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Consider the following statements: (I) CH3-O-CH2+ is more stable than CH3-CH2+ (II) (CH3)2CH+ is more stable than CH3-CH2-CH2+ (III) CH2=CH-CH2+ is more stable than CH3-CH2-CH2+ (IV) CH2=CH+ is more stable than CH3-CH2+ Which of these statements are correct?

1. I and II only
2. III and IV only
3. I, II and III
4. II, III and IV

Correct answer: I, II and III

Solution

Adjacent oxygen lone pairs (I) and allylic resonance (III) strongly stabilize a cation; a secondary cation is more stable than a primary one (II). A vinyl cation (IV), with the positive charge on an sp carbon and no resonance/hyperconjugation, is LESS stable than the ethyl cation, so (IV) is wrong.

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