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Define Organic Reactive Intermediates

The short-lived (\({10^{ – 6}}\) seconds to few seconds) highly reactive chemical species through which the majority of the organic reactions occur are called reaction intermediates. These are generated by the breaking of the covalent bond of the substrate. Reaction intermediates quickly convert into a more stable molecule when produced in a chemical process. Only in exceptional circumstances, such as low temperatures or matrix isolation, reaction intermediate compounds can be isolated and kept.

For example,

Tert-butyl carbocation, ethyl carbocation, acetaldehyde carbanion, chlorine-radical, benzyl radical, phenyl radical, dichlorocarbene, etc.

Reaction Intermediates in Organic Chemistry: Types of Reaction Intermediates

There are six types of reaction intermediates. They are:

1. Carbocations

A chemical species bearing a positive charge on carbon and carrying six electrons in its valence shell are called carbocations or carbenium ions. Carbocations have a planar structure.

Structure of carbocation

Preparation

Carbocation may be formed in several ways:

1. By the heterolysis of \({\text{C}} – {\text{X}}\) bond in alkyl halide:

Carbocations are formed by heterolytic cleavage of covalent bonds in which the leaving group takes away the shared pair of electrons of covalent bond with it.

For example,

2. By the dehydration of alcohol: At \(430 – 440{\text{ K}}\), carbocations are generated when alcohols are dehydrated in the presence of strong sulphuric acid.

Types of Carbocations

Carbocations are classified into three types, these are primary, secondary, and tertiary carbocations.

Orbital Structure of Carbocation

A carbocation has a \(\mathrm{sp}^{2}\) hybridised positively charged carbon atom. The \(\mathrm{sp}^{2}\) hybridised orbitals interact with the orbitals of other monovalent atoms or groups to produce sigma (\(\sigma\)) bonds. No electron exists in the unhybridized orbital. The three hybridised orbitals are arranged in a plane with a bond angle of \(120^{\circ}\). The orbital is perpendicular to the plane when it is not hybridised. The unhybridized \(2{\text{p}}\)-orbital is directed perpendicular to the plane. The bond angle is \(120^{\circ}\). Thus, a carbocation has a planar structure.

Reactivity Nature of Carbocation

Carbocations are highly reactive chemical species because the positive-charged carbon atom has only six electrons in its valence shell and thus has a strong tendency to complete its octet. The order of reactivity of any chemical species is usually the inverse of its stability. As a result, the order of reactivity of carbocations is as follows: \(1^{\circ}>2^{\circ}>3^{\circ}\).

2. Carbanions

A chemical species bearing a negative charge on carbon and possessing eight electrons in its valence shell is called carbanion.

Structure of carbanion

Preparation

Carbanions are formed when covalent bonds are cleaved heterolytically, leaving the shared pair of electrons with the carbon atom.

For example,

Classification

Carbanions are classified into primary, secondary, and tertiary according to the negative charge present on primary, secondary, and tertiary carbon atoms, respectively.

Orbital Structure of Carbanion

In a carbanion, the negatively charged carbon atom is \(\mathrm{sp}^{3}\) hybridised. It is expected to have tetrahedral geometry. The three hybridised orbitals, each with one electron, are involved in \(\sigma\)-bonds with the orbitals of other atoms or groups. The fourth hybridised orbital has a single pair of electrons and is not involved in the orbital overlap. It is responsible for the negative charge on the carbanion as well as the distortion of its geometry. The actual shape of the carbanion is pyramidal.

Reactivity Nature of Carbanion

The carbanions are highly reactive species and tend to donate the electron pair to some electron-deficient species. The relative order of reactivity of the alkyl carbanions is:

Primary < Secondary < Tertiary

The order of relative stabilities is the reverse.

Tertiary < Secondary < Primary

3. Radicals

Atom or group of atoms having an odd or unpaired electron is known as radicals.

Structure of radical

Examples of radicals are;

Preparation

A radical can be formed in a number of ways:

i. A radical is produced when a covalent bond is homolytic or symmetrically cleaved, either by heating or in the presence of ultraviolet radiation, which is also a source of energy.

ii. The attack of a radical on a covalent molecule produces a radical.

Classification

radicals, like carbocations and carbanions, are classified as primary \(\left({1^\circ } \right)\), secondary \(\left({2^\circ } \right)\), or tertiary \(\left({3^\circ } \right)\), depending on which type of carbon atom carries the unpaired electron.

Orbital Structure of Alkyl Radical

The orbital structure of an alkyl radical is not well defined. The carbon atom with the odd electron, on the other hand, is \(\mathrm{sp}^{2}\) hybridised. The three hybrid orbitals interact with the orbitals of the other atoms to produce a sigma bond. The unhybridized orbital, which is perpendicular to the plane of the hybrid orbitals, contains the odd electron. As a result, a radical is planar in nature and paramagnetic due to an unpaired electron.

Reactivity Nature of Radicals

A radical with an odd electron is expected to be unstable in nature because the carbon atom involved has an incomplete octet: It tries to form an octet with another species.

The order of reactivity of the alkyl radicals is:

Primary > Secondary > Tertiary

The order of stability is the reverse. It is:

Tertiary > Secondary > Primary

4. Carbene

A neutral divalent carbon species in which the carbon atom is bonded to two monovalent atoms or groups and contains two non-binding electrons is called carbenes. Like carbocations, carbenes are highly reactive chemical species that are short-lived because the central carbon atom has only six electrons in its valence shell and thus has a strong tendency to complete its octet by gaining two more electrons. As a result, carbenes behave as Lewis acids or electrophiles.

Structure of carbene

For example,

Preparation

i. Methylene carbene can be synthesised by decomposing diazomethane or ketene in the presence of heat or light.

ii. Dichlorocarbene is formed by the action of \({\text{NaOH}}\) on chloroform.

5. Nitrenes

Nitrenes are neutral monovalent nitrogen species that have two unshared pairs of electrons and are linked to only one monovalent atom or group. Like carbenes, nitrenes exist in both singlet and triplet states. The triplet state is the more stable of the two. Because nitrene is an electron-deficient species, it is highly reactive.

i. Thermal decomposition (thermolysis) of alkyl azide results in the formation of alkyl nitrene as follows:

ii. The photolysis of alkyl isocyanate can also prepare nitrene.

6. Benzynes

\(1, 2\)-Didehydrobenzene, \({{\text{C}}_6}{{\text{H}}_4}\), and its derivatives are called benzyne. It is a neutral reactive intermediate derived from the benzene ring by removing two substituents of ortho positions, one in the form of an electrophile and the other in the form of a nucleophile, leaving behind two electrons to be distributed between two orbitals. Benzyne intermediate is aromatic in character.

Preparation

When halobenzene is heated with sodamide, the formation of benzyne takes place.

Features of Reactive Intermediates

These intermediates do not satisfy the Lewis octet rule, with the exception of carbanions, resulting in significant reactivity.
This species is frequently generated during the chemical decomposition of a chemical compound.
Often possible to confirm the existence of this species using spectroscopic methods.
Cage effects must be considered.
It can be difficult to differentiate from a transition state due to conjugation or resonance stabilisation.
Chemical trapping is used to verify existence.

Summary

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, radicals, carbenes, nitrenes, and benzyne. These intermediates are often generated during the chemical decomposition of a chemical compound. They can be difficult to distinguish from a transition state due to conjugation or resonance stabilisation.

Frequently Asked Questions on Intermediates in Organic Reactions

We have provided some frequently asked questions about Reaction intermediates here:

Q.1. Which species is a reaction intermediate?Ans: A reaction intermediate is a transient species formed in a previous stage and consumed in a future phase to produce the final reaction product in a multi-step reaction process.

Q.2. What is the slowest step in a reaction mechanism?Ans: The rate-determining step is the slowest step in a reaction process. The rate-determining step determines the rate law for the overall reaction by limiting the overall rate.

Q.3. What is the function of reaction intermediates?Ans: A reaction intermediate is a transient species formed in a previous stage and consumed in a future phase to produce the final reaction product in a multi-step reaction process.

Q.4. What are intermediates in a chemical reaction?Ans: The short-lived (\({10^{ – 6}}\) seconds to few seconds) highly reactive chemical species through which the majority of the organic reactions occur, are called reaction intermediates.

Q.5. How do you find the intermediates in a reaction?Ans: A species that appear in the reaction mechanism but not in the overall balanced equation is referred to as an intermediate. In every mechanism, an intermediate is created in the first step and consumed in the later step.

Q.6. How many types of reaction intermediates are there?Ans: The types of reaction intermediates are as follows:1. Carbocations2. Carbanion3. radicals4. Carbene5. Nitrene6. Benzyne

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