JEE Mains Chemistry · Teaching notes
Organic Chemistry: Some Basic Principles and Techniques — JEE Mains Chemistry
Organic Chemistry – Some Basic Principles and Techniques has 243 past-year questions from 2021 to 2026, and 70 of them ask for a number rather than an option. The chapter falls into two kinds of work. About half the questions are about how organic molecules are built: naming them, counting their isomers and ranking them by electron shifts, where one rule applied in the right order settles almost every question. The other half is laboratory method, and most of the numerical answers sit there: which technique suits which mixture, what colour each test gives, and a percentage from a mass or a gas volume, where the marks go to arithmetic done without a slip.
Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.
Subtopic notes
Structure, Classification and IUPAC Nomenclature
34 PYQsA carbon's hybridisation follows from its bonds, a molecule's σ and π bonds can be counted from its formula, and an IUPAC name is built by picking the senior functional group, choosing and numbering the parent chain, and listing the other groups as prefixes in alphabetical order.
Structural Isomerism
13 PYQsStructural isomers share a molecular formula but join the atoms differently: a different carbon skeleton, a group in a different place, a different functional group, different alkyl groups on either side of one group, or a ring in place of a double bond.
Stereoisomerism and Conformations
32 PYQsStereoisomers have the same bonds but a different arrangement in space: cis and trans forms about a C=C that cannot rotate, mirror-image enantiomers from a carbon with four different groups, and conformations that differ only by rotation about a single bond.
Electronic Effects, Resonance and Acidity
24 PYQsElectrons in a molecule shift through σ bonds (the inductive effect), through π systems (resonance), on demand when a reagent attacks (the electromeric effect) and from C–H bonds into a neighbouring empty or π orbital (hyperconjugation); these shifts rank resonance structures and set the strength of acids and bases.
Reaction Intermediates, Bond Fission and Reagents
24 PYQsA covalent bond breaks evenly into two free radicals or unevenly into a carbocation and an anion; how stable these short-lived species are, and whether a reagent gives or takes an electron pair, decides how a reaction runs.
Methods of Purification
25 PYQsEach purification method exploits the one property in which the compound and its impurity differ: boiling point for the distillations, volatility in steam for steam distillation, solubility for crystallisation and extraction, and a direct change from solid to vapour for sublimation.
Chromatography
20 PYQsChromatography separates a mixture as a mobile phase carries it over a stationary phase: a component that clings more to the stationary phase travels less, so it has a lower Rf on a plate and leaves a column later.
Lassaigne's Test and Estimation of Nitrogen
38 PYQsFusing an organic compound with sodium turns its nitrogen, sulphur and halogens into cyanide, sulphide and halide ions that give coloured tests; nitrogen is then measured either as N₂ gas (Dumas' method) or as ammonia (Kjeldahl's method).
Estimation of Carbon, Hydrogen, Halogens, Sulphur and Phosphorus
33 PYQsEach element is turned into a product that can be weighed (CO₂, water, a silver halide, barium sulphate or magnesium pyrophosphate), and its percentage is the element's mass fraction in that product times the product's mass over the sample's mass.
Formula & revision sheet
14 formulas · 9 reference tables · 70 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
14 formulas · 9 reference tables · 70 gotchas across all subtopics — the exam-eve cheat-sheet
Formulas (2)
Reference tables (1)
Seniority of functional groups in IUPAC names12 rows
| Class | Group | Suffix when senior | Prefix when not senior |
|---|---|---|---|
| Carboxylic acid | -oic acid | carboxy | |
| Sulphonic acid | -sulphonic acid | sulpho | |
| Ester | alkyl …-oate | alkoxycarbonyl | |
| Acid chloride | -oyl chloride | chlorocarbonyl | |
| Amide | -amide | carbamoyl | |
| Nitrile | -nitrile | cyano | |
| Aldehyde | -al | formyl, or oxo when its carbon is in the chain The aldehyde outranks the ketone: a compound with both is named as an -al with an oxo prefix. | |
| Ketone | -one | oxo | |
| Alcohol | -ol | hydroxy | |
| Amine | -amine | amino | |
| Alkene, alkyne | C=C, C≡C | -ene, -yne | Never a prefix; always part of the parent name |
| Halide, nitro, ether | –X, , –OR | Never a suffix | halo, nitro, alkoxy |
Watch out for (9)
- Branch carbons count too→ Hybridisation and counting sigma and pi bonds
