A 100 MHz 1_H NMR spectrum of methyl crotonate (sec structure below) was run. a)

Question

A 100 MHz 1_H NMR spectrum of methyl crotonate (sec structure below) was run. a) Use a tree diagram to predict what the signal of H_a would look like in this spectrum. Sketch and label your tree diagram. Then sketch and label the spectrum showing only the Ha signal. Make sure you include a labeled chemical shift axis. On your sketch of the spectrum, show how the actual coupling constants associated with Ha can be measured from the spectrum. Use the following data: J_ab = 16 Hz delta H_a = 5.90 ppm J_ac = 2 Hz delta H_c = 6.90 ppm J_bc = 6Hz delta H_c = 1.90 ppm b) In what way would the spectrum change if it were run on a 400 MHz spectrometer? Illustrate your answer by sketching the Ha portion of the 400 MHz spectrum on a labeled frequency axis having the same ppm scale as your original spectrum. c) What would be the expected appearance/multiplicity of the H_b signal?

Explanation / Answer

a) Drawn below is the tree diagram for the Ha

b) If the spectrum is run at 400 MHz instrument instead, we would see multiplet for Ha, Ha here would also couple with CH3 protons along with the Hb proton.

c) Ha with one neighbouring proton would appear as doublet.

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