Cinnamomum loureiroi, also known as Vietnamese cinnamon, is an evergreen tree grown in the central a — Organic Chemistry Chemistry Question
Vietnamese cinnamon
Cinnamomum loureiroi, also known as Vietnamese cinnamon, is an evergreen tree grown in the central and northern regions of Vietnam. Aromatic bark of the tree has medicinal and culinary value. Essential oil of the bark contains cinnamaldehyde ((2E-3-phenylprop-2-enal) as the main component.
Mild oxidation of cinnamaldehyde with NaClO2 affords an acid A. A undergoes subsequent esterification with ethanol to form ethyl cinnamate (B). Refluxing ethyl cinnamate and 80% hydrazine hydrate solution for 10 h gives C (C9H10N2O). Refluxing C and p-nitrobenzaldehyde in ethanol for 12 h affords D (C16H13N3O3).
1H NMR spectra of C and D are given below (the multiplicity and relative intensity is shown for each signal).
[VISUAL]
Give the structures for A, B, C.
Model Answer
A: Cinnamic acid ((2E-3-phenylprop-2-enoic acid)
B: Ethyl cinnamate
C: (2E-3-phenylprop-2-enehydrazide (cinnamic acid hydrazide)
Assign 1H NMR signals in the first spectrum to appropriate proton groups of C.
Model Answer
9.0 ppm, s: He
7.4-7.3 ppm, m: C6H5
5.5 ppm, s: Hd
4.2 ppm, t: Hc
2.8 ppm, dd: Ha
2.6 ppm, dd: Hb
Propose a reaction mechanism for the formation of C from B.
Model Answer
The mechanism involves the nucleophilic attack of hydrazine (NH2NH2) on the ester carbonyl carbon of ethyl cinnamate (B) to form a tetrahedral intermediate, followed by the collapse of this intermediate and the elimination of an ethoxide group (which is protonated to ethanol) to yield the cinnamic acid hydrazide (C).
Among four given below structures, select one for D and give the reasons for your selection.
[VISUAL]
Model Answer
Selected structure: D3
Reasons for selection:
- D3 derives from C and corresponds with the given 1H NMR spectrum.
- D1 and D4 do not derive from C.
- D1 does not correspond with the second 1H NMR spectrum (for example, in the spectrum there are not two ethylenic protons).
- D2 and D4 seem corresponding to the given 1H NMR spectrum, but D2 contains three-membered and D4 contains four-membered cycles, which cannot exist after refluxing for 12 h.
Assign 1H NMR signals in the second spectrum to appropriate proton groups of D.
Model Answer
9.6 ppm, s: Hd
8.3 ppm, d: 2 Hf
7.6 ppm, d: 2 He
7.4-7.3 ppm, m: C6H5
3.0 ppm, t: Hc
2.9 ppm, dd: Ha
2.6 ppm, dd: Hb