Carbocation is a molecule in which a carbon atom has a positive charge and three bonds. It is an electron-deficient species with an incomplete octet and is stabilized by adding a nucleophile, forming a new covalent bond. Carbocations are very reactive and unstable due to their incomplete octet.

Carbocation

It is also known as carbonium ion. Let’s know more about Carbocation and their type, Formation, Order and Stability in detail below.

What is Carbocation

A carbocation is a molecule in which a carbon atom bears three bonds and a positive charge. It is an electron-deficient species with an incomplete octet and is stabilized by adding a nucleophile, forming a new covalent bond. Carbocations are very reactive and unstable due to their incomplete octet.

They are classified into primary, secondary, and tertiary carbocations based on the number of carbon atoms bonded to the positively charged carbon. Carbocation rearrangements are common in organic chemistry reactions and are defined as the movement of a carbocation from one carbon atom to another.

Carbocation Definition

Carbocations are electron-deficient species with an incomplete octet and a positive charge on the carbon atom.

Carbocation Structure

A carbocation is a positively charged carbon atom that is part of a molecule with three sigma bonds. The following features characterize the carbocation’s structure:

• Sp² hybridized carbon, resulting in a trigonal planar geometry.
• Bond angles of 120° between the three substituents.
• An unoccupied p orbital perpendicular to the plane of the substituents.
• Electron deficiency, with six valence electrons used to form three sigma bonds.

Carbocation Structure

Hybridization of Carbocation

Carbocations are sp² hybridized, which results in a trigonal planar geometry around the carbocation carbon.

Properties of Carbocations

The properties of carbocations are mentioned below:

• The positive charge of a carbocation is delocalized over all the atoms in the molecule, and the extent of this delocalization increases with increased resonance.
• Carbocations are classified into primary, secondary, and tertiary based on the number of carbon atoms bonded to the positively charged carbon.
• They are sp² hybridized, and the three sigma bonds formed lie in one plane, creating a trigonal planar geometry about the carbon nucleus.
• Carbocations are highly reactive and react quickly with nucleophiles to form new bonds.
• The stability of carbocations increases with the number of alkyl groups attached to the positively charged carbon.
• If the carbocation is trigonal planar and chiral, it can lead to a racemic mixture due to its lack of preference for forming one enantiomer over the other.

Types of Carbocations

They are classified based on the number of carbon groups bonded to the carbon carrying the positive charge. The types of carbocations include:

• Primary Carbocation
• Secondary Carbocation
• Tertiary Carbocation
• Methyl Carbocation

Primary (1°) Carbocation

In Primary Carbocation, carbon with the positive charge is attached to one other alkyl group.

Secondary (2°) Carbocation

In Secondary Carbocation, carbon with the positive charge is attached to two alkyl groups.

Tertiary (3°) Carbocation

In Tertiary carbocation, carbon with the positive charge is attached to three alkyl groups.

Methyl Carbocation

In Methyl Carbocation, no carbon atoms are attached to the carbon with the positive charge.

Formation of Carbocation

Carbocations can be formed by either heterolytic bond cleavage by losing a leaving group or adding π electrons to an electrophile.

Mechanism of Formation of Carbocations

There are two mechanism by which carbocations are formed. The two mechanisms are discussed below:

• Heterolytic Bond Cleavage by the Loss of a Leaving Group
• Addition of π Electrons to an Electrophile

Heterolytic Bond Cleavage by the Loss of a Leaving Group

Initiation: The reaction is initiated by departing a leaving group (X) from the substrate, leaving a carbocation behind.

RX → R⁺ + X−R X → R⁺ + X⁻

Formation of Carbocation: The leaving group takes away the shared electrons, leaving the carbon atom electron-deficient and forming a carbocation.

R⁺ + Nu⁻ → R NuR⁺ + Nu⁻ → R Nu

Stabilization: The positive charge on the carbocation is stabilized by adding a nucleophile, forming a new covalent bond, stabilizing the carbocation.

Rearrangement: The bonding electrons of a carbocation can be shifted between adjacent atoms to form a more stable carbocation through hydride or alkyl shifts.

Addition of π Electrons to an Electrophile

Electrophilic Attack: An electrophile attacks an unsaturated point (double or triple bond), breaking the π bond and forming a carbocation.

RCHCH₂ + H⁺ → R CH₂⁺ + H−C=CH₂R CH CH₂ ​+ H⁺ → R CH₂⁺​ + H−C=CH₂​

Stabilization: The formed carbocation can be stabilized by adjacent pi bonds, adjacent atoms with lone pairs, and adjacent electron-donating groups.

Rearrangement: Similar to the heterolytic bond cleavage, the carbocation formed can undergo rearrangement to a more stable carbocation through hydride or alkyl shifts.

Stability of Carbocation

Stability of a carbocation refers to its tendency to exist in a certain molecular environment and its resistance to undergoing rearrangements or reactions. Carbocations are positively charged carbon species that contain three bonds and an empty p orbital.

The stability of carbocations is influenced by several factors, which can be summarized as follows:

Factors influencing Carbocations stability

The formation of carbocations are affected by following factors.

• Number of Adjacent Carbon Atoms: Increasing the number of adjacent carbon atoms stabilizes the carbocation. The stability follows the order: methyl < primary < secondary < tertiary, with tertiary carbocations being the most stable due to the increased electron-donating effect of the alkyl groups.

