In organic reactions, certain unstable species are formed as intermediates. One such important intermediate is the carbocation, in which a carbon atom carries a positive charge. Due to the presence of only six electrons in its valence shell, it is electron-deficient and highly reactive. Carbocations play a significant role in many organic reaction mechanisms.

Structure of Carbocation

Carbocation has sp² hybridisation, a trigonal planar shape, and an empty p-orbital, making it electron-deficient. In a carbocation, the positively charged carbon atom is sp² hybridised. It forms three sigma (σ) bonds with three atoms or groups.

• The bond angle is approximately 120°.
• The carbon atom has an empty unhybridized p-orbital, which is perpendicular to the plane of the molecule.
• Due to the presence of only six electrons, the carbocation is electron-deficient and unstable.

Characteristics of Carbocations

Carbocations exhibit certain characteristic properties due to the presence of a positive charge and electron deficiency.

• Electron-deficient nature: Carbocations have only six electrons in their valence shell, so they are electron-deficient.
• Positive charge: The carbon atom carries a positive charge (C⁺).
• sp² hybridisation: The positively charged carbon is sp² hybridised and forms three σ bonds.
• Planar structure: Carbocations have a trigonal planar geometry with bond angle around 120°.
• Presence of empty p-orbital: They contain an empty unhybridized p-orbital, which makes them reactive.
• Highly reactive and unstable: Due to electron deficiency, carbocations are unstable intermediates and react quickly.
• Electrophilic nature: Carbocations act as electrophiles because they accept electron pairs.

Formation of Carbocation

Carbocations are formed when a carbon atom loses a pair of electrons and becomes positively charged (C⁺). This generally occurs by heterolytic bond cleavage or during electrophilic addition reactions.

1. Formation by Heterolytic Bond Cleavage

• In heterolytic cleavage, a covalent bond breaks unequally.
• Both electrons of the bond are taken by one atom (usually more electronegative).
• The carbon atom loses electrons and becomes a carbocation.

R–X → R⁺ + X⁻

• R⁺ = Carbocation
• X⁻ = Leaving group

Example: CH₃–Cl → CH₃⁺ + Cl⁻

Mechanism

• The bond between carbon and chlorine breaks.
• Chlorine takes both electrons (due to higher electronegativity).
• Carbon is left with only six electrons, forms CH₃⁺ (carbocation).

2. Formation during Electrophilic Addition (from π bonds)

• Occurs in alkenes or alkynes containing π bonds.
• The π electrons are electron-rich and attack an electrophile (like H⁺).
• This leads to breaking of the double bond and formation of a carbocation intermediate.

Example: Ethene reacts with HBr (or HCl, HI)

CH₂=CH₂ + HBr → CH₃−CH₂Br

Mechanism

Step 1: Formation of Carbocation

CH₂​=CH₂ ​+ H⁺ → CH₃−CH₂⁺

• π electrons attack H⁺
• One carbon gets bonded to H, the other carbon becomes positively charged (carbocation)

Step 2: Reaction of Carbocation

CH₃−CH₂⁺ ​+ Br^- → CH₃​−CH₂Br

• The carbocation formed is highly reactive and is attacked by a nucleophile (like Br⁻).

Types of Carbocations

Carbocations are classified on the basis of the number of alkyl groups attached to the positively charged carbon atom.

1. Primary (1°) Carbocation

• The positively charged carbon is attached to one alkyl group.
• Structure: R–CH₂⁺

Example: CH₃–CH₂⁺

2. Secondary (2°) Carbocation

• The positively charged carbon is attached to two alkyl groups.
• Structure: R₂CH⁺

Example: (CH₃)₂CH ⁺

3. Tertiary (3°) Carbocation

• The positively charged carbon is attached to three alkyl groups.
• Structure: R₃C⁺

Example: (CH₃)₃C ⁺

4. Methyl Carbocation

• The positively charged carbon is attached to no alkyl group.
• Structure: CH₃⁺

Example: CH₃ ⁺

Stability of Carbocation

Carbocations show different stability depending on their structure and substituents. The stability of a carbocation depends on the ability of groups attached to the positively charged carbon to donate electron density and reduce its electron deficiency. Carbocations attached to π systems are stabilized by resonance.

Order of Stability:

3° > 2° > 1° > CH₃ ⁺

• Tertiary (3°) carbocation is most stable
• Methyl (CH₃⁺) carbocation is least stable

Reasons for Stability

1. +I Effect (Inductive Effect)

• Alkyl groups donate electron density (+I effect) towards the positively charged carbon
• More alkyl groups, more electron donation and more stability

2. Hyperconjugation

• It is the Delocalisation of σ-electrons (C–H bonds) into the empty p-orbital.
• More alkyl groups, more hyperconjugation and greater stability

3. Resonance Effect

• Carbocations attached to a double bond or benzene ring are stabilised by resonance because the positive charge gets delocalised.
• Example include Allylic carbocation and Benzylic carbocation
• Resonance stabilised carbocations are more stable than ordinary alkyl carbocations.

Related Articles:

• Mesomeric Effect
• Electrophile and Nucleophile
• Electrophilic Substitution Reaction