Deshielding alkenyl carbons

When the fluorine substituent is located at the 2-position or on any alkyl-substituted alkenyl carbon, it experiences the usual deshielding of 30-40 ppm (Scheme 3.38). Note the interesting variation in the chemical shifts and coupling constants for the 1-fluorocycloalkenes. 

The carbon-13 NMR chemical shifts of alkenes that are contained in this section illustrate the low field chemical shifts of the alkenyl carbon atoms (100-150ppm) and the weakly deshielding effect of alkene linkages on the chemical shifts of adjacent aliphatic groups. The aliphatic additivity constants for two forms are given below. 

The hydrogens on methyl groups attached to alkenyl carbons resonate at about 6 = 1.6 ppm (see Table 10-2). Explain the deshielding of these hydrogens relative to hydrogens on methyl groups in alkanes. (Hint Try to apply the principles in Figure 11-9.)... 

The proton chemical shifts of representative acyclic and cyclic dienamines are collected in Tables 14-16. Examination of the Tables show that alkenyl substitution at C(l) or C(2) of an enamine produces also deshielding of the olefinic protons, as seen by comparing the <5H(2) values of compound 149 (3 3.75, 3.85 ppm Table 11) with those of the analogous cross-conjugated dienamine 105 ( 3.85, 4.05 ppm Table 14). The chemical shift of the vinylic protons of dienamines correlate only partially with the charge calculated for the corresponding olefinic carbons . In the linear conjugated... 

Why is deshielding so pronounced for alkenyl hydrogens Although the electron-withdrawing character of the p -hybridized carbon is partly responsible, another phenomenon is more important the movement of the electrons in the ir bond. When subjected to an external magnetic field perpendicular to the double-bond axis, these electrons enter into a circular motion. This motion induces a local magnetic field that reinforces the external... 

Alkenyl hydrogens (and carbons) are deshielded and give rise to relatively low-field NMR signals compared with those in saturated alkanes (Section 11-4). In contrast, alkynyl hydrogens have chemical shifts at relatively high field, mnch closer to those in alkanes. Similarly, the sp-hybridized carbons absorb in a range between that recorded for alkenes and alkanes. Alkynes, especially terminal ones, are also readily identified by IR spectroscopy. Finally, mass spectrometry can be a useful tool for identification and structure elucidation of alkynes. 

Unlike alkenyl hydrogens, which are deshielded and give NMR signals at 5 = 4.6-5.7 ppm, protons bound to ip-hybridized carbon atoms are found at S = 1.7-3.1 ppm (Table 10-2). For example, in the NMR spectrum of 3,3-dimethyl-l-butyne, the alkynyl hydrogen resonates at 5 = 2.06 ppm (Figure 13-3). 

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