Fission of Covalent Bond — Revision Notes

⚡ 30-Second Revision

Covalent Bond Fission: — Breaking of a covalent bond.
Homolytic Fission: — Symmetrical breaking, each atom gets one electron.

- Forms: Free radicals (e.g., $ext{R}cdot$) - Conditions: High T, UV light, peroxides. - Arrow: Fish-hook ($curvearrowright$) - Stability: $3^circ > 2^circ > 1^circ > \text{methyl}$ (similar to carbocations)

Heterolytic Fission: — Unsymmetrical breaking, one atom gets both electrons.

- Forms: Ions (carbocations $ext{R}^+$, carbanions $ext{R}^-$) - Conditions: Polar solvents, good leaving groups, acids/bases. - Arrow: Curved ($curvearrowright$) - Carbocation Stability: $3^circ > 2^circ > 1^circ > \text{methyl}$ - Carbanion Stability: $ext{methyl} > 1^circ > 2^circ > 3^circ$

Key Factors: — Electronegativity, bond strength, solvent, temperature, light.

2-Minute Revision

Covalent bond fission is the essential first step in most organic reactions, determining the reactive intermediates. It occurs in two main ways: homolytic and heterolytic.

Homolytic fission is the symmetrical breaking of a bond, where each atom takes one electron, forming neutral, highly reactive free radicals (e.g., $ext{Cl}cdot$, $ext{CH}_3cdot$). This process is typically initiated by high energy like heat or UV light, or by radical initiators such as peroxides.

Electron movement is shown by fish-hook arrows ($curvearrowright$). The stability of free radicals increases with alkyl substitution ($3^circ > 2^circ > 1^circ > \text{methyl}$) due to hyperconjugation and inductive effects.

Heterolytic fission is the unsymmetrical breaking of a bond, where one atom takes both bonding electrons, forming ions (a cation and an anion). If carbon becomes positive, it's a carbocation ($ext{R}^+$); if negative, it's a carbanion ($ext{R}^-$).

This fission is favored by polar solvents that stabilize the resulting ions and by the presence of good leaving groups. Electron movement is shown by curved arrows ($curvearrowright$). Carbocation stability follows the order $3^circ > 2^circ > 1^circ > \text{methyl}$, similar to free radicals.

Carbanion stability, however, is generally opposite for simple alkyl systems: $ext{methyl} > 1^circ > 2^circ > 3^circ$, as electron-donating alkyl groups destabilize the negative charge. Resonance stabilization is a powerful factor for all three types of intermediates.

5-Minute Revision

The breaking of a covalent bond, known as bond fission, is the gateway to all organic transformations. There are two fundamental modes, each leading to distinct reactive intermediates and reaction pathways.

1. Homolytic Fission (Homolysis):

This is the symmetrical cleavage of a covalent bond, where the shared electron pair is split equally, with one electron going to each atom. The result is the formation of free radicals, which are neutral species possessing an unpaired electron. These are highly reactive due to their incomplete octet.

Conditions: — Favored by high temperatures, ultraviolet (UV) light, or the presence of radical initiators like peroxides. Non-polar solvents also tend to favor homolysis.
Representation: — Single-headed or 'fish-hook' arrows ($curvearrowright$) are used to show the movement of individual electrons.
Stability of Free Radicals: — Generally, the more substituted the carbon radical, the more stable it is. This is due to electron-donating inductive effects from alkyl groups and hyperconjugation. The order is: Tertiary ($3^circ$) > Secondary ($2^circ$) > Primary ($1^circ$) > Methyl ($ext{CH}_3cdot$). Allylic and benzylic radicals are further stabilized by resonance.

*Example:* $ext{CH}_3-\text{CH}_3 xrightarrow{\text{UV light}} 2\text{CH}_3cdot$

2. Heterolytic Fission (Heterolysis):

This involves the unsymmetrical cleavage of a covalent bond, where one atom retains both electrons from the shared pair, while the other atom gets none. This leads to the formation of ions – a positively charged species (cation) and a negatively charged species (anion).

Conditions: — Favored by polar solvents (which stabilize ions through solvation), the presence of good leaving groups (stable anions or neutral molecules), and sometimes by acid/base catalysis.
Representation: — Double-headed or 'curved' arrows ($curvearrowright$) are used to show the movement of an electron pair.
Types of Ions:

* **Carbocations ($ext{R}^+$):** Positively charged carbon. Electrophilic. Stability order: $3^circ > 2^circ > 1^circ > \text{methyl}$ (due to +I effect and hyperconjugation). Resonance also significantly stabilizes carbocations (e.

g., allylic, benzylic). * **Carbanions ($ext{R}^-$):** Negatively charged carbon. Nucleophilic. Stability order for simple alkyl carbanions: $ext{methyl} > 1^circ > 2^circ > 3^circ$ (opposite to carbocations, as electron-donating alkyl groups destabilize the negative charge).

