Mechanisms of organic reactions

Types of Chemical Bond Cleavage

Any chemical transformation begins with the disruption of old bonds in the starting molecules. In organic chemistry, this process can proceed via one of two fundamental mechanisms:

1. Homolytic cleavage. In this scenario, the shared electron pair forming the chemical bond is divided equally between the atoms. As a result, each atom takes exactly one electron. This process leads to the formation of free radicals—highly reactive species with unpaired electrons. A radical can be either a single atom or an entire molecular fragment. Reactions proceeding via this pathway are called radical reactions and are denoted by the symbol R.

Process scheme: $C - X \rightarrow C(\cdot) + X(\cdot)$.

1. Heterolytic cleavage. This mechanism involves an asymmetrical distribution of electrons. One of the atoms takes the entire shared electron pair completely, while the second is left with an empty orbital.

As a result, two species with different properties are formed:

• Nucleophile (acting as a base) — a species that retains the electron pair.
• Electrophile (acting as an acid) — a species with an empty orbital, lacking an electron pair.

Process scheme: $C - X \rightarrow C(:) + (empty orbital)X$.

Features of Multiple Bond Cleavage

In organic molecules containing multiple (double or triple) bonds, heterolytic cleavage has its own specific patterns. The $\pi$-bond is most susceptible to such cleavage.

Meanwhile, the main single framework of the molecule, formed by the $\sigma$-bond, remains unchanged. This leads to a redistribution of electron density and the formation of adjacent charged centers, which can be represented by the scheme: $C=C \rightarrow C^+ - C^-$.

Electrophilic Reactions

If, during the interaction, a carbon atom in the molecule donates its electron pair (most often from its own $\pi$-system) to the attacking reagent, and then forms a new bond with an atom possessing an electron pair (filling its own vacant $C^+$ orbital), the reaction is called electrophilic. The attacking reagent in this case acts as an electrophile.

Depending on the final outcome, two main varieties of such processes are distinguished:

• $A_E$ — Electrophilic Addition (Addition Electrophilic).
• $S_E$ — Electrophilic Substitution (Substitution Electrophilic).

Nucleophilic Reactions

The opposite type of interaction occurs when a carbon atom loses its initial electron pair while accepting a new electron pair from a suitable reagent. Such a reaction is called nucleophilic, and the reagent itself is called a nucleophile.

By analogy with electrophilic processes, nucleophilic reactions are divided into:

• $A_N$ — Nucleophilic Addition (Addition Nucleophilic).
• $S_N$ — Nucleophilic Substitution (Substitution Nucleophilic).

Energetics and Dynamics of Processes

It is important to understand that in reality, heterolytic cleavage of an old bond and the formation of a new one do not always occur as separate, successive steps.

Very often this is a single concerted process. The weakening and gradual cleavage of the initial bond proceed synchronously with the formation of the new one. The main energy advantage of such a mechanism is that the activation energy of a concerted process is significantly lower than that of a stepwise reaction involving intermediate free ions.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of Electrophilic Addition ($A_E$)
• Features of Electrophilic Substitution ($S_E$)
• Nucleophilic Addition Reactions ($A_N$)
• Mechanisms of Nucleophilic Substitution ($S_N$)
• Properties and Stability of Free Radicals (R)

Other topics in "Chapter 14. Theoretical Foundations of Organic Chemistry"

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