Fission of Covalent Bond — Definition

Definition

Imagine two atoms, let's call them A and B, joined together by a covalent bond. This bond is essentially a shared pair of electrons holding them together. For any chemical reaction to occur, existing bonds must break, and new ones must form. The process of breaking this covalent bond is what we call 'fission' or 'cleavage'.

There are two main ways a covalent bond can break, and the distinction is crucial because it dictates the type of reactive intermediates formed and, consequently, the reaction pathway:

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Homolytic Fission (or Homolysis): — Think of 'homo' meaning 'same' or 'equal'. In homolytic fission, the shared pair of electrons is divided equally between the two atoms. Each atom gets one electron from the original shared pair. When an atom ends up with an unpaired electron, it becomes a highly reactive species called a 'free radical'. These free radicals are electrically neutral but possess a single unpaired electron, making them very eager to react and achieve a stable electron configuration. Homolytic fission typically occurs under harsh conditions like high temperatures, in the presence of ultraviolet (UV) light, or with initiators like peroxides. It's common in gas-phase reactions and non-polar solvents. The movement of a single electron is depicted by a 'fish-hook' or 'half-headed' arrow ($curvearrowright$).

*Example:* If a bond between two chlorine atoms ($ext{Cl-Cl}$) breaks homolytically, each chlorine atom gets one electron, forming two chlorine free radicals ($ext{Cl}cdot$).

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Heterolytic Fission (or Heterolysis): — Now, think of 'hetero' meaning 'different' or 'unequal'. In heterolytic fission, the shared pair of electrons is not divided equally. Instead, one of the atoms involved in the bond takes *both* electrons from the shared pair, while the other atom gets none. This unequal distribution leads to the formation of ions: one atom becomes positively charged (because it lost an electron), and the other becomes negatively charged (because it gained an electron). These charged species are called 'carbocations' (if the carbon atom is positive) or 'carbanions' (if the carbon atom is negative), or other types of ions depending on the atoms involved. Heterolytic fission usually occurs in polar solvents, which can stabilize the resulting ions, and is often facilitated by the presence of nucleophiles or electrophiles. The movement of an electron pair is depicted by a 'curved' or 'double-headed' arrow ($curvearrowright$).

*Example:* If a bond between a carbon atom and a chlorine atom ($ext{R-Cl}$) breaks heterolytically, and chlorine is more electronegative, the chlorine atom takes both electrons, forming a chloride ion ($ext{Cl}^-$) and a carbocation ($ext{R}^+$).

Understanding which type of fission is likely to occur in a given reaction is fundamental to predicting the reaction mechanism and the final products in organic chemistry. It's the very first step in analyzing how molecules transform.