JEE Mains Chemistry · Teaching notes
Hydrocarbons — JEE Mains Chemistry
Hydrocarbons has 165 past-year questions from 2021 to 2026, and 31 of them ask for a number rather than an option. Most of them come down to one question: which carbon does the new group go to? For an alkene the answer is the carbon that leaves the more stable carbocation, after any hydride or methyl shift, or the more stable radical when HBr has a peroxide; for a substituted benzene it is the position the group already on the ring directs to. The rest is careful counting, of products, of carbonyl fragments or of π electrons in a ring, and each count has a routine that works every time if you apply it in full.
Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.
Subtopic notes
Alkanes: Preparation, Structure and Conformations
19 PYQsAlkanes (CₙH₂ₙ₊₂) are made by adding hydrogen to a C=C or C≡C, by removing a halogen or a carboxyl group, or by joining two alkyl groups; their carbons are classed 1°, 2°, 3° or 4°, and rotation about each C–C bond gives staggered and eclipsed conformations.
Free-Radical Halogenation of Alkanes
11 PYQsIn light or heat, a halogen replaces the hydrogens of an alkane one at a time by a free-radical chain; each set of equivalent hydrogens gives one monohalo product, and bromine strongly prefers a tertiary hydrogen.
Alkene Stability and Addition of HX and Water
29 PYQsMore alkyl groups on the C=C make an alkene more stable; HX and water add through the more stable carbocation (Markovnikov), which may rearrange first, while HBr with a peroxide adds the other way through a radical.
Halogen Addition, Oxidation and Ozonolysis of Alkenes
26 PYQsBromine adds anti across a C=C through a bromonium ion; cold dilute KMnO₄ turns it into a diol while hot acidic KMnO₄ cuts it; ozonolysis cuts it into two carbonyl compounds whose structures reveal where the double bond was.
Alkynes: Preparation, Acidity, Reduction and Addition
20 PYQsAlkynes are made by removing two HX from a dihalide; a terminal alkyne's C–H is acidic enough to give an acetylide, which builds longer chains; H₂ with Lindlar's catalyst gives the cis alkene and Na in liquid NH₃ the trans; water adds with Hg²⁺ to give a ketone.
Benzene and Aromaticity
22 PYQsBenzene's six π electrons are spread over a planar ring of six sp² carbons, which makes it far more stable than three separate double bonds; any ring that is cyclic, planar, fully conjugated and holds 4n + 2 π electrons shares that stability.
Electrophilic Substitution: Reactivity and Directing Effects
19 PYQsBenzene keeps its aromatic ring by swapping an H for an electrophile made by an acid or a Lewis acid; a group already on the ring speeds or slows that swap and sends the new group ortho and para or meta.
Friedel–Crafts, Side-Chain Oxidation and Arene Synthesis
19 PYQsFriedel–Crafts alkylation goes through a carbocation that may rearrange, while acylation does not; hot KMnO₄ cuts any side chain with a benzylic H down to –COOH; and the order in which groups are put on a ring decides where they end up.
Formula & revision sheet
9 formulas · 14 reference tables · 55 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
9 formulas · 14 reference tables · 55 gotchas across all subtopics — the exam-eve cheat-sheet
Alkanes: Preparation, Structure and Conformations
Reference tables (2)
Preparing alkanes and what each route does to the carbon count7 rows
| Route | Reagents | Carbon count of product | Example |
|---|---|---|---|
| Hydrogenation | with Pt, Pd or Ni | Same as the alkene or alkyne | Propene gives propane |
| Reduction of R–X | Zn and dilute HCl | Same as the halide | gives ethane |
| Wurtz | Na in dry ether | Twice the alkyl group | gives butane |
| Kolbe electrolysis | Electrolysis of the aqueous sodium salt | Twice the alkyl group | Sodium propanoate gives butane |
| Soda-lime decarboxylation | NaOH with CaO, heat | One carbon fewer than the salt | Sodium propanoate gives ethane |
| Grignard + acidic H | , ROH or | Same as the alkyl group | gives ethane |
| Clemmensen reduction | Zn–Hg and conc. HCl | Same; C=O becomes CH₂ | Propanone gives propane |
Wurtz and Kolbe double the chain, so neither can make methane; decarboxylation removes one carbon.
