Chemistry Carbocation Stability: Attracting Inductive vs Mesomeric Effects

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SUMMARY

The discussion centers on the stability of carbocations, specifically comparing the stability of the 1st and 4th carbocations in the presence of a carbonyl group. The 4th carbocation is confirmed to be less stable due to the attracting inductive effect of the carbonyl group, which destabilizes the carbocation despite potential resonance stabilization. The resonance structure C=C-O+ is less stable because it places the positive charge on the more electronegative oxygen atom. Therefore, the 1st carbocation, an allyl cation, is more stable than the 4th.

PREREQUISITES
  • Understanding of carbocation stability and classification (primary, secondary, tertiary)
  • Knowledge of inductive and mesomeric (resonance) effects in organic chemistry
  • Familiarity with resonance structures and their implications on charge distribution
  • Basic principles of polar covalent bonds and electronegativity
NEXT STEPS
  • Study the stability of allyl cations in organic chemistry textbooks
  • Research the effects of inductive and resonance stabilization on carbocation stability
  • Learn about the mechanisms of resonance and how they affect molecular stability
  • Explore the comparative stability of secondary and tertiary alkyl carbocations
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Chemistry students, organic chemists, and anyone interested in understanding carbocation stability and the effects of functional groups on molecular structures.

duchuy
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Homework Statement
determine the stability of these following carbocations
Relevant Equations
x
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Hi,
Sorry in advance if the vocab is incorrect, I'm translating this from french.
In the correction, it's written that the 4th carbocation is the less stable due to the attracting inductif effect of the carbonyle group.
But isn't this carbocation stabilised by the mesomer effect of the C=O bond? And if it isn't stabilised by the mesomer effect can you please explain to me why?
And in case it is, how do we determine the stability between the 1st and 4th carbocation?
Thank you so much!
 
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The carbonyl group is polarised due to the resonance C=O ↔ C+-O-. The carbocation is destabilised by being next to a δ+ carbon.
Resonance with the carbonyl group is possible, but it would give a structure C=C-O+, which is less stable because it has the + charge on the more electronegative O atom.
So 4 is definitely less stable than 1. Whether it's less stable than 2 or 3 I'm not so sure.
 
#1 is an allyl cation. A good organic textbook will have an explanation of allyl group stability. The other two are secondary and tertiary alkyl carbocations, and a good textbook should also explain their order of stability.
 

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