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Carbocation: Carbocation, Stability of carbocation, Bredt’s Rule, Rearrangement of Carbocation, Practice Problems, FAQs

Carbocation: Carbocation, Stability of carbocation, Bredt’s Rule, Rearrangement of Carbocation, Practice Problems, FAQs

A carbocation is a molecule with three bonds and a positively charged carbon atom. A carbocation is a general term for them. It was previously known as carbonium ion. Any even-electron cation with a large positive charge on the carbon atom is now known as a carbocation.

Because of the incomplete octet, the carbon cations are extremely reactive and unstable. Carbocations lack eight electrons and so do not satisfy the octet rule.

A carbocation is commonly seen in reactions, elimination reactions, and other chemical reactions.

 

Table of contents

  • Carbocation
  • Classification of Carbocations
  • Formation of Carbocation
  • Stability of Carbocation
  • Overall Stability order of Carbocations
  • Bredt’s Rule
  • Stability of Alicyclic Cations
  • Rearrangement of Carbocation
  • Practice Problems
  • Frequently Asked Questions

Carbocation

It is a reaction intermediate that contains three bond pairs and a positive charge on it. Therefore, it has a total of six valence electrons. Carbocations can be (methyl carbocation) or sp hybridized (vinyl carbocation).

Classification of carbocations

The number of carbon groups bound to the carbon atom of the carbocation determines the name of the carbocation. The number of carbon atoms connected to the carbocation determines whether it is primary, secondary, or tertiary:

  1. Primary Carbocation: In this carbocation, only one carbon is attached to the carbon atom which contains +ve charge.

  1. Secondary Carbocation: In this carbocation, two carbon atoms are attached to the carbon atom which contains +ve charge.

  1. Tertiary Carbocation: In this carbocation, three carbon atoms are attached to the carbon atom which contains +ve charge.

There are some other examples of carbocations which are given as:

  1. Methyl Carbocation
  • It has 6 electrons in the outermost shell.
  • All electrons are paired, hence it is diamagnetic.
  • It behaves as a Lewis acid.

  1. Allylic Carbocation
  • Carbon-containing positive charge next to the carbon-carbon double bond is called allylic carbocation.

  1. Vinylic Carbocation
  • When one of the carbon-carbon double bond having a +ve charge is called vinylic carbocation.

  1. Aryl Carbocation
  • When a carbon with a positive charge is a part of a benzene ring is called aryl carbocation.

  1. Benzylic Carbocation
  • A benzylic carbocation is formed when a positively charged carbon is present next to a benzene ring.

Formation of Carbocation

There are two fundamental steps involved in the formation of a carbocation:

  1. Heterolytic Bond Cleavage of Carbon
  2. Electrophilic Addition

Heterolytic Bond Cleavage

When the bond between carbon and the atom bound to it is heterolytically cleaved, the leaving group leaves with the shared pair of electrons, and leaves the carbon as a carbocation which is an electron-deficient species.

It is formed as an intermediate in many organic reactions.

The lower the activation energy, the greater the chances of bond cleavage or formation of a more stable carbocation. 

Electrophilic Addition

In electrophilic addition, an electrophile attacks an unsaturated point (double or triple bond), the pi bond breaks, and carbocation forms. The lower the activation energy, the faster the addition, and the more stable the carbocations are.

Stability of carbocation

The factors affecting the stability of carbocations are as follows:

  • Aromatic Effect
  • Mesomeric effect (+M)
  • Delocalisation of charge/ pi electrons (resonance)
  • Hyperconjugation (+H)
  • Inductive effect (+I)

Overall Stability order of Carbocations

[1] 

[2]

Important points for the stability order of carbocations

  • Generally, Aromatic carbocations are the most stable due to the delocalization of charge.

Example: The seven-membered aromatic tropylium cation, containing six pi electrons is the most stable in the given order followed by cyclopropenyl carbocation (double bond in conjugation with a positive charge) having two pi electrons. 

