Carbocation Stability: Attracting Inductive vs Mesomeric Effects

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In summary, the conversation discusses the stability of different carbocations and their determining factors. The carbonyl group polarizes the resonance, but it also destabilizes the carbocation due to the presence of a delta positive carbon. While resonance with the carbonyl group is possible, it results in a less stable structure. The stability of the 4th carbocation is questioned, and it is determined to be less stable than the 1st carbocation, and the stability of the 2nd and 3rd carbocations is still unclear. The stability of these carbocations is dependent on their structure, with the 1st being an allyl cation and the others being secondary and tertiary alkyl carbocations.
  • #1
duchuy
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Homework Statement
determine the stability of these following carbocations
Relevant Equations
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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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  • #2
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.
 
  • #3
#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.
 

1. What is a carbocation?

A carbocation is a positively charged carbon atom that has only three bonds. It is a highly reactive species and is often formed as an intermediate in chemical reactions.

2. What factors affect the stability of carbocations?

The stability of carbocations is affected by the number of alkyl groups attached to the positively charged carbon atom. The more alkyl groups, the more stable the carbocation. Additionally, the presence of electron-donating or electron-withdrawing groups can also impact the stability of a carbocation.

3. What is the difference between inductive and mesomeric effects?

Inductive effects refer to the polarization of bonds due to differences in electronegativity between atoms. This can result in the distribution of electron density towards or away from a carbocation, affecting its stability. Mesomeric effects, on the other hand, involve the delocalization of electrons through pi bonds, which can also impact the stability of a carbocation.

4. Which effect, inductive or mesomeric, has a greater impact on carbocation stability?

Mesomeric effects generally have a greater impact on carbocation stability compared to inductive effects. This is because mesomeric effects involve the delocalization of electrons, which can stabilize a carbocation by distributing the positive charge over multiple atoms.

5. How can the stability of a carbocation be determined experimentally?

The stability of a carbocation can be determined experimentally by measuring the rate of a reaction in which the carbocation is formed. A more stable carbocation will have a slower rate of reaction, as it is less likely to undergo further reactions. Additionally, techniques such as NMR spectroscopy can also be used to study the stability of carbocations.

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