Aldehydes deshielding

The induced field of a carbonyl group (C=0) deshields protons in much Ihe same way lhal a carbon-carbon double bond does and Ihe presence of oxygen makes il even more eleclron wilhdrawmg Thus protons attached to C=0 m aldehydes are Ihe leasl shielded of any protons bonded to carbon They have chemical shifts m Ihe range 8 9-10... 

NMR Like other carbonyl groups the carbon of the —CO2H group of a car boxylic acid is strongly deshielded (8 160-185) but not as much as that of an aldehyde or ketone (8 190-215)... 

Protons of substrueture B and C are assigned by means of the mesomerie effeet of the aldehyde group whieh deshields the protons in o-position of the attaehed p-disubstituted benzenoid ring and in p-position of the eentral CC double bond ort/io-protons of the monosubstituted benzenoid ring D split into a doublet beeause of one ortho eoupling ( 7.5 Hz) while the meta-protons split into a triplet beeause of two ortho eouplings. 

The impact of a carboxylic acid function upon the chemical shift of a CF2H group is almost indistinguishable from the impact of a ketone, whereas secondary CF2 groups next to an acid or ester function are slightly deshielded relative to those next to a ketone or aldehyde (Scheme 4.35). 

H and 13C NMR Data. The ester function of ethyl difluoroac-etate deshields the CF2H proton slightly (about 0.1 ppm), whereas as was the case for ketones and aldehydes, it shields the carbon of either a CF2H or a CF2-alkyl group significantly (by about lOppm) (Scheme 4.37). 

So what about aromatic protons (<56.0-9.5) aldehyde protons (<59.5—9.6), or even protons oh double, nay triple bonds (<52.5-3.1) All these protons are attached to carbons with n bonds, double or triple bonds, or aromatic systems. The electrons in these n bonds generate their own little local magnetic field. This local field is not spherically symmetric — it can shield or deshield protons depending on where the protons are — it s anisotropic. In Fig. 137, the shielding regions have plusses on them, and deshielding regions have minuses. 

Unlike the 13C-NMR method, H-NMR spectra are not applicable to 3-alkylaldehydes. For 3-arylaldehydes, the chemical shifts of 2-H, 3 -H, and 5-H appear deshielded in oxazolidines derived from chiral aldehydes with configuration A, where Rz is the aryl group. 

Earlier work had revealed [70JCS(C)980] that the 3-arylidene derivatives possess the Z-configuration. Gallina and Liberatori have confirmed this. In addition, they have isolated both Z and E alkylidene derivatives from condensation with aliphatic aldehydes. In the H-NMR spectrum, the vinylic proton is deshielded by 0.65 ppm in the Z isomer (88) compared to the E isomer (89). Other workers have confirmed these observations [80JCS(P1)419],... 

Electron-withdrawing groups, such as aldehydes and carboxylic acids, cause deshielding to both Ca and Cp, so that Cp is usually more deshielded, while electron-donating groups cause deshielding of Ca but shielding of Cp. 

Carbonyl carbons, such as those in esters, carboxylic acids, amides, ketones and aldehydes, in which the sp carbon is bonded by a ir-bond to the highly electronegative O, have a characteristic chemical shift of S 160-220 (Box 4.13). Within this range, the sp carbon atoms of aldehydes and ketones are most deshielded and usually resonate at 8 185-220, while those of carboxylic acids and their derivatives resonate about 160-180. 

Electron withdrawing substituents such as aldehyde or imonium functions at the other end of the alkene or the 1,3-diene transform enamines and 1,3-dienamines into push-pull systems characteristic of dyes. Electron release (push) of the donor group shields the / , S, e,... carbons and electron withdrawal (pull) of the acceptor group (carbonyl oxygen or imonium nitrogen) deshields in the a, y,... position [343],... 

The fact that the H2 ( aldehydic ) proton of 112 is more deshielded than the corresponding proton in 113 is in favor of the dithiolylidene-aldehyde formula for 112, whereas the chemical shifts in 113 favor the bicyclic formula with a relatively strong ring current.13 49... 

---