Question 18.37: Based on the following C-13 NMR data, deduce the structures ......
Based on the following C-13 NMR data, deduce the structures of isomeric compounds having the molecular formula C₄H₈O.
(a) 25.7 ppm, 68.0 ppm
(b) 15.5 ppm, 41.0 ppm, 204.9 ppm
(c) 7.9 ppm, 29.4 ppm, 36.9 ppm, 209.2 ppm
(d) 13.7 ppm, 15.7 ppm, 45.8 ppm, 207.6 ppm
Compounds with the formula C₄H₈O must contain a ring or a double bond. Possibilities include cyclic ethers, cyclic alcohols, unsaturated alcohols, unsaturated ethers, and ketones.
(a) A compound with only two C-13 resonance must has two sets of nonequivalent carbon atoms. The signals correspond to an internal methylene group, d = 25.7 ppm, and a methylene group bonded to oxygen, d = 68.0 ppm. The compound is tetrahydrofuran.
(b) Since there are three resonances and four carbon atoms, two of the carbons must be equivalent. The carbon with a chemical shift of 204.9 ppm is an aldehyde carbon. The resonance at 41.0 ppm corresponds to a methylene group bonded to the carbonyl carbon; the carbon with a chemical shift of 15.5 ppm corresponds to two equivalent methyl group. The compound is 2-methylpropanal.
(c) Since there are four resonances and four carbon atoms, there must be four nonequivalent carbon atoms. The carbons with chemical shifts of 29.4 and 36.9 ppm are alpha to a carbonyl carbon with a chemical shift of 209.2 ppm. The resonance at 7.9 ppm corresponds to a methyl group bonded to an alpha carbon. The compound is 2-butanone.
(d) Since there are four resonances and four carbon atoms, there must be four nonequivalent carbon atoms. The carbons with chemical shifts of 45.8, 15.7, and 13.7 are C-21, C-3, and C-4; C-1 is an aldehyde carbon. The compound is butanal.