Carbocation
The primary job of the carbocation is to stop being a carbocation and it consists of two approaches. Either by getting rid of the positive charge or by gaining a negative charge. Both methods involve distributing the missing electrons to the electrons that lack carbon. It is essential to differentiate a carbocation from other cations.
A carbocation is short of electrons and has a positive charge, where the carbon itself is capable of accepting another two. This makes it a lewis acid and makes a carbocation different from other cations.
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 into 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 ions. 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.
Three Types of Carbocations
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.
Factors Influencing the Stability of Carbocations
Resonance: With the increasing number of resonances, the stability of carbocations increases. More number of resonating structures results in more stability of the carbocation. The delocalization of the positive charge is the main reason. Due to delocalization, the electron deficiency is decreased and the stability is increased.
In comparison, the resonance effect is a more dominating factor than substitution. The structures with resonance are better stabilized than others. Cyclopropane carbocation is remarkably very stable because of dancing resonance. Hence, tricyclo-propane carbocation is the most stable carbocation.
Hyperconjugation and Inductive Effect: Increasing substitution results in an increase of hyperconjugation and thus an increased instability. The higher the hyperconjugation, the higher will be its stability. The Carbocation stability as a whole depends on the number of carbon groups linked to the carbon that carries the positive charge.
Electronegativity: Electronegativity represents the ability of an atom to attract electrons. The more the electronegativity is, the more is the electron attraction towards the atom. The stability of the carbocation gets affected directly due to the electronegativity of the carbon that has a positive charge. If the electronegativity of the carbon atom increases, the stability of the carbocation decreases. sp > sp2 > sp3. In the vinylic carbocation, the hybridization of the positively charged carbon is sp, whose electronegativity is greater than the sp2 alkyl carbocation's hybridized carbon. Due to this, the stability of a primary vinylic carbocation is lesser than a primary alkyl carbocation.
Classification of Carbocation
The carbocation is termed as methyl, primary, secondary or tertiary based on the number of carbon atoms attached to it:
Methyl carbocation: If there is no carbon attached with the positively charged carbon it is called methyl carbocation.
If one carbon is attached to the positively charged carbon it is called the primary carbocation.
If two carbon is attached to the positively charged carbon it is called secondary carbocation.
If three-carbon is attached to the positively charged carbon it is called the tertiary carbocation.
If there is a presence of a carbon-carbon double bond near to the positively charged carbon it is termed allylic carbocation.
Similarly, if the positively charged carbon is attached to a double bond, the carbocation is called vinylic carbocation. Here, hybridization of the positively charged carbon is sp and geometry is linear.
Whenever the positively charged carbon is part of a benzene ring, then the carbocation is said to be aryl carbocation.
If the positively charged carbon is immediately next to a benzene ring, it is named a benzylic carbocation.
FAQs on Carbocation Stability
1. What is a carbocation, and what is its fundamental structure?
A carbocation is an organic chemical species that contains a carbon atom with a positive formal charge and only six valence electrons, making it electron-deficient. The positively charged carbon atom is typically sp² hybridized, resulting in a trigonal planar geometry with a vacant, unhybridized p-orbital perpendicular to the plane of the other three bonds.
2. What is the general order of stability for simple alkyl carbocations?
The stability of simple alkyl carbocations increases with the number of alkyl groups attached to the positively charged carbon. The accepted order of stability is:
- Tertiary (3°) carbocation (most stable)
- Secondary (2°) carbocation
- Primary (1°) carbocation
- Methyl carbocation (least stable)
Therefore, the order is 3° > 2° > 1° > Methyl.
3. What are the key electronic effects that determine carbocation stability?
The stability of a carbocation is primarily determined by effects that help disperse its positive charge and reduce its electron deficiency. The main factors are:
- Inductive Effect (+I Effect): Electron-donating groups, like alkyl groups, push electron density towards the positive carbon, reducing its charge and increasing stability.
- Hyperconjugation: The delocalisation of sigma (σ) electrons from adjacent C-H or C-C bonds into the empty p-orbital of the carbocation. More adjacent alkyl groups mean more hyperconjugative structures and greater stability.
- Resonance (Mesomeric Effect): Delocalisation of pi (π) electrons, which is particularly effective in allylic and benzylic carbocations. This spreads the positive charge over multiple atoms, leading to significant stabilisation.
4. What is the difference between an allylic and a benzylic carbocation?
The main difference lies in the system the carbocation is adjacent to. An allylic carbocation has the positive charge on a carbon atom that is directly attached to a carbon-carbon double bond (C=C-C⁺). A benzylic carbocation has the positive charge on a carbon atom that is directly attached to a benzene ring. Both are significantly stabilised by resonance.
5. Why is a tertiary (3°) carbocation more stable than a primary (1°) carbocation?
A tertiary carbocation, such as the tert-butyl carbocation ((CH₃)₃C⁺), is more stable than a primary one, like the ethyl carbocation (CH₃CH₂⁺), for two main reasons:
- Greater Inductive Effect: The three alkyl groups on a tertiary carbocation exert a stronger electron-donating inductive effect (+I) compared to the single alkyl group on a primary carbocation, neutralising the positive charge more effectively.
- More Hyperconjugation: A tert-butyl carbocation has nine α-hydrogens available for hyperconjugation, creating nine stabilising structures. An ethyl carbocation has only three α-hydrogens, resulting in fewer hyperconjugative forms and less stability.
6. How does resonance provide exceptional stability to a benzylic carbocation?
In a benzylic carbocation, the empty p-orbital on the benzylic carbon can overlap with the pi-electron system of the adjacent benzene ring. This allows the positive charge to be delocalised across the ring, specifically at the ortho and para positions, in addition to the benzylic carbon itself. This distribution of charge over multiple atoms, creating several resonating structures, drastically reduces the electron deficiency and makes the carbocation highly stable.
7. Why are vinylic and aryl carbocations considered very unstable?
The instability arises from the hybridization of the positively charged carbon. In both vinylic (C=C⁺) and aryl (benzene ring-C⁺) carbocations, the carbon bearing the positive charge is sp hybridized. An sp-hybridized carbon has more s-character (50%) than an sp²-hybridized carbon (33.3%), making it more electronegative. This high electronegativity makes the carbon atom less willing to bear a positive charge, leading to significant instability.
8. Which is more stable: a tertiary butyl carbocation or a benzyl carbocation? Explain the reasoning.
This is a classic comparison where two different stabilising effects compete. The benzyl carbocation is stabilised by extensive resonance, while the tertiary butyl carbocation is stabilised by nine hyperconjugative structures and the inductive effect. Generally, resonance is a more powerful stabilising effect than hyperconjugation. Therefore, the benzyl carbocation is typically considered more stable than the tertiary butyl carbocation. However, substitutions on the benzene ring can alter this order.
9. What is 'dancing resonance' and how does it explain the stability of a cyclopropylmethyl carbocation?
'Dancing resonance' is a term used to describe the unique orbital overlap that explains the extraordinary stability of the cyclopropylmethyl carbocation. The C-C bonds in a cyclopropane ring are strained and have significant p-character, behaving somewhat like π-bonds. These bent 'banana bonds' can effectively overlap with the adjacent empty p-orbital of the carbocation. This delocalises the positive charge into the ring structure, providing a degree of stabilisation that is even greater than that seen in a benzyl carbocation.
