Why Bonds Break — The Starting Point of Every Reaction
Chemical reactions involve the breaking of one or more of the existing chemical bonds in the reactant molecules and the formation of new bonds leading to products. In organic chemistry this fundamental event has a name: the breaking of a covalent bond is known as bond fission. A covalent bond is formed by the sharing of two electrons contributed by two atoms, so the only question that matters at the moment of breaking is how that shared pair is divided.
During bond fission the two shared electrons can be distributed in just two ways — equally or unequally between the two bonded atoms. These two modes are not minor variations of one another; they generate entirely different reactive species and therefore drive entirely different families of reactions. There are accordingly two types of bond fission: homolytic and heterolytic.
| Mode of fission | How the shared pair is split | Species formed |
|---|---|---|
| Homolytic | Equal sharing — one electron to each atom | Two neutral free radicals |
| Heterolytic | Unequal sharing — both electrons to one atom | A carbocation and a carbanion (ions) |
Homolytic Fission — Equal Sharing, Free Radicals
Homolytic fission is the fission of a covalent bond with equal sharing of the bonding electrons. Each of the two atoms walks away with exactly one of the two electrons that formerly made up the bond. For a hypothetical molecule $\ce{A-B}$, homolysis is written as the bond splitting into $\ce{A^.}$ and $\ce{B^.}$, where the dot denotes the single retained electron.
A concrete NIOS example is the carbon–carbon bond of ethane. Under the influence of heat or light, the bond breaks symmetrically:
$\ce{H3C-CH3 ->[\Delta][h\nu] {^.CH3} + {^.CH3}}$
The neutral species so formed are known as free radicals. A free radical is a neutral but highly reactive species that carries an unpaired electron, and like ions it can initiate a chemical reaction. Because the fragments are uncharged, homolysis is favoured in non-polar environments where there is no solvent to stabilise charge.
Heterolytic Fission — Unequal Sharing, Ions
Heterolytic fission is the fission of a covalent bond involving unequal sharing of the bonding electrons. Here the entire shared pair stays with one of the two atoms; the other atom leaves with nothing. For a hypothetical molecule, the heterolysis is written as $\ce{A:B -> A^+ + B^-}$, the colon emphasising that both electrons depart with B.
This type of bond fission results in the formation of ions. The ion that has a positive charge on the carbon atom is called the carbonium ion or, in modern usage, the carbocation; the ion with a negative charge on the carbon atom is the carbanion. NIOS gives clear examples of each:
| Carbocations (positive C) | Carbanions (negative C) |
|---|---|
| $\ce{CH3CH2^+}$ — ethyl carbocation | $\ce{CH3CH2^-}$ — ethyl carbanion |
| $\ce{(CH3)2CH^+}$ — isopropyl carbocation | $\ce{CH3^-}$ — methyl carbanion |
The charged species obtained by heterolytic fission go on to initiate chemical reactions, and they are classified as electrophiles and nucleophiles. An electrophile is an electron-deficient species — positively charged or neutral — that seeks electrons; examples include $\ce{H+}$, $\ce{Br+}$, $\ce{Cl+}$, $\ce{Ag+}$ and $\ce{BF3}$, and it attacks a position of high electron density. A nucleophile is a negatively charged or electron-rich neutral species such as $\ce{OH-}$, $\ce{H2O}$ and $\ce{:NH3}$, and it attacks a position of low electron density.
Arrow Conventions — Reading the Mechanism
The arrows used in mechanisms are not decoration; they encode exactly how many electrons move and where they go. Confusing the two arrow types is one of the most common errors in mechanism questions, so the convention is worth fixing firmly.
| Arrow | What it shows | Used for |
|---|---|---|
| Single-barbed half-head (fish-hook) | Movement of one electron | Homolytic fission → free radicals |
| Double-barbed full curved arrow | Movement of an electron pair | Heterolytic fission → carbocation + carbanion |
In homolysis you draw two fish-hook arrows, one starting from the bond to each atom, because the pair is being split into two single electrons. In heterolysis you draw one full curved arrow from the bond to the atom that keeps both electrons (the one that becomes the anion). Count the arrowheads to recover the mechanism: half-headed means radicals, full-headed means ions.
Homolytic → radical · Heterolytic → ion
Students routinely swap the two outcomes under exam pressure. Anchor it to the prefix: homo = same/equal, so each atom gets the same share (one electron) and you obtain neutral radicals; hetero = different/unequal, so the share is uneven and you obtain charged ions.
Homolytic fission → equal split → free radicals (fish-hook arrows). Heterolytic fission → unequal split → carbocation + carbanion (full curved arrow). Never write radicals for heterolysis or ions for homolysis.
Homolytic vs Heterolytic — Side by Side
With the two modes defined, the comparison that NEET examiners lean on is the contrast between them. The following table consolidates the arrow type, the intermediate produced and the conditions that favour each, all grounded in the NIOS treatment.
| Feature | Homolytic fission | Heterolytic fission |
|---|---|---|
| Sharing of bonding electrons | Equal — one electron to each atom | Unequal — both electrons to one atom |
| Arrow used | Two single-barbed fish-hook arrows | One double-barbed full curved arrow |
| Species formed | Two neutral free radicals | A carbocation and a carbanion (ions) |
| Net charge of fragments | Neutral, with an unpaired electron | Oppositely charged ions |
| Favoured for | Non-polar / weakly polar bonds | Polar bonds |
| Typical conditions | Heat or UV light; gas phase or non-polar solvent | Polar solvents that stabilise ions |
| Representative example | $\ce{H3C-CH3 -> 2{^.CH3}}$ | $\ce{A:B -> A^+ + B^-}$ |
The electron-deficient and electron-rich ions made by heterolysis are the actors in every polar mechanism. See Electrophiles and Nucleophiles for how they attack.