- A carbonyl or nitrile carbon is not sp³→ Hybridisation and counting sigma and pi bonds
- One ring double bond does not make a ring aromatic→ Hybridisation and counting sigma and pi bonds
- The ketone does not outrank the aldehyde→ Seniority of functional groups in IUPAC names
- The nitrile sits between the amide and the aldehyde→ Seniority of functional groups in IUPAC names
- The junior group never takes the suffix→ Seniority of functional groups in IUPAC names
- Prefixes go in alphabetical order, not locant order→ Choosing and numbering the parent chain
- Compare the whole locant set→ Choosing and numbering the parent chain
- The OH carbon is C-1 in a ring→ Choosing and numbering the parent chain
Reference tables (1)
Kinds of structural isomerism6 rows
| Type | What differs | Example pair (one formula) |
|---|---|---|
| Chain | The carbon skeleton | Pentane and 2-methylbutane (C₅H₁₂) |
| Position | Where the group or multiple bond sits | Propan-1-ol and propan-2-ol (C₃H₈O) |
| Functional group | The functional group itself | Ethanol and methoxymethane (C₂H₆O) |
| Metamerism | The alkyl groups on either side of one group | Methoxypropane and ethoxyethane (C₄H₁₀O) Two ethers can be metamers. An ether and an alcohol cannot: they are functional isomers. |
| Ring-chain | A ring in place of a C=C | Cyclopropane and propene (C₃H₆) |
| Tautomerism | The position of a mobile hydrogen; the forms interconvert | Propanone and prop-1-en-2-ol (keto and enol) |
Watch out for (6)
- A moved double bond is position isomerism→ Kinds of structural isomerism
- Metamers keep one functional group→ Kinds of structural isomerism
- No α-hydrogen, no tautomer→ Kinds of structural isomerism
- Isopropyl and propyl are different side chains→ Counting structural isomers of a formula
- A renumbered chain is the same compound→ Counting structural isomers of a formula
- Nitrogen adds and halogen subtracts in the DoU formula→ Counting structural isomers of a formula
Formulas (3)
Watch out for (9)
- Two identical groups on one carbon cancel it→ Geometrical isomerism and conformations
- The trans isomer usually melts higher→ Geometrical isomerism and conformations
- Conformations are not separable isomers→ Geometrical isomerism and conformations
- Deuterium is different from hydrogen→ Chiral centres, meso compounds and optical purity
- Two chiral centres do not guarantee optical activity→ Chiral centres, meso compounds and optical purity
- Compare configurations, not drawings→ Chiral centres, meso compounds and optical purity
- 2ⁿ is a ceiling, not the answer→ Counting stereoisomers
- A double bond is a stereogenic unit too→ Counting stereoisomers
- Include stereoisomers only when asked→ Counting stereoisomers
Reference tables (2)
Inductive, resonance, electromeric and hyperconjugation effects4 rows
| Effect | Electrons move through | Permanent or temporary | Typical example |
|---|---|---|---|
| Inductive (I) | σ bonds, weakening with distance | Permanent | Cl pulls electrons along the chain in |
| Resonance (R or M) | π bonds and lone pairs on adjacent atoms | Permanent | pushes its lone pair into the ring of aniline |
| Electromeric (E) | One π bond, shifted completely to one atom | Temporary; only while the reagent is present | The C=O of propanone as attacks The only one of the four that disappears when the reagent is taken away. |
| Hyperconjugation | A C–H σ bond into an adjacent empty p or π orbital | Permanent | The three C–H bonds of the group stabilise the C=C of propene |
Rules for the stability of resonance structures5 rows
| Rule | More stable contributor | Less stable contributor |
|---|---|---|
| Neutral beats charge-separated | ||
| Every atom with a complete octet | (carbon with a sextet) | |
| Negative charge on the more electronegative atom | ||
| Opposite charges close, like charges apart | Unlike charges on neighbouring atoms | Like charges on neighbouring atoms (the worst case) |
| No atom beyond an octet | Nitrogen with four bonds and a + charge, as in | Nitrogen with five bonds: not a valid structure A 'resonance structure' with five bonds to N or C is simply wrong, however it is charged. |
Watch out for (9)
- Hyperconjugation is a permanent effect→ Inductive, resonance, electromeric and hyperconjugation effects
- H⁺ shows a +E effect, not −E→ Inductive, resonance, electromeric and hyperconjugation effects
- Iodine is the weakest −I group of the halogens→ Inductive, resonance, electromeric and hyperconjugation effects
- Five bonds to nitrogen is never allowed→ Rules for the stability of resonance structures
- Charge separation costs stability→ Rules for the stability of resonance structures
