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Question

Which is the most stable carbocation ?

  1. enter image description here

  2. enter image description here

  3. enter image description here

  4. enter image description here

Answer

3. enter image description here

My attempt

I answered as 1 as the first one had +M of -O- and hyperconjugation effect as the positive charge is at para position. Why is the answer 3? It just has +I and +H of the second chain and +M is stronger than +I

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  • $\begingroup$ When asking problem statements, could you try to conform to this template? You seem to be someone who would stick through SE for an extended period of time and we would be glad to keep it that way. $\endgroup$
    – Safdar Faisal
    Jul 31, 2020 at 13:09
  • $\begingroup$ As for the answer, a hint would be the strength of oxygen in direct extended conjugation is much stronger than that of carbon. think on that front. $\endgroup$
    – Safdar Faisal
    Jul 31, 2020 at 13:13
  • $\begingroup$ @Safdar okat ill stick to the template. about the hint, i dont understand what direct extended conjugation means. Could you please use a simpler term ?my chem level is that of a high school student $\endgroup$
    – Shaurya Goyal
    Jul 31, 2020 at 13:27
  • $\begingroup$ if you try to draw resonance structures, youll find that 3 has the most number of those. $\endgroup$
    – Aniruddha Deb
    Jul 31, 2020 at 16:05

1 Answer 1

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I think the third option as greatest number of resonance structures can be alluded to it.

The following may suffice:

  • In the first structure, the carbocation is isolated except for the presence of a single double bond in conjugation beside it. The oxygen and the double bond beside it play no role in stablizing it.
  • In the second structure, the lone pair on oxygen, the double bond and the carbocation are in conjugation. Hence, it is somewhat stable.
  • In three, similar conjugation like two is present, but is it extended over another double bond i.e. the lone pair of oxygen, two double bonds and the carbocation are all in conjugation and an extended resonance comes into play. Thus, it is more stable than two, since more number of resonance structures may be drawn for it than 2.
  • The fourth one is the least stable among them, as no conjugation or resonace structures may be constructed. Moreover the I- effect of the neighbouring oxygen tends to destablise the carbocation.

Hence, the order of stablity:

3>2>1>4

An arrow-pushing representation of resonance in the third option can be made as follows: Resonance diagram for third option

Hope it helps.

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