Isomerisation, aromatisation and oxidation of alkanes6 rows
| Reaction | Conditions | What changes | Example |
|---|---|---|---|
| Isomerisation | Anhydrous , HCl gas, heat | Chain branches; formula unchanged | n-Hexane → 2-methylpentane and 3-methylpentane |
| Aromatisation | or on alumina, 773 K, 10–20 atm | Six-carbon ring closes; H₂ is lost | n-Hexane → benzene |
| KMnO₄ oxidation | Tertiary C–H becomes C–OH | 2-Methylpropane → 2-methylpropan-2-ol | |
| Controlled oxidation | Cu at 523 K and 100 atm, or | Methane becomes methanol or methanal | |
| Steam reforming | , Ni, 1273 K | Methane becomes CO and H₂ | |
| Pyrolysis | Strong heat, no air | Chain breaks into smaller alkanes and alkenes | Hexane → butene + ethane, among others |
Aromatisation keeps the carbon count: count the carbons of the arene to find the alkane.
Watch out for (8)
- Kolbe electrolysis of sodium ethanoate gives ethane→ Preparing alkanes and what each route does to the carbon count
- A mixture of two salts or two halides gives three alkanes→ Preparing alkanes and what each route does to the carbon count
- Every acidic H counts for a Grignard reagent→ Preparing alkanes and what each route does to the carbon count
- A ring has two hydrogens fewer→ Alkane formula, carbon classes and conformations
- The fully eclipsed form of butane is the highest in energy→ Alkane formula, carbon classes and conformations
- Conformers are not isolable→ Alkane formula, carbon classes and conformations
- Isomerisation does not change the formula→ Isomerisation, aromatisation and oxidation of alkanes
- KMnO₄ needs a tertiary hydrogen→ Isomerisation, aromatisation and oxidation of alkanes
Free-Radical Halogenation of Alkanes
Reference tables (1)
Radical selectivity and multiple halogenation5 rows
| Substrate and conditions | What happens | Product | Reason |
|---|---|---|---|
| 2-Methylpropane, , light | Bromine takes the tertiary H | 2-Bromo-2-methylpropane (major) | Br· is highly selective for the most stable radical |
| Propane, , light | Chlorine attacks both kinds of H | 1-Chloropropane and 2-chloropropane in similar amounts | Cl· is fast and less selective |
| Methane, excess , light | Substitution continues | Each product still has H to replace | |
| Ethane, excess , light | Every degree of substitution forms | 9 bromoethanes, from to | Counts per formula: 1, 2, 2, 2, 1, 1 |
| Cyclopropane, , light | One Br replaces one H when the data show one Br per molecule | Bromocyclopropane, | One used, the second Br leaves as HBr |
Use the product's C : X ratio to tell substitution (one X per used) from addition (two X).
Watch out for (5)
- Do not count equivalent methyls twice→ Counting monohalogenation products
- Read whether stereoisomers are counted→ Counting monohalogenation products
- Cyclic isomers do not decolourise KMnO₄→ Counting monohalogenation products
- Substitution puts one halogen in the product→ Radical selectivity and multiple halogenation
- Chlorination does not pick only the tertiary H→ Radical selectivity and multiple halogenation
Alkene Stability and Addition of HX and Water
Reference tables (2)
Stability of alkenes and carbanions6 rows
| Alkene | Alkyl groups on C=C | α-H count | Place in stability order |
|---|---|---|---|
| 2,3-Dimethylbut-2-ene | 4 | 12 | Most stable of this list |
| 2-Methylbut-2-ene | 3 | 9 | Second |
| trans-But-2-ene | 2 | 6 | Third |
| cis-But-2-ene | 2 | 6 | Fourth (steric crowding of the cis groups) |
| Propene | 1 | 3 | Fifth |
| Ethene | 0 | 0 | Least stable |
Stability follows the number of alkyl groups on the C=C; at equal substitution, trans beats cis.
