For a lot of A‑level students, organic mechanisms are the moment chemistry stops feeling like memorising facts and starts feeling like a foreign language of curly arrows. Nucleophiles, electrophiles, homolytic, heterolytic — it can seem like a wall of jargon.
Here’s the secret I share with every student: mechanisms aren’t something to memorise — they’re something to understand. Once you grasp the handful of ideas underneath them, you can work out almost any mechanism instead of trying to recall it. Let’s build that understanding.
The one idea that explains everything: electrons move
Every organic mechanism is just a story about where electrons go. Chemical bonds are pairs of electrons, and reactions happen when those electron pairs move to form new bonds and break old ones.
We show that movement with curly arrows. And there’s one golden rule:
A curly arrow always shows the movement of a pair of electrons, starting from where the electrons are (a bond or a lone pair) and pointing to where they’re going.
Master what a curly arrow means and you’re already halfway there.
Two characters in every story: nucleophiles and electrophiles
Almost every mechanism is a relationship between two types of species:
- Nucleophile (“nucleus‑loving”) — electron‑rich. It has a lone pair or a negative charge and donates electrons. Think: OH⁻, :NH₃, CN⁻.
- Electrophile (“electron‑loving”) — electron‑poor. It has a positive charge or a partial positive (δ+) and accepts electrons.
The whole plot: electrons flow from the nucleophile (rich) to the electrophile (poor). The curly arrow always starts at the electron‑rich species. Once you internalise this, you can predict the direction of almost any mechanism.
The main mechanisms you need (and the logic behind each)
At A‑level, most mechanisms are variations on a few themes:
1. Nucleophilic substitution
A nucleophile attacks an electron‑poor carbon and kicks out a leaving group.
- Example: a halogenoalkane reacting with OH⁻ to form an alcohol.
- Logic: the carbon bonded to the halogen is δ+ (the halogen is more electronegative), so the electron‑rich nucleophile is attracted to it and swaps in.
2. Electrophilic addition
An electron‑rich C=C double bond attacks an electrophile, opening up the double bond.
- Example: an alkene reacting with bromine or HBr.
- Logic: the double bond is a region of high electron density, so it seeks out the electron‑poor electrophile.
3. Elimination
A small molecule (like water or a hydrogen halide) is removed, forming a C=C double bond.
- Example: a halogenoalkane forming an alkene.
- Logic: essentially the reverse idea of addition.
4. Free‑radical substitution
Bonds break evenly (homolytic fission) to form radicals, driven by UV light.
- Example: methane reacting with chlorine in sunlight.
- Logic: here we use single‑headed “fish‑hook” arrows because single electrons move, not pairs.
Notice the pattern: name the electron‑rich and electron‑poor parts, and the mechanism almost writes itself.
How to actually get good at them
Understanding is the foundation — but fluency comes from doing. My advice:
- Draw them by hand, repeatedly. Mechanisms are a motor skill as much as a mental one. Redraw each one until the arrows feel natural.
- Say the logic out loud as you draw: “The lone pair on the nucleophile attacks the δ+ carbon…”
- Get your arrows precise. Examiners are strict here — an arrow that starts or ends in the wrong place loses marks even if the overall idea is right.
- Practise on past‑paper questions, then check against the mark scheme.
Examiner’s tip
The most common way students lose mechanism marks isn’t getting the wrong products — it’s sloppy curly arrows. The arrow must start clearly from a bond or a lone pair and point exactly to where the new bond forms. Drawing it from the wrong atom, or from a positive charge instead of an electron pair, costs marks. Precision is everything.
The bottom line
Organic mechanisms become simple when you stop memorising and start thinking in terms of electron movement:
- Curly arrows show pairs of electrons moving.
- Electrons flow from nucleophiles (rich) to electrophiles (poor).
- Most A‑level mechanisms are variations on substitution, addition, and elimination.
- Draw them by hand and keep your arrows precise.
Get this framework, and mechanisms turn from your most‑feared topic into a satisfying, logical puzzle.
If you’d like to work through mechanisms live on the whiteboard — drawing them together until they genuinely click — that’s exactly how I teach them.
👉 Book a free intro call and let’s make organic chemistry make sense.