Conditions That Decide the Mode
Whether a given bond breaks homolytically or heterolytically is governed largely by the polarity of the bond and the environment in which it breaks. A non-polar or only weakly polar bond, such as the C–C bond of ethane, has no built-in preference for either atom to keep the electrons, so it tends to split evenly into radicals — and this is most readily achieved by supplying energy as heat or ultraviolet light, conditions associated with the gas phase or non-polar solvents.
A polar bond, by contrast, already has its electron density pulled towards the more electronegative atom. Under suitable conditions the bond simply completes that displacement, the electron pair ending up on the more electronegative partner to give ions. Polar solvents promote heterolysis because they surround and stabilise the newly formed charges, lowering the energy needed to separate them.
Q. Predict the likely mode of fission when (a) chlorine gas is irradiated with UV light, and (b) tert-butyl chloride ionises in a polar solvent.
(a) The $\ce{Cl-Cl}$ bond is perfectly non-polar, and UV light supplies the energy to break it symmetrically: homolysis gives two chlorine radicals, $\ce{Cl2 ->[h\nu] 2Cl^.}$.
(b) The $\ce{C-Cl}$ bond is polar and a polar solvent stabilises the resulting ions, so heterolysis is favoured: the electron pair leaves with chlorine to give a carbocation and a chloride ion, $\ce{(CH3)3C-Cl -> (CH3)3C^+ + Cl^-}$.
Reaction Intermediates — Radicals, Carbocations, Carbanions
The fragments produced by fission are short-lived, highly reactive species called reaction intermediates. They do not appear in the overall balanced equation but exist transiently along the path from reactants to products. The three intermediates that emerge directly from bond fission are the free radical, the carbocation and the carbanion.
| Intermediate | Origin | Electronic feature on carbon | Charge |
|---|---|---|---|
| Free radical | Homolytic fission | One unpaired electron | Neutral |
| Carbocation | Heterolytic fission | Sextet — electron-deficient | Positive |
| Carbanion | Heterolytic fission | Lone pair — electron-rich | Negative |
A free radical is neutral and reactive and can initiate a chain reaction. A carbocation is electron-deficient and therefore behaves as an electrophile, while a carbanion is electron-rich and behaves as a nucleophile. The relative stabilities of these intermediates — which decide which one forms preferentially and how fast a reaction proceeds — are studied in detail separately, but the link back to fission is direct: homolysis is the gateway to radicals, heterolysis to the two ions.
Why is a tertiary carbocation more stable than a primary one? Build the full ordering in Stability of Carbocations, Carbanions & Radicals.
Types of Organic Reactions — Where Fission Leads
Once a bond has broken and an intermediate has formed, the reaction proceeds along one of four broad routes. NIOS classifies the different ways organic reactions occur into substitution, addition, elimination and molecular rearrangements, and understanding a reaction mechanism means knowing the detailed steps by which reactants become products.
| Reaction type | What happens | NIOS example |
|---|---|---|
| Substitution | One atom or group is displaced by another | $\ce{R-X + Nu^- -> R-Nu + X^-}$ |
| Addition | A reagent adds across a multiple bond of an unsaturated compound | $\ce{CH2=CH2 + Br2 -> CH2Br-CH2Br}$ |
| Elimination | A small molecule is removed from adjacent carbons, forming a double bond | $\ce{CH3CH2OH ->[H2SO4][403\,K] CH2=CH2 + H2O}$ |
| Rearrangement | An atom or group migrates, changing the carbon skeleton | $\ce{CH3CH2CH2CH2Cl ->[AlCl3] CH3CH2CHClCH3}$ |
In aliphatic nucleophilic substitution a haloalkane is converted to many different compounds by replacing the halogen with nucleophiles such as $\ce{-OH}$, $\ce{-NH2}$, $\ce{-CN}$ or $\ce{-OR'}$. In aromatic systems the ring, being electron-rich, is attacked by an electrophilic reagent and one ring hydrogen is the leaving group — for example, nitration replaces a hydrogen of benzene with the $\ce{-NO2}$ group. Addition occurs because the weaker π bond of an alkene or alkyne breaks readily; elimination is the reverse, expelling a small molecule to create unsaturation; and rearrangement, illustrated by the AlCl₃-catalysed conversion of 1-chlorobutane to 2-chlorobutane, fundamentally alters the skeleton.
Lock these before the exam
- Bond fission is the breaking of a covalent bond; the shared pair is divided either equally or unequally.
- Homolytic fission = equal sharing → two neutral free radicals, each with an unpaired electron; shown by two single-barbed fish-hook arrows; favoured by heat/UV light and non-polar bonds.
- Heterolytic fission = unequal sharing → a carbocation ($\ce{C^+}$) plus a carbanion ($\ce{C^-}$); shown by one double-barbed curved arrow; favoured by polar bonds and polar solvents.
- The ions from heterolysis are classified as electrophiles (electron-deficient, e.g. $\ce{H+}$, $\ce{BF3}$) and nucleophiles (electron-rich, e.g. $\ce{OH-}$, $\ce{:NH3}$).
- The four reaction types are substitution, addition, elimination and molecular rearrangement.