Substituent size, equilibrium

These same authors (16) also compared experimental values as a function of x for a series of bulk equilibrates of the structure -+R(CH3)Si-0—J-x wherein R equalled H, CH3, CH3CH2, CH3CH2CH2, and CF3CH2CH2 in order to assess the effect of the substituent size on the equilibrium distribution. The values... 

Other substituted cyclohexanes are similar- to methylcyclohexane. Two chair confonnations exist in rapid equilibrium, and the one in which the substituent is equatorial is more stable. The relative fflnounts of the two confor-rnations depend on the effective size of the substituent. The size of a substituent, in the context of cyclohexane confor-rnations, is related to the degree of branching at the atom connected to the ring. A single... 

The equilibrium concentrations of many disubstituted benzenes (containing alkyl and halogen substituents) show that the meta isomer is in nearly all cases the most thermodynamically stable. It is not obvious why this should be so. Shine182 had discussed this problem in terms of the relative sizes of the standard enthalpy and entropy changes between any pair of isomers. 

This review will restrict itself to boron-carbon multiple bonding in carbon-rich systems, as encountered in organic chemistry, and leave the clusters of carboranes rich in boron to the proper purview of the inorganic chemist. Insofar as such three-dimensional clusters are considered at all in these review, interest will focus on the carbon-rich carboranes and the effect of ring size and substituents, both on boron and carbon, in determining the point of equilibrium between the cyclic organoborane and the isomeric carborane cluster. A typical significant example would be the potential interconversion of the l,4-dibora-2,5-cyclohexadiene system (7) and the 2,3,4,5-tetracarbahexaborane(6) system (8) as a function of substituents R (Eq. 2). 

The diastereomeric pentacoordinated bromostannanes 49 and 50 were found to equilibrate at — 13°C in toluene to a mixture of diastereomers27. The composition of the equilibrium mixture is dependent upon the R substituent (Table 4). As the size of this group is increased, the equilibrium is shifted in favor of the sterically less congested diastereomer 49. 

An NMR study on the conformation of glucopyranosylammonium compounds showed that the general tendendency of many electronegative substituents at C(i) to adopt an axial conformation was prevalent in this case too, as depicted in equilibrium 36 for R groups of various sizes. These results disclaim the importance of the so-called reverse anomeric effect 404. 

Barriers to rotation about the aryl-to-fluorene bond in 9-arylfluorene (63), where the aryl group is an o-alkylphenyl, were examined by two groups of investigators (93,105). Results given by Nakamura and Oki are shown in Table 13. Both the free energy of activation for rotation and the equilibrium constant are affected by the size of the substituent. Especially noteworthy is the case of 9-(2-rm-butylphenyl)fluorene, where the equilibrium is quite lopsided and the barrier for the process sp -> ap is very large. The ap form of this compound was prepared by protonation of the corresponding lithio compound, and equi-... 

Only electrical effects should be of major importance. The equilibrium constants were correlated with the Hammett equation (equation 5). This was necessary, due to the small size of the data set which included both meta and para substituents. The application of the Hammett equation to data sets including both meta and para substituents is most successful when the geometry of the system resembles that of the benzoic acids from which the Hammett constants were obtained. That is not the case in this reaction. The regression equation is equation 33 ... 

The preference of 2,7-disubstituted oxepins for this tautomeric form at equilibrium may be rationalized in terms of a steric substituent effect. The eclipsing interactions of the 2,7-substituents in the arene oxide form will be diminished by isomerization to the oxepin. When the 2,7-substituents form part of an annelated ring system, e.g. (22)-(24), the tautomeric preference will be determined by the size of the methylene bridge (67AG(E)385). Thus when n = 5 the annelated oxepin (24) was present in approximately equal proportions with the arene oxide form. However with n =4. (23) tetralin 9,10-oxide was dominant. The... 

By analogy with the tetrahydro-l,3,5-oxadiazine system, the ae conformer 466 of 3,5-dialkyltetrahydro-l,3,5-thiadiazines (465) should predominate in the conformational equilibrium. Investigation of a series of 3,5-dialkyl derivatives by H-NMR disclosed ring-inversion barriers that decrease with increasing size of the substituent (465 R = Me, AG 12.1 + 0.3 R = Et, AG 12.0 + 0.2 R = iPr, AG 10.5 + 0.4 kcal mol ). No further changes in the H-NMR spectra on cooling were found. Dipole-moment data indicated that all the compounds existed predominantly in the ae conformation.356 It is difficult to draw conclusions for Jtem in this series. 

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