Three ways to add water to an alkene3 rows
| Route | Orientation | Rearrangement | Product from 3,3-dimethylbut-1-ene |
|---|---|---|---|
| Markovnikov | Yes (methyl shift here) | 2,3-Dimethylbutan-2-ol | |
| ; | Markovnikov | No | 3,3-Dimethylbutan-2-ol |
| ; | Anti-Markovnikov, syn | No | 3,3-Dimethylbutan-1-ol |
One alkene, three different alcohols: the acid route is the only one that can move a methyl group.
Watch out for (7)
- Carbanions run the opposite way to carbocations→ Stability of alkenes and carbanions
- A weaker π bond does not make C=C weaker than C–C→ Stability of alkenes and carbanions
- Look for a shift before placing X→ Markovnikov and anti-Markovnikov addition of HX
- Peroxide changes only HBr→ Markovnikov and anti-Markovnikov addition of HX
- Count stereocentres in the product, not the alkene→ Markovnikov and anti-Markovnikov addition of HX
- Oxymercuration does not rearrange→ Three ways to add water to an alkene
- Hydroboration is anti-Markovnikov without any peroxide→ Three ways to add water to an alkene
Halogen Addition, Oxidation and Ozonolysis of Alkenes
Formulas (2)
Reference tables (1)
Adding halogens across C=C, and allylic substitution5 rows
| Reagent and conditions | Type of reaction | Product from cyclohexene | Stereochemistry |
|---|---|---|---|
| in , dark | Electrophilic addition | 1,2-Dibromocyclohexane | trans (anti addition) |
| in water | Addition of Br and OH | 2-Bromocyclohexan-1-ol | trans (anti addition) |
| in , dark | Electrophilic addition | 1,2-Dichlorocyclohexane | trans (anti addition) |
| , light or 500 °C (low concentration) | Radical allylic substitution | 3-Chlorocyclohexene | C=C kept; racemic at C-3 |
| NBS, light or peroxide | Radical allylic substitution | 3-Bromocyclohexene | C=C kept; racemic at C-3 |
The same halogen adds in the dark and substitutes at the allylic carbon in light: the conditions decide.
Watch out for (7)
- Anti addition to trans gives meso→ Adding halogens across C=C, and allylic substitution
- Light turns addition into substitution→ Adding halogens across C=C, and allylic substitution
- =CH₂ gives CO₂, not methanal, with hot KMnO₄→ KMnO₄: cold gives a diol, hot cuts the C=C
- Baeyer's reagent gives a diol, not cleavage→ KMnO₄: cold gives a diol, hot cuts the C=C
- A ring alkene gives one product, not two→ Ozonolysis: predicting products and working back to the alkene
- Zn decides aldehyde or acid→ Ozonolysis: predicting products and working back to the alkene
- cis and trans isomers give the same products→ Ozonolysis: predicting products and working back to the alkene
Alkynes: Preparation, Acidity, Reduction and Addition
Formulas (2)
Reference tables (1)
Reducing alkynes to cis or trans alkenes3 rows
| Reagent | How H adds | Product from pent-2-yne | Dipole of product |
|---|---|---|---|
| H₂, Lindlar's catalyst | Syn, stops at the alkene | cis-Pent-2-ene | Non-zero |
| Na in liquid NH₃ | Anti, stepwise | trans-Pent-2-ene | Close to zero |
| Excess H₂, Pt or Ni | Syn, twice | Pentane | Close to zero |
Lindlar gives cis, sodium in ammonia gives trans, and an unpoisoned catalyst with excess H₂ goes to the alkane.
