Intuition The big pictureA reaction is just electrons moving . Atoms get dragged along, but it's the electrons that decide what breaks and what forms. Curved arrows are the grammar of organic chemistry — they let you track every electron from start to finish. If you can push arrows correctly, you can predict products you've never memorized.
Definition Reaction mechanismA mechanism is the step-by-step account of which bonds break , which bonds form , and how electrons move to turn reactants into products.
Definition Curved-arrow notationA curved arrow shows the movement of electrons , not atoms.
A double-barbed arrow (↷, full arrowhead) = movement of an electron pair (2 electrons).
A single-barbed / fishhook arrow (⇀, half arrowhead) = movement of a single electron (1 electron).
The arrow starts at the electron source (a bond or a lone pair) and points to where the electrons end up.
A covalent bond is a shared pair of electrons (·:·). When the bond breaks, those 2 electrons have to go somewhere . There are only two possibilities:
Definition Heterolysis (heterolytic cleavage)The bond breaks unevenly — both electrons go to ONE atom .
A : B ⟶ A + + : B − A\!:\!B \longrightarrow A^{+} + \;:\!B^{-} A : B ⟶ A + + : B − double-barbed arrow. Produces ions (a cation and an anion). The atom that keeps the electrons becomes negative; the one that loses them becomes positive.
Definition Homolysis (homolytic cleavage)The bond breaks evenly — each atom keeps ONE electron .
A : B ⟶ A ∙ + B ∙ A\!:\!B \longrightarrow A^{\bullet} + B^{\bullet} A : B ⟶ A ∙ + B ∙ two fishhook arrows. Produces free radicals (neutral species with an unpaired electron).
Intuition WHY does which one happen?
Heterolysis is favored when the two atoms differ a lot in electronegativity (a polar bond) and/or the solvent stabilizes ions (polar/protic). The more electronegative atom is happy to grab both electrons.Homolysis is favored for non-polar bonds (similar electronegativity) and/or high energy input with no ion-stabilizing help — like UV light or heat in the gas phase. Splitting evenly avoids charge separation that nothing can stabilize.
An arrow is a moving electron pair. You can't move electrons from an empty place — there'd be nothing to move. So the tail must sit on something that actually has electrons.
A new bond forms when a lone pair (or π/σ bond) attacks an electrophilic atom . The arrowhead lands between the two atoms = the new shared pair = the new bond. Bond breaking and bond forming are the same act of electron motion viewed from the two ends of the arrow.
Worked example 1 — Heterolysis of a C–Br bond (forming a carbocation)(CH 3 ) 3 C–Br ⟶ (CH 3 ) 3 C + + : Br − \text{(CH}_3)_3\text{C–Br} \longrightarrow \text{(CH}_3)_3\text{C}^{+} + \;:\!\text{Br}^{-} (CH 3 ) 3 C–Br ⟶ (CH 3 ) 3 C + + : Br − Arrow: tail on the C–Br bond, head onto Br.
Why this step? Br is more electronegative, so it pulls both electrons. The arrow's head landing on Br turns it into : B r − :\!Br^- : B r − − 1 -1 − 1 + 1 +1 + 1 carbocation ).Charge check: reactant neutral (0) → products ( + 1 ) + ( − 1 ) = 0 (+1) + (-1) = 0 ( + 1 ) + ( − 1 ) = 0
Worked example 2 — Bond formation: nucleophile attacks the carbocation(CH 3 ) 3 C + + : O − H ⟶ (CH 3 ) 3 C–OH \text{(CH}_3)_3\text{C}^{+} + \;:\!\overset{-}{\text{O}}\text{H} \longrightarrow \text{(CH}_3)_3\text{C–OH} (CH 3 ) 3 C + + : O − H ⟶ (CH 3 ) 3 C–OH Arrow: tail on the lone pair of O H − OH^- O H − C + C^+ C +
Why this step? The carbocation is electron-poor (+ + + is the new C–O bond.Charge check: ( + 1 ) + ( − 1 ) = 0 (+1)+(-1) = 0 ( + 1 ) + ( − 1 ) = 0
Worked example 3 — Homolysis of Cl–Cl by UV lightCl–Cl → h ν Cl ∙ + Cl ∙ \text{Cl–Cl} \xrightarrow{h\nu} \text{Cl}^{\bullet} + \text{Cl}^{\bullet} Cl–Cl h ν Cl ∙ + Cl ∙ Arrows: two fishhooks, one electron of the bond to each Cl.
Why this step? Cl–Cl is non-polar (same atom, same electronegativity) — neither side wants both electrons, so they split evenly. UV supplies the energy; no ions form because there's nothing to stabilize charge in the gas phase.Result: two neutral chlorine radicals , each with 7 electrons and one unpaired electron.
Worked example 4 — Reading a mechanism backwards (forecast practice)Given: H 2 C = C H 2 + H − B r → C H 3 − C H 2 − B r H_2C{=}CH_2 + H{-}Br \to CH_3{-}CH_2{-}Br H 2 C = C H 2 + H − B r → C H 3 − C H 2 − B r
The π bond is electron-rich → tail of arrow on the C=C π bond, head to the H of H–Br.
Simultaneously the H–Br bond breaks heterolytically : arrow tail on H–Br bond, head onto Br.
Why? The π electrons grab the partially positive H (δ + \delta^+ δ + C H 3 − C H 2 + CH_3{-}CH_2^+ C H 3 − C H 2 + : B r − :Br^- : B r − Charge check: neutral + neutral → ( + 1 ) + ( − 1 ) = 0 (+1)+(-1)=0 ( + 1 ) + ( − 1 ) = 0
Common mistake Drawing the arrow from the atom instead of the electronsWhy it feels right: we think in terms of atoms moving ("Cl leaves"), so we draw the arrow from the Cl atom.
