Question

The chemical shifts of the C-2 hydrogen in the spectra of pyrrole, pyridine, and pyrrolidine are 2.82 ppm, 6.42 ppm, and 8.50 ppm. Match each heterocycle with its chemical shift.

Accepted Answer

Understand the problem: The task is to match the given chemical shifts (2.82 ppm, 6.42 ppm, and 8.50 ppm) to the correct heterocyclic compounds (pyrrole, pyridine, and pyrrolidine) based on their electronic environments and aromaticity. Step 1: Recall the structure and properties of each heterocycle. Pyrrole is an aromatic five-membered ring with a nitrogen atom contributing a lone pair to the aromatic π-system. Pyridine is an aromatic six-membered ring with a nitrogen atom that does not contribute its lone pair to the π-system. Pyrrolidine is a non-aromatic five-membered ring with a nitrogen atom in an sp³ hybridized state. Step 2: Analyze the chemical shift trends. In general, aromatic protons experience deshielding due to the ring current effect, leading to higher chemical shifts. Non-aromatic systems, like pyrrolidine, typically have lower chemical shifts due to the absence of this effect. Step 3: Match the chemical shifts to the heterocycles. The lowest chemical shift (2.82 ppm) corresponds to pyrrolidine, as it is non-aromatic and has less deshielding. The intermediate chemical shift (6.42 ppm) corresponds to pyrrole, which is aromatic but has a less electronegative nitrogen atom. The highest chemical shift (8.50 ppm) corresponds to pyridine, as its nitrogen atom is more electronegative and contributes to greater deshielding. Step 4: Verify the reasoning by considering the electronic environments of the C-2 hydrogens in each compound. Pyrrolidine's C-2 hydrogen is in a relatively shielded environment, pyrrole's C-2 hydrogen is in an aromatic environment with moderate deshielding, and pyridine's C-2 hydrogen is in an aromatic environment with significant deshielding due to the electronegative nitrogen atom.

The chemical shifts of the C-2 hydrogen in the spectra of pyrrole, pyridine, and pyrrolidine are 2.82 ppm, 6.42 ppm, and 8.50 ppm. Match each heterocycle with its chemical shift.

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Key Concepts

Chemical Shift

Heterocycles

Electronegative Atoms and Their Effects