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For the following pair of cations, state which one is more stable: (I) the benzyl cation, C6H5-CH2(+), and (II) the allyl cation, CH2=CH-CH2(+).

1. (I) benzyl cation is more stable because it is stabilised by an aromatic ring (resonance over the ring)
2. (II) allyl cation is more stable
3. Both are equally stable
4. Neither is resonance stabilised

Correct answer: (I) benzyl cation is more stable because it is stabilised by an aromatic ring (resonance over the ring)

Solution

The benzyl cation C6H5-CH2(+) delocalises its positive charge into the benzene ring, giving four important resonance structures (charge on the benzylic carbon plus the ortho and para ring carbons). The allyl cation CH2=CH-CH2(+) delocalises charge over only two carbons (two equivalent resonance structures). More extensive delocalisation lowers the energy more, so the benzyl cation is more stable than the allyl cation.

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