Watch out for (7)
- Na gives half a mole of H₂ per acidic H→ Making alkynes and using the acidic terminal H
- Convert moles to millilitres carefully→ Making alkynes and using the acidic terminal H
- Only a terminal alkyne has the acidic H→ Making alkynes and using the acidic terminal H
- Aqueous KOH substitutes, alcoholic KOH eliminates→ Reducing alkynes to cis or trans alkenes
- The cis isomer is the more polar one→ Reducing alkynes to cis or trans alkenes
- An enol is not the final product→ Adding water, halogens and ozone to alkynes
- A terminal alkyne gives a methyl ketone, not an aldehyde→ Adding water, halogens and ozone to alkynes
Benzene and Aromaticity
Reference tables (2)
The structure of benzene4 rows
| Evidence | Kekulé cyclohexatriene predicts | Benzene shows | Conclusion |
|---|---|---|---|
| C–C bond lengths | Three of 154 pm and three of 133 pm | Six equal bonds of 139 pm | Electrons are delocalised |
| Heat of hydrogenation | About 3 × 120 = 360 kJ mol⁻¹ | About 208 kJ mol⁻¹ | Extra stability of about 150 kJ mol⁻¹ |
| Reaction with Br₂ | Quick addition like an alkene | Substitution, and only with a Lewis acid | The π system resists addition |
| Isomers of o-dibromobenzene | Two (Br across a single or a double bond) | Only one | The two Kekulé forms are one molecule |
Every measurement says the bonds are equal: the two Kekulé structures are resonance forms, not isomers.
Aromaticity decides stability and acidity7 rows
| Species | π electrons in the ring | Verdict | Consequence |
|---|---|---|---|
| Cyclopentadienyl anion | 6 | Aromatic | Cyclopentadiene is unusually acidic |
| Tropylium cation | 6 | Aromatic | Tropylium salts are ionic and stable |
| Cyclopropenyl cation | 2 | Aromatic | A stable carbocation |
| Cyclopropenyl anion | 4 | Antiaromatic | Very hard to form |
| Cyclopentadienyl cation | 4 | Antiaromatic | Very hard to form |
| Cycloheptatrienyl anion | 8 | Antiaromatic if planar | Cycloheptatriene is not especially acidic |
| Cyclobutadiene | 4 | Antiaromatic | Exists only at very low temperature |
Ask what the ion would be; an aromatic ion is easy to make, an antiaromatic one is not.
Watch out for (7)
- Kekulé forms matter for a substituted benzene→ The structure of benzene
- Benzene does add, but only under force→ The structure of benzene
- Look-alike drawings with one double bond missing→ Deciding aromaticity: Hückel's rule
- The right count is not enough without planarity→ Deciding aromaticity: Hückel's rule
- An exocyclic C=O does not add to the ring's count→ Deciding aromaticity: Hückel's rule
- Judge the ion, not the neutral molecule→ Aromaticity decides stability and acidity
- Antiaromatic is worse than non-aromatic→ Aromaticity decides stability and acidity
Electrophilic Substitution: Reactivity and Directing Effects
Reference tables (3)
Making the electrophile, and when Friedel–Crafts fails5 rows
| Reaction | Reagents | Electrophile | Product from benzene |
|---|---|---|---|
| Nitration | Conc. HNO₃ + conc. H₂SO₄ | (nitronium) | Nitrobenzene |
| Chlorination | Cl₂ with anhydrous AlCl₃ or FeCl₃ | Chlorobenzene | |
| Sulphonation | Fuming H₂SO₄ (oleum) | Benzenesulphonic acid | |
| Friedel–Crafts alkylation | CH₃Cl with anhydrous AlCl₃ | Toluene | |
| Friedel–Crafts acylation | CH₃COCl with anhydrous AlCl₃ | (acylium) | Acetophenone |
Every electrophile is made by an acid or a Lewis acid; the ring then loses H⁺ to stay aromatic.
