Bond dependent

In order to develop a quantitative interpretation of the effects contributing to heats of atomization, we will introduce other schemes that have been advocated for estimating heats of formation and heats of atomization. We will discuss two schemes and illustrate them with the example of alkanes. Laidler [11] modified a bond additivity scheme by using different bond contributions for C-H bonds, depending on whether hydrogen is bonded to a primary (F(C-H)p), secondary ( (C-H)g), or tertiary ( (C-H)t) carbon atom. Thus, in effect, Laidler also used four different kinds of structure elements to estimate heats of formation of alkanes, in agreement with the four different groups used by Benson. 

Whether an element is the source of the cation or anion in an ionic bond depends on several factors for which the periodic table can serve as a guide In forming ionic compounds elements at the left of the periodic table typically lose electrons giving a cation that has the same electron configuration as the nearest noble gas Loss of an elec tron from sodium for example yields Na which has the same electron configuration as neon... 

The strength of this bonding depends on the kind of ether Simple ethers form relatively weak complexes with metal ions but Charles J Pedersen of Du Pont discovered that cer tain polyethers form much more stable complexes with metal ions than do simple ethers Pedersen prepared a series of macrocyclic polyethers cyclic compounds contain mg four or more oxygens m a ring of 12 or more atoms He called these compounds crown ethers, because their molecular models resemble crowns Systematic nomencla ture of crown ethers is somewhat cumbersome and so Pedersen devised a shorthand description whereby the word crown is preceded by the total number of atoms m the ring and is followed by the number of oxygen atoms... 

In indazoles there are two possibilities for ring closure by creation of an N—C bond, depending on whether the bond is the N(2)—C(3) or the N(l)—C(7a) bond. Both are classical methods of indazole synthesis (types A and C (67HC(22)l)). An example of each class is shown in Scheme 50 (78S633). 

The strength of a glue bond depends on various parameters ... 

The results of the reductions of some steroidal a,)3-unsaturated ketones have been summarized by Brown. " The carbonyl group is usually reduced to the hydrocarbon, but the behavior of the double bond depends on the structure of the compound undergoing the reduction. Cholest-4-en-3-one gives chol-est-4-ene. Addition of aluminum chloride to a solution of a 4-ene-3,6-dione followed by treatment with LiAIH4 gives the 4-ene-6-one. Steroid 4,6-dien-3-ones yield mixtures of dienes. When the ketone and double bond are in different rings the results become even more complex dienes as well as mono-enes are obtained. 

The strength of this bonding depends on the kind of ether. Simple ethers fonn relatively weak complexes with metal ions, but Charles J. Pedersen of Du Pont discovered that certain polyethers fonn much more stable complexes with metal ions than do simple ethers. 

The various name reactions that have appeared in the literature over the years make use of a common retrosynthetic strategy, namely, disconnection at the C-N bond. Depending on how and at what time in the synthesis these bonds are formed dictates how the synthetic approach is classified, thus the name applied to the procedure (see Figure 8.1.5). 

In contrast to S l reactions, the Sn2 type represent a one-step process. The rate-determining stage, and hence the formation of the new bond, depends on the nucleophilicity of the anion (the nucleo-philicity is essentially a function of the polarizability ). In the Sn2 reaction under discussion, Eq, (7), the nitrogen in CH3—is replaced by the atom of the mesomeric anion which possesses the greatest nucleophilicity. 

With cyclic substrates, the formation of the new double bond depends on the availability of a c i -/3-hydrogen, which is required for the yw-elimination... 

The initial step is the coordination of the alkyl halide 2 to the Lewis acid to give a complex 4. The polar complex 4 can react as electrophilic agent. In cases where the group R can form a stable carbenium ion, e.g. a tert-buiyX cation, this may then act as the electrophile instead. The extent of polarization or even cleavage of the R-X bond depends on the structure of R as well as the Lewis acid used. The addition of carbenium ion species to the aromatic reactant, e.g. benzene 1, leads to formation of a cr-complex, e.g. the cyclohexadienyl cation 6, from which the aromatic system is reconstituted by loss of a proton ... 

During hydrogenation, intermediate aromatic hydroxylamines may undergo various cyclization reactions in molecules containing a suitably disposed carbonyl group, or carbonyl derivative, such as an oxime (13). The cyclized product may or may not maintain the N—OH bond, depending on the solvent, the catalyst, and the electrophilicity of the carbonyl (27,28,29,32,67,68). 

It has been noted5,10 that the length of a C—S bond depends also on the nature of the atoms or groups bonded to the carbon atoms. In this context, the sulfones and sulfoxides show greater sensitivity than the sulfides10, as is demonstrated by the following examples of C—S bond lengths (A) ... 

Triple bonds can be reduced, either by catalytic hydrogenation or by the other methods mentioned in the following two sections. The comparative reactivity of triple and double bonds depends on the catalyst. With most catalysts, (e.g., Pd) triple bonds are hydrogenated more easily, and therefore it is possible to add just 1 mol of hydrogen and reduce a triple bond to a double bond (usually a stereoselective syn addition) or to reduce a triple bond without affecting a double bond present in the same molecule. A particularly good catalyst for this purpose is the Lindlar catalyst (Pd-CaCOs—PbO). An alternative catalyst used for selective hydrogena-... 

Cumulated sulfur-sulfur bonds as in homocycles show a unique interdependence in so far as the length of a particular bond depends on the arithmetic mean of the lengths of the two neighboring bonds [77]. This is demonstrated in Fig. 9 for the bonds in the homocycles S (n = 6, 7, 8, 10, 12) and the sulfur-rich oxides S 0 (n = 7, 8). 

The metallation of 1,3-diselenanes is complex. When potassium diisopropylamide is used as base, deprotonation and alkylation affords the 2-equatorially substituted derivative <96TL2667>. However, with rertbutyllithium, Se-Li exchange is observed in preference to H-Li exchange in the reaction with 2-ox-methylseleno derivatives <96TL8015>. The reaction with nBuLi either forms the anion or cleaves a C-Se bond depending on the substituents present at the 2-, 4- and 6- positions <96TL8011>. 

It seems clear that the acidity of an intra-annular acid depends on its ability to form intramolecular hydrogen bonds. By how much the pAg values are changed by hydrogen bonds depends strongly on the medium and whether it can offer alternative hydrogen bonds or not. Therefore in even less polar media, an intra-annular acid should be even less acidic than the analogues. 

Figure 6.8. Summary of molecular orbital theory for homonuclear molecules. Note how the stability of a chemical bond depends both on the interaction strength and the filling of the orbitals.

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