How to predict the major product of an organic reaction
Ask five questions in order. What is the functional group? What is the reagent — nucleophile, base, electrophile, oxidant or reductant? Which mechanism does that pairing imply? Which selectivity rule governs that mechanism — Markovnikov, Zaitsev, ortho/para versus meta, retention versus inversion? And does the answer change at a stereocentre? Answer those five and the major product falls out; skip one and you are guessing.
Why "what is the product" is really five questions
Students who are good at product prediction are not remembering more reactions than you. They are running a short, fixed routine, and the routine is what makes an unfamiliar reagent survivable. The reason it works is that organic chemistry has perhaps a dozen mechanisms and hundreds of reagents — so if you can classify the reagent, you have already narrowed hundreds of possibilities down to one or two.
Question 1 — what is the functional group?
Draw a circle round it. An alkene reacts because it has a π bond full of accessible electrons. A carbonyl reacts because its carbon is electron-poor. An alkyl halide reacts because it has a leaving group. An aromatic ring reacts because it is electron-rich but unwilling to lose its aromaticity.
Each of those is a different kind of reactivity, and each one can only do a small number of things. Naming the group narrows the field before you have even read the reagent.
Question 2 — what is the reagent?
Do not try to remember what the reagent does. Classify what it is.
| If the reagent is… | Recognise it by | It will… |
|---|---|---|
| A nucleophile | Negative charge or a lone pair: CN⁻, RO⁻, RMgBr, RS⁻, NH₃ | Attack an electron-poor carbon |
| A base | Negative charge on a small, hard atom; bulky alkoxides; amide bases | Remove a proton, usually β to a leaving group |
| An electrophile | Positive charge or a strongly polarised bond: H⁺, Br₂, NO₂⁺, AlCl₃ adducts | Be attacked by π electrons or a lone pair |
| An oxidant | Contains O–O, Cr(VI), Mn(VII): PCC, KMnO₄, mCPBA, O₃ | Add bonds to oxygen or remove bonds to hydrogen |
| A reductant | Contains hydride or H₂: NaBH₄, LiAlH₄, H₂/Pd | Add bonds to hydrogen |
Two reagents that look nothing alike often behave identically because they are the same class. Cyanide and azide are both strong nucleophiles that are weak bases; both give clean SN2 on a primary halide.
Question 3 — which mechanism does that pairing imply?
Functional group plus reagent class usually gives exactly one mechanism:
- Alkene + electrophile → electrophilic addition
- Alkyl halide + nucleophile or base → substitution or elimination (see the SN1/SN2/E1/E2 guide)
- Carbonyl + nucleophile → nucleophilic addition, or acyl substitution if there is a leaving group on the carbonyl carbon
- Arene + strong electrophile → electrophilic aromatic substitution
- Alcohol + oxidant → oxidation to aldehyde, ketone or acid depending on the alcohol class and the oxidant's strength
Question 4 — apply that mechanism's selectivity rule
This is where the major product is actually decided, and where most marks are lost. Each mechanism has exactly one rule you need.
| Mechanism | The question it answers | The rule |
|---|---|---|
| Electrophilic addition | Which carbon gets the H? | Markovnikov — via the more stable carbocation. Flipped by peroxides or borane. |
| Elimination | Which alkene forms? | Zaitsev — the more substituted alkene. Flipped by a bulky base (Hofmann). |
| Aromatic substitution | Where on the ring? | Directing effects — donors go ortho/para, withdrawers go meta. |
| SN2 | What happens at the stereocentre? | Inversion, every time. |
| SN1 | What happens at the stereocentre? | Racemisation, via the planar cation. |
| Carbonyl reduction / Grignard | What is left after workup? | The tetrahedral alkoxide is protonated to an alcohol. |
Question 5 — did anything happen at a stereocentre?
Ask three sub-questions: did a stereocentre form, was one destroyed, and was one inverted? A carbocation intermediate means racemisation. A concerted backside attack means inversion. Adding a nucleophile to a flat carbonyl creates a new stereocentre as a racemic pair, unless something chiral is directing it.
Two worked examples
Propene + H₂O, H₂SO₄ catalyst
- Functional group: alkene.
- Reagent: acid — an electrophile, with water as a weak nucleophile.
- Mechanism: acid-catalysed electrophilic addition, via a carbocation.
- Rule: Markovnikov. The proton adds to the CH₂ end so the positive charge lands on the middle carbon, which is secondary rather than primary.
- Stereochemistry: a new stereocentre would be racemic, but propan-2-ol's central carbon carries two methyls, so it is not a stereocentre at all.
Major product: propan-2-ol.
Toluene + HNO₃ / H₂SO₄
- Functional group: aromatic ring with a methyl substituent.
- Reagent: mixed acid generates NO₂⁺, a strong electrophile.
- Mechanism: electrophilic aromatic substitution.
- Rule: methyl is a weak activator and an ortho/para director.
- Stereochemistry: none involved.
Major products: 2-nitrotoluene and 4-nitrotoluene. Methyl is small enough that the two ortho positions still get the larger share of the product overall; a bulkier alkyl group (t-butyl, say) pushes the mixture strongly toward para. Meta stays minor either way.
Common mistakes
- Pattern-matching on the reagent alone. "NaBH₄ makes alcohols" fails the moment the question puts NaBH₄ next to an ester, which it barely touches.
- Answering with the thermodynamic product when the question is kinetic. Low temperature usually means kinetic control; heat and long reaction times mean thermodynamic.
- Forgetting the workup step. Grignard and hydride reactions give an alkoxide; the alcohol only appears after the H₃O⁺ step written on the second arrow.
- Drawing a product that violates an octet or loses a carbon. Count atoms on both sides before you commit.
How Organic Chemistry AI helps here
The routine only becomes automatic through repetition with feedback. When you photograph or type a "predict the major product" question, the solve names the reaction type, works through the steps that identify the intermediate, and states the rule that chose between the candidates — so you can compare its reasoning against your own at the exact point where they diverged. The offline quiz bank has product-prediction questions you can drill without using a solve.
Product-prediction questions from the offline bank of 397, with the right answer revealed the moment you choose and a hint available on every question.
Frequently asked
What does 'major product' actually mean?
The product formed in the greatest amount when more than one is possible. Exam questions ask for it because real reactions give mixtures — the question is testing whether you know which pathway is favoured, not whether you can list every conceivable outcome.
How do I handle a reagent I have never seen?
Classify it rather than recall it. Look for a negative charge or lone pair (nucleophile or base), a positive charge or strongly polarised bond (electrophile), an O–O bond or a high-oxidation-state metal (oxidant), or a hydride source (reductant). The class tells you what it does even when the name means nothing to you.
When do I need to worry about rearrangements?
Whenever a carbocation forms and a hydride or methyl shift would produce a more stable one. If your mechanism generates a secondary cation with a tertiary carbon next door, expect a 1,2-shift and a product that looks like it came from the wrong carbon.
Practise product prediction with feedback
Photograph a problem set, read the reasoning, then drill the same skill offline with the quiz bank.
Free to download · calculators, flashcards and quizzes free forever · no free trial