Organic reactions can be classified into a small number of types. Understanding the type tells you the mechanism and the products. CBSE Class 12 and NEET test this heavily — two to three questions a year.
Core Concepts
Substitution
One atom or group replaces another. Nucleophilic (SN1, SN2) in haloalkanes. Electrophilic aromatic substitution in arenes (nitration, halogenation, sulphonation, Friedel-Crafts).
Nucleophilic substitution at a glance:
- SN2: one step, backside attack, inversion. Primary substrates, polar aprotic solvents.
- SN1: two steps, carbocation intermediate, racemisation. Tertiary substrates, polar protic solvents.
Electrophilic aromatic substitution (EAS):
The benzene ring is electron-rich (pi electrons). An electrophile (E+) attacks the ring, temporarily breaking aromaticity, then a proton leaves to restore it.
| EAS Reaction | Electrophile | Catalyst | Product |
|---|---|---|---|
| Nitration | NO2+ | H2SO4 + HNO3 | Nitrobenzene |
| Halogenation | Cl+ or Br+ | AlCl3 or FeBr3 | Chlorobenzene/Bromobenzene |
| Sulphonation | SO3 | Fuming H2SO4 | Benzenesulphonic acid |
| Friedel-Crafts alkylation | R+ | AlCl3 | Alkylbenzene |
| Friedel-Crafts acylation | RCO+ | AlCl3 | Acyl benzene (ketone) |
Friedel-Crafts acylation is preferred over alkylation because: (1) acylation does not give rearrangement products (carbocations do not rearrange in acylation), (2) the product (ketone) is deactivated, preventing polysubstitution. Alkylation often gives a mix of products.
Addition
Atoms add across a multiple bond. Markovnikov rule — H adds to the carbon with more Hs. Electrophilic addition of HX to alkenes. Catalytic hydrogenation to alkanes. Addition polymerisation.
When HX adds to an unsymmetrical alkene, H goes to the carbon with more hydrogen atoms (less substituted), and X goes to the more substituted carbon.
Why? The more substituted carbocation intermediate is more stable (hyperconjugation and inductive stabilisation by alkyl groups).
Exception: In the presence of peroxide (Kharash effect), HBr adds anti-Markovnikov through a radical mechanism. Only HBr shows this — not HCl or HI.
Types of addition reactions:
| Reaction | Reagent | Product | Rule |
|---|---|---|---|
| HX addition | HCl, HBr, HI | Haloalkane | Markovnikov (ionic) |
| H2O addition | H2O/H+ | Alcohol | Markovnikov |
| H2 addition | H2/Ni or Pt | Alkane | Catalytic hydrogenation |
| Br2 addition | Br2/CCl4 | Dibromoalkane | Anti addition |
| HBr + peroxide | HBr/ROOR | Haloalkane | Anti-Markovnikov |
Elimination
Removal of atoms or groups to form a multiple bond. E1 and E2 mechanisms. Zaitsev’s rule — the more substituted alkene is the major product.
When elimination can give more than one alkene, the more substituted alkene is the major product.
Why? The more substituted alkene is more stable due to hyperconjugation. More alkyl groups on the double-bonded carbons = more hyperconjugative structures = lower energy.
Exception: Bulky bases like tert-butoxide give the less substituted (Hofmann) product because steric hindrance prevents access to the more hindered hydrogen.
Oxidation and reduction
Oxidation — loss of H or gain of O. Alcohol → aldehyde → acid. Reduction — gain of H or loss of O. Ketone → alcohol. Aldehyde → alcohol.
The oxidation ladder:
Each step represents an increase in oxidation state. The reagent determines where you stop:
- PCC (pyridinium chlorochromate): stops at aldehyde (mild oxidant)
- KMnO4 or K2Cr2O7: goes all the way to carboxylic acid (strong oxidant)
- Tollen’s reagent: oxidises aldehyde to acid (mild, selective)
For reduction:
- NaBH4: reduces aldehyde/ketone to alcohol (selective, does not reduce C=C)
- LiAlH4: reduces aldehyde, ketone, acid, ester to alcohol (powerful, non-selective)
- Clemmensen (Zn-Hg/HCl): reduces C=O to CH2 (removes oxygen completely)
- Wolff-Kishner (NH2NH2/KOH): same result as Clemmensen but in basic medium
Rearrangement
Carbocations can rearrange to more stable forms (tertiary > secondary > primary). Important in SN1, E1 and some addition reactions.
A classic example: treatment of neopentyl alcohol with HBr does not give neopentyl bromide. Instead, the primary carbocation rearranges (methyl shift) to a more stable tertiary carbocation, giving tert-pentyl bromide.
Condensation
Two molecules combine with loss of a small molecule (usually water). Ester formation, amide formation, aldol condensation.
Aldol condensation: Two molecules of an aldehyde (or ketone with alpha-H) combine in the presence of dilute alkali.
The product (aldol) is a beta-hydroxy aldehyde. On heating, it dehydrates to give an alpha-beta unsaturated aldehyde (crotonaldehyde). This reaction forms a new C-C bond — essential for building larger molecules from smaller ones.
Worked Examples
HCl adds to propene. H goes to CH2 (more Hs), Cl goes to CH. Product is 2-chloropropane, not 1-chloropropane.
With HBr and peroxide, the radical mechanism gives anti-Markovnikov product. H goes to CH, Br to CH2. Only HBr shows this — HCl and HI do not.
2-Bromobutane with alcoholic KOH: two possible alkenes are but-2-ene and but-1-ene. By Zaitsev’s rule, but-2-ene (more substituted) is the major product. But-2-ene itself has cis and trans forms, with trans being more stable and therefore the major product.