  • The benzyl cation () is more stable than the allyl cation ( ) due to +M effects of the benzene ring in the benzyl cation.
  • Among the alkyl carbocations, the tertiary carbocation is the most stable due to the three electron-releasing alkyl groups. The primary carbocation is the least stable because it has only one alkyl group.
  • Since the electronegativity of the hybridized carbon atoms is less than that of the sp hybridized carbon atoms, thus methyl carbocation is more stable than the vinyl cation 

().

  • Also, the phenyl cation is less stable than the vinyl cation. The phenyl cation is rarely seen because of its instability.

Bredt’s Rule

Small ring bicyclic molecules cannot have carbocations or double bonds at the bridgehead position because they are hybridized.

If one or two-membered bridges are there in the bicyclic molecule, then these are known as small ring bicyclic compounds.

The bridgehead carbons in the norbornane molecule are shown in the following figure:

Example:

Possibility 1 Possibility 2

In the molecule, a 

positive charge on a bridgehead

carbon makes the system much more

strained and unstable

Can not be formed

In the given molecule, a

positive charge is 

not present on the bridgehead carbon, so it is comparatively stable carbocation

Can be formed

Stability of Alicyclic Cations

Each hybridized carbon atom has some bond angles associated with it. When that bond angle is not attained, the molecule gets an angular strain. For the hybridized carbon of the carbocation, the expected bond angle is .

Angular strain is the result of deviation from ideal bond angles caused by inherent structural constraints (such as ring size).

Carbocation 

Molecule

Expected Bond

Angle

Actual Bond

Angle

Angular Strain
0

Bond angle and strain involved for various carbocations

The cyclic rings with the least angular strain are the most stable. Thus, the order of stability of carbocations is

Rearrangement of Carbocation

Whenever a carbocation intermediate is formed in a reaction, it may rearrange to gain stability. Only those carbocations which can produce a more stable form of carbocation will rearrange.

There are five types of possible rearrangements.

  1. Shifting of H (1,2 shift) or hydride shift
  • When the shift of hydrogen with a pair of electrons is involved in the rearrangement, it is known as a hydride shift.
  • In this reaction, the initial intermediate is a primary carbocation, and according to the order of stabilities, we know that a primary carbocation is the least stable carbocation (compared to secondary and tertiary carbocations).
  • The hydride shift results in the formation of a more stable carbocation i.e., tertiary carbocation, from the primary carbocation.
  • To achieve this, a hydrogen attached to carbon (which is next to the carbon having a positive charge) is shifted to the carbon having the positive charge.

  1. Shifting of an alkyl group (1,2 shift)

In this case, a methyl group shifts with a pair of electrons to the adjacent positively charged carbon to form a more stable tertiary carbocation. This kind of shift is known as a 1,2-methyl shift.

[3] 

  1. Shifting of aryl group (1,2 shift)

In this case, a phenyl group can shift with a pair of electrons to the adjacent positively charged carbon to form a more stable carbocation. This kind of shift is known as a 1,2-phenyl shift.

  1. Ring expansion

Ring expansion is also a type of rearrangement.

The trick for ring expansion/contraction

Here, the carbon having the positive charge is taken as the alpha carbon (just to remember the trick).

  1. Break the bond between the beta and gamma carbons.
  2. Join the alpha and the gamma carbons via a bond.
  3. Remove the positive charge on the alpha carbon.
  4. Add a positive charge to the beta carbon.

Example:

1.

2.

  1. Ring contraction

Ring contraction will only happen if a more stable carbocation is formed after contraction. A similar trick is followed in making a stable carbocation.

Example:

A cyclopropyl methyl carbocation is formed, which is highly stable.

Migratory aptitude

The tendency of an atom or group of atoms to migrate with their bonded electrons from one atom to another is called migratory aptitude of that atom or group of atoms.

Migratory Tendency of groups in the rearrangement of Carbocation is as follows:

Practice Problems

Q. 1. Which of the following is/are the most stable carbocation(s) ?

  1. [4] 

Answer: D 

Solution:

In option (A), the molecule has a primary allylic carbocation.

In option (B), the molecule has a tertiary carbocation. However, it is not in conjugation with the double bond.

In option (C), the molecule has a tertiary carbocation, which is also allylic.

In option (D), the molecule has a tertiary carbocation with two allylic groups. Hence, it is most stable.

