A covalent bond is two shared electrons, so when it breaks there are only two ways to divide them. Split them evenly and you get radicals. Give both to one atom and you get ions — and those ions are exactly the nucleophiles and electrophiles that drive organic chemistry.
📚 What you need to know
Heterolytic fission is the breaking of a covalent bond so that both bonding electrons go to one atom, normally the more electronegative one.
It produces a positive ion (left with neither electron) and a negative ion (which gained both).
It is shown with a double-headed curly arrow starting at the bond and ending on the atom that takes the pair.
Homolytic fission is the alternative: one electron each, shown with single-headed fishhook arrows, producing radicals.
Polar bonds tend to break heterolytically; non-polar bonds tend to break homolytically.
The negative ion is electron-rich and acts as a nucleophile; the positive ion is electron-deficient and acts as an electrophile.
Running the process backwards — a nucleophile donating a pair to an electrophile — forms a coordinate covalent bond.
Two ways to divide a shared pair
You met homolytic fission with the halogenation of alkanes: a Cl–Cl bond breaks down the middle, each chlorine keeps one electron, and you get two radicals. That works because the bond is non-polar — neither atom has any claim on the pair, so an even split is the natural outcome.
A polar bond is different. One atom is already pulling the pair harder than the other, so when the bond gives way that atom simply takes the lot.
The prefixes are worth unpicking: homo means the same, hetero means different. An even split gives two species of the same kind; an uneven one gives two opposites.
The curly arrow rule
A double-headed curly arrow means “this pair of electrons moves”. It has a fixed grammar, and marks are lost for getting it wrong more often than for getting the chemistry wrong.
🧩 Drawing the arrow correctly
The tail starts on the thing that is moving — the bond itself for fission, or a lone pair when a nucleophile attacks.
The head lands where the pair ends up — on the atom that keeps them, or in the space where a new bond forms.
Use two heads. A single-headed hook means one electron and belongs in radical mechanisms.
Check the charges afterwards: the atom that gained the pair goes negative, the one that lost it goes positive.
The charges are not arbitrary. Hydrogen started with one electron in that bond and ends with none, so it is 1+; chlorine ends with one more than it started with, so it is 1–.
Where nucleophiles and electrophiles come from
Look at what heterolytic fission has just produced and the two great families of organic reagents fall out of it automatically.
Product of the fission
Electron situation
What it does
Name
the negative ion
electron-rich, has a spare pair
donates a pair of electrons
nucleophile
the positive ion
electron-deficient, short of a pair
accepts a pair of electrons
electrophile
This is why the last page worked the way it did. Breaking a C–X bond heterolytically leaves a carbocation, which is an electrophile, and a halide ion, which is a nucleophile. The whole of nucleophilic substitution is one heterolytic bond break and one bond formation.
Running it backwards: the coordinate bond
Heterolytic fission gives one atom a lone pair it did not have before. Bond formation is the same idea in reverse: a nucleophile hands a lone pair to an electrophile, and the two electrons of the new bond both came from the same place. A bond made that way is a coordinate covalent bond — sometimes called a dative bond.
The finished water molecule cannot tell you which of its two O–H bonds was made this way. “Coordinate” describes how the bond was formed, not what it is like afterwards.
If the names will not stick, read them literally. A nucleophile is a nucleus-lover, and a nucleus is positive — so a nucleophile is drawn to positive charge, which means it must be the one carrying the spare electrons. An electrophile loves electrons, so it must be the one that has not got enough.
WORKED EXAMPLE
The H–Br bond breaks heterolytically. State the products, explain which atom keeps the electrons, and state where the curly arrow starts and ends.
Which atom keeps the pairBromine is more electronegative than hydrogen, so it was already pulling the shared pair towards itself.bromine takes both electronsThe productsHBr → H⁺ + :Br⁻The arrowThe tail sits on the H–Br bond and the head lands on the bromine atom. It has two heads, because a whole pair is moving.Check the charges add up: neutral HBr gives 1+ and 1−, which cancel.
WORKED EXAMPLE
Explain why a Cl–Cl bond tends to break homolytically, while a C–Cl bond tends to break heterolytically.
Cl–ClThe two atoms are identical, so the electronegativity difference is zero and the bonding pair is shared equally. Neither atom has a reason to take both.splits evenly, giving two chlorine radicalsC–ClChlorine is the more electronegative atom, so the pair is already displaced towards it. Breaking the bond simply completes a shift that had begun.chlorine takes both, giving a carbocation and Cl⁻The general rule: polar bond, heterolytic; non-polar bond, homolytic.
WORKED EXAMPLE
Ammonia reacts with a hydrogen ion to form the ammonium ion. Identify the nucleophile and the electrophile, and name the type of bond formed.
The nucleophileNitrogen in NH₃ has a lone pair and is electron-rich.NH₃The electrophileH⁺ has no electrons at all and an empty orbital to fill.H⁺The bondBoth electrons in the new N–H bond came from the nitrogen.a coordinate covalent bondAll four N–H bonds in NH₄⁺ are equivalent once it has formed.
💡 Exam tip
Define heterolytic fission as both bonding electrons going to one atom, and say which atom — the more electronegative one.
State the products as ions, with charges shown, and show the lone pair on the anion.
Say double-headed curly arrow for a pair, single-headed fishhook for one electron.
Start the arrow at the bond or the lone pair, never at an atom label.
Link the two ideas: heterolytic fission creates nucleophiles and electrophiles, and the reverse process forms a coordinate bond.
Use electronegativity to justify which way round the fission goes — do not just assert it.
⚠️ Common mix-up
Swapping the definitions of nucleophile and electrophile. The nucleophile has the electrons; the electrophile wants them.
Drawing one curly arrow for homolytic fission. That needs two fishhooks, one for each electron.
Giving the electrons to the less electronegative atom, which produces the wrong charges.
Writing radicals as products of heterolytic fission. Radicals come from the homolytic route.
Treating a coordinate bond as weaker than a normal covalent bond. It is identical once formed.
Up next: Electrophilic Addition Reactions — so far the electron-rich species has been an ion with a lone pair. In an alkene it is a whole bond, sticking out of the molecule and waiting to be attacked.
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