Question

What structural or electronic aspect of (2E)-1-(4-Nitrophenyl)-3-(2-pyridinyl)-2-propen-1-one prevents the resonance argument and empirical parameters from adequately predicting the H-NMR chemical shifts of the alpha C-H?

Answer 1

Predicting the chemical shift of the olefinic hydrogens(double bond hydrogens), specially alpha hydrogen ( from carbonyl) is very tedious. Several electronic and structural effects play a role:

(a) had it been pure sp² olefinic hydrogens, the cemical shift would occur in the ranfe of 5.4-5.5. But, there, the olefinc group is attached from both sides with groups which are usually withdraw electron density from the environment. O is electroneagtive and phenyl ring also have electronic effects which deshield the olefinic hydrogens. These withdraw electron density from the hydrogen, thus deshielding the hydrogen which make them difficult to predict.

(b) Anisotropy of phenyl and carbonyl gropus also furthur deshield the alpha hydrogen, which makes the chemical shift more downfield.

(c) alpha hydrogen is a itself sp² hybridized which is deshielded in nature.

Hence, all these factors lead to a decrease in presence of electron near the hydrrogen, hence value of chemical shift becomes more downfield ( greater than 8).

Since other hydrogens of phenyl rings occur in the same range, it becomes difficult to identity the peak corresponding to the alpha hydrogen,