- Conjugation shortens the single bond→ Rules for the stability of resonance structures
- Basicity runs opposite to acidity→ Acid strength from conjugate-base stability
- An alkoxide is a stronger base than hydroxide→ Acid strength from conjugate-base stability
- Rank C–H acidity by s-character first→ Acid strength from conjugate-base stability
Formulas (1)
Reference tables (1)
Bond fission, free radicals, carbanions and reagent types3 rows
| Species | Formed by | Carbon: hybridisation and shape | Electrons on the carbon | Behaves as |
|---|---|---|---|---|
| Carbocation | Heterolysis; carbon loses the pair | sp², trigonal planar | 6 (a sextet) | Electrophile |
| Carbanion | Heterolysis; carbon keeps the pair | sp³, pyramidal | 8, with one lone pair | Nucleophile and base |
| Free radical | Homolysis | sp², nearly planar | 7, with one unpaired electron | Neutral, very reactive; starts chain reactions Radical and carbocation stability follow the same alkyl order; carbanion stability runs the other way. |
Watch out for (7)
- More stable means LOWER hydride affinity→ Carbocation stability, hydride affinity and rearrangement
- A meta donor cannot reach the charge→ Carbocation stability, hydride affinity and rearrangement
- The vinyl cation is not allylic→ Carbocation stability, hydride affinity and rearrangement
- The methyl cation has no hyperconjugation→ Carbocation stability, hydride affinity and rearrangement
- Carbanion stability runs opposite to carbocation stability→ Bond fission, free radicals, carbanions and reagent types
- Ionic reactions come from heterolysis→ Bond fission, free radicals, carbanions and reagent types
- A π bond can make a nucleophile→ Bond fission, free radicals, carbanions and reagent types
Reference tables (2)
Choosing a distillation method5 rows
| Method | Use when | Classic example |
|---|---|---|
| Simple distillation | Boiling points far apart, or a liquid from a non-volatile solid | Chloroform from aniline |
| Fractional distillation | Liquids with close boiling points | Crude oil into petrol, kerosene and diesel fractions |
| Distillation under reduced pressure | The liquid decomposes at its normal boiling point | Glycerol from spent lye |
| Steam distillation | Steam-volatile substance, immiscible with water | Aniline from an aniline–water mixture; o-nitrophenol from p-nitrophenol The substance distils below its own boiling point because the two vapour pressures add up. |
| Azeotropic distillation | A constant-boiling mixture that fractional distillation cannot split | Water removed from ethanol by adding benzene |
Crystallisation, sublimation and extraction5 rows
| Method | Property exploited | Example |
|---|---|---|
| Crystallisation | Solubility in one solvent, hot against cold | Benzoic acid recrystallised from hot water |
| Fractional crystallisation | A small difference in solubility, used repeatedly | Two solids of similar solubility separated in stages |
| Sublimation | Solid changes directly to vapour | Naphthalene or camphor separated from sodium chloride |
| Differential extraction | Different solubility in two immiscible solvents | An organic compound extracted from water into ether |
| Acid–base extraction | An acid or base turned into a water-soluble salt | Phenol pulled from a toluene solution into aqueous NaOH NaOH extracts acids and phenols; dilute HCl extracts amines. Neither extracts a neutral compound. |
Watch out for (6)
- Lower pressure means a lower boiling point→ Choosing a distillation method
- The rising vapour gets richer in the lower-boiling liquid→ Choosing a distillation method
- Steam distillation needs a water-immiscible substance→ Choosing a distillation method
- Sublimation is about solid to vapour, not a low melting point→ Crystallisation, sublimation and extraction
- NaOH does not extract an amine→ Crystallisation, sublimation and extraction
- The impurity matters as much as the compound→ Crystallisation, sublimation and extraction
Reference tables (1)
Adsorption and partition chromatography3 rows
| Technique | Principle | Stationary phase | Mobile phase |
|---|---|---|---|
| Column chromatography | Adsorption | Silica gel or alumina packed in a glass column | A solvent (the eluant) run down the column |
| Thin-layer chromatography (TLC) | Adsorption | A thin layer of silica gel or alumina on a glass plate | A solvent rising up the plate by capillary action |
| Paper chromatography | Partition | Water held in the pores of the paper | A solvent rising up the paper The paper itself is only the support, not the stationary phase. |
Watch out for (6)
- The paper is not the stationary phase→ Adsorption and partition chromatography
