The 1H NMR signal for a proton on an alkene adjacent to an electron-withdrawing group typically appears downfield or upfield?

• A. Upfield
• B. Midfield
• C. Downfield
• D. Not shifted

Answer: (C)

In NMR, shielding depends on the local electron environment. Electron-withdrawing groups pull electron density away from nearby protons, reducing shielding and pushing the resonance to a higher chemical shift. A proton on an alkene that is directly adjacent to such a withdrawing group feels strong deshielding because the nearby electronegative group and the adjacent pi system withdraw electron density from that vinylic proton. As a result, its signal appears at a higher ppm, i.e., downfield. This contrasts with protons that are more shielded (near electron-donating groups or farther from withdrawing effects), which would appear upfield. So the proton next to an electron-withdrawing group on an alkene shows a downfield signal.