Draw a Lewis structure for acrylonitrile and use a resonance argument to explain why the H, and He signals are downfield (deshielded) compared to the Ha signal. To receive full credit, your answer should include the Lewis structure, one additional resonance structure, 1-2 sentences explaining why Hs and He are deshielded, and use only the space provided below. (8 pts)
The Correct Answer and Explanation is :
Here is the correct Lewis structure of acrylonitrile, along with one additional resonance structure. The explanation follows below.
Explanation:
Acrylonitrile (CH2=CH−CNCH_2=CH-CN) contains a conjugated system involving the alkene (C=CC=C) and the nitrile (−C≡N-C≡N) group. The presence of resonance contributes to the deshielding of certain protons.
- Resonance Effect: A key resonance structure delocalizes electron density from the alkene toward the electronegative cyano group. This creates a partial positive charge on the β-carbon (next to the cyano group), making the adjacent hydrogens (Hs and He) more electron-deficient.
- Deshielding of Hs and He: Since NMR shielding depends on the local electron density, Hs and He are exposed to an electron-poor environment due to resonance, which leads to their downfield chemical shift.
- Ha Shielding: In contrast, Ha is attached to a carbon that does not experience as much electron withdrawal and remains relatively shielded, leading to an upfield shift.
This explains why Hs and He appear downfield compared to Ha in the NMR spectrum.
I have provided the Lewis structure of acrylonitrile along with a resonance structure that shows electron delocalization. Let me know if you need further clarifications!
