REACTION
INTERMEDIATES
DEEPSHIKHA
DEPARTMENT OF CHEMISTRY
A chemical reaction is a process that results in the chemical transformation of one set of chemical substances into another set of chemical substances. Chemical reactions are typically defined as changes in the positions of electrons in the formation and breaking of chemical bonds between atoms, with no change in the nuclei (no change in the elements present), and can be explained using a chemical equation.
The short-lived, highly reactive chemical species through which most organic reactions occur are called reaction intermediates. There are six types of reaction intermediates:
Carbocations
Carbanions
Free radicals
Carbenes
Nitrene
Benzyne
Reaction intermediates:
For example, consider this hypothetical stepwise reaction:
A + B → C + D
The reaction includes these elementary steps:
A + B → X*
X* → C + D
The chemical species X* is an intermediate.
Carbocations
A positively charged carbon atom bearing three covalent bonds and an empty orbital is called a carbocation
Both carbocations and neutral boron compounds generally have
Like boron compounds, carbocations are electron-deficient Lewis acids that will readily combine with Lewis bases, resulting in a tetrahedral, sp3-hybridized atom with a full octet of electrons.
Due to their trigonal planar geometry, carbocations can undergo attack from either face of the empty p-orbital.
Formation of Carbocations
Lacking a full octet of electrons around carbon and bearing a positive charge, carbocations are higher in energy and more unstable than neutral carbon compounds.
The carbocation formation occurs when the leaving group is lost from the site.
Dehydration of alcohols: The hydroxyl group (OH) acts as the leaving group, forming a carbocation. The carbocation undergoes a 1,2-hydride shift to form a highly stable tertiary carbocation intermediate. Alkene is formed in the final step.
Unimolecular elimination reactions
Addition reactions
Halides are inserted in unsaturated carbon double bonds in the presence of hydrochloric acid. The formation of the carbocation intermediate occurs in the course of the reaction. An alkyl halide is produced as the product of the reaction.
Unimolecular nucleophilic substitution reactions
The unimolecular nucleophilic substitution reactions or SN1 reactions are slow. The step in which the carbocation intermediate is formed is the rate-determining step of the reaction. The formation of the carbocation facilitates the inversion of the products. The carbocation formation takes place when the leaving group is lost from the site.
The experimentally measured stability of carbocations shows the following trend:
Methyl (least stable) < primary < secondary < tertiary (most stable)
�
Factors That Stabilize Carbocations – Substitution
One way to rationalize this trend is through applying inductive effects.
�
A more satisfying explanation comes from hyperconjugation.
No overlap means no delocalization, which means no added stability.
CARBANIONS
Heterolytic cleavage of a bond, where carbon retains both the shared pair of electrons results into the formation of a carbanion (i.e, carbon atom having negative charge).In these species, carbon atom carrying negative charge has eight electrons in the valence shell- six from three covalent bonds and two from lone pair of electrons.
Carbanion are generated as intermediate in various organic reactions. Some of the methods for the generation of carbanion are:
• Proton abstraction
• Decarboxylation
• Addition of nucleophile to alkene
• Formation of organometallic compounds
Proton abstraction
When proton is abstracted from a carbon centre then the resulting anion is called a carbanion.
The acidic hydrogen of an organic substrate can be abstracted by an appropriate base. For example carbanion generated from carbonyl compounds. Here, are some examples showing generation of carbanion by abstraction of the acidic proton using a base (OH─ , NH2 ─ , RO─ ).
Decarboxylation
Decarboxylation of carboxylates leads to formation of carbanion intermediate.
Formation of organometallic compounds
Metals which are less electronegative than carbon (such as magnesium, lithium, potassium, sodium, zinc, mercury, lead, thallium) react with alkyl halides under appropriate conditions to form a carbon-metal bondwhere the carbon carries negative charge and metal positive charge. Although the carbon does not carry full negative charge but it acts like a carbanion in its reactions. Thus, metallation reverse the polarity of the carbon from positive in reactant to negative in the organometallic compound this is known as umpolung
Features of Carbanion
• The negatively charged carbon is trivalent
• If all the sustituents on the negatively charged carbon are different the carbanion will be chiral
• Carbanion act as base or nucleophile
• Carbanion has a pyramidal geometry
STRUCTURE OF CARBANION
The structure of a carbanion is as follows-
Carbanions are trivalent species and have sp3 hybridization. The lone pair of electrons occupies one of the sp3 orbitals. Therefore, the geometry of carbanions is tetrahedral.
Stability of Carbanion
Factors which can stabilize or disperse the negative charge on carbon will stabilize a carbanion. The stability of carbanion depends on the following factors:
• Inductive effect
• Extent of conjugation of the anion
• Hybridization of the charge-bearing atom
• Aromaticity
Inductive effect
If the groups attached to carbanion are electron releasing in nature they will increase the negative charge on carbon and thus destabilize it. However, electronegative atoms or electron withdrawing groups adjacent to the negatively charged carbon will stabilize the carbanion.
The alkyl groups are electron releasing in nature due to inductive effect (+I). More the number of alkyl groups attached lesser will be the stability.
Carbanions prefer a lesser degree of alkyl substitution. Therefore the order of stability order of alkyl carbanion is methyl>1o >2o >3o .
Presence of electronegative atoms (F, Cl, Br) or electron withdrawing groups (NO2, CN, COOH, CO) close to the negatively charged carbon will stabilize the charge. Thus more the number of such groups in a carbanion greater will be the stability.
Extent of conjugation of the anion
Which is more stable amongst benzyl and allyl carbanions?
The negative charge is delocalized through resonance in both benzyl and allyl anions. But benzyl anion has more number of contributing structures. Thus, benzyl anion is more stable than allyl anion
If negatively charged carbon is in conjugation with a double bond the resonance effects will stabilize the anion by spreading out the charge by rearranging the electron pairs.
3 Hybridization of the charge bearing atom
Stability of anion will depend upon the s character of carbanion i.e. more the s character, higher will be the stability of anion. The percentage s character in the hybrid orbitals is as follows: sp(50%)> sp2 (33%)>sp3 (25%).
Aromaticity
In some carbanions, the lone pair of electrons of the negative charge is involved in delocalization to add on to the aromatic character of the molecule which gives them extra stability. For example, in cyclopentadienyl anion there are 6 π electron and thus it obeys Huckel rule, (4n+2) π electron. It has pKa value 16. This anion is stabilized by aromatization
cyclopentadienyl anion
Cyclooctatetraene on reaction with potassium gets converted to cyclooctatetraenyldianion potassium salt. This is 10 π electron system which is stable due to aromaticity
THANKS