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Selenium — like sulfur — is a chalkogen/member of group 16. It is heavier than sulfur, and thus more metalloid. In this regard similar to the trend in the adjacent group 17 (the halogens) where iodine has more metal-like character than the other members, or group 15 (the tetrels) with carbon; yet silicon, germanium, tin, and lead. Thus, formally, ...


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Species 1 is your seventh structure. The C=N=N atoms are linear and a long reach for the remaining nitrogen. Electrocyclization of 1 to 2 restores aromaticity. Nitrene species 3 allows for a 1,2-shift to an electron-deficient center with concommitant loss of strain in the 4-membered ring to produce 1H-benzo[d][1,2,3]triazole 4. Cyclization of 5 $\rightarrow$ ...


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Demethylation of phenolic ethers generally requires some pretty extreme conditions. In verifying the structure of vanillin Tiemann and Haarmann (page 620) demethylated it successfully with dilute hydrochloric acid at up to 200°C. To illustrate the "usual" extreme conditions for demethylation, the authors first tried fuming hydroiodic acid at up to ...


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The Zeisel reaction as described here reads like submitting the substrate to refluxing $\ce{HI}$ with $\ce{HOAc}$. This seems much more aggressive than the oxymercuration conditions in the first scheme, where you need a bit of $\ce{H+}$ to transfer the intermediate enol into the more stable ketone by keto-enol tautomerism. Compare e.g., with an entry on ...


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