Question 18.38: Based on the following carbon NMR data, deduce the structure......
Based on the following carbon NMR data, deduce the structures of isomeric ketones having the molecular formula C₅H₁₀O.
(a) 18.1 ppm, 27.3 ppm, 41.5 ppm, 211.7 ppm
(b) 13.5 ppm, 18.5 ppm, 29.3 ppm, 45.2 ppm, 206.6 ppm
(c) 7.3 ppm, 35.3 ppm, 209.3 ppm
Compounds with the formula C₅H₁₀O must contain a ring or a double bond. Possibilities include cyclic ethers, cyclic alcohols, unsaturated alcohols, unsaturated ethers, and ketones.
(a) The compound has four nonequivalent carbon atoms. The carbons with chemical shifts of 27.3 and 41.5 ppm are alpha to a carbonyl carbon with a chemical shift of 211.7 ppm. The signal with a chemical shift of 18.1 ppm corresponds to two equivalent methyl groups. The compound is 3-methyl-2-butanone.
(b) Since there are five resonances and five carbon atoms, there must be five nonequivalent carbon atoms. The carbons with chemical shifts of 29.3 and 45.2 ppm are alpha to a carbonyl carbon with a chemical shift of 209.2 ppm. The resonances at 18.5 and 13.5 ppm corresponds to a methylene group and a methyl group. The compound is 2-pentanone.
(c) Since there are three resonances and five carbon atoms, there must be three nonequivalent carbon atoms. A carbonyl carbon with a chemical shift of 209.3 ppm is bonded to two equivalent ethyl groups. The compound is 3-pentanone.