Alkenes carbon-13 chemical shifts

NMR was equally helpful (consult Table 10-6 for typical carbon chemical shifts), as it clearly revealed the two C=0 carbons at 6 = 171.0 and 173.4 ppm and indicated the presence of eight alkene and benzene carbons (8 > 95 ppm). The three tetrahedral carbons attached to oxygen showed up as peaks between 8 = 61 and 81 ppm, and the two remaining tetrahedral carbon signals occurred at S = 28.9 and 37.4 ppm All of these were identified further by establishing the number of attached hydrogens by techniques equivalent to DEPT NMR. 

Comparison of C NMR Absorptions of Alkenes Table 11-3 with the Corresponding Alkane Carbon Chemical Shifts (in ppm)... 

The carbon NMR absorptions of the alkenes also are highly revealing. Relative to alkanes, the corresponding alkenyl carbons (with similar substituents) absorb at about 1(X) ppm lower field (see Table 10-6). Two examples are shown in Table 11-3, in which the carbon chemical shifts of an alkene are compared with those of its saturated counterpart. Recall that, in broad-band decoupled NMR spectroscopy, all magnetically unique carbons absorb as sharp single lines (Section 10-9). It is therefore very easy to determine the presence of sp carbons by this method. 

In general, the factors that affect the chemical shifts of carbons are the same as for protons (i.e. electron density around the nucleus in question, and anisotropy effects). Carbon chemical shifts can be readily calculated from tables of shift effects found in many texts. However, unlike protons attached to sp carbons, sp carbons attached to sp carbons exhibit only a small shift difference. There are also few good substituent parameters available for calculating the chemical shifts of alkene carbons bearing polar groups, unlike the calculation of NMR chemical shifts near polar groups. However, in systems where resonance is present, some predictions can be made of relative shift differences in the carbons (see Figure 1). 

The proton chemical shifts of the protons directly attached to the basic three carbon skeleton are found between 5.0 and 6.8 ppm. The J(H,H) between these protons is about -5 Hz. The shift region is similar to the region for similarly substituted alkenes, although the spread in shifts is smaller and the allene proton resonances are slightly upfield from the alkene resonances. We could not establish a reliable additivity rule for the allene proton shifts as we could for the shifts (vide infra) and therefore we found the proton shifts much less valuable for the structural analysis of the allene moiety than the NMR data on the basic three-carbon system. 

Substituents on both sides of the double bond are considered separately. Additional vinyl carbons are treated as if they were alkyl carbons. The method is applicable to alicyclic alkenes in small rings carbons are counted twice, i.e., from both sides of the double bond where applicable. The constant in the equation is the chemical shift for ethylene. The effect of other substituent groups is tabulated below. 

In contrast to H shifts, C shifts cannot in general be used to distinguish between aromatic and heteroaromatic compounds on the one hand and alkenes on the other (Table 2.2). Cyclopropane carbon atoms stand out, however, by showing particularly small shifts in both the C and the H NMR spectra. By analogy with their proton resonances, the C chemical shifts of k electron-deficient heteroaromatics (pyridine type) are larger than those of k electron-rieh heteroaromatic rings (pyrrole type). 

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). 

Proton and Carbon NMR Data. Some representative chemical shift and coupling constant data are provided in Scheme 3.48 for alkenes with vicinal fluorines. 

This is not the case for trisubstituted alkenes. A study on the 13C chemical shifts of oe,/ -disubstituted methyl vinyl ethers showed that the /(-carbon is strongly deshielded (Ills ppm) in the Z- compared to the -isomer268. It is believed268 that in the Z-isomer there is a reduced conjugation within the vinyloxy 7r-system due to non-coplanarity induced by steric interference between the methoxy group and R2. 

The / -methylene of an enone is typically 10-15 ppm downfield from the corresponding alkene (6). This general observation is attributed to a reduction of electron density at the / -methylene carbon due to conjugation with the carbonyl. In the case of 2,4,4-trimethylpentene-3-one, the / -methylene carbon of the enone resonates only 3.5 ppm downfield from the / -methylene carbon of the analogous alkene. Table I contains the carbon-13 chemical shift data for a series of enones and their corresponding alkenes that were measured under the same conditions. These C-13 NMR data indicate that 2,4,4-trimethylpentene-... 

Carbon-13 chemical shifts of some representative alkenes [90, 232 236] are collected in Table 4.10. Inspection of the data shows that the shift value increases with increasing alkylation of the observed olefinic carbon atom. 

Substituents attached to the olefinic carbon atoms exert a substantial effect on the chemical shift of both of these carbon atoms. These effects are exemplified by the chemical shift values for monosubstituted alkenes shown in Appendix 3, Table A3.13. 

Numerous tables, empirical equations, and even computer programs are available that enable the chemical shifts of the carbons of most compounds to be predicted rather accurately. However, it is possible to assign the carbons responsible for the peaks in many spectra based only on the limited information presented here. For example, the peak at 198.0 S in Figure 14.11 is assigned to the carbonyl carbon, and the peak at 26.1 S is due to the methyl carbon. The two alkene carbons appear at 137.5 and 128.5 S. 

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