Fix: the tail must sit on a bond or lone pair . "Cl leaves" is modeled as the bond's electrons moving onto Cl: tail on the C–Cl bond, head on Cl.
Common mistake Using a full arrow for radical reactionsWhy it feels right: the double-barbed arrow is the one you use most.
Fix: radicals move single electrons → use fishhook (half) arrows, and you need two of them for a homolysis.
Common mistake Forgetting that net charge is conservedWhy it feels right: you focus on getting the product skeleton and ignore charges.
Fix: sum the charges on both sides. If they don't match, an arrow is wrong or missing.
Common mistake "Heterolysis always makes a carbocation"Why it feels right: the first heterolysis you learn is C–Br → C⁺.
Fix: which atom keeps the pair depends on electronegativity/stability. C–MgBr heterolyzes to give a carbanion (C − C^- C −
Recall Feynman: explain to a 12-year-oldTwo kids share two candies (that's a chemical bond). When they split up there are two fair-ish ways:
Selfish split (heterolysis): one kid takes both candies and walks off rich (negative), the other walks off broke (positive). This happens when one kid is greedy (more electronegative).
Even split (homolysis): each kid takes one candy — perfectly fair. This happens when the kids are equal and someone (the Sun = UV light) pushes them apart.
The curved arrow is just an arrow showing where the candies go . A full arrow = both candies move together; a fishhook = one candy moves.
Mnemonic Remember the splits"HOMO = HOMOgeneous = same/even → radicals (each gets ONE)."
"HETERO = different → ions (one greedy atom takes BOTH)."
Arrowheads: Full arrow = Full pair. Half (fishhook) arrow = Half the pair (one electron).
Which arrow shows movement of a single electron?
Heterolysis produces what kind of species? Homolysis?
From which feature of a molecule does an arrow's tail begin?
Why does Cl–Cl prefer homolysis over heterolysis?
A curved arrow represents the movement of what (not atoms)? Electrons (an electron pair for a full arrow, one electron for a fishhook).
A double-barbed (full) arrow shows movement of how many electrons? Two (an electron pair).
A single-barbed fishhook arrow shows movement of how many electrons? One electron.
Define heterolysis. Uneven bond cleavage where both bonding electrons go to one atom, producing a cation and an anion (ions).
Define homolysis. Even bond cleavage where each atom keeps one electron, producing two neutral free radicals.
Heterolysis produces which species? Ions (a cation + an anion).
Homolysis produces which species? Free radicals (species with an unpaired electron).
Where must the tail of a curved arrow start? On a source of electrons — a lone pair or a bond.
Where does the head of a curved arrow point? Toward an electron-poor site (positive charge, δ⁺, or an atom that can accept electrons).
Which conditions favor homolysis? Non-polar bonds, gas phase, high energy input like UV light or heat with no ion stabilization.
Which conditions favor heterolysis? Polar bonds (large electronegativity difference) and ion-stabilizing polar/protic solvents.
What quantity is always conserved across a mechanism? Total charge (and total number of electrons).
In C–Br heterolysis, which atom becomes negative and why? Br, because it is more electronegative and keeps both bonding electrons → :Br⁻.
How many fishhook arrows are needed for a homolysis? Two (one for each electron of the bond).
Does heterolysis always give a carbocation? No — the atom keeping the electrons depends on electronegativity; e.g. C–Mg gives a carbanion.
Carbocations — stability and structure Nucleophiles and Electrophiles Inductive effect and electronegativity Free radical substitution (halogenation of alkanes) Electrophilic addition to alkenes Bond dissociation energy Resonance and arrow pushing
Bond breaking and forming
Polar bond and polar solvent
Non-polar bond and UV or heat
Start at electrons, point to electron-poor, conserve charge
Intuition Hinglish mein samjhoDekho, organic chemistry ka asli khel electrons ka movement hai, atoms ka nahi. Curved arrow ek tarah ki grammar hai jo batati hai ki electron pair kahan se chal kar kahan ja raha hai. Full arrow (double-barbed) matlab poora electron pair (2 electrons) move kar raha hai, aur fishhook (half arrow) matlab sirf ek electron move kar raha hai. Arrow ki tail hamesha electrons par hoti hai — yaani kisi bond par ya lone pair par — aur arrow ka head electron-poor jagah ki taraf jaata hai, jaise positive charge ya δ⁺.
Jab koi bond tootta hai to do shared electrons ko kahin na kahin jaana hi padta hai. Agar dono electrons ek hi atom ke paas chale jaayein, to use heterolysis kehte hain — isse ions bante hain (ek cation, ek anion). Ye tab hota hai jab bond polar ho (electronegativity ka antar zyada) ya solvent ions ko stabilize kare. Agar har atom ek-ek electron le le, to use homolysis kehte hain — isse free radicals bante hain. Ye tab hota hai jab bond non-polar ho (jaise Cl–Cl) aur UV light ya heat se energy mile.
Yaad rakhne ka trick: HOMO = homogeneous = barabar baant = radicals , HETERO = alag = ek lalchi atom dono le le = ions . Aur ek bohot important rule — total charge hamesha conserve hota hai . Reactant aur product dono taraf charge add karke check karo; agar match nahi karta to koi arrow galat hai. Bas yahi 20% concept padh lo to aap GOC ke 80% mechanism khud predict kar paoge — carbocation, nucleophilic attack, radical halogenation, sab isi pe based hai.
