Features of Reactive Intermediate

What are reaction intermediates?

In chemistry, a reactive intermediate is a short-lived, high energy, reactive species molecule. 

 These species can only be isolated and preserved in extreme circumstances, such as low temperatures or matrix isolation.  Reactive intermediates can help explain how a chemical reaction works when its presence is indicated. A response mechanism is made up of a succession of steps.

A reactive intermediate is distinguished from a reactant, product, or simple reaction intermediate solely in that it cannot usually be isolated and is sometimes only visible with quick spectroscopic approaches. 

It is steady in the sense that a fundamental reaction produces the reactive intermediate, which must be destroyed by an elementary reaction in the following step. When a reactive intermediate isn’t observable, its existence must be deduced through trial and error. Changing reaction conditions, such as temperature or concentration, and using chemical kinetics, chemical thermodynamics, or spectroscopic techniques are common examples to detect their presence.  

Characteristics of reactive intermediate:

Reactive intermediates have several characteristics:

• Low concentrations in relation to the reaction substrate and final reaction product 

• Frequently generated during the chemical decomposition of a chemical compound.

• Frequently it is possible to prove the existence of this species using spectroscopic methods. 

• Cage effects must be considered. 

• Stabilisation by conjugation or resonance is often difficult to distinguish from a transition state.

• Chemical trapping can also be used to prove their existence in a chemical reaction.

Detection of intermediates:

It’s challenging to spot the difference between a transition stage and a full-fledged stage. It’s a state with a reaction coordinate and a higher-level correlation to potential energy. It’s a type of chemical reaction in which a specific configuration can be found along the reaction coordinate.

• Labelling of Isotopes

Information about the reaction process can be gained by following the path of the reaction using molecules that have been isotopically tagged. 

Tracers can be made from both radioactive and stable isotopes. It can be detected using mass spectroscopy.

• Evidence from Stereochemistry

If a reaction’s products can exist in many stereoisomeric forms, the form that is acquired may reveal information about the mechanism.

• Evidence of Kinetics

Kinetic studies can provide a variety of mechanistic information, such as the order of the reaction, the rate-determining step, and so on. The rate constant derived from kinetic data is crucial because it indicates how changes in the structure of the reactants, the solvent, ionic strength, catalyst addition, and other factors affect the reaction rate.

Types of reactive intermediates

There are 6 types of reactive intermediates:

1. Carbocation: 

It is a 6 electron species, which is electron deficient in nature. It is hybridised and has a planar structure having a bond angle of .

2. Carbanion

It has 8 electrons in the valence shell. It is electronically rich. It has a tetrahederal structure and the carbon atom is hybridized.

3. Free radical

A species with one or more unpaired electrons is known as a free radical. The total magnetic moment is 0 in species where all electrons are coupled.

4. Carbene

Carbenes are divalent carbon species that are neutral and contain two non-binding electrons. They are connected to two monovalent atoms or groups. Carbenes, like carbocations, are highly reactive chemical entities with short half-lives due to the core carbon atom’s valence shell having only six electrons and hence a strong tendency to complete its octet by obtaining two more electrons. 

Carbenes behave as Lewis acids or electrophiles as a result of this.

5. Nitrene

A nitrene is the nitrogen equivalent of a carbene in chemistry. Because the nitrogen atom only has 6 electrons accessible, it is classified as an electrophile. A nitrene is a reactive intermediate that is used in a variety of chemical processes.

6. Benzyne

an unsaturated cyclic hydrocarbon formed from benzene and similar to it, but with one of the benzene’s double bonds substituted by a triple bond.

Example:

Reimer-Tiemann Reaction

It is also known as an ortho-formylation reaction.

This reaction takes place in the presence of chloroform and a strong base.

The intermediate formed in this reaction is carbene which is electron deficient in nature.

The reaction mechanism can be shown as: