Aldehydic hydrogens, chemical shift

Structural enviromnent. The hydrogen chemical shifts typical of standard organic structural units are listed in Table 10-2. It is important to be famihar with the chemical shift ranges for the stractural types described so far alkanes, haloalkanes, ethers, alcohols, aldehydes, and ketones. Others will be discussed in more detail in subsequent chapters. 

The two bridgehead hydrogens Ha and Hb in the anisaldehyde dithioace-tal (130) have a large difference in their acidities, as indicated by the NMR chemical-shift difference (4.88 and 5.03 ppm). Preparation of the monocarbanion (1 BuLi in THF at -78°C) and quenching with DC1 removed only the higher field hydrogen. The carbanion can be reacted with electrophiles such as primary halides, acid halides, or aldehydes to produce (135). Carbanion generation and alkylation can be repeated on (135) to yield the disubstituted derivative (136) as shown in Scheme 44. 

The carbonyl carbon atoms of aldehydes and ketones have chemical shifts around 200 ppm in the carbon NMR spectrum. Because they have no hydrogens attached, ketone carbonyl carbon atoms usually give weaker absorptions than aldehydes. The a carbon atoms usually absorb at chemical shifts of about 30 to 40 ppm. Figure 18-2 shows the spin-decoupled carbon NMR spectrum of heptan-2-one, in which the carbonyl carbon absorbs at 208 ppm, and the a carbon atoms absorb at 30 ppm (methyl) and 44 ppm (methylene). 

Proton magnetic resonance techniques have been used for the measurement of rates of hydrogen-deuterium exchange of pyrazine (in CHsOD-CHsONa at 164.6") (591) for a study of protonation of pyrazine (1472) for analysis of the reaction mixture from quatemization of 2-substituted pyrazines with methyl iodide (666) for elucidation or confirmation of the structures of alkylpyrazines obtained by alkylation of pyrazines with aldehydes and ketones in the presence of a solution of an alkali or alkaline earth metal in liquid ammonia, or a suspension of these metals in other solvents (614) for a study of changes in chemical shifts produced on ionization of 2-methyl and 2-amino derivatives of pyrazine in liquid ammonia (665) for characterization of methoxymethylpyrazines (686) for the determination of the position of the A -oxide function in monosubstituted pyrazine V-oxides and the analysis of V-oxidation reactions (838) for a study of the structure of the cations of fV-oxides of monosubstituted pyrazines (1136) and for the determination of the structure of the products of peroxyacetic and peroxysulfuric acid iV-oxidation of phenyl- and chlorophenylpyrazines (733b). 

Of importance in understanding chemical shift patterns of phenols is, of course, also the effect of taking part in hydrogen bonding as, e.g., in salicylaldehyde, o-hydroxyaceto-phenones etc. Firstly, the anisotropy caused by the OH group but also the anisotropy effects of the other substituent (aldehyde, ketone etc.) lead to extensive non-additivity if using the standard values mentioned above. 

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... 

The hydrazones which are primarily used as derivatives for the characterization of ketones and aldehydes produce rather wide ranges of chemical shifts for the —CH=N— proton and for the various types of NH hydrogens. Both groups are quite sensitive to the substituent and its position on adjacent aromatic rings, various nitro-phenyl hydrazones being a common variety. 

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