- TLC works by adsorption, not partition→ Adsorption and partition chromatography
- No visualising agent in the mobile phase→ Adsorption and partition chromatography
- Measure from the base line, not the plate edge→ Retardation factor and order of elution
- More polar means lower Rf on silica→ Retardation factor and order of elution
- The first compound out has the higher Rf→ Retardation factor and order of elution
Formulas (2)
Reference tables (1)
Lassaigne's test for nitrogen, sulphur, halogens and phosphorus8 rows
| Element | In the extract as | Reagent | Positive result |
|---|---|---|---|
| Nitrogen | NaCN | , boil, then conc. | Prussian blue, |
| Sulphur | Sodium nitroprusside | Violet, | |
| Sulphur | Ethanoic acid and lead acetate | Black precipitate of PbS | |
| Nitrogen and sulphur together | NaSCN | (iron(III) chloride) | Blood red, With excess sodium the thiocyanate breaks down, and the separate cyanide and sulphide tests work instead. |
| Chlorine | NaCl | Boil with , then | White AgCl, soluble in ammonia |
| Bromine | NaBr | Boil with , then | Pale yellow AgBr, sparingly soluble in ammonia |
| Iodine | NaI | Boil with , then | Yellow AgI, insoluble in ammonia |
| Phosphorus | (after oxidation with ) | and ammonium molybdate | Yellow |
Watch out for (9)
- No carbon, no cyanide→ Lassaigne's test for nitrogen, sulphur, halogens and phosphorus
- Nitric acid, not hydrochloric acid→ Lassaigne's test for nitrogen, sulphur, halogens and phosphorus
- Lassaigne's test cannot detect oxygen→ Lassaigne's test for nitrogen, sulphur, halogens and phosphorus
- Subtract the aqueous tension first→ Dumas method for nitrogen
- Nitrogen gas is 28, not 14→ Dumas method for nitrogen
- Copper gauze, not copper oxide, reduces the oxides→ Dumas method for nitrogen
- Sulphuric acid is dibasic→ Kjeldahl method for nitrogen
- Not for nitro, azo or ring nitrogen→ Kjeldahl method for nitrogen
- Use the acid actually neutralised→ Kjeldahl method for nitrogen
Formulas (3)
Watch out for (9)
- Hydrogen is 2/18 of water, not 1/18→ Combustion analysis for carbon, hydrogen and oxygen
- Oxygen is never weighed directly→ Combustion analysis for carbon, hydrogen and oxygen
- Match the tube to the gas→ Combustion analysis for carbon, hydrogen and oxygen
- Divide by the silver halide's molar mass→ Carius method for halogens
- Keep the three silver halides apart→ Carius method for halogens
- Carius does not estimate nitrogen→ Carius method for halogens
- Two phosphorus atoms per pyrophosphate→ Estimation of sulphur and phosphorus
- Use the molar mass the question gives→ Estimation of sulphur and phosphorus
- Sulphur is weighed as a barium salt→ Estimation of sulphur and phosphorus
PYQ weightage by concept
23 concepts · 243 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
23 concepts · 243 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| Seniority of functional groups in IUPAC names | 13 | 5% |
| Choosing and numbering the parent chain | 11 | 5% |
| Hybridisation and counting sigma and pi bonds | 10 | 4% |
| Concept | PYQs | Share |
|---|---|---|
| Kinds of structural isomerism | 8 | 3% |
| Counting structural isomers of a formula | 5 | 2% |
| Concept | PYQs | Share |
|---|---|---|
| Chiral centres, meso compounds and optical purity | 14 | 6% |
| Geometrical isomerism and conformations | 9 | 4% |
| Counting stereoisomers | 9 | 4% |
| Concept | PYQs | Share |
|---|---|---|
| Inductive, resonance, electromeric and hyperconjugation effects | 10 | 4% |
| Rules for the stability of resonance structures | 9 | 4% |
| Acid strength from conjugate-base stability | 5 | 2% |
| Concept | PYQs | Share |
|---|---|---|
| Carbocation stability, hydride affinity and rearrangement | 16 | 7% |
| Bond fission, free radicals, carbanions and reagent types | 8 | 3% |
| Concept | PYQs | Share |
|---|---|---|
| Choosing a distillation method | 16 | 7% |
| Crystallisation, sublimation and extraction | 9 | 4% |
| Concept | PYQs | Share |
|---|---|---|
| Adsorption and partition chromatography | 10 | 4% |
| Retardation factor and order of elution | 10 | 4% |
| Concept | PYQs | Share |
|---|---|---|
| Lassaigne's test for nitrogen, sulphur, halogens and phosphorus | 18 | 7% |
| Dumas method for nitrogen | 12 | 5% |
| Kjeldahl method for nitrogen | 8 | 3% |
| Concept | PYQs | Share |
|---|---|---|
| Combustion analysis for carbon, hydrogen and oxygen | 11 | 5% |
| Carius method for halogens | 11 | 5% |
| Estimation of sulphur and phosphorus | 11 | 5% |
Test yourself on Organic Chemistry - Some Basic Principles and Techniques
20 past JEE Mains questions from this chapter, timed at 48 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.