Reaction intermediates are
- high energy
- short-lived and
- highly reactive molecule.
When a reaction intermediate is formed, it converts to form the product (a stable molecule).
During an organic reaction, the stability of the reactive intermediate decides the major and the minor product formed during the reaction. It all boils down to this fact
Higher is the energy of the intermediate, lower its stability will be.
The reaction mechanism can be understood very well when one understands the intermediate formed. Based on the reactants, there are various types of intermediates:
- Carbocations
- Carbanions
- Free radicals
- Carbenes
- Nitrenes
- Benzyne
But before we get into detail about all the intermediates, let us discuss the two types of fission that take place.
A single covalent bond is made by sharing two electrons between two atoms. Based on the distribution of electrons when bond fission takes place.
Homolytic Fission
In this type of fission, the two electrons are distributed in such a way that each atom gains one electron. The fission is denoted by half headed arrow.
Heterolytic Fission
In this type of fission, the two electrons are distributed in such a way that one atom gains both the electrons making it electron-rich whereas the other atom becomes electron deficient. The fission is denoted by a full-headed arrow. The atom that gains the electrons is more electronegative.
An electronegative atom is the one that pulls the electrons of the covalent bond to itself.
Carbocation
A carbocation is a positively charged ion. When heterolytic fission of a bond between C-X takes place and X is more electronegative than carbon, the shared pair of electrons is taken by the electronegative atom. This leaves the carbon atom with a positive charge. The carbon is trivalent and has an even number of electrons.
The carbon atom is sp² hybridised. The empty p-orbital is perpendicular to the plane of the other bonds. The carbocation has a trigonal coplanar shape with a bond angle of 120°.
Stability of Carbocations
Primary, secondary and tertiary carbocation
The primary, secondary and tertiary carbocation differ based on the number of alkyl groups the central carbon atom is bound to.
Two effects are affecting the stability of the carbocation:
- Inductive effect- Alkyl group is electron releasing in nature. So an alkyl group helps stabilise the positive charge on the carbon atom. The more the number of alkyl groups, more is the stability of the carbocation.
- Hyperconjugation- The positive charge of the carbon atom is getting delocalised between the p orbital of the positively charged carbon and the sigma bond between the beta C-H bond. More the number of alkyl groups implies more number of hyperconjugation structures. When the number of hyperconjugation structures increases, the stability increases.
So the order,
Tertiary carbocation> Secondary carbocation> Primary carbocation
Presence of Electron withdrawing groups
Electron withdrawing groups pull the electrons. So, when such groups are attached to a carbocation, they destabilise the carbocation.
Benzylic Carbocation
A benzylic carbocation is a carbocation attached to a benzene ring. The carbon atom of the benzene ring is not the one bearing the positive charge.
The positive charge of the carbon atom is stabilised by the resonance structures. As we know, more the number of resonance structures, more is the stability. So the benzylic carbocation is stabilised.
Allylic Carbocation
An allylic carbocation consists of a carbon-carbon double bond adjacent to the carbon-containing positive charge.
The allylic carbocation is resonance stabilised too.
Triphenyl methyl carbocation
Remember the benzylic carbocation? It’s the same but only the remaining hydrogen atoms of the positively charged carbon atom are replaced by benzene rings to form a triphenyl methyl carbocation.
Due to the high number of resonance structures formed, this carbocation is so stable that its solid salts have been isolated.
Requirements for the stability of a carbocation
- A planar structure helps for the effective delocalisation of electrons.
- The presence of conjugated double bond will increase the number of resonance structures formed
- The presence of heteroatom having an unshared pair of electrons adjacent to the cationic centre will help increase stability by resonance.
Examples of heteroatoms- Oxygen, nitrogen, halogen etc.
Carbanion
A carbanion is a negatively charged ion. It is derived by heterolytic fission of the C-X bond where the carbon atom is more electronegative than X. So, the carbon takes the bonding electron pair.
The negatively charged carbon atom has eight electrons so it is not electron deficient.
The carbon atom is trivalent and sp³ hybridised. The carbanion has a pyramidal shape. Unshared electron pair of electrons occupy the apex of the tetrahedron.
