Vicinal groups

Richardson, the extent of dipolar repulsion (and ease of displacement) was dependent upon (a) the proximity, (b) the angular orientation, and (c) the numbers of permanent dipoles, with the most significant dipolar repulsions occurring when vicinal groups and the developing transitory dipoles in the S/y2 transition state are parallel and aligned (Scheme 42). 

Undoubtedly, the vicinal groups play a fundamental role in the outcome of this reaction, especially with the sugars, where the favored conformation of the molecule at equilibrium is controlled at the outset by groups that determine whether the molecule exists as a cyclic or acyclic structure. The deamination of cyclic and acyclic amino sugar derivatives by nitrous acid will be considered in turn. 

Having documented that we are perplexed by the differences we found for the enthalpies of formation of simple cyclopropyl and phenyl derivatives, it is premature and perhaps foolish to go from singly to doubly substituted species and expect any better understanding. Steric repulsion between vicinal groups on a cyclopropane may be expected, and hence the trans-isomQv of 1,2-dimethylcyclopropane is expected to be more stable than the cis. This is correct the enthalpies of formation of trans-1,2- and cis-1,2-dimethylcyclopropane (20c and 20b, X = Me) respectively, are (-30.7 0.8) and (-26.3 0.6) kJ moF for the liquids, and-3.8 and 1.7 kJmoF for the gases. The 1,1-dimethyl isomer (20a, X = Me) is more stable yet (-33.3 0.7) and (-8.2 1.2) kJ moF for the liquid and gas, respectively. What comparisons with benzene derivatives can we make With the 1,1-disubstituted cyclopropane isomer, seemingly none. With the cw-l,2-isomer, let s try the o-disubstituted benzene, o-xylene (47a, X = Me). With the trans-l,l-isomcr, let s try the -xylene isomer (47b,... 

When two groups, for example two hydroxyl groups, are adjacent to each other, they are known as vicinal groups, whilst two groups attached to the same atom are referred to as geminal groups. 

Figure 10. Silanol groups and siloxane bridges on the surface of colloidal silicas. Characteristic infrared bands at 3750 and 3660 cm-1 are shown for single and vicinal groups. Q1 terminology is used in NMR n indicates the number of bridging oxygens (-O-Si) bonded to the central silicon (n = 0-4).

In this chapter, we discuss reactions that either add adjacent (vicinal) groups to a carbon-carbon double bond (addition) or remove two adjacent groups to form a new double bond (elimination). The discussion focuses on addition reactions that proceed by electrophilic polar (heterolytic) mechanisms. In subsequent chapters we discuss addition reactions that proceed by radical (homolytic), nucleophilic, and concerted mechanisms. The electrophiles discussed include protic acids, halogens, sulfenyl and selenenyl reagents, epoxidation reagents, and mercuric and related metal cations, as well as diborane and alkylboranes. We emphasize the relationship between the regio-and stereoselectivity of addition reactions and the reaction mechanism. 

From a consideration of detailed results on the conformational equilibria of aldopentopyranose derivatives, it has been pointed out92- 9 that a more sophisticated model is required before conformational populations can be reliably predicted, at least with acylated derivatives. Even with adjustment of the original parameters in order to take revised values for the anomeric equilibrium of D-lyxopyranose tetraacetate and the conformational equilibrium of /3-D-arabinopyranose tetraacetate into account, the observed data cannot be accommodated within the framework of this model, except on a very broad, qualitative basis. Other possible factors that should be considered " include polar contributions from substituents other than that on C-1, attractive interactions between syn-diaxial acyloxy groups, non-bonded interactions between atoms that have unshared pairs of electrons,repulsive interactions between gauche-vicinal groups, the effect of solvent pressure, and differences between the molar volume of conformers. 

Other vicinal groups which may be involved in quenching of the tyrosine emission are anoide and peptide links a-amino groups... 

As noted earlier, three types of functional silanol groups exist on the silica surface isolated hydroxyl groups, geminal groups (two -OH groups on one Si atom), and vicinal groups (see Figure 9.6). The silanization reaction is illustrated in Eq. (9.5). 

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