Alkenes proton chemical shifts

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. 

Note that the lack of rotation about the double bond means that E and Z isomers are distinct entities in the same way that cis and trans isomers are distinct in conventional alkenes. It is not really feasible to give a comprehensive guide to the chemical shifts of these protons but expect them to be somewhat lower field (approx. 1 ppm) than for comparable alkenes, with chemical shifts being driven largely by the anisotropy of the substituents. 

The key chemical shifts for a munber of pharmacologically important 1,4-benzodiazepines have been tabulated <80jhci483, 85MRC280>. However, since for the most part, these compounds have an aryl substituent in the 5-position and a carbonyl group in the 2-position data on the diazepine ring protons are restricted to the 3-protons, which when methylene, are found between <5 4.3 and 4.9. In the less heavily substituted benzodiazepines (14) and (15) the 5-benzylic alkenic proton is shifted downfield to <5 8.35 <87CPB4ll0>. 

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). 

Hashimoto et al. [122-126] have used 13C-NMR as a tool for the determination of sulfonic acid and sulfonate species in AOS and its intermediates. The Na salts of C16 1-alkenesulfonic acid, Cu 2-alkenesulfonic acid, C18 9-alkene-sulfonic acid, 1-propylenesulfonic acid, and C16 3-hydroxyalkanesulfonic acid showed that alkenyl protons in 1-alkenesulfonic acids had NMR chemical shifts at 6.1-6.7 ppm, while other acids had chemical shifts at 5.4-5.9 ppm. The results were useful for rapid determination of 1-alkenesulfonic acid content. 

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

Table 5.6 Estimation of chemical shifts for alkene protons.

Enol ether protons are interesting in that their chemical shifts are unusually high field in comparison with other alkenes on account of lone pair donation into the double bond from oxygen (Structure 5.5). No special precautions are necessary when dealing with them as this is reflected in the values obtained using Table 5.6. 

It should always be remembered of course, that the NMR spectrum reflects a compound s behaviour in solution. It is quite possible for a compound and a weak acid to crystallise out as a stoichiometric salt and yet in solution, for the compound to give the appearance of a free base. For this reason, care should be taken in attempting to use NMR as a guide to the extent of protonation. If the acid has other protons that can be integrated reliably, e.g., the alkene protons in fumaric or maleic acid, then there should be no problem but if this is not the case, e.g., oxalic acid, then we would council caution Do not be tempted to give an estimate of acid content based on chemical shift. With weak acids, protonation may not occur in a pro rata fashion though it is likely to in the case of strong acids. 

Tables 5.2 and 5.3 give characteristic shifts for nuclei in some representative organic compounds. Table 5.4 gives characteristic chemical shifts for protons in common alkyl derivatives. Table 5.5 gives characteristic chemical shifts for the olefinic protons in common substituted alkenes. To a first approximation, the shifts induced by substituents attached an alkene are additive. So, for example, an olefinic proton which is trans to a -CN group and has a geminal alkyl group will have a chemical shift of approximately 6.25 ppm [5.25 + 0.55(tra .s-CN) + 0.45(gew-alkyl)].

The circulating electrons in the 7t-system of aromatic hydrocarbons and heterocycles generate a ring current and this in turn affects the chemical shifts of protons bonded to the periphery of the ring. This shift is usually greater (downfield from TMS) than that expected for the proton resonances of alkenes thus NMR spectroscopy can be used as a test for aromaticity . The chemical shift for the proton resonance of benzene is 7.2 ppm, whereas that of the C-1 proton of cyclohexene is 5.7 ppm, and the resonances of the protons of pyridine and pyrrole exhibit the chemical shifts shown in Box 1.12. 

The chemical shifts of H-3 and H-4 in coumarin (19) are similar to those in pyran-2-one (Figure 4). The values are closely related to the shifts of the a and /3 protons of o-coumaric acid (56), implying that the heteroring has little or no aromatic character (62pia(A)(56)71). The signal for H-3 appears at higher field than that from H-4 and distinction between 3-and 4-substituted isomers is usually possible. Coupling between the alkenic protons is typical of a m-alkene (/3t4 = 9.8 Hz) and the pair of doublets is a characteristic feature of the spectra of coumarins. 

In proton nmr spectra, the chemical shifts of alkenic hydrogens are toward lower fields than those of alkane hydrogens and normally fall in the range 4.6-5.3 ppm relative to TMS (see Section 9-10E and Table 9-4). Spin-spin couplings of alkenic hydrogens are discussed in Section 9-10G and 9-10J. 

Structures in which a methyl group is held over an alkene double bond pose a special problem. Rapid rotation of the methyl group results in the NMR chemical shift representing the average environment of the three protons. Both GIAO and the... 

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