Reactive Intermediates

A reactive intermediate is a molecule that is a product in an intermediate step of a chemical reaction. It is typically very energetic: it usually exists only for a short time, as a product of an earlier step in the reaction, and quickly stabilizes. These reactive intermediates provide a basis for understanding how complex reactions are possible.

Interestingly, identifying the presence of these intermediates is not always simple. Unlike typical reactants and products, reactive intermediates typically cannot be isolated and may only be seen using spectrometry or through using experimentation to infer their existence.

Any organic reaction is characterized by the breaking of one or more covalent bonds and the formation of new ones. The reaction is completed successfully if the bonds formed are stronger than the bonds broken. In other words, the reaction is successful if the products are more stable than the reactants. The breaking of bonds can occur through two different methods. The two types of bond cleavage are thrombolytic bond cleavage and electrolytic bond cleavage.

Homolytic cleavage is the cleavage of a covalent bond between two species A and B such that after the cleavage both A and B acquire an unpaired electron. In organic chemistry, homolytic cleavage of a covalent bond containing carbon in a reaction occurs only under some specific conditions. The conditions are listed below:

1) The two atoms bonded must have a small difference in electronegativity: that is, the bond between the two atoms must be non-polar.

2) The reaction mixture should contain free radical initiators. For example, peroxides, non polar mediums (carbon tetrachloride, etc.), light, and extreme temperature are all free radical initiators.

Note: Homolytic cleavage is sure to happen if you see anyone of the following initiators:

H \(=\) Heat, E \(=\) Electricity, L \(=\) Light, P \(=\) Peroxide, R \(=\) Radicals.

In short, HELPR helps the initiating of a homolytic fission.

Heterolytic cleavage is the cleavage of a covalent bond between two species A and B such that after cleavage one of the atoms acquires positive charge, and the other negative charge. In organic chemistry, heterolytic cleavage of a covalent bond containing carbon in a reaction occurs only under some specific conditions. The conditions are listed below:

1) The two atoms bonded must have large change in electro negativity, that is, the bond between the two atoms must be polar.

2) Factors like polar solvents, low temperature, etc. favor heterolytic bond cleavage.

A series of steps involved in the transformation of reactants into products is called reaction mechanism. \(_\square\)

The organic reactions and their mechanisms are classified into three groups:

1. Substitution reaction
2. Addition reaction
3. Elimination reaction

Carbon compounds bearing a positive charge on carbon and carrying six electrons in its valence shell are called carbocations.

These are formed by heterolytic cleavage of the covalent bonds.

Types of carbocations: Depending on the nature of the carbon bearing the positive charge, carbocations are classified as primary \((1^\circ)\) meaning that the carbon is bonded to just one other carbon, secondary \((2^\circ)\) meaning that the carbon is attached to two other carbons, and tertiary \((3^\circ)\) meaning that the carbon is attached to three different carbons.

Although there are quaternary carbons, there is no possibility of quaternary carbocations. That is because their octet is filled and if there is cleavage between \(\ce{C}-\ce{C}\) bond, it will turn into a tertiary carbocation, hence ruling out the possibility of quaternary carbocations.

The reaction intermediate formed due to the heterolytic cleavage of a covalent bond such that the electron pair of the bond is taken by the carbon atom, is called a carbanion.

They are important chiefly as chemical intermediates—that is, as substances used in the preparation of other substances. Important industrial products, including useful plastics, are made using carbanions. The simplest carbanion is the methide ion \((\ce{CH3^{-}})\), which is derived from Methane \((\ce{CH4})\) by the loss of a proton. Carbanions exist in a trigonal pyramidal geometry.

Similarly, here too are three types of carbanions, depending on the nature of carbon which bears the negative charge.

They are primary, secondary, and tertiary, named so on the number of other carbons they are bonded to.

There are no quaternary carbanions.

Free radicals are reaction intermediates formed due to the homolytic cleavage of a covalent bond containing carbon such that carbon gets an unpaired electron.

In this process each atom takes away one of the two electrons forming a single covalent bond. It will produce two new species having an unpaired electron. This chemical species with only one unpaired electron are called free radicals. Free radicals actively take part in chemical reactions and are highly reactive.

\[\huge\ce{A-B->A\cdot \ +\ B^\cdot }\]

How does \(\ce{Cl2}\) divide under an influence?

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Since both chlorine atoms in \(\text{Cl}_2\) have equal electro negativity, they will undergo homolytic fission when energy is applied. The reaction will produce \(\ce{2Cl\cdot}\) which is highly reactive. \(_\square\)

Usually, when reactions are written, and when there is a possibility of homolytic fission, in other words, formation of free radicals, we draw two queer arrows, also called as fish hook arrow. This represents that the bond between the two atoms in question is going to cleave homolytically. This can be observed in the above example where there is a homolytic fission between \(\ce{Cl}-\ce{Cl}\) bond.

Similarly, there are three types of free radicals. They are primary, secondary, and tertiary.

They have been classified as such depending upon the nature of carbon carrying the single odd/unpaired electron.

• Electrophiles and Nucleophiles

• Inductive Effect, Electromeric Effect, Resonance, and Hyperconjugation

• Octet Rule

• Valence Shell Electron Pair Repulsion Theory