15.5 Terminology of Organic Reactions

This section covers the fundamental terminology used to describe the participants and processes in organic chemical reactions, including the types of reagents and how chemical bonds are broken.

1. Types of Reagents in Chemical Reactions

Organic reactions involve interactions between different chemical species known as reagents. These can be broadly classified based on their electron affinity. In a typical reaction, the organic molecule being attacked is called the substrate, while the attacking species is the reagent.

i. Free Radical

A free radical is an atom or a group of atoms that possesses an unpaired electron.

• They are highly unstable and very reactive.
• Formed by the homolytic fission of a covalent bond.
• Examples: Chlorine radical (${\mathrm{Cl}}^{\cdot }$), Methyl radical (${\mathrm{CH}}_3^{\cdot }$).

ii. Electrophile

An electrophile (meaning "electron-loving") is a reagent that is deficient in electrons.

• It is attracted to electron-rich centers.
• An electrophile can be a positively charged ion or a neutral molecule with an empty orbital that can accept a pair of electrons.
• Example: Carbocation (${\mathrm{CH}}_3^{+}$), ${\mathrm{H}}^{+}$, ${\mathrm{AlCl}}_3$.

iii. Nucleophile

A nucleophile (meaning "nucleus-loving") is a reagent that is rich in electrons.

• It is attracted to electron-deficient centers (like the nucleus).
• A nucleophile can be a negatively charged ion or a neutral molecule with a lone pair of electrons to donate.
• Example: Hydroxide ion ($\mathrm{O}{\mathrm{H}}^{-}$), Ammonia (${\mathrm{NH}}_3$), Water (${\mathrm{H}}_2\mathrm{O}$).

---

Table 1: Comparison of Chemical Reagents

---

2. Types of Bond Breakage in Chemical Reactions

The breaking, or fission, of covalent bonds is a critical step in any chemical reaction. There are two primary ways a bond can break. These processes often result in a reaction intermediate, which is a short-lived, high-energy species formed during the reaction.

i. Homolytic Fission

Homolytic fission is the symmetrical cleavage of a covalent bond where each atom involved in the bond retains one of the shared electrons.

• This process results in the formation of two free radicals.
• It typically occurs between atoms with a zero or very small difference in electronegativity, often triggered by heat or light.
• A free radical carbon has seven electrons in its valence shell and no formal charge.

The homolytic fission of a chlorine molecule:

${\mathrm{Cl}}_2⟶2{\mathrm{Cl}}^{\cdot }$

ii. Heterolytic Fission

Heterolytic fission is the unsymmetrical cleavage of a covalent bond where one atom takes both of the shared electrons.

• This process results in the formation of a positive ion (cation) and a negative ion (anion).
• It occurs between atoms with a significant difference in electronegativity. The more electronegative atom takes the electron pair.

The heterolytic fission of a methyl chloride molecule:

${\mathrm{CH}}_3\mathrm{Cl}⟶{\mathrm{CH}}_3^{+}+{\mathrm{Cl}}^{-}$

Here, the more electronegative chlorine atom takes both electrons, forming a chloride ion (${\mathrm{Cl}}^{-}$) and a methyl carbocation (${\mathrm{CH}}_3^{+}$).

---

Possible Questions/Answers

• Q: What is the key difference between homolytic and heterolytic fission?
  A: In homolytic fission, the bond breaks evenly, with each atom getting one electron, forming free radicals. In heterolytic fission, the bond breaks unevenly, with one atom taking both electrons, forming ions.

• Q: Is a water molecule (${\mathrm{H}}_2\mathrm{O}$) more likely to act as an electrophile or a nucleophile?
  A: A water molecule is more likely to act as a nucleophile. The oxygen atom has two lone pairs of electrons that it can donate to an electron-deficient species.