The natural compound l-carvone is found in spearmint and gingergrass oil. l-Carvone has a negative o — Analytical Chemistry — Spectroscopy Chemistry Question
Carvone
The natural compound l-carvone is found in spearmint and gingergrass oil. l-Carvone has a negative optical rotation. Its enantiomer d-carvone which has a positive optical rotation is present in caraway seeds. Carvone has been analyzed and consists of 80.00 % carbon, 9.33 % hydrogen and 10.67 % oxygen. Mass spectroscopy indicates that the molecular mass of carvone = 150. The NMR and IR spectra of carvone are shown below. [VISUAL] In the UV spectrum of carvone there is a strong absorption maximum at 238 nm.
Calculate the molecular formula of carvone.
Model Answer
Number of C-atoms: NC = (Mr, × %C) / 12 = (150 × 0.8) / 12 = 10
Number of H-atoms: NH = (Mr, × %H) / 1 = (150 × 0.0933) / 1 = 14
Number of O-atoms: NO = (Mr, × %C) / 16 = (150 × 0.1067) / 16 = 1
Calculate the number of unsaturation of carvone.
Model Answer
A fully saturated hydrocarbon with 10 carbon atoms has the formula C10H22.
Carvone has the formula C10H14O. For calculation of the unsaturation, the O is not relevant. Subtraction gives a shortage of 8H for carvone. This is equivalent to 4 unsaturated sites (either double bonds or rings).
Which functional group is responsible for the strong absorption at 1680 cm -1 in the IR spectrum?
Model Answer
C═O group.
In the IR spectrum there is no absorption above 3000 cm -1. Indicate which type of functional group is absent in carvone.
Model Answer
–OH (–COOH is not a correct answer. Carvone has one oxygen atom only.) There is no relevant strong absorption above 3000 cm –1. That means that no –OH group is present.
Propose the most likely structure for carvone when it is given that it is a 1,2,4-trisubstituted 6-membered ring system. (The overlapping multiplets consist of a CH and a CH2 signal.)
Model Answer
Carvone contains a 6-membered ring. This leaves three more unsaturated sites.
The IR indicates the presence of a C═O group, and this leaves 2 more unsaturated sites and they must C═C bonds.
The strong UV-absorption suggests a conjugated system, most likely C═C–C═O.
The singlets at 1.63 and 1.68 ppm in the 1 H-NMR are two –CH3 groups with no vicinal coupling.
The multiplet from 1.9 – 2.2 ppm consists of a –CH and a –CH2– groups (from the question).
The multiplet from 2.2 – 2.5 ppm is most likely a –CH2– group with many neighbouring H-atoms.
The dublets at 4.75 and 4.93 ppm are indicative of two =CH– groups, it might even be =CH2 given the small and identical coupling constants (see the enlarged area).
The triplet at 6.73 ppm is indicative of a =CH– which is situated next to –CH2– group. Combining all these information with the given 1,2.4 substitution pattern gives the structure below as the most likely structure of carvone. [VISUAL]