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Why does pyridine require the lowest temperature for hydrogenation, even though it has it has the highest resonance energy amongst pyridine, pyrrole and furan? Is it just because of the increase in the absolute pressure or does the HCl also effect the reaction somehow?


for pyridine


for furan, pyrrole


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  • $\begingroup$ There's higher pressure though, and different catalyst, Pt is better I think. $\endgroup$ – Mithoron Aug 12 '17 at 15:48
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    $\begingroup$ In your example, you're not reducing pyridine, but pyridinium hydrochloride. The difference is quite large, pyridine requires 70 bar and up to 100 degrees Celsius to form piperidine at an effective rate, using nickel. $\endgroup$ – Beerhunter Aug 12 '17 at 17:05
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When we reduce pyridine using the $\ce{H2/Pt/HCl}$ reagent-catalyst combination, we require a significantly lower temperature and pressure compared to other similar reductions (but without $\ce{HCl}$), because $\ce{H+}$ (and other Lewis acids for that matter) will go and add to the nitrogen atom, forming pyridinium salts.

This makes the compound more reactive towards reduction (as a partial positive charge will develop in the ring).

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