Electronegative substituents, effect

Table Bl.11.1. Effect of an electronegative substituent upon methyl shifts in X- C H. ...

The decreased shielding caused by electronegative substituents is primarily an inductive effect and like other inductive effects falls off rapidly as the number of bonds between the substituent and the proton increases Compare the chemical shifts of the pro tons m propane and 1 mtropropane... 

The deshieldmg effects of electronegative substituents are cumulative as the chem ical shifts for various chlorinated derivatives of methane indicate... 

The effect of structure on acidity was introduced m Section 1 15 where we developed the generalization that electronegative substituents near an lomzable hydrogen increase Its acidity Substituent effects on the acidity of carboxylic acids have been extensively studied... 

An electronegative substituent adjacent to a ring oxygen atom also shows a preference for an axial orientation. This is known as the anomeric effect , and is particularly significant to the conformations of carbohydrates (B-71MI20100, B-83MI20100). 

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). 

Substituent effects (electronegativity, configuration) influence these coupling constants in four-, five- and seven-membered ring systems, sometimes reversing the cis-tmns relationship so that other NMR methods of structure elucidation, e.g. NOE difference spectra (see Section 2.3.5), are needed to provide conclusive results. However, the coupling constants of vicinal protons in cyclohexane and its heterocyclic analogues (pyranoses, piperidines) and also in alkenes (Table 2.10) are particularly informative. 

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... 

Carbanion-stabilizing effects have been calculated at several levels of theory. Table 7.6 gives some gas-phase data. The AMI and PM3 semiempirical calculations have also been done in water. The order NO2 > CH=0 > CN > Ph > CH2=CH is in accord with the experimental trends and reflects charge delocalization. The electronegative substituents F, OH, and NH2 are stabilizing by virtue of polar effects. The small stabilization provided by CH3 is presumabfy a polarization effect. 

Anomeric effect (Section 25.8) The preference for an electronegative substituent, especially a hydroxyl group, to occupy an axial orientation when bonded to the anomeric carbon in the pyranose form of a carbohydrate. 

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