H (a) Assign the C (lowercase Roman numerals - i, ii, iii, etc.) and H (lowercase Roman letters a, b, c, etc.) atoms in the given structures using the "C (top) and 'H (bottom) NMR spectra provided. 6 200 iii 150 100 PPM a PPM 2 50
C13 NMR determination:
In this molecule 4 types of C-atoms are present. They are designated by i, ii, iii, iv.
The C-atom i.e. C-iv is the most deshielded carbon as it is a ketone carbonyl and the shift value is nearly at 208 ppm.
The carbonyl carbon of ester group (C-iii) is somewhat shielded than C-iv. The electron donating nature of O-atom(+R effect) in ester group decreases the electron deficiency of carbonyl carbon. So it appears at nearly 170 ppm.
The carbon atom that is C-ii is directly attached to electronegative O-atom (-I effect) and is adjacent to carbonyl group. So it is also deshielded but it is a sp3 carbon. Therefore C-13 NMR value is smaller than sp2 carbons like C-iv and C-iii. So it appears at nearly 78 ppm.
The carbon atom C-i is also sp3 hybridized and it is adjacent to two carbonyl groups. But it is not directly connected to any electronegative atom. Therefore it is also deshielded but not like C-ii. It appears at nearly 50 ppm.
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