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Which of the following carbocations is the most stable?

1. An alpha-keto carbocation bearing an adjacent -OCH3 group
2. A cross-conjugated carbocation with an -OCH3 group and the positive charge on a terminal carbon
3. A methoxy-substituted allylic carbocation (CH2=CH-CH(+)-OCH3 type)
4. A methoxy-substituted linearly conjugated carbocation with the positive charge on a terminal carbon

Correct answer: A methoxy-substituted linearly conjugated carbocation with the positive charge on a terminal carbon

Solution

A carbocation is most stabilized when its empty orbital receives the most electron donation. A methoxy oxygen lone pair gives powerful resonance (+M) stabilization (forming an oxocarbenium ion). When the positive charge also sits at a terminal position of an extended/linear conjugated pi system, both the oxygen lone pair and the conjugated pi electrons can delocalize onto the cationic carbon, giving the greatest number of stabilizing resonance structures. An adjacent carbonyl (option A) is destabilizing (-I/-M of C=O withdraws density), and a simple allylic methoxy cation (option C) has less extended conjugation than option D.

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