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Organic chemistry often turns into a list: reagent, conditions, product, hundreds of times over. Lists work until the exam shows you a molecule you have not seen. Mechanisms are the shortcut, because a few ideas explain much of the list.
Why does memorising reactions break down?
The syllabus itself is built around ideas, not lists. The JEE (Main) 2026 syllabus names carbocations, free radicals, electrophiles and nucleophiles, inductive and electromeric effects, resonance and hyperconjugation, and then asks for the mechanisms of electrophilic addition, electrophilic substitution and substitution in haloalkanes.
Research on how students draw mechanisms shows what goes wrong when they are learnt as patterns. Summarising earlier studies, Houchlei, Bloch and Cooper (2021) note that students often "decorate" structures with curved arrows, before or after writing the product, or use them simply to "get to the product" rather than to reason about how electrons move. In their own study, students in a course that treated mechanisms as explanations were over three times more likely to draw mechanistically reasonable steps and a plausible product for an unfamiliar reaction than students from a traditional course.
The lesson for you: an arrow is a claim that a pair of electrons moves from somewhere to somewhere. If you can say why, you can predict.
The four questions to ask about any reaction
Where are the electrons rich? Lone pairs, π bonds and negative charges. This is your nucleophile.
Where are they poor? Positive charges, and carbon atoms bonded to more electronegative atoms (δ+). This is your electrophile.
What intermediate forms, and what stabilises it? Carbocations and radicals are stabilised by more alkyl groups (via hyperconjugation and inductive effects) and by resonance.
What leaves? A group that is stable on its own, such as a halide ion or water.
Draw every arrow from electrons (a lone pair or bond) to an atom. Never start an arrow from a positive charge.
Worked example 1: adding HBr to an alkene
Propene reacts with HBr. Which product forms?
The π bond is electron-rich; the H in H–Br is electron-poor. The π electrons attack H.
That H can go to either end of the double bond. Adding it to the end CH₂ leaves a positive charge on the middle carbon, a secondary carbocation. Adding it to the middle carbon gives a primary carbocation.
The secondary carbocation is more stable, so it forms faster.
Br⁻ attacks the positive carbon. Product: 2-bromopropane.
That is Markovnikov's rule, but you derived it rather than remembering it. Now try something you have not memorised: 2-methylpropene with HBr. The same logic gives a tertiary carbocation and 2-bromo-2-methylpropane.
With peroxides, HBr adds the other way round (anti-Markovnikov). The syllabus lists this "peroxide effect" separately, and it makes sense once you see that the mechanism changes: a bromine radical adds first, and it adds to give the more stable radical intermediate. Different mechanism, different product, same stability question.
Worked example 2: SN1 or SN2?
Compare two reactions with hydroxide or water:
Bromomethane with OH⁻: the carbon is not crowded, and OH⁻ is a strong nucleophile. It attacks from the side opposite Br in one step while Br⁻ leaves. That is SN2. Because the attack comes from the back, SN2 inverts the configuration, which matters when the carbon is chiral.
2-bromo-2-methylpropane in water: three methyl groups crowd the carbon, so a backside attack is hard. But if Br⁻ leaves first, a stable tertiary carbocation forms. Water then attacks it. That is SN1: two steps, and forming the carbocation is the slow step.
You do not need two lists. You need one question: is the carbon crowded, and would its carbocation be stable?
How should you practise organic chemistry?
Group reactions by mechanism, not by chapter. Electrophilic addition to alkenes and electrophilic substitution in benzene start the same way: π electrons attack an electrophile.
Explain each step out loud. Our post on the self-explanation technique shows why this helps.
Keep a small set of named reactions on cards. Some facts, such as the reagents in named reactions, do need memory. Use retrieval and spacing for them, as in our guide to learning inorganic chemistry.
Test on unfamiliar molecules. If you can only do the textbook example, you have learnt the example, not the mechanism.
If an idea such as hyperconjugation or resonance is shaky, the Fermi Concept Library has a lesson for every concept in chemistry, each with its place in the map. For practice, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer.
Next step: pick five reactions from your notes, cover the products, and predict each one using only the four questions.