Electron-withdrawing groups and resonance stabilize carbanions.

Key Takeaways for NEET: Focus on identifying the type of fission based on conditions, predicting the resulting intermediates, and accurately ranking their stabilities. Understanding arrow pushing is crucial for visualizing electron flow in mechanisms.

Prelims Revision Notes

Fission of Covalent Bond: NEET Revision Notes

1. Basic Concept:

Bond fission is the breaking of a covalent bond, initiating chemical reactions.
Two main types: Homolytic and Heterolytic.

2. Homolytic Fission (Homolysis):

Definition: — Symmetrical breaking; each atom gets one electron from the shared pair.
Products: — Free radicals (neutral species with an unpaired electron, highly reactive).

* Notation: $ext{R}cdot$ (e.g., $ext{CH}_3cdot$, $ext{Cl}cdot$)

Conditions:

* High temperature (heat) * Ultraviolet (UV) light * Presence of radical initiators (e.g., peroxides $ext{R-O-O-R}$, azo compounds $ext{R-N=N-R}$) * Non-polar solvents

Electron Movement: — Depicted by fish-hook (half-headed) arrows ($curvearrowright$).
Stability of Free Radicals:

* Order: Tertiary ($3^circ$) > Secondary ($2^circ$) > Primary ($1^circ$) > Methyl ($ext{CH}_3cdot$) * Factors: Hyperconjugation (more $alpha$-hydrogens), electron-donating inductive effect (+I) of alkyl groups. * Resonance stabilization: Allylic and benzylic radicals are highly stable.

3. Heterolytic Fission (Heterolysis):

Definition: — Unsymmetrical breaking; one atom takes both electrons from the shared pair.
Products: — Ions (a cation and an anion).

* Carbocation: Positively charged carbon ($ext{R}^+$), $sp^2$ hybridized, planar, empty p-orbital, electrophilic. * Carbanion: Negatively charged carbon ($ext{R}^-$), typically $sp^3$ hybridized, pyramidal, lone pair, nucleophilic.

Conditions:

* Polar solvents (stabilize ions via solvation, e.g., water, ethanol, DMSO) * Presence of good leaving groups (stable anions or neutral molecules, e.g., $ext{Cl}^-$, $ext{Br}^-$, $ext{H}_2\text{O}$) * Acid/base catalysis

Electron Movement: — Depicted by curved (double-headed) arrows ($curvearrowright$).
Stability of Carbocations:

* Order: Tertiary ($3^circ$) > Secondary ($2^circ$) > Primary ($1^circ$) > Methyl ($ext{CH}_3^+$) * Factors: Electron-donating inductive effect (+I) of alkyl groups, hyperconjugation (overlap of $sigma$ bonds with empty p-orbital). * Resonance stabilization: Allylic and benzylic carbocations are highly stable.

Stability of Carbanions:

* Order (simple alkyl): Methyl ($ext{CH}_3^-$) > Primary ($1^circ$) > Secondary ($2^circ$) > Tertiary ($3^circ$) * Factors: Electron-withdrawing inductive effect (-I) of groups, resonance stabilization (delocalization of negative charge), higher s-character of the carbon orbital ($sp > sp^2 > sp^3$). Alkyl groups (electron-donating) destabilize carbanions.

4. Key Distinctions (Summary):

Aspect	Homolytic Fission	Heterolytic Fission
Electron Split	Equal	Unequal
Intermediates	Free Radicals	Ions (Carbocations, Carbanions)
Arrow Type	Fish-hook ($curvearrowright$)	Curved ($curvearrowright$)
Conditions	High T, UV, Peroxides	Polar Solvents, Good LGs

5. NEET Focus Areas:

Identify fission type from conditions/products.
Rank stability of free radicals, carbocations, and carbanions (including resonance-stabilized ones).
Understand the role of bond fission in reaction mechanisms (e.g., SN1, E1, free radical substitution).

Vyyuha Quick Recall

Homo Radicals UV Temp Peroxides, Hetero Ions Polar Leaving Groups.

Homo: Homolytic fission
Radicals: Forms Free Radicals
UV Temp Peroxides: Conditions (UV light, high Temperature, Peroxides)

Hetero: Heterolytic fission
Ions: Forms Ions (Carbocations, Carbanions)
Polar Leaving Groups: Conditions (Polar solvents, Good Leaving Groups)

For stability: Carbocations & Radicals are 321M (3° > 2° > 1° > Methyl). Carbanions are M123 (Methyl > 1° > 2° > 3°).