Deshielding substituents

Spectrum 5.5 shows the effect of a single deshielding substituent (carboxylic acid) on the benzene ring. 

This time, we observe a pronounced downheld shift of the protons ortho- to the deshielding substituent and note that the signal is dominated by the large ortho-coupling and that it also bears a smaller meta-one. The signal is however both roofed and is composed of more lines than you might naively expect. 

Thus, a substituent (X) that is more strongly shielding than H (ctx > cth) has a negative value of A8X (corresponding to an upfield shift), while a deshielding substituent (ctx < cth) has a positive A8X (corresponding to a downfield shift). ... 

Substitution of the cyclic alkanes by a deshielding substituent leads to a characteristic chemical shift for the hydrogen attached to the alpha carbon depending upon the size of the ring and the deshielding effect of the substituting group as listed below. 

The primary amine group acts as a weak deshielding substituent on methyl, methylene and methine groups, but has a strong shielding effect upon the ortho and para aromatic hydrogens. 

The nitrite group is one of the most strongly deshielding substituents in its effect on the alpha aliphatic groups. Methylene groups are deshielded to about 4.5 ppm and methines to about 5.5 ppm. Such extremes of chemical shift are characteristic of only a few substituents making the identification of a Nitrite compound a relatively easy matter. 

In comparison to the sulfoxides (-S(=0)-), the sulfones (-S(=0)2-) are a more strongly deshielding substituent in their effect on both the adjacent aliphatic groups and on the ortho aromatic protons. Some of the relative deshielding effect of the sulfur containing functional groups are displayed. 

This section contains the carbon-13 NMR chemical shifts of fluorinated hydrocarbons. The chemical shifts of these compounds illustrate not only that fluorine is one of the most strongly deshielding substituents in its effect on the alpha carbon (alkanes F- C =+69.9ppm, benzenes F-a C = +34.9ppm) but that fluorine 1... 

The chemical shift effect exerted by all of the different valence slates in which phosphorus can exist varies from that of a weakly deshielding substituent on adjacent aliphatic carbons to either a weakly shielding or weakly deshielding substituent on aromatic phenyl C-1 carbons. 

The chemical shift effect of the disulfide functional group displayed by the carbon-13 NMR chemical shifts in this section is that of a moderately strong deshielding substituent in relation to adjacent aliphatic carbons. A comparison of the aliphatic additivity constants for the sulfide and disulfide groups is given below. 

The deshielding effect of the anhydride group in relation to adjacent aliphatic carbons is that of a weakly deshielding substituent. 

The chemical shift effect of the carboxylic acid group is that of an intermediate deshielding substituent in its effect on aliphatic groups and a weakly deshielding moiety in its effect on adjacent aromatic carbons. 

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). 

The reversed polarity of the double bond is induced by a n electron-accepting substituent A (A = C=0, C=N, NO2) the carbon and proton in the p-position are deshielded (-A/effect, larger shifts). These substituents have analogous effects on the C atoms of aromatic and heteroaromatic rings. An electron donor D (see above) attached to the benzene ring deshields the (substituted) a-C atom (-/ effect). In contrast, in the ortho and para positions (or comparable positions in heteroaromatic rings) it causes a shielding +M effect, smaller H and C shifts), whereas the meta positions remain almost unaffected. 

An electron-accepting substituent A (see above) induces the reverse deshielding in ortho and para positions (-M effect, larger //and shifts ), again with no significant effect on meta positions. 

The methoxy group is a +M substituent, and so shields ortho protons and C atoms in ortho positions the protons at 5 = 6.67 and 6.79 reflect this shielding. The carbonyl group as a -M substituent deshields ortho protons, and is ortho to the proton at <5// = 7.97. With the additional doublebond equivalent for a ring, 6-methoxytetralin-l-one (E) results. 

The deshielding effects of electronegative substituents are cumulative, as the chemical shifts for various chlorinated derivatives of methane indicate. 

In general, and all other things being equal, the huorines of a tri-huoromethyl group are more deshielded than those of a CF2H or R—CF2—R group, which are themselves more deshielded than a single huorine substituent (Scheme 2.1). 

Those of the readers who are already quite familiar with proton NMR spectroscopy are aware that, as the most electronegative element, fluorine substituents deshield proximate hydrogens more than any other atomic substituent because of their unique inductive influence. This fact is exemplified below in Scheme 2.11. 

This trend is consistent with the trends for both proton and carbon chemical shifts, with the proton on the most highly substituted carbon, and the carbon with the most alkyl substituents being the most highly deshielded. 

When CH2F is a substituent on most alicyclic rings, such as a cyclohexane ring, the 19F chemical shift of this group is not significantly altered from that of an acyclic system (Scheme 3.2). On the other hand, when it is attached to a cyclopropane ring, a unique deshielding influence is observed. 

In going from a secondary to a tertiary cycloalkyl fluoride, one observes the usual deshielding effect as is exemplified by the isomeric 1 -lluoro-1 -mcthyl-4-t-butylcyclohcxancs. Of course, these two isomers exist essentially in the single conformation given, because of the presence of the 4-t-butyl substituent. 

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