Lewis structures summary

Begin with the Lewis structure, and then survey each inner atom for its steric number and hybridization. Here is a summary of the information ... 

In summary, sometimes we need double or triple bonds to satisfy the octet rule. Writing Lewis structures is a trial-and-error process. Start with single bonds between the bonded atoms and add multiple bonds as needed. 

In Summary Lewis structures describe bonding by the nse of electron dots or straight lines. Whenever possible, they are drawn so as to give hydrogen an electron duet and other atoms an electron octet. Charges are assigned to each atom by evaluating its electron connt. 

In Summary Some molecules cannot be described accurately by one Lewis structure but exist as hybrids of several resonance forms. To find the most important resonance contributor, consider the octet rule, make sure that there is a minimum of charge separation, and place on the relatively more electronegative atoms as much negative and as little positive charge as possible. 

A more representative polyatomic example is given by CH3OH (cf. 1/0-3.3). In this case, the default NBO search again required only one cycle and returned an optimal Lewis structure of very high accuracy (99.56% of the total electron density), containing two CR-type, five BD-type, and two LP-type NBOs. The NBO summary for occupancies and energies of these nine L-type orbitals and the final five (valence BD ) NL-type orbitals (i.e., neglecting the 58 RY -type orbitals in this basis set) are shown in I/0-4.3. 

The final summary of NBOs is shown in I/0-4.6, including the important BD -type NL orbitals that are indicated to play the leading role in delocalization effects, but excluding the many remaining RY -type orbitals. The L-type NBOs correspond to the Lewis structure diagram... 

FIGURE 10-8 Summary scheme for drawing Lewis structures... 

Summary of the Relationship between Diastereoselectivity and the Transition Structure. In this section we considered simple diastereoselection in aldol reactions of ketone enolates. Numerous observations on the reactions of enolates of ketones and related compounds are consistent with the general concept of a chairlike TS.35 These reactions show a consistent E - anti Z - syn relationship. Noncyclic TSs have more variable diastereoselectivity. The prediction or interpretation of the specific ratio of syn and anti product from any given reaction requires assessment of several variables (1) What is the stereochemical composition of the enolate (2) Does the Lewis acid promote tight coordination with both the carbonyl and enolate oxygen atoms and thereby favor a cyclic TS (3) Does the TS have a chairlike conformation (4) Are there additional Lewis base coordination sites in either reactant that can lead to reaction through a chelated TS Another factor comes into play if either the aldehyde or the enolate, or both, are chiral. In that case, facial selectivity becomes an issue and this is considered in Section 2.1.5. 

Summary of Facial Stereoselectivity in Aldol and Mukaiyama Reactions. The examples provided in this section show that there are several approaches to controlling the facial selectivity of aldol additions and related reactions. The E- or Z-configuration of the enolate and the open, cyclic, or chelated nature of the TS are the departure points for prediction and analysis of stereoselectivity. The Lewis acid catalyst and the donor strength of potentially chelating ligands affect the structure of the TS. Whereas dialkyl boron enolates and BF3 complexes are tetracoordinate, titanium and tin can be... 

In summary, there now exists a body of data for the reactions of carbocations where the values of kjkp span a range of > 106-fold (Table 1). This requires that variations in the substituents at a cationic center result in a >8 kcal mol-1 differential stabilization of the transition states for nucleophile addition and proton transfer which have not yet been fully rationalized. We discuss in this review the explanations for the large changes in the rate constant ratio for partitioning of carbocations between reaction with Bronsted and Lewis bases that sometimes result from apparently small changes in carbocation structure. 

In summary, the detailed electronic character of dihapto metal-acetylene complexes depends strongly on the Lewis-acceptor capacity of the metal. Formal two-versus four-electron rp ligation to a transition metal can lead to breaking of one or both 7T bonds, dramatically altering the structure and reactivity of the alkynyl... 

In summary, we can say that, because of the unique absence of angular and radial nodes in the H-atom valence shell, the hydride oah orbital is uniquely suited to strong n-a donor-acceptor interactions with Lewis bases. In turn, the unique energetic and angular features of nB-aAH interactions (or equivalently, of B H—A <—> B—H+ A covalent-ionic resonance) can be directly associated with the distinctive structural and spectroscopic properties of B - H—A hydrogen bonding. 

Table 6.3 provides a summary of the different microscopic techniques that have been applied to hydrate studies and the type of information that can be obtained from these tools. The following discussion provides a brief overview of the application of diffraction and spectroscopy to study hydrate structure and dynamics, and formation/decomposition kinetics. For information on the principles and theory of these techniques, the reader is referred to the following texts on x-ray diffraction (Hammond, 2001), neutron scattering (Higgins and Benoit, 1996), NMR spectroscopy (Abragam, 1961 Schmidt-Rohr and Spiess, 1994), and Raman spectroscopy (Lewis and Edwards, 2001). 

In summary, then, analysis of the structures of product alkyl-aromatics, patterns of substrate reactivity, and side reaction pathways in modified faujasite-catalyzed alkylations reveals great similarity to the corresponding features commonly reported for electrophilic aromatic substitutions in the presence of strong protonic acids or promoted Lewis acids. 

Interaction between the glycoprotein E-selectin (expressed on the surface of endothelial cells) and the sialyl Lewis 1 ligand structures on the surface of neutrophils is a major event in inflammation, infection, and metastasis. Both chemical and enzymatic syntheses have been described for the synthesis of the sialyl Lewisx structure. A summary of the general strategy used in chemoenzymatic synthesis is shown here. Extensive in situ regenerations of the sugar nucleotides are not shown in the scheme.97... 

The formation of a complex prior to reaction is well supported in the case of bromination. Molecular bromine forms charge transfer complexes with benzene even in the absence of Lewis acid catalysts. The complexes can be detected spectroscopically and can even be crystallized for structure determination. Complexes of Br with the aromatic compound may also be formed. For a summary of experimental data in this area, see the discussion in reference 178. 

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