piperidine and l-(l-hexenyl)piperidine have been determined in 95% ethanol at 20°C 13). The values for the first-order rate constants are 4 x 10 sec and approximately 10 sec , respectively. Apart from steric effects the difference in rate may be interpreted in terms of resonance stabilization by the phenyl group on the vinyl amine structure, thus lowering the nucleophilic reactivity of the /3-carbon atom of that enamine. 

The DTBS group is probably the most useful of the bifunctional silyl ethers. Dimeihylsilyl and diisopropylsilyl derivatives of diols are very susceptible to hydrolysis even in water and therefore are of limited use, unless other structurally imposed steric effects provide additional stabilization. 

Secondary steric effects of nitro groups are more easily detected by comparing the reactivities with those of aza derivatives. For example, in structure 20 the rate depression on passing from methyl to -butyl is only 2.5-fold and can be attributed to an inductive effect, whereas in structure 21 a similar change involves the factor 16, which can be attributed in part to steric inhibition of resonance (S.I.R.) of thep-N02 group (reaction with piperidine). 

A few comments on the polar effects of the substituents reported in Tables IX—XI are now relevant. With the exception of 4-chloro-5-nitroquinoline (see Section IV, C, l,c), they involve only positions not subject to primary steric effects. The relations to the reaction center are of the conjugative cata, amphi) as well as of the non-conjugative class meta, epi, pros) as shown in Chart 3 by structures 45 and 46. 

With reactive aldehydes an early transition state is probably involved and therefore the steric demands of the aldehyde substituents are not highly influential. On the other hand, with less reactive ketones, the carbon-carbon bond formation is established further along the reaction coordinate, permitting the steric effects to play a greater role in the determination of the transition stale structure. 

Polk et al. reported27 that PET fibers could be hydrolyzed with 5% aqueous sodium hydroxide at 80°C in the presence of trioctylmethylammonium bromide in 60 min to obtain terephthalic acid in 93% yield. The results of catalytic depolymerization of PET without agitation are listed in Table 10.1. The results of catalytic depolymerization of PET with agitation are listed in Table 10.2. As expected, agitation shortened the time required for 100% conversion. Results (Table 10.1) for the quaternary salts with a halide counterion were promising. Phenyltrimethylammonium chloride (PTMAC) was chosen to ascertain whether steric effects would hinder catalytic activity. Bulky alkyl groups of the quaternary ammonium compounds were expected to hinder close approach of the catalyst to the somewhat hidden carbonyl groups of the fiber structure. The results indicate that steric hindrance is not a problem for PET hydrolysis under this set of conditions since the depolymerization results were substantially lower for PTMAC than for die more sterically hindered quaternary salts. 

Since steric effects can change catalysis (e.g., the above mentione trypsin case), one may still argue that such effects do influence the correl tion between structure and function. However, this case is not so relevant t structure-function correlation since the steric effects establish new structui and the activity associated with this structure is the main subject of ot... 

Transition state theory, 46,208 Transmission factor, 42,44-46,45 Triosephosphate isomerase, 210 Trypsin, 170. See also Trypsin enzyme family active site of, 181 activity of, steric effects on, 210 potential surfaces for, 180 Ser 195-His 57 proton transfer in, 146, 147 specificity of, 171 transition state of, 226 Trypsin enzyme family, catalysis of amide hydrolysis, 170-171. See also Chymotrypsin Elastase Thrombin Trypsin Plasmin Tryptophan, structure of, 110... 

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