Noting electronegativity

The two elements have similar electronegativity. (Note electronegativity is the power of an element to attract electrons to itself when present in a molecule or in an aggregate of unlike atoms it is a different property from the electrode potential, which depends on the free energy difference between an element in its standard state and a compound or ion in solution (see Section 20.1).) In addition a metal of a lower valency tends to dissolve a metal of a higher valency more readily than vice versa. 

The W—W bond energy should be about one-sixth of the sublimation energy (note Section III-IB), and there are various schemes for estimating electronegativities, of which Mulliken s [151,152] is perhaps the most fundamental. 

Figure 1.35 shows the second-row elements Li through F in their compounds with hydrogen. Note the transformation of hydrogen from hydride in LiH to a much smaller, protonlike entity in HF as the electronegativity of the heavier atom increases. 

Interesting stabilization and structural trends have been noted using MP2/6-31G calculations on the effect of substituents on imines. The data below give AE for the isodesmic reaction and show that stabilization tends to increase with Xbe 1 gi (>up electronegativity of the substituent. The X—N=CH2 bond angle decreases with Xbe-... 

The importance of solvent participation in the borderline mechanisms should be noted. Nucleophilic participation is minimized by high electronegativity, which reduces the Lewis basicity and polarizability of the solvent molecules. Trifluoroacetic acid and perfiuoro alcohols are among the least nucleophilic of the solvents used in solvolysis studies. These solvents are used to define the characteristics of reactions proceeding without nucleophilic solvent participation. Solvent nucleophilicity increases with the electron-donating capacity of the molecule. The order trifluoroacetic acid < trifluoroetha-nol <acetic acid < water < ethanol gives a qualitative indication of the trend in solvent nucleophilicity. More will be said about solvent nucleophilicity in Section 5.5. 

Strategy Identify the types of hydrogens in the molecule, and note whether each is alkyl, vinylic, or next to an electronegative atom. Then predict where each absorbs, using Table 13.3 if necessary. 

Closely related to the carboxylic acids and nitriles discussed in the previous chapter are the carboxylic acid derivatives, compounds in which an acyl group is bonded to an electronegative atom or substituent that can net as a leaving group in a substitution reaction. Many kinds of acid derivatives are known, but we ll be concerned primarily with four of the more common ones acid halides, acid anhydrides, esters, and amides. Esters and amides are common in both laboratory and biological chemistry, while acid halides and acid anhydrides are used only in the laboratory. Thioesters and acyl phosphates are encountered primarily in biological chemistry. Note the structural similarity between acid anhydrides and acy) phosphates. 

Variation of c/s-bond lengths has been noted in some cases and is believed to reflect steric interaction. Therefore, in the series PtX(PEt3)3 (X = H, Cl, F) the trans-bond to X shows a dependence upon the electronegativity of X, while the cz s-Pt—P bond shows no such dependence (Figure 3.91) [148], instead increasing as X becomes larger. 

It is worth noting in Fig. 2.19 how the presence of a strong electropositive adsorbate (K+) on the catalyst surface causes NO to behave as an electronegative adsorbate. Thus while on the clean surface NO behaves at high coverages as an electron donor it always behaves as an electron acceptor on the K+ promoted surface. 

It is important to notice that the work function, , of a given solid surface changes significantly with chemisorption. Thus oxygen chemisorption on transition metal surfaces causes up to 1 eV increase in while alkali chemisorption on transition metal surfaces causes up to 3 eV decrease in . In general electronegative, i.e. electron acceptor adsorbates cause an increase in 0 while electropositive, i.e. electron donor adsorbates cause a decrease in 0. Note that in the former case the dipole vector P formed by the adsorbate and the surface points to the vacuum while in the latter case P points to the surface (Fig. 4.20). 

Allylic radical are relatively stable, and the pentadienyl radical is particularly stable. In such molecules, (E), E), (E),(Z), and (Z),(Z) stereoisomers can form. It has been calculated that (Z),(Z)-pentadienyl radical is 5.6 kcal mol less stable than the ( ),( )-pentadienyl radical. ° It is noted that vinyl radical have (E) and (Z) forms and the inversion barrier from one to the other increases as the electronegativity of substituents increase. Enolate radicals are also known. ... 

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