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How to Revise A-Level Organic Chemistry Mechanisms

Revise A-level organic chemistry mechanisms by tracing electrons, drawing from blank and fixing arrow errors, with a worked bromoethane example.

Flat illustration of a navy and orange ball-and-stick molecule with two curved arrows beside a round-bottom flask of orange liquid and a rack of test tubes, with open cream space on the left.

Organic chemistry mechanisms can feel like a wall of arrows to copy. Students often learn them as pictures, then freeze when a question gives an unfamiliar molecule. The better plan is to learn what the arrows mean, then practise drawing them from a blank page until each step makes sense. This guide shows how to do that for A-level chemistry.

Know what the exam asks for

Start with your own specification, because the mechanisms you need depend on your course. On AQA's A-level Chemistry specification at a glance page, organic chemistry (section 3.3) is assessed in Paper 2, a two-hour written paper worth 35% of the A-level. Paper 3 is listed as assessing any content, so organic questions can turn up there too. The course is linear, so everything is examined at the end.

If you follow another board, open its specification and write the list of required mechanisms on one sheet. That sheet becomes your revision map, and it stops you spending time on reactions you do not need.

Learn the rules behind the arrows

The Royal Society of Chemistry's article Teaching curly arrows at post-16 gives a simple set of ideas. A curly arrow shows the movement of an electron pair, or a single electron in a free radical step. A nucleophile has a lone pair or a negative charge, so the arrow starts there. An electrophile accepts an electron pair, so the arrow points to it.

The same article lists mistakes that cost understanding: drawing arrows as if atoms are moving, being vague about where an arrow starts and ends, missing the lone pairs that are not drawn, and trying to memorise mechanisms instead of understanding them. Keep these in mind as you practise.

A five-step routine for each mechanism

Use this routine for every mechanism on your list.

  1. Redraw the reactants with every lone pair and charge shown, including the ones the textbook leaves out.
  2. Mark the dipoles on the key bond, for example a partial positive carbon bonded to a partial negative halogen.
  3. Say in words where the electron pair starts and where it ends up. Avoid saying only that the nucleophile attacks.
  4. Draw the arrow from the lone pair or bond to the target, then draw the next step one change at a time.
  5. Check that the charges balance before and after, and name the type of mechanism.

Worked example: bromoethane and hydroxide

Take the reaction of bromoethane with aqueous hydroxide ions, a nucleophilic substitution.

  1. Redraw the reactants: CH3CH2Br and OH-, with three lone pairs on the oxygen and the negative charge shown.
  2. Mark the dipole: the carbon bonded to bromine is partly positive and the bromine is partly negative.
  3. Say it in words: a lone pair on the oxygen of OH- starts the arrow and ends at the partly positive carbon.
  4. Draw a second arrow from the carbon-bromine bond to the bromine atom, so the bromine leaves as Br-.
  5. Check the charges: the left side has a total charge of minus one, and the right side has a bromide ion with minus one and a neutral ethanol molecule, so the charge is balanced.

The product is ethanol, CH3CH2OH. Now try the same routine for a different reagent, such as ammonia or cyanide ions, and note what stays the same.

Practise from blank, not from the page

Reading a mechanism feels like knowing it. Retrieval practice, which means recalling information with little prompting, asks more of you. Cover your notes, draw the mechanism from a blank page, then check it against your textbook or mark scheme. Our guide to active recall explains how to turn this into a routine.

Try a weekly pattern:

  1. Day one: learn two mechanisms using the five steps.
  2. Day three: draw both from a blank page and fix every error in a different colour.
  3. Day seven: draw them again, then mix in one mechanism from an earlier week.

Find your own mistakes

The RSC article suggests giving students incorrect mechanisms to spot and correct. You can do this yourself. Ask a friend to hide one error in a mechanism, or redraw an old attempt and look for a charge that does not balance, an arrow that starts at the wrong place, or a missing lone pair. Keep a short error log with the mistake and the rule that fixes it.

A short checklist before the exam

  • You have the list of mechanisms from your specification, and each has been drawn from blank at least twice on different days.
  • Every arrow starts at a lone pair or a bond and ends at an atom or a new bond.
  • You can explain each step in words, not only draw it.
  • You have checked your board's mark schemes for how arrows, dipoles and charges are credited.

Mechanisms become manageable when you trace the electrons, not the shapes. Start with one reaction tonight, draw it from memory tomorrow, and build from there.