Organic Chemistry/Introduction to reactions/Carbocations



Carbocations are carbon atoms in an organic molecule bearing a positive formal charge. Therefore they are carbon cations. Carbocations have only six electrons in their valence shell making them electron deficient. Thus, they are unstable electrophiles and will react very quickly with nucleophiles to form new bonds. Because of their reactivity with heteroatoms, carbocations are very useful intermediates in many common organic reactions.

Carbocation Structure
The orbitals of carbocations are generally sp2 hybridized so that the three full orbitals are arranged in a trigonal planar geometry about the carbon nucleus. The remaining p orbital is empty and will readily accept a pair of electrons from another atom. Because of the symmetry of this geometric arrangement, nucleophilic attack is equally favorable above or below the plane formed by the full orbitals.Result give racemic mixture but extent of racemisation depends upon the nature of solvent. polar protic solvents favours formation of carbocation and polar aprotic solvent take more time to give carbocation while non polar solvent does not give carbocation. Just after formation of carbocation it exist in intimate ion pair, after some time it converts into solvent separated ion pair then it give free form of carbocation. formation of free form depend upon solvent, if it is very good polar protic solvent ( that helps to separate carbocation and it's negative counter part ) then in a very small time, it will give free carbocation.

Carbocation Stability
Carbocations are generally unstable and fairly hard to form. They usually cannot be isolated from a reaction as they will react immediately to fill their empty p orbital. Because they are electron deficient, attaching electron donating groups (such as alkyl groups) to the carbocation will help stabilize the carbocation. In general:

Clearly, the tertiary carbocation is the most stable, as it is surrounded by three other carbon atoms that share the burden of its positive charge. Primary and especially methyl carbocations are rarely seen in organic reactions except under special circumstances like in the case of benzylic or allylic cations.

Carbocations can also be stabilized through resonance by neighboring lone pairs or pi-electrons. In general, this stabilization is greater than one degree of substitution, so a secondary carbocation stabilized by resonance will be more stable than a tertiary carbocation with no resonance stabilization, and a primary carbocation stabilized by resonance will be more stable than a secondary carbocation with no resonance stabilization.

Formation of Carbocations
y Carbocation intermediates are formed in three main types of reactions: additions to pi bonds, unimolecular eliminations, and unimolecular nucleophilic substitution. On a bridge head a positive carbon is rare. The 3-cyclopropyl carbocation is the most stable carbocation.

Reactions of Carbocations
In general, carbocations will undergo three basic types of reactions:

1. Nucleophile Capture
Carbocations will react with even mild nucleophiles (such as water) to form a new bond.

2. Elimination to form a pi bond
Carbons alpha to the carbocation will often lose a proton to form a double (or, in some cases) triple bond from the carbocation. Such a reaction requires only a mild base (e.g. chloride) to remove the proton.

3. Rearrangement
A secondary carbocation may rearrange to form a tertiary carbocation before the ion is stabilized using one of the above-mentioned reactions. Since a cation constitutes a deficiency of electrons, the empty orbitals do not move; rather, a hydrogen atom bonded to a nearby carbon is moved to stabilize the secondary carbocation, of the hydrogen atom creates a new tertiary carbocation, which is more stable and will be substituted to lead to the final product. See carbocation rearrangement.