Activating, deactivating, ortho-para and meta directors8 rows
| Group | Main electronic effect | Rate compared with benzene | Directs to |
|---|---|---|---|
| –NH₂, –NR₂ | +R (strong) | Much faster | ortho and para |
| –OH, –OCH₃ | +R (strong) | Much faster | ortho and para |
| –NHCOCH₃ | +R (moderate; the lone pair is shared with C=O) | Faster | ortho and para |
| –CH₃, –C₂H₅ | +I and hyperconjugation | Slightly faster | ortho and para |
| –Cl, –Br | –I stronger than +R | Slightly slower | ortho and para |
| –CHO, –COR, –COOH, –COOR | –R and –I | Slower | meta |
| –CN, –SO₃H, –CF₃ | –R and –I (–CF₃ by –I only) | Much slower | meta |
| –NO₂ | –R and –I (strongest) | Much slower | meta |
Every activator directs ortho and para; every meta director deactivates; halogens deactivate yet direct ortho and para.
Ranking rings by rate of electrophilic substitution8 rows
| Compound | Group | Effect on the ring | Place in rate order |
|---|---|---|---|
| N,N-Dimethylaniline | –N(CH₃)₂ | Strong +R | Fastest of this list |
| Anisole | –OCH₃ | Strong +R | Second |
| Toluene | –CH₃ | +I and hyperconjugation | Third |
| Benzene | –H | Reference | Fourth |
| Chlorobenzene | –Cl | –I beats +R | Fifth |
| Benzaldehyde | –CHO | –R and –I | Sixth |
| Benzonitrile | –CN | –R and –I | Seventh |
| Nitrobenzene | –NO₂ | Strongest –R and –I | Slowest of this list |
Rank by the strongest group on each ring; alkyl groups add up, and each extra withdrawing group slows the ring further.
Watch out for (7)
- AlCl₃ is a Lewis acid, not a Lewis base→ Making the electrophile, and when Friedel–Crafts fails
- Chlorobenzene still reacts→ Making the electrophile, and when Friedel–Crafts fails
- Halogens deactivate but direct ortho and para→ Activating, deactivating, ortho-para and meta directors
- –OH and –OCH₃ are never meta directors→ Activating, deactivating, ortho-para and meta directors
- Nitro activates for the other kind of substitution→ Activating, deactivating, ortho-para and meta directors
- Halogenobenzenes are slower than benzene→ Ranking rings by rate of electrophilic substitution
- Count the alkyl groups→ Ranking rings by rate of electrophilic substitution
Friedel–Crafts, Side-Chain Oxidation and Arene Synthesis
Reference tables (2)
Friedel–Crafts alkylation and acylation5 rows
| Reagent with benzene and AlCl₃ | Cation formed | Does it rearrange? | Main product |
|---|---|---|---|
| No | Toluene | ||
| (isobutyl chloride) | Primary, shifts to tertiary | Yes (hydride shift) | tert-Butylbenzene |
| Cyclohexene with HF | Cyclohexyl cation | No | Cyclohexylbenzene |
| Acylium ion | No | Butyrophenone (1-phenylbutan-1-one) | |
| , then Zn–Hg/HCl | Acylium ion | No | n-Butylbenzene |
Alkyl halides can rearrange before they attack; acyl chlorides never do, so acylation then reduction gives a straight chain.
Choosing the order of steps for a disubstituted benzene5 rows
| Target | Order of steps | Why this order | Wrong order gives |
|---|---|---|---|
| m-Bromonitrobenzene | HNO₃/H₂SO₄, then Br₂/FeBr₃ | –NO₂ sends Br meta | o- and p-bromonitrobenzene |
| p-Bromonitrobenzene | Br₂/FeBr₃, then HNO₃/H₂SO₄; separate para | –Br sends NO₂ ortho and para | m-Bromonitrobenzene |
| m-Nitroacetophenone | CH₃COCl/AlCl₃, then HNO₃/H₂SO₄ | Acylation fails on nitrobenzene; –COCH₃ sends NO₂ meta | No reaction at the acylation step |
| 3-Bromobenzoic acid (from toluene) | KMnO₄, then Br₂/FeBr₃ | –COOH sends Br meta | 2- and 4-bromobenzoic acid |
| 4-Bromobenzoic acid (from toluene) | Br₂/FeBr₃, separate para, then KMnO₄ | –CH₃ sends Br ortho and para | 3-Bromobenzoic acid |
Work back from the target: the relationship of the two groups tells you which one went on first.
