Chemical shift allylic hydrogens

Chemical Shift. The position of the resonance signal of a proton (or of a group of identical protons) is an indication of the molecular environment of that proton and permits, e.g., the distinction of aliphatic, allylic, olefinic, and aromatic hydrogens. 

The enamines derived from aldehydes (Table 3) can be divided into two types. First, for those compounds where a hydrogen atom is bound to the -carbon atom the spectroscopic behaviour parallels that for the enamines derived from ketones. When the jS-carbon atom bears two substituents, its chemical shift increases. In addition to the chemical shift difference, for the allylic and )8-carbon atoms differences were noted for the coupling constants between the a-hydrogen and the allylic carbon atoms. The magnitude of the vicinal coupling constant changes with configuration and can... 

A similar, although much smaller, effect of tt electron systems is seen in the chemical shifts of benzylic and allylic hydrogens. The methyl hydrogens in hexamethylben-zene and in 2,3-dimethyl-2-butene are less shielded than those in ethane. 

The double and triple resonance experiments with roxburghine-D establish that the hydrogen atom at 0-15 Hg) is coupled to two hydrogens at C-14 and one at C-20 but there is also a small coupling between Hb and the hydrogen atom (H2) at C-17 (t/ 15,17 = 0.6 Hz). Such an allylic coupling is reasonable on the basis of structure 80 but is much less easily rationalized on the basis of structure 81. Further, the chemical shifts observed for the protons at C-15 and C-20 (S 2.15 and 1.76, respectively) are consistent with an allylic position for the former and an attachment of C-20 to saturated carbon atoms only. 

Alkenes have two types of hydrogens vinyl (those attached directly to the double bond) and allylic hydrogens (those attached to the a carbon, the carbon atom attached to the double bond). Each type has a characteristic chemical shift region. 

Two of the carbons of isomer B have chemical shifts characteristic of 5p -hybridized carbon. One of these bears two protons 8 118.8 ppm) the other bears one proton (8 134.2 ppm). The remaining carbon is p -hybridized and bears two hydrogens. Isomer B is allyl bromide. 

An alkyne with a triple bond at the end of a chain is called a terminal alkyne and the hydrogen atom at the end of the triple bond is referred to as an acetylenic hydrogen. This terminal proton is shielded by the anisotropy of the triple bond r-electrons, as shown in Figure 3.9, and so absorbs at about 1.8 ppm. The protons on the carbon next to the triple bond are affected in the same way as allylic protons in alkenes and absorb in the same chemical shift range. 

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