• Adjacent Pi Bonds: Carbocations are stabilized by adjacent pi bonds that allow the carbocation p-orbital to be part of a conjugated pi-system, increasing stability.

• Adjacent Atoms with Lone Pairs: Adjacent atoms with lone pairs can provide the carbon with a full octet, stabilizing the carbocation.

• Electron-withdrawing Groups: Adjacent electron-withdrawing groups that cannot donate a lone pair, such as CF₃ or NO₂, decrease the stability of carbocations.

• Alkyl Groups: Alkyl groups are electron-donating and stabilize nearby carbocations, with more substituted carbocations being more stable than less substituted ones.

• Electronegativity: The carbocation’s stability decreases as the carbon atom’s electronegativity carrying the positive charge increases.

Carbocation Stability Order

The order of stability of carbocations is as follows:

• Tertiary (3°) carbocation: Most stable due to the increased electron-donating effect of the alkyl groups.
• Secondary (2°) carbocation: More stable than primary carbocations due to the presence of two alkyl groups.
• Primary (1°) carbocation: Less stable than secondary and tertiary carbocations due to the presence of only one alkyl group.
• Methyl carbocation: Least stable due to the absence of adjacent alkyl groups.

In short, the stability of carbocations follows the order:

tertiary > secondary > primary > methyl

Forms of Carbocation

Carbocations are positively charged carbon species stabilized by adjacent alkyl groups, resonance, and hyperconjugation. Allylic, benzyl, phenyl, and vinyl carbocations are specific carbocations stabilizing by resonance.

• Allylic Carbocation: This is a carbocation adjacent to a carbon-carbon double bond. It is stabilized by resonance, as the positive charge can be delocalized into the adjacent pi system, making it more stable than a typical alkyl carbocation.

• Benzyl Carbocation: This is a carbocation where the positive charge is on a carbon atom directly attached to a benzene ring. It is highly stabilized by resonance, as the positive charge can be delocalized over the benzene ring, making it more stable than a typical alkyl carbocation.

• Phenyl Carbocation: Phenyl carbocation is not a commonly discussed or stable species. The positive charge on the carbon directly attached to the benzene ring would be highly unstable due to the lack of adjacent substituents to stabilize the positive charge.

• Vinyl Carbocation: This is a carbocation where the positive charge is on the carbon of a carbon-carbon double bond. It is highly unstable and not commonly observed due to the lack of adjacent alkyl groups to stabilize the positive charge.

Rearrangement of Carbocation

Carbocation rearrangements are common in organic chemistry and involve the movement of a carbocation from an unstable state to a more stable state. There are two main types of carbocation rearrangements: hydride shift and alkyl shift:

• Hydride shift: A hydrogen atom moves over to a neighboring carbon, which is less substituted and more stable. This process is typically observed in reactions involving alcohols and hydrogen halides.

• Alkyl shift (alkyl group migration): An alkyl group moves to a neighboring carbon, which is less substituted and more stable. This process is observed when a carbocation does not have a suitable hydrogen atom on an adjacent carbon for rearrangement.

Rearrangements usually occur in reactions that involve carbocations, such as E₁ elimination reactions and certain electrophilic additions.

Reactions Involving Carbocations

Carbocations are highly reactive intermediates that can undergo various reactions. Some of the reactions involving carbocations are:

Nucleophilic Substitution SN1 Reaction

In SN1 Reaction reaction, a nucleophile attacks the carbocation, forming a new covalent bond and the displacement of the leaving group. The general reaction is:

R X → R⁺ + X−R X→ R⁺ + X⁻

R⁺ + Nu⁻ → R NuR⁺ + Nu⁻ → R Nu

This reaction is commonly observed with tertiary carbocations, which are more stable and thus more likely to form.

Elimination (E₁) Reaction

In this reaction, a proton is removed from the carbocation, forming a double bond and losing a leaving group. The general reaction is:

R⁺ + B → R=B + H⁺

This reaction is commonly observed with secondary and tertiary carbocations.

Rearrangement Reaction

Carbocations can undergo rearrangement to a more stable carbocation through hydride or alkyl shifts. The general reaction is:

R⁺ → R′ + R⁺ → R′⁺

This reaction is commonly observed with primary and secondary carbocations.

Electrophilic Addition Reaction

In this reaction, an electrophile attacks an unsaturated point (double or triple bond), forming a carbocation. The general reaction is:

RCHCH₂ + H⁺ → RCH2⁺ + H−C=CH2R CH CH2​ + H⁺ → RCH2⁺​ + H−C=CH2​

This reaction is commonly observed with unsaturated hydrocarbons.

Lewis Acid Catalyzed Reactions

Carbocations can be formed by reacting an alkyl halide with a Lewis acid, such as AlCl3 or BF3. The general reaction is:

RX + Lewis Acid → R⁺ + X−R X + Lewis Acid → R⁺ + X⁻

This reaction is commonly observed in Friedel-Crafts reactions.

Carbocations and Carbonations

Carbocations and carbanions are two ionic forms of carbon which are formed during organic synthesis reactions. These are fundamentals in Organic Reaction Mechanism. A detailed comparison between the two is tabulated below: