Fundamental Concepts in Organic Reaction Mechanism: In an organic reaction, the substrate is the organic molecule that reacts with an appropriate attacking reagent to form one or more intermediate(s) and, finally, product(s). A sequential account of each step, describing details of electron movement, energetics involved during bond formation and cleavage, and the kinetics of transforming reactants into products, is referred to as a reaction mechanism. Let’s understand some of the fundamental concepts involved in the organic reaction mechanism.

Fission as a Covalent Bond

A covalent bond undergoes cleavage either by (i) homolytic cleavage or by (ii) heterolytic cleavage.

Homolytic Cleavage

In this type of cleavage, the bond cleavage occurs in such a fashion that the shared pair of electrons are equally shared among the bonded atoms, i.e. one of the electrons of the shared pair goes with each of the bonded atoms. In homolytic cleavage, the movement of a single electron takes place instead of an electron pair which is represented by a ‘half-headed curved arrow. Neutral species (atom or group) that contains an unpaired electron are formed in this cleavage. These neutral species are called radicals and are very reactive. A homolytic cleavage can be shown as: 

Alkyl radicals are classified as primary \(\left( {{1^{\rm{o}}}} \right)\), secondary \(\left( {{2^{\rm{o}}}} \right)\),  or tertiary \(\left( {{3^{\rm{o}}}} \right)\).

Alkyl radical stability decreases as we proceed from tertiary to primary. Organic reactions that involve homolytic fission are called radical or homopolar, or nonpolar reactions.

Heterolytic Cleavage

In this type of cleavage, the bond cleavage occurs in such a fashion that the shared pair of electrons is retained with one of the fragments. The fragment that withdraws the shared pair of electrons is more electronegative than the fragment from which the shared pair of electrons is withdrawn.

In the above example, bromine is more electronegative than carbon; hence the shared pair of electrons in the \({\rm{C – Br}}\) bond is withdrawn by the bromine atom. The atom from which the shared pair of electrons is withdrawn has a sextet electronic structure (\(6\) electrons in its outermost shell) and a positive charge. In contrast, the other fragment that withdraws the shared pair of electrons has an octet electronic structure with at least one lone pair and a negative charge. The organic reactions that involve heterolytic bond cleavage are called ionic or heteropolar, or just polar reactions.

If carbon bears the positive charge, then it is called carbocation. Carbocations are classified as primary \(\left( {{1^{\rm{o}}}} \right)\), secondary \(\left( {{2^{\rm{o}}}} \right)\),  or tertiary \(\left( {{3^{\rm{o}}}} \right)\),  depending on whether one, two or three carbon atoms are directly attached to the positively charged carbon.

Carbocations are highly unstable species and are stabilised by alkyl groups due to inductive and hyperconjugation effects.

The heterolytic cleavage can also give a species in which carbon gets the shared pair of electrons, and the carbon-bearing negative charge is called carbanion.

Heterolytic fission generally occurs in polar covalent molecules, but in nonpolar molecules, it takes place in the presence of catalysts like \({\rm{AlC}}{{\rm{l}}_{\rm{3}}}\) (anhydrous), \({\rm{FeC}}{{\rm{l}}_{\rm{3}}}\) (anhydrous) etc.

Attacking Reagent

Attacking reagents are of two types:

(i) Electrophiles or Electrophilic Reagents, and

(ii) Nucleophiles or Nucleophilic Reagents

Electrophiles are positively charged electron seeking species, whereas nucleophiles are nucleus seeking species. Electrophiles being electron deficient behave as Lewis acids while nucleophiles behave as Lewis bases. The following species act as electrophiles:

During a polar organic reaction, a nucleophile attacks an electrophilic centre of the substrate. The attack takes place at that specific atom or part of the electrophile that is electron deficient. Similarly, the electrophiles attack the nucleophilic centre of the substrate, which is electron-rich. Thus, the electrophiles receive electron pairs from the nucleophiles. A curved arrow denotes the movement of an electron pair from the nucleophile to the electrophile.

Some examples of nucleophiles with lone pair of electrons and negative charges are cyanide \(\left( {{\rm{N}}{{\rm{C}}^ – }} \right)\), hydroxide \(\left( {{\rm{O}}{{\rm{H}}^ – }} \right)\), ions and carbanions \(\left( {{{\rm{R}}_3}{\rm{C}}{:^ – }} \right)\).

Neutral molecules also act as nucleophiles due to the presence of lone pair of electrons. For example-

\( – {\rm{N}}{{\rm{H}}_3},\,{\rm{RN}}{{\rm{H}}_2},\,{\rm{H}} – {\rm{O}} – {\rm{H}},\,{\rm{R}} – {\rm{OH}},\,{\rm{R}} – {\rm{O}} – {\rm{R}},\,{\rm{R}} – {\rm{S}} – {\rm{R}}\)

Examples of electrophiles include carbocations \(\left( {{\rm{CH}}_3^ + } \right)\)

Neutral molecules that have carbonyl \(\left( { > {\rm{C}} = {\rm{O}}} \right)\) functional groups also act as nucleophiles. Alkyl halides \(\left( {{{\rm{R}}_3}{\rm{C}} – {\rm{X}}} \right)\) due to the polarity of the \({\rm{C}} – {\rm{X}}\) bond, a partial positive charge is generated and acts as nucleophiles.

Electron Movement in Organic Reactions

The general reaction involving organic molecules is depicted as follows:

A curved-arrow notation denotes the movement of electrons in organic reactions. The tail of the curved arrow begins from the point from where an electron pair is shifted, and the head ends at a location to which the electron pair is shifted.

Electron Displacement Effects in Covalent Bonds

The electron displacement in an organic molecule may take place either in the ground state or under the influence of an appropriate attacking reagent. Due to the influence of an attacking reagent, the electron displacement may lead to the permanent polarisation of the bond, such as the inductive effect and resonance effects. Temporary electron displacement effects are also seen in a molecule when a reagent attacks it, such as the electromeric effect.

Inductive Effect

The inductive effect is the polarisation of a \(\sigma \)-bond caused by the polarisation of an adjacent \(\sigma \)-bond.

The electron density between two unlike atoms having electronegativity difference is denser towards the atom having higher electronegativity than the other. This uneven distribution of electrons causes the bond’s polarisation and affects the adjacent bonds.

The inductive effect is a distance-dependent phenomenon and causes permanent polarisation of the \(\sigma \)-bond. This is illustrated as below:

\({{\rm{C}}^{\partial + }} – {{\rm{X}}^{\partial – }}\)

The atom \({\rm{X}}\) is more electronegative than the carbon atom and acquires a slightly negative charge \((\partial – )\), and the carbon atom a slightly positive charge \((\partial + )\), which means the bond is polarised:

The positive charge on the carbon atom is relayed to the neighbouring carbon atoms. \({\rm{C}}1\), the carbon atom adjacent to the \({\rm{X}}\) atom, with its positive δ charge, exerts a pull on the electrons of \({\rm{C}}2\)​, but the pull is weaker than it is between \({\rm{X}}\) and \({\rm{C}}1\)​. The effect rapidly dies out and is usually not significant after the \(2{\rm{nd}}\) carbon atom, or at most the \(3{\rm{rd}}\).

It is of two types:

-I effect: Electron-withdrawing effect

\({\rm{ – I}}\) effect is caused when an electronegative atom, such as a halogen, is introduced to a chain of carbon atoms. Due to unequal sharing of electrons, the halide ions develops a negative charge, and the carbon atom develops a positive charge. This positive charge is relayed and transmitted through the chain of carbon atoms.

+I effect: Electron releasing or donating effect

\({\rm{ + I}}\) effect is caused when a chemical species such as an alkyl group that can release or donate electrons is introduced to a carbon chain. The negative charge is relayed and transmitted through the chain of carbon atoms. This effect is called the Positive Inductive Effect or the \({\rm{ + I}}\) Effect.

Resonance or Mesomeric Effect

The phenomenon in which a molecule is represented in more than one form is known as resonance. The most common example of a molecule exhibiting the phenomenon of resonance is that of benzene. The cyclic structure of benzene contains alternating \({\rm{C – C}}\) single and \({\rm{C = C}}\) double bonds.

Benzene has a uniform \({\rm{C – C}}\) bond distance of \(139\;{\rm{ pm}}\), intermediate between the \({\rm{C – C}}\) single \(\left( {154\;{\rm{ pm}}} \right)\) and \({\rm{C = C}}\) double \(\left( {134\;{\rm{ pm}}} \right)\) bonds. Thus, benzene is represented equally well by the energetically identical structures I and II.

The resonance effect is of two types \({\rm{ + R}}\) or \({\rm{ – R}}\).

(+R) or Positive Resonance Effect – In this effect, electrons’ displacement occurs away from an atom or substituent group attached to the conjugated system. It results in certain positions in the molecule that possess high electron density. For example-

(-R) or Negative Resonance Effect– Negative resonance effect involves the displacement of electrons towards an atom or substituent group attached to the conjugated system. It results in certain positions in the molecule that possess a positive charge. For example-

Electromeric Effect

When an attacking reagent approaches a substrate, the intramolecular movement of electrons in the substrate occurs from a pi bond to another atom in the molecule. This effect is known as the electromeric effect and is temporary. As soon as the attacking reagent is removed from the domain of the reaction, the effect is annulled. This effect is observed only in substrates that contain at least one multiple bonds.

It is of two types-

+E Effect or Positive Electromeric effect

When an electrophile approaches the substrate, the \({\rm{ + E}}\) effect occurs. In this effect, the electron pair of the pi bond is transferred to that atom to which the reagent gets attached.

-E Effect or Negative Electromeric effect

When a nucleophile approaches the substrate, the \({\rm{ – E}}\) effect occurs. In this effect, the electron pair of the \({\rm{pi}}\) bond is transferred to that atom to which the attacking reagent does not get attached.

Hyperconjugation

Hyperconjugation or No bond resonance is a permanent effect and is very similar to resonance.

Hyperconjugation involves the delocalisation of σ electrons of the \({\rm{C – H}}\) bond of an alkyl group. The alkyl group should be directly attached to an atom with an unshared p orbital or to an unsaturated system. The σ electrons of the \({\rm{C – H}}\) bond of the alkyl group is involved in partial conjugation with the unshared p orbital or the unsaturated system.

For example,  in ethyl cation \(\left( {{\rm{C}}{{\rm{H}}_3}{\rm{CH}}_2^ + } \right)\), the positively charged carbon atom has an empty \({\rm{p}}\) orbital. One of the \({\rm{C – H}}\) bonds of the methyl \({\left( {{\rm{C}}{{\rm{H}}_3}} \right)}\) group can align in the plane of this empty \({\rm{p}}\) orbital, and the electrons constituting the \({\rm{C – H}}\) bond can then delocalise into the empty \({\rm{p}}\) orbital.

The hyperconjugation effect helps in the dispersal of positive charges, hence stabilising the carbocation. Hence, the greater the number of alkyl groups attached to a positively charged carbon atom, the greater the hyperconjugation effect and the higher the stabilisation of the carbocation. The relative stability based on hyperconjugation is given as,

Summary

Reaction mechanisms provide a sequential detail of each step taking place in an organic reaction. It describes details of electron movement, energetics involved during bond formation and bond cleavage, and the kinetics of transforming reactants into products. This page explains the fundamental concepts involved while dealing with organic reactions. It also describes the various effects due to electron movements.

Frequently Asked Questions (FAQs)

Q.1. What is the mechanism of organic reaction?
Ans: An organic reaction mechanism is a complete, step-by-step description of how a reaction of organic compounds takes place.

Q.2. What are the five types of organic reactions?
Ans: There are mainly five types of organic reactions. These are-
1. Substitution reaction
2. Elimination reaction
3. Addition reaction
4. Radical reactions
5. Oxidation-Reduction reactions.

Q.3. How is the rate of a reaction determined?
Ans: The rate of the slowest step determines the overall rate of a reaction, called the rate-determining step.

Q.4. What is the first step to understanding the mechanisms of organic reactions?
Ans: The first step to understanding mechanisms of organic reactions is to have knowledge about the structure, bonding, and other attributes of the molecule in question.

Q.5. What is the difference between homolytic and heterolytic cleavage?
Ans:

Homolytic Cleavage	Heterolytic Cleavage
In this type of cleavage, the bond cleavage occurs in such a fashion that the shared pair of electrons are equally shared among the bonded atoms, i.e. one of the electrons of the shared pair goes with each of the bonded atoms.	In this type of cleavage, the bond cleavage occurs in such a fashion that the shared pair of electrons is retained with one of the fragments.
Both the bonded atoms get one electron of the shared pair of electrons.	One of the bonded atoms gets both of the shared pair of electrons.

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