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Types of reaction intermediates in organic chemistry

Intermediate is an unstable compound formed during a chemical reaction . This intermediate is very unstable and get converted  into product as soon as it forms . Reaction intermediate forms in one step of a chemical reaction and get converted into product in the next step . 

There are basically four types of reaction intermediates in organic chemistry

◆Carbocation
◆carbanion
◆free radical
◆carbene

Carbocation

Carbocation is an intermediate in which  carbon atom carries a positive charge . Carbocation is obtained during heterolytic fission. 

Heterolytic fission is defined as break down of covalent bond in such a way that more electronegative atom attract  both the electrons . Carbocation is formed when more electronegative atom is directly attached to carbon atom during fission .

Carbocations are sp2 hybridized having trigonal planar geometry

Stability of carbocation depends on how easily electron density of carbocation can be increased . More the electron donating or releasing  groups more will the stability of carbocation. Alky groups are electron releasing group , therefore they increases the stability of carbocation . For example ,
Increasing order of the Stability of carbocation 

 Tertiary carbocation > secondary carbocation > primary carbocation 

 Carbanion

Carboanion is an intermediate in which carbon atom has a  negative charge . Carboanion is formed by heterolytic fission of Covalent bond . But in this case , more electropositive group is directly attached to carbon atom . Carbon accept both electron of Covalent bond and form carboanion . 
Carbanions are sp3 hybridized and have a tetrahedral geometry . 

Stability of carboanion depends on how the electron density of carboanion can be decreased. Electron withdrawing groups increases the stability of carbanion by decreasing the electron density of carbanion. Electron releasing groups(like alky group) increases the electron density of carboanion and therefore making it more unstable . For example 
Increasing order of the stability of carboanion 

Primary carbanion>secondary
carbanion>tertiary carbanion

Free radical

Free radical is an intermediate in which carbon atom has  an unpaired electron in valence shell . Free radicals are formed by homolytic fission between two atoms . Atoms with almost same electronegativity break their covalent bond with each atom carries one electron . 
Free radicals are sp2 hybridized and have a trigonal planar geometry .

Stability of free radical can be explained on the basis of hyperconjugation and follows same order as that's of carbocation. For example
Increasing order of the stability of free radical

Tertiary > secondary > primary

Carbene

Carbene is an intermediate obtain by thermal decomposition of diazoalkanes Or it can also obtain from chloroform by removing hydrogen halide from it. 
Carbene contains two unshared valance electron . 

Carbenes are of two type , singlet carbene and triplet carbene . In singlet carbene unshared valance electrons are spin paired but in case of triplet carbene , both the electrons have parallel spin . Generally , it has been found that triplet carbenes are more stable than singlet carbenes. Singlet carbenes have sp2 hybridization whereas triplet carbenes have sp hybridization
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