Carbocation Stability

In 1902, Norris and Kehrman independently discovered that colorless triphenylmethanol gives deep-yellow solutions in concentrated sulfuric acid. Later on, in 1902, a scientist named Adolf von Baeyer was the first to recognize the salt-like characteristics of the compounds formed.

Carbocation stability order acts as reactive intermediates in most of the organic reactions. This idea, first proposed by Julius Stieglitz in 1899, was further developed by Hans Meerwein in his 1922 study of the Wagner-Meerwein rearrangement. We can also find that carbocation's stability order was involved in the SN1 reaction. A carbocation stability’s job is not being a carbocation, and there are two ways to do it. The first method is to get rid of the positive charge, and the second is to gain a negative charge. Now we will look at the definition of carbocation stability order.

Definition of Carbocation Stability

Until the year the 1970s, we all knew carbocations as carbonium ions. We usually look at carbocation as the positively charged carbon atom that it is. According to the valence of the charged carbon, we can classify it in two main categories. These two categories are protonated carbenes and Protonated alkane.

The heterolytic cleavage in an organic molecule where the carbon donates the shared pair of electrons to the leaving group. Which, in turn, results in the development of a positive charge on the carbon atom. Such species where covalency of carbon is three are called carbonium ions or carbocations (e.g., CH3+). The carbon atom of the carbocation is sp2 hybridized, and it uses the three hybridized orbitals for single bonding to three substituents; the remaining p-orbital is empty. However, the pentacoordinate positively charged species such as CH5+is called carbonium ion. G. A. Olah was the first person to propose this nomenclature. We can synthesize some stable carbocation as solid salts, and we can also synthesize some into a solution (free or ion pair). Now we will look at the classification of carbocations.

Classification of Carbocations Stability Order

Primary carbocation: The carbon bearing the positive charge is attached to one carbon of an alkyl or aryl group.

Secondary carbocation: Here, carbon with a positive charge is attached to two other carbons.

Tertiary carbocation: In this case, the positively charged carbon is attached to three carbon atoms.

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The image depicts the Primary, Secondary, Tertiary, and Methyl Carbocation Stability Order

Stability and Reactivity of Carbocations Order

If electrons were money, the carbocation stability order would be the beggars of organic chemistry. Since carbocation has only six valence electrons, these electron-deficient elements have a high tendency to take part in many reactions. Since it is electron-deficient, it is effortless to form a carbocation in these reactions.

So, now we can look at some of the ways we can stabilize carbocations. Like the chemical law, which states that opposite charges attract, and as charges repel, we gain the stability of carbocation order from the near electron-donating groups.

1. Stability of Carbocation Order by a Neighbouring Carbon Atom.

When we go from primary to secondary and then to tertiary carbons, the stability of carbocations increases, which is the result, as carbons belong to the electron-releasing groups and are connected to hydrogen, it will be electron-rich. Therefore, it can give away some of those electrons to the next carbocation. The second reason for this phenomenon to happen is hyperconjugation.

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This image is an example of the stability of carbocation order by neighbouring Carbon atoms.

2. Stability of Carbocation order by Carbon-Carbon Multiple Bonds.

In this method, the carbocations join together to form a carbon-carbon double or triple bond. Due to the formation of these bonds, they tend to have unique stability. It is mainly due to the overlap caused by the p orbitals of the π bond and the empty p orbital of the carbocation. Therefore, and it gives rise to the charge, which is shared between the atoms.  

3. Stability of Carbocation Order Formed by Adjacent Atoms Bearing Lone Pairs.

In this method, we focus on the atom that gives away a pair of electrons to the electron-deficient carbocation, which influences the stabilization process. It results in forming a double bond, and thus the charge will move to the atom that is donating the electron pair.

Solved Problems

Question 1. Rank the Carbocations in the Order of Decreasing Stability?

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Answer 

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As we all know, the carbocation will be the most stable if the carbocation is most substituted. The adjacent carbon atoms game away some of their electron density to the electron-deficient carbocations, making them more neutral and stable. So, the tertiary carbocation is the most stable, and the least is the methyl carbocation.

Question 2. Form 1-methylcyclohexane from the Two Carbocations?

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Answer

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From the above equations, we can see that only the tertiary carbocation is formed. The main reason is that tertiary carbocations are more stable than secondary carbocations.

FAQ (Frequently Asked Questions)

Q1. What are the Criteria for the Stability of Carbocations?

Ans. 

Some of the factors that we take into considerations are as follows:

  • Stability is directly proportional to the +I effect, that is the reason tertiary carbocation is more stable than secondary (explained by hyperconjugation or dispersal of charge)

  • Carbocation stability increases with an increase in the percentage of p character in hybridization.

  • Carbocation stability increases with increasing alpha hydrogen atoms due to hyperconjugation.

  • Adjacent lone pairs of electrons stabilize carbocation due to resonance.

  • Carbocations are stabilized by neighboring carbon multiple bonds due to resonance.

Q2. Which Carbocation is More Stable: Benzyl or Tertiary?


Ans. In the 1-degree benzylic carbocation, the comparison is made between tertiary carbocation and Benzyl carbocation. It is applicable only for methyl-substituted ones.

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Image of a conjugative structure with nine alpha hydrogen.

The maximum number of hyper conjugative structures dominates over octet resonance. Therefore, the tertiary carbocation is more stable than benzyl carbocation. In contrast, when it becomes 2nd-degree benzylic carbocation, it acts differently. The number of resonating structures increases drastically to make it dominate over nine alpha hydrogens, which is the case of the tertiary carbocation.