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SolutionsBenzylic Position

1. Benzylic positions

Mark the benzylic positions in the following compounds.

Twelve compounds: toluene, p-cymene, a methylated tetrahydrophenanthrene, 9,10-dihydroanthracene, 2-ethylpyridine, a tetrahydroquinoxaline, a cyclopentafuran, 9,10-dihydrophenanthrene, a tetrahydrobenzothiophene, methyltropylium, isopropylferrocene and a tetrahydroindene
Benzylic positions circled: they are the sp3 carbons directly bonded to an aromatic ring; the tetrahydroindene has none, because its cyclopentadiene is not aromatic

A position is benzylic when the carbon is directly bonded to an aromatic ring, whether a benzene, an aromatic heterocycle, the tropylium cation or the cyclopentadienyl anion of ferrocene.

2. Radical stability

Rank the following radicals by stability: methyl, benzyl, isopropyl and diphenylmethyl.

Diphenylmethyl > benzyl > isopropyl > methyl. Benzylic radicals are stabilized by delocalization into the ring, and diphenylmethyl into two rings.

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