Question

Broadband-decoupled CNMR spectra for three compounds with the molecular formula C3H8O are shown in Figure 23.15 (Mohrig 4th ed). Deduce the structure of each compound, estimate the chemical shift of each of its carbon atoms using the additive parameters in Table 23.3 and 23.4, and assin the NMR signals to their respective carbon atoms. The measured chemical shifts of the carbon atoms follow. (a) 10.2, 25.8,64.4 ppm, (b) 25.3, 64.2 ppm, (c) 15.0, 58.2, and 67.9 ppm

Answer 1

Given Molecular formula C3H8O.

Corresponding hydrocarbon formula C3H8 (no compansation for O removal). For a saturated hydrocarbon formula with 3 C atoms is C3H8 (general formula C_nH_2n+2 where n = number of C atoms.)

Hence there is unsaturation or cyclic structure in given unknown comound. Based on this and given chemical shifts structures of unknown compounds is suggested.

(a) 10.2, 25.8,64.4 ppm

3 kinds of CNMR signals indicates 3 different kinds of C in unknown as per chemical shifts,

64.4 ppm suggests aliphatic sp3 C attached to electron withdrawing atom i.e O here so -CH2-O- is possible segment.

25.8 ppm suggests aliphatic sp3 C attached to electronwithdrawer at beta postion i.e -CH2-CH2-O- segemnt possible.

10.2 ppm suggest aliphatic sp3 C i.e. -CH3 group possible struture proposed is,

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(b) 25.3, 64.2 ppm,

Only 2 chemical shifts indicates 2 kinds of protons.

64.2 ppm indicative of secondary sp3 C atom bonded to electronwithdrawer like OH.

25.3 ppm indicate sp3 C i.e -CH3 groups with -OH at beta position.

Suggested structure is,

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(c) 15.0, 58.2, and 67.9 ppm

3 different chemical shifts indicates 3 different kinds of protons.

58.2, and 67.9 ppm both indicate sp3 C bonded to electron withdrawer like O so possible segment is -CH2-O-CH3.

terminal methyl here will be upfield to intermediate -CH2- hence according chemical shifts are asigned in structure.

(c) 15.0,ppm indicate -CH3 group.

Hence structure proposed is

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