Chemical shift of monosubstituted benzenes

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). 

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

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."... 

The physico-chemical properties of monosubstituted benzenes were discussed in terms of inductive and resonance effects [9-1 ], the last being very important in the interpretation of electronic spectra and of dipole moments. In vapour phase, the near ultraviolet spectra of monosubstituted benzenes are shifted towards longer wave lengths as compared to the benzene spectrum [15]. Sponer et al [16] have established such a shift of, 035 cm for aniline. We established the position of the electronovibrational component max vapour phase, at 39,6 0 cm for benzene, and at 3, 870 cm... 

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

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).

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. 

CHEMICAL SHIFTS OF PROTONS ON MONOSUBSTITUTED BENZENE RINGS... 

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... 

Recently Nelson et al. have shown that the chemical shifts of the carbon atoms of monosubstituted phenyl rings are proportional to the CNDO/2 calculated total charge densities (178). It was concluded that it is possible to estimate the total charge density at the meta- or para-carbon atoms for a monosubstituted benzene to 0.005 charge unit simply by measuring the NMR spectrum. A similar, but less accurate linear relationship exists for all the carbons in the phenyl ring. There is also a linear relationship between the chemical shift for the para position and the Hammett a value for the substituent. This linear relationship has previously been reported for phenylphosphorus derivatives (193). 

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-... 

Table 3.10 Chemical Shifts of Protons on Monosubstituted Benzene Rings ...

Thus, in estimating various a constants for heteroaromatic groups, the most reliable and convenient data are those on reactivity and physical parameters, especially the relative chemical shifts of meta and para carbon atoms of the benzene ring in C-NMR spectra of monosubstituted benzenes. Heteroaromatic groups are assumed to be substituents. 

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. )...

The chemical shifts of multiply substituted benzenes and naphthalenes can be estimated using the substituent effects in the corresponding monosubstituted hydrocarbons. 

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. 

---