Electronegativity series

The SE1 mechanism, equations (24) and (25), implies that dialkylmercurys should ionise according to equation (24) in solvent ethanol, even in the absence of hydrogen chloride. But dialkylmercurys are quite stable in ethanolic solution and thus reaction (24) would seem to be unrealistic Winstein and Traylor27 have already pointed out that the typical dialkylmercurys used in the studies of Kharasch do not follow an SE1 mechanism. There is thus very little theoretical basis for regarding (23) as an electronegativity series. 

The intuitive type of chemical reaction involves the participation of chemically dissimilar atoms, traditionally referred to as electropositive atoms reacting with electronegative atoms. The mutual affinity between such types is self-evident. The electronegativity series has been defined to arrange all elements in sequence, starting from the most electronegative at the highest value to the most electropositive, at the minimum. 

Looking at the electronegative series of the elements, we see that the noble metals, Hg, Au, Pt, Ir, Pd, Os, Rh and Ru, all form metal ions which are classified as soft. Their heats of hydration are all less than expected and their inertness is thus explained. 

In Pauling s electronegativity series, the most electronegative element, fluorine, with E = 4.0, acts as a standard or reference element. In this scheme, carbon has a value of E = 2.5 and hydrogen, a value of 2.1. Every combination of elements with electronegativity higher than 2.5 with those of... 

Pauling electronegativity series based on bond energies is the best known. 

Sachet E, Currie JF (1988) A comparison of electronegativity series. J Electr Spectr Relat Phenom 46 173-177... 

Grandberg I, Faizova GK, Kost AN. Gomparative basicities of substituted pyridines and electronegativity series for substituents in the pyridine series. 

An easy way to keep track of the effect of the s character of carbon is to associ ate It with electronegativity As the s character of carbon increases so does that carbon s electronegativity (the electrons m the bond involving that orbital are closer to carbon) The hydrogens m C—H bonds behave as if they are attached to an increasingly more electronegative carbon m the series ethane ethylene acetylene... 

In general, the dissection of substituertt effects need not be limited to resonance and polar components, vdiich are of special prominence in reactions of aromatic compounds.. ny type of substituent interaction with a reaction center could be characterized by a substituent constant characteristic of the particular type of interaction and a reaction parameter indicating the sensitivity of the reaction series to that particular type of interactioa For example, it has been suggested that electronegativity and polarizability can be treated as substituent effects separate from polar and resonance effects. This gives rise to the equation... 

The ease with which an atom gains or loses electrons is termed die electronegativity of die element. Tabulation of die elements in order of ease hy which diey lose electrons is called die electrochemical series and is shown in Table 6.10. Chapter 4 explains die importance of diis to die formation and control of coiTosion, and Chapter 6 discusses die relevance to predicting reactivity of metals towards water and their potential to become pyrophoric. 

Electronegativity. The effect of electronegativity on acidity is evident in the following series involving bonds between hydrogen and the second-row elements C, N, O, and F. 

Using the relationship from the preceding section that the effective electronegativity of carbon in a C—H bond increases with its s character (sp < sp < sp), the order of hydrocarbon acidity behaves much like the preceding methane, ammonia, water, hydrogen fluoride series. 

The bonds become shorter and stronger in the series as the electronegativity increases N—H longest and weakest, H—F shortest and strongest. 

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