Stability of Carbanion
Since carbanions are electron-rich, they are highly reactive and can be readily attacked by an electrophile.
Primary, secondary tertiary carbanions
Alkyl groups are electron releasing in nature. The more the number of alkyl groups, the nature the stability of the carbanions.
So the order of stability,
Primary carbanion > Secondary carbanion > Tertiary carbanion
Presence of Electron Releasing Groups
Electron releasing groups destabilise the carbanion.
Presence of Electrons Withdrawing Groups
Electron withdrawing groups increase the stability of carbanion.
Benzyl carbanion
Like the benzyl carbocation, benzyl carbanion is resonance stabilised too. The negative charge is delocalised in the benzene ring.
Allylic Carbanion
An allylic carbanion consists of a carbon-carbon double bond adjacent to the carbon-containing negative charge.
The allylic carbanion is resonance stabilised too.
Free Radicals
Free radicals are formed by homolytic fission of the bond between the two atoms. Each atom gets one electron from the electrons of the bond. Free radicals have an odd number of electrons.
It is a highly reactive neutral intermediate. The high reactivity of free radicals is due to the need for electrons to pair up. Free radicals are paramagnetic and are observed by electron spin resonance (E.S.R).
Two structures have been postulated:
There is no chemical evidence for the planar or the pyramidal structure. According to electron spin resonance, ultraviolet spectroscopy and infrared spectroscopy shows that simple radicals are planar.
Stability of Free Radicals
Primary, Secondary and Tertiary free radicals
The order of stability of free radicals is similar to that of carbocations.
Tertiary free radicals > Secondary free radicals > Primary free radicals
This is due to hyperconjugation. The more the number of hyperconjugation structures, the more is the stability. Tertiary free radicals have more hyperconjugation structures than the other two, and so it is more stable.
Allylic and Benzylic free radicals
Allylic and benzylic free radicals are stabilised by resonance. Benzylicfree radicals are more stable than allylic free radicals because of more number of resonance structures in benzylic free radicals.
The stability above can also be explained by resonance structures. The more the number of possible resonance structures, the more is the stability.
Like carbocations, more the number of phenyl groups attached to the carbon free radical, more is the stability (Due to resonance).
Carbenes
Carbenes are formed by homolytic fission of two bonds formed by the carbon atom. These are highly reactive and have a lifetime of less than a second. The carbon atom has two nonbonding electrons.
Since carbon atoms have 6 electrons, it is electron-deficient and can act as a strong electrophile.
Carbenes can exist in two states: singlet and triplet state.
Stability of Carbene
Generally, the singlet carbene is more stable than triplet due to resonance.
When the carbon atom of carbene is bound to an atom containing lone pair of electrons, its stability increases.
Nitrene
Nitrenes are the nitrogen analogues of carbene that contain a monovalent nitrogen atom.
The nitrogen atom has a sextet of electrons. Like carbene, nitrene exists as a singlet and triplet state.
Stability of Nitrenes
Alkyl nitrenes are more reactive than aryl nitrene. So, alkyl nitrenes have been isolated by trapping in a matrix at 4K but aryl nitrenes can be trapped at 77K.
Like carbenes, nitrenes are also affected by the substituents attached. Pi electron-donating substituents stabilise the nitrene that causes it to show nucleophilic character.
Nitrene reacts with CO to form isocyanate.
Benzyne
This is a highly reactive neutral intermediate that consists of a benzene ring. The aromaticity is not disturbed because the pi-electron system is not disturbed.
Structure and Stability of Benzyne
Benzyne can exist as ortho, meta and para isomers.
Hope the post helped you understand reaction intermediates better. The reaction intermediates helps decide the major product in a chemical reaction. Usually, the more stable reactant gives the major product.
References
Ahluwalia, V. K., & Parashar, R. K. (2011). Reaction Intermediates. In Organic reaction mechanisms, Narosa.
Singh, M. S. (2014). Nitrenes. In Reactive intermediates in organic chemistry: Structure, mechanism, and reactions, Wiley-VCH.
