Substituent electronegativity

Substituent effects (substituent increments) tabulated in more detail in the literature demonstrate that C chemical shifts of individual carbon nuclei in alkenes and aromatic as well as heteroaromatic compounds can be predicted approximately by means of mesomeric effects (resonance effects). Thus, an electron donor substituent D [D = OC//j, SC//j, N(C//j)2] attached to a C=C double bond shields the (l-C atom and the -proton (+M effect, smaller shift), whereas the a-position is deshielded (larger shift) as a result of substituent electronegativity (-/ effect). 

Haasnoot, C. A. G., De Leeuw, F. A. A. M., De Leeuw, H. P. M., Altona, C. Relationship between proton-proton NMR coupling constants and substituent electronegativities. III. Conformational analysis of proline rings in solution using a generalized Karplus equation. Biopolymers 1981, 20,1211-1245. 

A simple model, known as the Internal equivalent field model (9) (see "Figure 1") accounts for the different p values of mono-substltuted benzene derivatives R-X, and relates them to the respective donor or acceptor strength of X. It Is assumed In this model that the substituent action on the it electrons of the ring Is equivalent to that of a D-C field EQ with direction and Intensity related to the substituent electronegativity. Identifying dipole expansions of R-X In presence of E and of R In presence of E0 + E yields ... 

Nearly four decades ago, American chemist Henry Bent40 formulated a remarkable principle that relates atomic hybridization to substituent electronegativity. This principle, now called Bent s rule, was originally expressed in the following words ... 

Equations (3.75a)-(3.75j) constitute a more quantitative formulation of the relationship between hybrid p character and substituent electronegativity, generalizing Bent s rule. The accuracy of these approximations is generally of the order of a few percent, sufficient to determine hybrid angles within 1-2° as illustrated in the following examples. 

The results of recent ab initio computations are shown in Table 39. As the substituent electronegativity increases the energy difference between (A) and (B) does indeed decrease. However, this is a limited trend since OH leads to a smaller difference than F. Additional factors must be involved. 

Effects on benzene geometry stemming from substituent electronegativity are dominated by changes in internal angles. Nonetheless, the idea that the electronegative character of an annelation can induce bond fixation has been forwarded. In order to resolve the situation, a direct test of 7t-donor character (i.e. 

There are excellent texts on proton chemical shifts listed in the references and this topic will not be discussed here. Specific aspects of substituent electronegativity, hyperconjugation and solvent effects are treated separately whenever a sufficient amount of data is available for the purpose of interpretation. 

Letcher and Van Wazer<1966,2,3 1967,11 have proposed to deal with s and p contributions, and especially their 7r-terms, as being by far the most important in relation to sizeable parameters, e.g. substituent electronegativities and bond angles, of phosphorus. As the latter are known in but a few compounds the use of derived formulae can only be qualitative as for phosphonate anions/1967,71 Of course, the consideration of only s and p orbitals is better when dealing with di- or tri- valent... 

Single-bond 13C- H coupling constants for six-membered heteroaromatic compounds lie in the approximate range 150-220 Hz, the magnitude varying with substituent electronegativity. Data for... 

If the central atom is in the third row or below in the periodic table, the lone pair will occupy a stereochemically inactive s orbital and the bonding will be through p orbitals and near 90° bond angles if the substituent electronegativity is —2.5... 

Recently, it has been suggested (285) on the basis of ab initio calculations that the second ionization band of dimolybdenum tetracarboxylates contains both the it and o ionizations as opposed to the assignment presented in the text. This interpretation must be viewed with caution because inter alia it requires the degeneracy of the 2EU and 2A,g ion states for a wide range of substituent electronegativity. 

A correlation between 13C chemical shifts and substituent electronegativities is found for the a carbons in many classes of compounds, as is illustrated for the halohexanes in Table 3.2. 

The bond polarization due to electronegative substituents should propagate along the carbon chain and decrease with the inverse cube of the distance. Much smaller inductive effects are thus expected in /I and y position. Table 3.2 shows, however, that the observed ft and y effects do not correlate at all with substituent electronegativities and that the influence of other effects must therefore be involved. 

Fig. 4.8. Substituent, electronegativities Ex versus 13C shifts (<5C in ppm) of substituted benzenoid carbons, corrected for magnetic anisotropy effects [386],...

Increments for a larger number of substituents in various positions permit 13C shift predictions of pyrroles [435], thiazoles [437], indoles [445], and pyridines [452], Similarly to benzene, substituted carbon shifts essentially are related to substituent electronegativity, while ortho- and para-like carbons are predominantly affected by mesomeric (resonance) effects of donor (( — )-M) or acceptor substituents as shown for methoxy... 

Fig. 5.11. I3C Chemical shift values versus substituent electronegativity, Et, for 5-fluoro-, 5-chloro-, 5-bromo- and 5-iodouracil and their different carbon atoms [756].

The pKa values of a series of conjugated acids of the arsonium ylides have been determined by potentiometric titration (73). A decrease in the basicity of the ylides with an increase in substituent electronegativity was observed. The effect of the substituents agrees well with the Hammett equation. Arsonium ylides are more basic (200-230 times) than the corresponding phosphonium compounds. 

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