Exam technique · 6 min read

How to answer mechanism questions in an exam

Examiners reward clear, correct electron pushing far more than clever guessing. A systematic routine gets you most of the marks even on a reaction you have never seen.

Step 1: label what you are given

Write down the starting material, the reagent and the conditions. Mark the functional groups. Many mistakes come from missing a group, such as a second ester, in the first thirty seconds.

Step 2: identify the reaction family

Alkyl halide plus a base or nucleophile? Think substitution or elimination.
A carbonyl plus a nucleophile? See carbonyl chemistry.
Alkene plus an electrophilic reagent? Consider addition and its stereochemistry.
A diene, heat and an electron-poor alkene? Pericyclic.
Light, peroxide, NBS? Radical.

Step 3: find the nucleophile and the electrophile

Circle the most electron-rich atom and the most electron-poor atom. The first arrow is almost always from one to the other. If the reagent is an acid, the first arrow is usually a lone pair or π bond picking up the proton.

Step 4: draw every arrow, every step

One step at a time, with a complete structure after each step.
Show lone pairs on atoms that use them, and formal charges everywhere they occur.
Include proton transfers, since they are often worth marks.
Do not combine two bond-making events in one arrow unless the reaction is concerted.

Step 5: check for common traps

Carbocation rearrangements (hydride and alkyl shifts) in SN1, E1 and acid-catalysed alkene reactions. See the Wagner–Meerwein shift.
Regiochemistry: Markovnikov or anti-Markovnikov, Zaitsev or Hofmann.
Stereochemistry: inversion, racemisation, syn or anti. Draw wedges where the question shows stereocentres.
Work-up: remember that "H₃O⁺" or "H₂O" at the end is a separate protonation step, not present during the main reaction.

Step 6: sanity-check the answer

Count atoms and charge. Check that the product has the formula the question implies. Confirm there is no five-valent carbon and that every step is chemically reasonable in the given solvent.

If you get stuck

Work backwards from the product: which bond is new, which bond is gone?
Write down any step you are sure of, even if you cannot complete the whole mechanism. Partial marks are common.
State your assumptions clearly, such as "assuming the more substituted alkene forms (Zaitsev)".

Practise the routine

Pick a reaction in the app, hide the answer by taking the Quiz, then draw the mechanism on paper using these six steps. Four or five times through any one family usually makes the pattern automatic.

Practise in the interactive app Browse all 345 reactions