Reaction Mechanism Fundamentals

Two ways a bond can break. Picture a covalent bond as a shared pair of electrons sitting between two atoms. When the bond snaps, those two electrons have to go somewhere, and there are exactly two possibilities. In homolytic fission (homolysis), the pair splits evenly: each atom walks away with one electron. This needs a real energy push — heat or ultraviolet light — because you are forcing a stable, paired bond to give up its symmetry. Curved-arrow notation shows this with a "fish-hook," a single-barbed arrow drawn for each electron, one arrow per atom. The products are free radicals: neutral species carrying one unpaired electron, and because an unpaired electron is energetically uncomfortable, radicals are highly reactive.

where: $h\nu$ is a photon of ultraviolet light supplying the energy, and $\text{Cl}^{\bullet}$ denotes a chlorine free radical, the dot marking its single unpaired electron. In heterolytic fission (heterolysis), the pair does not split — both electrons leave together with the atom better able to hold them, usually the more electronegative one. A single full (double-barbed) curved arrow shows both electrons moving as a pair. This produces two oppositely charged fragments in one step:

where: $\text{R}^+$ is a carbocation — a carbon holding only six valence electrons, positively charged, $sp^2$-hybridised and planar around that carbon — and $\text{X}^-$ is the anion carrying the full electron pair. The mirror case, where carbon keeps the pair instead of losing it, gives a carbanion $\text{R}_3\text{C}^-$: electron-rich, $sp^3$-hybridised, roughly pyramidal, with a lone pair where the fourth bond would be.

Sorting species by what they seek. Once a fragment exists — whether from fission or simply as an added reagent — it can be classified by its electron appetite. An electrophile ("electron-lover") is electron-deficient: it carries a positive charge or an empty orbital and goes looking for electron-rich centres to bond with. Examples include $\text{H}^+$, $\text{NO}_2^+$, $\text{BF}_3$ and any carbocation. A nucleophile ("nucleus-lover") is electron-rich: negatively charged, or neutral with a lone pair to donate, and it seeks out electron-deficient centres. Examples include $\text{OH}^-$, $\text{CN}^-$, $\text{NH}_3$ and $\text{H}_2\text{O}$. Note that neutrality does not disqualify a species from being a nucleophile — what matters is whether a lone pair is available to donate, not the formal charge.

Four broad reaction categories. With the fission types and species named, Class 11 also names — without kinetics — the four umbrella categories every organic transformation falls under: substitution (one atom or group replaced by another), addition (two groups add across a multiple bond, lowering bond order), elimination (groups removed from adjacent carbons, usually forming a multiple bond), and rearrangement (the skeleton reorganises, often by migration to an electron-deficient centre, with no change in molecular formula).

There is no rate-law formula at this level — Class 11 stops at classification, not kinetics — so the reasoning that matters is why electronegativity and octet stability decide how a bond splits, and why lone pairs versus empty orbitals decide reactivity. Heterolysis is driven by electronegativity and stability, not by chance. When a bond between carbon and a more electronegative atom (like halogen) breaks under ionising conditions (polar solvent, or an electron-withdrawing group already present), the electron pair prefers to sit on the atom that can hold negative charge more comfortably — usually the more electronegative one, or the one whose resulting anion is resonance- or inductively stabilised. Carbon is left electron-deficient, a carbocation, precisely because it was the worse place for two extra electrons to sit. This is why heterolysis dominates in polar, ionic environments, while homolysis — which needs no stabilising solvent, only enough energy to break the bond symmetrically — dominates under heat or light with weak, non-polar bonds like $\text{Cl}{-}\text{Cl}$ or $\text{O}{-}\text{O}$. The electrophile/nucleophile labels follow the same logic in reverse. A species reacts as an electrophile because it has somewhere for new electrons to go — an empty orbital or a full positive charge creates a genuine electronic vacancy. A species reacts as a nucleophile because it has electrons ready to give away — a lone pair or a formal negative charge is electron density sitting idle, waiting for an electron-deficient partner. Reactivity in organic chemistry, at this foundational stage, is nothing more than matching an electron-rich site to an electron-poor one; every later mechanism (substitution, addition, elimination) is this same matching game played out with specific structures.

Classify the bond-breaking step $(\text{CH}_3)_3\text{C}{-}\text{Br} \longrightarrow (\text{CH}_3)_3\text{C}^+ + \text{Br}^-$, name every species formed, and state which one would attack next as an electrophile and which as a nucleophile.

Step 1. Track where the bonding electrons end up. Both electrons of the original C–Br bond are now on bromine — $\text{Br}^-$ carries a full octet and a negative charge — leaving the carbon atom with only six valence electrons.

Step 2. Classify the fission. Since both electrons went to a single atom (Br) rather than splitting one-each, this is heterolytic fission, shown by a single full curved arrow from the C–Br bond onto Br.

Step 3. Name the carbon species. Carbon retains three bonds (to three methyl groups) and an empty $p$ orbital with a positive charge — this is a carbocation, specifically the tert-butyl cation, a tertiary carbocation (three carbon substituents on the charged carbon).

Step 4. Classify each fragment's reactivity. $\text{Br}^-$ has a complete octet and spare electron density — it behaves as a nucleophile (here it is also the leaving group). The tert-butyl cation is electron-deficient with an empty orbital, so in any subsequent step it behaves as an electrophile, ready to accept a pair of electrons from an incoming nucleophile.

Answer: heterolytic fission produces the tert-butyl carbocation (electrophile) and bromide ion (nucleophile).

Competitive stretch

Identify whether $\text{NH}_3$ and $\text{AlCl}_3$ each act as an electrophile or a nucleophile. Nitrogen in $\text{NH}_3$ carries a lone pair available for donation, so despite being neutral it is a nucleophile. Aluminium in $\text{AlCl}_3$ has only six electrons around it — three bonds to Cl and no lone pair — so it has an empty orbital and acts as an electrophile; this is why $\text{AlCl}_3$ is the classic Lewis-acid catalyst in Friedel–Crafts reactions studied later in Hydrocarbons. The rule that decides both cases is the same: lone pair or negative charge means nucleophile, empty orbital or positive charge means electrophile — formal charge alone is not the test.

• Assuming heterolysis always sends both electrons to the more electronegative atom "by rule" — reason from resonance/inductive stability of the resulting charged species, not rote memory.
• Confusing carbocations (electron-deficient, positive, $sp^2$, planar) with carbanions (electron-rich, negative, $sp^3$, pyramidal, has a lone pair) — remember "cat-ion" is missing electrons, not gaining them.
• Classifying a species as electrophile or nucleophile purely from formal charge and ignoring lone pairs — neutral molecules like $\text{NH}_3$ and $\text{H}_2\text{O}$ are nucleophiles because of an available lone pair, not because of a negative charge.
• Reaching for SN1/SN2/E1/E2 rate laws or detailed step-by-step mechanisms — Class 11 covers only classifying bond fission and naming reaction categories; the kinetics belong to Class 12.

Ask the tutor: want to practise classifying a batch of reagents as electrophiles or nucleophiles, or walk through another homolysis-versus-heterolysis example step by step?