Shifts in Monosubstituted Benzene

Cp) of several monosubstituted benzenes are linearly related to the total 7t-electron density at the para position. Furthermore, the corrected shifts (5Cp — are linearly related to/>-carbon 7t-electron density due to resonance interaction with the substituent. These conclusions, based on comparisons with F studies in similar compounds, indicate that both shifts in monosubstituted benzenes and F shifts of />ara-substituted fluorobenzenes are useful parameters for determining the 7t-electron density at the />-carbon atoms. 

Effect of Substituents on the C-Chemical Shifts in Monosubstituted Benzenes (6 in ppm relative to TMS)... 

Table 5 Effect of substituents on the chemical shifts in monosubstituted benzenes ...

The observed regioselectivity of the addition of asymmetrically substituted olefins RCH=CH2 (R = Me, OH, CO2H, CN, Cl, etc.) was rationalized in terms of the magnitude of the electronic effect, calculated by using the "C NMR chemical shifts for monosubstituted benzene and polarizability."... 

Substituent increments Z, obtained from 13C shifts of numerous monosubstituted benzenes according to eq. (4.15) have been tabulated [383]. They permit prediction of benzene ring carbon shifts in multisubstituted benzenes according to eq. (4.16). These increments and their practical application will be summarized in Section 4.16. 

Most of the substituent increments presented in Table 4.82 can be derived from 3C shifts of benzenoid carbons in monosubstituted benzenes as listed in Table 4.53. Additional substituent increments are available for fused aromatic rings such as naphthalene and... 

Since chemical shift increments are approximately additive, it is possible to calculate the ring proton shifts in polysubstituted benzene rings from the monosubstituted values in Appendix Chart D.l. The chemical shift increments for the ring protons of m-diacetylbenzene ... 

A number of workers have reported the NMR spectra of mono-substituted phenyl groups and correlated the shifts vdth molecular parameters. It has been shown for organic compounds that the chemical shift of the para-carbon in monosubstituted benzenes is linearly related to the total TT-electron density at the para position in these compounds. Also the shift separation of the meta- and para-carbons appears to be linearly related to the rr-electron density on the para-carbon due to resonance interaction with the substituent 49, 156). Spiesecke and Schneider have reported a good linear relationship between the para-carbon chemical shift of monosubstituted benzenes and the Hammett, o-para constant, but no such relationship appears to exist for the other carbon chemical shifts, except between the chemical shift for the substituted carbon atom (corrected for magnetic anisotropy effects of the substituent) and the electronegativity of the substituent 210). 

The chemical shifts of meta and para (but not ortho carbon atoms in monosubstituted benzenes correlate well with the inductive and resonance constants of the substituents " . Examination of and a° constants obtained from NMR data (Table 14) shows that for the groups containing a penta-coordinate silicon atom the cr-donor component is enhanced and the rc-acceptor one is lowered. Similar conclusions have been reached by other measurements e.g., the in-... 

Fig. 1. Differences in n electron density, calculated by the "n inductive model , versus chemical shift of the para carbon in monosubstituted benzenes. (From Clark. )...

Charge Distributions in Monosubstituted Benzenes and in Meta- and Para-Substituted Fluoroberuenes, Ah Initio Calculations of Comparison with H. andSubstituent Shifts (Hetue, Taft,... 

Effect of Substituents on Chemical Shifts of Monosubstituted Benzenes (in ppm)... 

First the five protons (integral) of the //NMR spectrum (Sfj = 7.50 - 7.94) in the chemical shift range appropriate for aromatics indicate a monosubstituted benzene ring with typical coupling constants 8.0 Hz for ortho protons, 1.5 Hz for meta protons.). The chemical shift values especially for the protons which are positioned ortho to the substituent Sn = 7.94) reflect a -M effect. Using the CH COLOC plot it can be established from the correlation signal hclS = 66.AI7.94 that it is a benzoyl group A. 

Fig. 12-1. Correlation between azo coupling rates of substituted benzenediazonium ions with the 2-naph-thoxide-3,6-disulfonate trianion and the chemical shifts of protons at the position of the diazonio group in NMR spectra of the corresponding monosubstituted benzene parent compounds (Diener and Zollinger, 1986).

Carbon-13 shift values of a small selection of monosubstituted benzenes [383] are collected in Table 4.53. Signal assignments are based on conventional techniques such as proton off-resonance and gated decoupling as well as comparative measurements of specifically deuterated compounds [384],... 

Table 4.53. 13C Chemical Shifts of Representative Monosubstituted Benzenes (<5C in ppm) [383].

A quantitative description of the reactivity of monosubstituted benzenes to electrophilic substitution based on considerations of inductive effect parameters and con-jugative effect parameters from the 13 C chemical shifts of the aromatic compounds has been proposed.3 MO calculations on the proton migration in the ipso adducts formed in the reaction of CH3+ and SiH3+ with benzene have been described.4 With SiH3+ the ipso adduct is the most stable of possible isomers, whereas for CH3+ the >ara-protonated isomer is the most stable. 

TABLE 4.12 Incremental Shifts of the Aromatic Carbon Atoms of Monosubstituted Benzenes (ppm from Benzene at 128.5 ppm). Carbon Atom of Substituents in parts per million from TMSa... 

FIGURE 4.8 Chemical shifts of protons ortho, meta, and para to the substituent in a series of monosubstituted benzenes. Chemical shifts are expressed relative to benzene as zero. Adapted from Spiesecke and Schneider.58... 

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