Be-X bonds

The broken bonds (boldface = recommended data reference in parentheses) Methods (reference in  

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Fig. 17.1 Optimized structures for complexes between BeX2 (X = H, F) and different Lewis basis B (B = NH3, PH3, H2O, SH2 from left to right) at B3LYP/aug-cc-pVTZ level of theory. The Be-X bond length in free BeX2 is 1.332 A and 1.379 A for X = H and X = F, respectively...

Conceptually the most simple syntheses of complex molecules involve the joining of structural units in which all functional groups and all asymmetric centres are preformed. This technique can usually only be applied to compounds in which these units are connected by —C—X— bonds rather than C—C. It is illustrated here by the standard syntheses of oligonucleotides, peptides, and polydentate macrocyclic ligands. 

In the preceding parts of Section 4.04.2.1.3 the electrophilic attack on pyrazolic nitrogen with the concomitant formation of different classes of N—R bond has been examined N—H (iv, v), N—metal (vi), N—C(sp ) (vii, viii, xi), N—C(sp ) (be, x, xi), N—SO2R (x), N—halogen (xii), N—O (xiii) and N—-N (xiv). In this last part the reaction with other Lewis acids leading to the formation of pyrazole N—metalloid bonds will be discussed, and the study of their reactivity will be dealt with in Section 4.04.2.3.lO(viii). 

Synthesis of heterocycles by forming C—X bonds by radical reactions is not a generally applicable method, and seems not to be useful for making small rings. However, the attack of thiol radicals on double bonds can be a practical synthetic route, such as in the conversion of 1-hexene-7-thiol to thiepane (Section 5.17.3.3.1). 

The heat of formation of 804X4 was determined to be AH°f = +163 kcal mor. " Thus 804X4 is even more endothermic than S4X4 (AH°f = +110 kcal mok ). The mean E-X bond energies in E4X4 were estimated to be 59 kcal mol (E = Se) " and 72 kcal mol (E = S) from the enthalpies of formation. 

Fonnation of C-X bonds is not normally a problem but the Grignaid route can occasionally be useful when normal halogen exchange fuls. Thus iodination of Me3(XH2CI cannot be achieved by reaction with Nal or similar reagents but direct iodinadon of the conesponding Grignaid effects a smooth conversion ... 

Nitrogen forms two series of oxohalides — the nitrosyl halides XNO and the nitryl halides XNO2. There are also two halogen nitrates FONO2 (bp -46°) and CIONO2 (bp 22.3°), but these do not contain N-X bonds and can be considered as highly reactive derivatives of nitric acid, from which they can be prepared by direct halogen ation ... 

One final point saying that one particular conformer is "more stable" than, another doesn t mean the molecule adopts and maintains only the more stable conformation. At room temperature, rotations around stable conformation than in a less stable one. 

Compounds that have the same number of carbon atoms can be compared by adding the number of C-O, C-N, and C-X bonds in each and then subtracting the number of C—H bonds. The larger the resultant value, the higher the oxidation level. 

What s so special about periplanar geometry Because the sp3 a orbitals in the reactant C-H and C-X bonds must overlap and become p it orbitals in the alkene product, there must also be some overlap in the transition state. This can occur most easily if all the orbitals are in the same plane to begin with—that is, if they re periplanar (Figure 11.181. 

The distinction between coordination polymerization and ionic polymerization is not sharp. Let us consider for example a C—X bond, X being a halogen or a metal. Winstein54 and Evans14 have demonstrated that in a compound containing this type of bond an equilibrium may be established in a suitable solvent between... 

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