To convert 1-propanol to propanal (aldehyde), use PCC in CH2Cl2. If you use KMnO4, you will get propanoic acid instead (over-oxidation). The choice of reagent determines where on the oxidation ladder you stop.
The -NO2 group is a strong deactivator (-I and -R effects). It makes the ring so electron-poor that the electrophile cannot attack. Friedel-Crafts requires an electron-rich (or at least neutral) ring. Strongly deactivated rings need harsher conditions (like fuming HNO3 for further nitration).
Common Mistakes
Confusing SN1 and SN2. SN1 is unimolecular, carbocation intermediate; SN2 is bimolecular, concerted.
Saying Markovnikov rule applies to all additions. It applies to HX addition to asymmetric alkenes.
Writing that oxidation is always with KMnO4. Many oxidising agents exist — CrO3, PCC, Tollen’s, etc.
Forgetting that Zaitsev’s rule has exceptions. Bulky bases give the Hofmann (less substituted) product.
Saying Friedel-Crafts works on all aromatic compounds. It fails on strongly deactivated rings (nitrobenzene) and rings with -NH2 (amine complexes with the Lewis acid catalyst).
Exam Weightage and Revision
Organic reaction types carry 2-3 NEET questions per year, primarily on Markovnikov addition, EAS directing effects, and oxidation/reduction reagent selection. JEE focuses on mechanism details, stereochemistry, and reagent selection. CBSE boards ask for reaction types with examples.
| Question Type | NEET Frequency | JEE Frequency |
|---|---|---|
| Markovnikov/anti-Markovnikov | Every year | Every year |
| EAS directing effects | Most years | Every year |
| Zaitsev’s rule product | Every 2 years | Most years |
| Oxidation reagent selection | Most years | Every year |
| Aldol condensation | Occasional | Most years |
The most reliable question: “What is the major product of HBr addition to propene (a) without peroxide (b) with peroxide?” Without: 2-bromopropane (Markovnikov). With: 1-bromopropane (anti-Markovnikov). Know the reasoning for both.
Practice Questions
Q1. Predict the major product: but-1-ene + HBr (no peroxide).
Markovnikov addition. H adds to C1 (more H atoms), Br adds to C2 (more substituted). Product: 2-bromobutane ().
Q2. Why does benzene undergo substitution rather than addition?
Benzene has aromatic stabilisation energy (~150 kJ/mol from delocalised pi electrons). Addition would destroy this aromaticity by breaking the pi system. Substitution preserves aromaticity — the electrophile replaces an H, and the aromatic ring remains intact. The driving force is the thermodynamic stability of the aromatic product.
Q3. Convert propan-1-ol to propanoic acid. Name the reagent.
Use KMnO4 (acidified) or K2Cr2O7/H2SO4. Both are strong oxidising agents that oxidise the primary alcohol all the way to the carboxylic acid (via the aldehyde intermediate).
FAQs
How do I remember which reactions are addition and which are substitution?
Addition increases the number of atoms bonded to carbon (double bond becomes single bond, gaining two atoms). Substitution keeps the same number of bonds but swaps one atom for another. Elimination removes atoms to create a double bond.
Why do alkenes undergo addition but benzene undergoes substitution?
Both have pi electrons, but benzene’s pi electrons are delocalised across the entire ring, giving exceptional stability (aromaticity). Adding across benzene would destroy this stability. Alkenes have localised pi electrons — addition is thermodynamically favourable because the sigma bond formed is stronger than the pi bond broken.
Learn the six reaction types with one example each. NEET questions assume you can classify instantly.
Reaction Classification Flowchart
When you see an organic reaction in a question, follow this decision tree:
Step 1: Count atoms. Does the product have more atoms than the starting material? → Addition. Fewer atoms? → Elimination. Same number but different arrangement? → Substitution or rearrangement.
Step 2: If substitution, is the substrate aromatic? → Electrophilic aromatic substitution. Is it a haloalkane? → Nucleophilic substitution (SN1 or SN2).
Step 3: If addition, is the substrate an alkene/alkyne? → Electrophilic addition. Is it a carbonyl? → Nucleophilic addition.
Step 4: Is oxygen being gained or hydrogen lost? → Oxidation. Reverse? → Reduction.
Named Reactions Quick Reference
| Named Reaction | Type | Key Feature |
|---|---|---|
| Wurtz | Coupling | 2 R-X + 2Na → R-R (doubles carbon chain) |
| Grignard | Addition | R-MgX + carbonyl → alcohol |
| Aldol | Condensation | 2 RCHO → beta-hydroxy aldehyde (new C-C bond) |
| Cannizzaro | Disproportionation | HCHO → acid + alcohol (no alpha-H aldehydes) |
| Clemmensen | Reduction | C=O → CH2 (Zn-Hg/HCl) |
| Wolff-Kishner | Reduction | C=O → CH2 (NH2NH2/KOH) |
| Kolbe electrolysis | Decarboxylation | 2 RCOO- → R-R + 2CO2 (electrolysis) |
| Sandmeyer | Substitution | ArN2+ → ArX (using CuX) |
Why Reaction Type Matters for Product Prediction
Knowing the type immediately tells you the product structure:
- Addition: All atoms from both reactants appear in the product (no byproduct, or minor byproduct)
- Substitution: One group enters, another leaves (leaving group is the byproduct)
- Elimination: Two groups leave, forming a double bond (the two groups are the byproduct)
- Oxidation/Reduction: The carbon’s oxidation state changes, but the skeleton usually stays intact
This framework means you never need to memorise thousands of individual reactions. Learn the six types, and you can predict the product of almost any reaction by identifying which type it is.
Organic reactions are a taxonomy. Classify the reaction and half the work is done — the mechanism flows from the type.