Thus, option (D) is the correct answer.

Q. 2. Why Cyclopropylmethyl cation is more stable than benzyl cation?

Solution: 

Each carbon atom of the ring of this carbocation is hybridized. Thus, the bond angle should be 28’. However, due to the cyclic ring, the bond angle is reduced to a greater extent. But in order to somewhat minimize this angular strain, the carbon-carbon bonds (cyclic ring) formed by the hybrid orbitals are off-centered bulging bonds.

The hydrogens of the carbocation are perpendicular to the plane of the vacant orbital. Since the empty p-orbital of the carbocation and the hybrid orbitals (involved in carbon-carbon bonds) of the cyclopropyl ring are in the same plane, conjugation between the bent orbital of the cyclopropyl ring and vacant atomic orbital of the carbocation is observed and hence the carbocation is stabilized through the conjugation.

Q. 3. Which of the following structures is the most stable carbocation?

  1. [5] 

Answer: A

Solution:

In option (A), all the three rings A, B, and C are involved in conjugation with carbocation.

In option (B), let us denote the carbon having a positive charge as X. Now, this is in conjugation with the double bonds present in ring B, and the pi electrons of ring B are in conjugation with ring A. Thus, only two rings A and B participate in conjugation in the whole system.

In option (c), the benzyl cation (also, a tertiary cation) is present in conjugation with two rings. Thus, only two rings B and C participate in conjugation in the whole system.

In option (d), only one ring C is in conjugation with the carbocation in the whole system.

Thus, option (A) is the correct answer.

Q. 4. Arrange the following carbocations in decreasing order of their stability.

Solution:

In structure (I), the positive charge is on the bridgehead carbon atom. Thus, it disobeys Bredt’s rule. So, it is the least stable.

In structure (II), the carbocation is a secondary carbocation that is more stable than the primary.

In structure (III), the carbocation is a tertiary carbocation. So, it is the most stable.

In structure (IV), the carbocation is a primary carbocation.

Thus, the correct order of stability of carbocations is as follows: (iii) > (ii) > (iv) > (i).

Q. 5. In which of the following pairs the second ion is more stable than the first one?

Answer: B and C

Solution:

The stability order for the species is: Aromatic > Non-aromatic > Anti-aromatic

In option (A), the first one is aromatic and the second is anti-aromatic.

In option (B), the first one is non-aromatic and the second is aromatic.

In option (C), the first one is anti-aromatic and the second is aromatic.

In option (D), the positive charge on carbon in the first compound is stabilized by the

+M effect of the oxygen atom of −OH. Thus it is more stable than the tertiary

carbocation in the second one.

Thus, options (B) and (C) are correct answers.

Q. 6. Rearrange (if applicable) the following carbocation into the more stable form.

Solution:

Hydride shift can occur either from the left or the right side. The possible ways of rearranging the carbocation are as follows:

  1. If a hydride shift occurs from the left carbon, then the carbocation formed is stabilized by the −OH group showing +M effect.

  1. If a hydride shift occurs from the right carbon, then the carbocation formed is stabilized by the resonance effect of the phenyl group.

A positive charge on the carbon is more stabilized when the −OH group is attached to it as it completes the octet of the carbocation (due to the formation of oxonium ion).

Hence, hydride shift takes place from the left carbon atom forming a more stable carbocation. So, the first possibility is preferred.

Frequently Asked Questions

Q.1. What is the shape of methyl carbocation?

Answer: Methyl carbocation is a hybridized cation having a trigonal planar shape.

Q. 2. Give one name reaction which involves the carbocation intermediate formation.

Answer: In Wagner-Meerwein rearrangement carbocation is formed as an intermediate.

Q. 3. Why is tertiary carbocation more stable than secondary and primary carbocation?

Answer: In tertiary carbocation, there are more alpha hydrogens than in secondary and primary carbocation, so by hyperconjugation and the inductive effect it is generally more stable.

Stability order: Tertiary > Secondary > Primary

Related Topics

Chlorination Friedel-crafts reaction 
Electrophilic Aromatic Substitution reactions of benzene Alkanes
Chemical Reactions of Alkynes Toluene
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