Watch out for (7)
- The alkyl group on the ring may differ from the halide→ Friedel–Crafts alkylation and acylation
- Polyalkylation is likely, not certain→ Friedel–Crafts alkylation and acylation
- The whole chain goes, however long→ Side-chain oxidation to benzoic acid
- Check for a benzylic H before oxidising→ Side-chain oxidation to benzoic acid
- –COOH directs the next group meta→ Side-chain oxidation to benzoic acid
- Friedel–Crafts must come before any strong deactivator→ Choosing the order of steps for a disubstituted benzene
- A later change of group can flip its direction→ Choosing the order of steps for a disubstituted benzene
PYQ weightage by concept
23 concepts · 165 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
23 concepts · 165 PYQs — where the marks actually sit, so you know what to drill first
Alkanes: Preparation, Structure and Conformations19 PYQs · 12%
| Concept | PYQs | Share |
|---|---|---|
| Alkane formula, carbon classes and conformations | 8 | 5% |
| Preparing alkanes and what each route does to the carbon count | 7 | 4% |
| Isomerisation, aromatisation and oxidation of alkanes | 4 | 2% |
Free-Radical Halogenation of Alkanes11 PYQs · 7%
| Concept | PYQs | Share |
|---|---|---|
| Counting monohalogenation products | 7 | 4% |
| Radical selectivity and multiple halogenation | 4 | 2% |
Alkene Stability and Addition of HX and Water29 PYQs · 18%
| Concept | PYQs | Share |
|---|---|---|
| Markovnikov and anti-Markovnikov addition of HX | 16 | 10% |
| Stability of alkenes and carbanions | 7 | 4% |
| Three ways to add water to an alkene | 6 | 4% |
Halogen Addition, Oxidation and Ozonolysis of Alkenes26 PYQs · 16%
| Concept | PYQs | Share |
|---|---|---|
| Ozonolysis: predicting products and working back to the alkene | 14 | 8% |
| KMnO₄: cold gives a diol, hot cuts the C=C | 7 | 4% |
| Adding halogens across C=C, and allylic substitution | 5 | 3% |
Alkynes: Preparation, Acidity, Reduction and Addition20 PYQs · 12%
| Concept | PYQs | Share |
|---|---|---|
| Reducing alkynes to cis or trans alkenes | 8 | 5% |
| Making alkynes and using the acidic terminal H | 6 | 4% |
| Adding water, halogens and ozone to alkynes | 6 | 4% |
Benzene and Aromaticity22 PYQs · 13%
| Concept | PYQs | Share |
|---|---|---|
| Deciding aromaticity: Hückel's rule | 13 | 8% |
| Aromaticity decides stability and acidity | 5 | 3% |
| The structure of benzene | 4 | 2% |
Electrophilic Substitution: Reactivity and Directing Effects19 PYQs · 12%
| Concept | PYQs | Share |
|---|---|---|
| Activating, deactivating, ortho-para and meta directors | 7 | 4% |
| Ranking rings by rate of electrophilic substitution | 7 | 4% |
| Making the electrophile, and when Friedel–Crafts fails | 5 | 3% |
Friedel–Crafts, Side-Chain Oxidation and Arene Synthesis19 PYQs · 12%
| Concept | PYQs | Share |
|---|---|---|
| Side-chain oxidation to benzoic acid | 7 | 4% |
| Choosing the order of steps for a disubstituted benzene | 7 | 4% |
| Friedel–Crafts alkylation and acylation | 5 | 3% |
Test yourself on Hydrocarbons
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.