Concept understanding — Homolytic and Heterolytic Bond Fission
The Core Intuition
Imagine a covalent bond as a shared pair of electrons — two people holding a rope. When the bond breaks, something has to happen to those two electrons. They can't just vanish. The entire story of homolytic and heterolytic fission is simply: what happens to the two electrons when the bond snaps?
If each atom walks away with one electron, that's homolytic fission. If one atom grabs both electrons and the other gets none, that's heterolytic fission. That's it. Everything else — free radicals, ions, reaction mechanisms — follows from this single difference.
Homolytic Fission: Each Atom Keeps One Electron
In homolytic fission, the bond breaks symmetrically. Each atom takes one electron from the shared pair.
A:B⟶A∙+B∙
The dot (∙) represents an unpaired electron. Species with an unpaired electron are called free radicals. They are highly reactive because having an unpaired electron makes them desperate to pair it up.
Note
Homolytic fission is typically caused by heat (thermolysis) or light (photolysis). It is common in non-polar bonds, like Cl−Cl or Br−Br.
Example: Chlorine gas exposed to sunlight.
Cl−ClhνCl∙+Cl∙
Each chlorine atom now has seven valence electrons — one unpaired. This is the first step in the free radical chlorination of methane, a classic reaction you will meet in organic chemistry.
Watch out
A common mistake: thinking homolytic fission always produces two identical radicals. It does not. The bond can be between two different atoms, like CH3−Br, giving CH3∙ and Br∙. The key is each atom gets one electron, not that the products are the same.
Heterolytic Fission: Both Electrons Go to One Atom
In heterolytic fission, the bond breaks asymmetrically. One atom takes both electrons; the other gets none.
A:B⟶A−+B+
The atom that takes both electrons becomes negatively charged (anion). The atom that loses both electrons becomes positively charged (cation). These are ions, not radicals — no unpaired electrons.
Note
Heterolytic fission is typically caused by polar solvents or polar reagents. It is common in polar bonds, like C−Br or H−Cl, where one atom is significantly more electronegative.
Example: Breaking a carbon-bromine bond in the presence of a polar solvent.
CH3−Br⟶CH3++Br−
Bromine is more electronegative than carbon, so it pulls both electrons toward itself. The result: a bromide ion and a carbocation (carbon with a positive charge).
Important
Heterolytic fission requires that the bond be polar enough for one atom to have a significantly stronger pull on the electrons. In a perfectly non-polar bond like Cl−Cl, heterolytic fission is extremely difficult because neither atom can dominate the electron pair.
In heterolytic cleavage, a covalent bond breaks unevenly — both bonding electrons go to the more electronegative/stabilised fragment, generating a cation and an anion.
Heterolytic cleavage (heterolysis) is the breaking of a covalent bond such that both electrons of the shared pair go to one of the two bonded atoms, rather than one electron going to each (which would be homolysis). This requires the presence of an electronegativity difference or a polar/polarisable bond, and typically occurs in polar solvents.
Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.
CBSE 2026Set ANNUAL1 markMCQ
Q.Homolytic fission leads to the formation of
(a) nucleophile
(b) carbanion
(c) free radical
(d) carbocation
›Reveal solutionSolution
Covalent bond cleavage can happen in two ways: homolytic (each atom keeps one electron -> free radicals) or heterolytic (one atom keeps both electrons -> ions like carbocations/carbanions).
In homolytic fission (or homolysis), a covalent bond A-B breaks symmetrically, with one electron from the shared pair going to each fragment:
A:B -> A. + B. (each fragment now has an unpaired electron)
Q.Free radical species are product of ______ bond cleavage.
›Reveal solutionSolution
Free radicals are formed by homolytic (not heterolytic) bond fission.
A covalent bond can break in two ways: Homolytic cleavage (homolysis) — the bonding electron pair splits equally, one electron going to each fragment, producing two neutral species each with an unpaired electron: these are free radicals (e.g. Cl-Cl → Cl• + Cl•, typically induced by heat or UV light). Heterolytic cleavage (heterolysis) — the bonding pair …
Homolysis breaks a covalent bond so that each fragment keeps one electron of the shared pair, producing neutral free radicals — never ions.
In CH3CH2−Cl, homolytic fission of the C-Cl bond gives one electron to carbon and one to chlorine: CH3CH2Cl→CH3CH2∙+Cl∙, i.e. an ethyl free radical and a chlorine free radical. (Heterolytic fission, by contrast, …
Q.Heterolytic cleavage of C - X bond may result in the formation of
(a) free radical
(b) carbocation
(c) carbene
(d) none of these
›Reveal solutionSolution
Heterolytic (unequal) bond cleavage sends both bonding electrons to one atom -- if they go to X, carbon is left short an electron pair, forming a carbocation.
Bond cleavage (fission) can happen in two ways:
Homolytic cleavage: the bonding pair splits evenly, one electron to each atom, producing two free radicals. …
Q.Homolytic fission of a covalent bond leads to the formation of
(a) Electrophile
(b) Nucleophile
(c) Free radical
(d) Carbocation
›Reveal solutionSolution
Homolytic (symmetrical) fission of a covalent bond gives two neutral species each with an unpaired electron — free radicals.
A covalent bond A-B is formed by a shared pair of electrons. It can break in two ways:
Homolytic fission: the bond breaks symmetrically, and each atom takes exactly one electron of the shared pair, i.e., A-B → A• + B•. Each product is a neutral species with one unpaired electron, called a free radical. This typically requires energy input such as heat, light (UV), or peroxides, and occurs in non-polar/gas-phase or radical-chain reactions.
Heterolytic fission (for contrast): the bond breaks unsymmetrically, and both electrons go to one atom, giving a cation and an anion (e.g. a carbocation/electrophile and a carbanion/nucleophile).
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