Study skills · 8 min read

How to learn reaction mechanisms (without memorising everything)

Most students meet organic chemistry as a wall of reactions and try to memorise them one by one. That works for about three weeks. The students who do well treat mechanisms as a small set of electron-pushing patterns that appear again and again in different clothes.

1. Learn the handful of moves, not hundreds of reactions

Nearly every polar mechanism you will meet is built from the same few moves:

• A nucleophile attacks an electrophile (a lone pair or a π bond forms a new σ bond).
• A leaving group departs and takes its bonding electrons with it.
• A proton is transferred (an acid–base step, often fast and reversible).
• A π bond is formed or broken as electrons shift along a conjugated system.
• A group migrates to an electron-poor neighbour (rearrangements).

Once you can spot these, a new reaction stops looking new. The aldol reaction, the Claisen condensation and the Michael addition are all "an enolate attacks a carbonyl-type electrophile" with a different electrophile each time.

2. Always ask the same three questions

1. Where is the electron-rich site? Look for lone pairs, negative charges and π bonds.
2. Where is the electron-poor site? Look for positive charges, carbonyl carbons, carbons attached to good leaving groups.
3. Which bond breaks to keep the electron count sensible? Every time a new bond forms to an atom that is already full, another bond has to break.

If you can answer those three for each step, you can usually derive the mechanism rather than recall it.

3. Group reactions into families

Instead of a flat list, build a map. A useful one has about eight families: substitution, elimination, addition to C=C, addition to C=O, acyl substitution, enolate chemistry, pericyclic reactions and metal-catalysed couplings. Our reaction index is organised along similar lines. When you meet a new reaction, put it in a family and note what is different from its closest relative. For example, compare SN2 with SN1, or Wittig reaction with the Horner–Wadsworth–Emmons reaction.

4. Draw it, do not just read it

Reading a finished mechanism creates a feeling of understanding that disappears the moment the page is closed. Use this routine instead:

1. Look at a mechanism for one minute, step by step.
2. Close it and redraw it from the starting materials, including every curved arrow.
3. Compare with the original and mark only the arrows you got wrong.
4. Repeat the next day, then a week later.

The interactive version of each reaction in Mechanism Lab lets you step through the arrows, and the Quiz mode asks you to choose the correct mechanism or drawing, which is a good warm-up before drawing it yourself.

5. Learn the reagents as electron behaviours

A reagent list is much easier to remember when each entry has a role. NaBH₄ is a hydride donor. LDA is a strong, bulky base that makes kinetic enolates. DCC activates a carboxylic acid so an amine can attack it. Our glossary of abbreviations gives a one-line role for each of the common ones.

6. Check your arrows

Before you move on, run a quick audit of every step:

• Does each arrow start at an electron source (a lone pair, a bond) and end at an atom or bond that can accept electrons?
• Do the formal charges add up to the overall charge at every step?
• Is any atom left with five bonds (for carbon) or a missing lone pair?
• Is each intermediate chemically reasonable, for example no primary carbocation unless there is a very good reason?

The guide on curved-arrow notation goes through the common mistakes in detail.

A realistic weekly plan

• After each lecture: redraw each mechanism you saw, from memory, once.
• Mid-week: do a short quiz on the family you are studying.
• Weekend: pick two reactions you find hardest and explain them aloud as if teaching a friend.

Explaining out loud is one of the best tests there is: a gap in your understanding shows up straight away.

Practise in the interactive app   Browse all 345 reactions

More study guides

• Curved-arrow notation: rules and the mistakes students makeHow to read and draw curved arrows in organic mechanisms: where they start and end, full-headed versus fish-hook arrows, and the most common errors.
• SN1, SN2, E1 or E2? How to decide which mechanism winsA clear decision guide for substitution versus elimination: substrate, nucleophile or base strength, solvent and temperature, with examples.
• Nucleophiles, electrophiles and leaving groups: what makes a reaction goWhat decides how reactive a nucleophile, an electrophile or a leaving group is, and how solvent, charge, size and resonance change reactivity.
• Carbonyl chemistry: addition, acyl substitution and enolatesThe three big patterns of carbonyl reactivity in one place: nucleophilic addition, addition-elimination on acid derivatives, and enolate chemistry.