JEE Mains Chemistry · Teaching notes
Haloalkanes and Haloarenes — JEE Mains Chemistry
Haloalkanes and Haloarenes has 92 past-year questions from 2021 to 2026, and only 6 of them ask for a number. It is a reasoning chapter: most questions give a halide and a reagent and ask what happens, so the work is choosing the pathway before writing the product. Can the carbon form a stable cation? Is it open to attack from behind? Is the reagent acting as a nucleophile or as a base? Ranking questions are common, and each ranks by one property only, so name that property first. The recall is small but exact: a few named reagents, a few physical trends and the uses of some polyhalogen compounds.
Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.
Subtopic notes
Classification, Structure and Physical Properties
13 PYQsAn organic halide is named by the carbon that holds the halogen: sp³ for alkyl, allylic and benzylic halides, sp² for vinylic and aryl halides, whose C–X bond is shorter, stronger and less polar; boiling point and density rise with the size and number of halogen atoms.
Preparation of Haloalkanes and Haloarenes
11 PYQsHaloalkanes come from alcohols (HX, PCl₅ or SOCl₂), from alkenes (HX, Markovnikov unless HBr meets a peroxide) and from C–H bonds by free-radical halogenation; haloarenes come from ring halogenation with a Lewis acid or from diazonium salts, never from phenol.
SN1 and SN2: Mechanism, Kinetics and Stereochemistry
14 PYQsSN2 is one step, second order and inverts the carbon; SN1 ionises first to a planar carbocation, is first order in the halide alone, racemises a stereocentre and can rearrange.
Reactivity Order in Nucleophilic Substitution
16 PYQsRank halides for SN1 by the stability of the carbocation they would form, and for SN2 by how open the carbon is to backside attack; vinylic, aryl and bridgehead halides cannot form a usable cation at all.
Nucleophiles and Ambident Reagents
13 PYQsA nucleophile is stronger when it is charged, when it is the stronger base among donors of the same atom, and, in a protic solvent, when its donor atom is larger; cyanide and nitrite are ambident, so the potassium and silver salts give different products.
Elimination Versus Substitution
15 PYQsHydroxide or alkoxide can attack carbon (substitution) or remove a β-hydrogen (elimination); water favours substitution, alcoholic KOH with heat, a bulky base or a tertiary halide favours the alkene, and the major alkene is the more substituted one unless another is conjugated.
Haloarenes and Reactions with Metals
10 PYQsA haloarene resists nucleophiles until nitro groups ortho or para to the halogen stabilise the intermediate anion, while its halogen slows electrophilic substitution yet directs it ortho and para; with magnesium an organic halide gives a Grignard reagent, and with sodium it couples.
Formula & revision sheet
7 formulas · 8 reference tables · 39 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
7 formulas · 8 reference tables · 39 gotchas across all subtopics — the exam-eve cheat-sheet
Reference tables (3)
Types of organic halides and the aryl C–X bond8 rows
| Class | Carbon that holds X | Example | What follows |
|---|---|---|---|
| Alkyl, 1° | sp³ carbon bonded to one other carbon | Reacts mainly by SN2 | |
| Alkyl, 3° | sp³ carbon bonded to three other carbons | Reacts mainly by SN1 or elimination | |
| Allylic | sp³ carbon next to a C=C | Ionises easily: the allyl cation is resonance-stabilised | |
| Benzylic | sp³ carbon attached to a benzene ring | , | Ionises easily: the benzyl cation is resonance-stabilised |
| Vinylic | sp² carbon of a C=C | Partial C=Cl character; no SN1 or SN2 under normal conditions | |
| Aryl | sp² carbon of a benzene ring | Partial C=Cl character; substituted only under harsh conditions or with nitro groups ortho or para | |
| Geminal dihalide | Both X on one carbon | Ethylidene chloride, (1,1-dichloroethane) | Common name ends in -idene |
| Vicinal dihalide | X on two adjacent carbons | Ethylene dichloride, (1,2-dichloroethane) | Common name ends in -ene |
Boiling point, melting point, density and polarity of halides8 rows
| Property | Trend | Reason |
|---|---|---|
| Boiling point, changing the halogen | A larger, more polarisable halogen gives stronger van der Waals forces | |
| Boiling point, longer chain | A larger surface gives stronger London forces | |
| Boiling point, branched isomers | Falls with branching | A branched molecule is more nearly spherical, with less contact area |
| Melting point of dichlorobenzenes | para (323 K) > ortho (256 K) > meta (249 K) | The symmetric para isomer packs best in the crystal |
| Boiling point of dichlorobenzenes | ortho (453 K) > para (448 K) > meta (446 K) | The ortho isomer has the largest dipole |
| Density | ; iodides densest | More and heavier halogen atoms in about the same volume |
| Dipole moment of | Charge × bond length is largest for C–Cl; the C–F bond is very short | |
| cis against trans 1,2-dihaloethene | cis is polar and boils higher; trans has almost no dipole | In the trans isomer the two C–X dipoles point opposite ways and cancel |
Polyhalogen compounds: formulas and uses9 rows
| Compound | Formula | Use or fact |
|---|---|---|
| Dichloromethane (methylene chloride) | Paint remover, solvent and aerosol propellant | |
| Trichloromethane (chloroform) | Solvent; stored in dark, full bottles because air and light turn it into phosgene | |
| Triiodomethane (iodoform) | Antiseptic, through the free iodine it releases | |
| Tetrachloromethane (carbon tetrachloride) | Fire extinguisher (earlier), solvent, feedstock for freons | |
| Freon-12 (dichlorodifluoromethane) | Refrigerant and aerosol propellant; a CFC with 2 Cl | |
| DDT (p,p′-dichlorodiphenyltrichloroethane) | Non-biodegradable insecticide; 5 Cl | |
| Gammaxene (lindane, BHC) | Insecticide; 6 Cl | |
| Chloropicrin (trichloronitromethane) | Insecticide and war gas; 3 Cl | |
| Chloral (trichloroethanal) | Raw material for DDT; 3 Cl |
Watch out for (8)
- Allylic is not vinylic→ Types of organic halides and the aryl C–X bond
- Ethylidene and ethylene dichloride→ Types of organic halides and the aryl C–X bond
- Resonance makes the aryl C–Cl bond shorter and less polar→ Types of organic halides and the aryl C–X bond
- Symmetry raises the melting point, not the boiling point→ Boiling point, melting point, density and polarity of halides
- Methyl fluoride is not the most polar methyl halide→ Boiling point, melting point, density and polarity of halides
- Iodides are the densest, not the lightest→ Boiling point, melting point, density and polarity of halides
- A freon needs both chlorine and fluorine→ Polyhalogen compounds: formulas and uses
- Chloroform is kept full and dark→ Polyhalogen compounds: formulas and uses
Reference tables (1)
Named reactions that make or couple organic halides7 rows
| Reaction | Reagent | Change | Example |
|---|---|---|---|
| Finkelstein | NaI in dry acetone | R–Cl or R–Br → R–I | |
| Swarts | AgF, , or | R–Cl or R–Br → R–F | |
| Sandmeyer | , or CuCN/KCN | → ArCl, ArBr or ArCN | Benzenediazonium chloride → chlorobenzene |
| Gattermann | Copper powder with HCl or HBr | → ArCl or ArBr | Benzenediazonium chloride → bromobenzene |
| Iodide from a diazonium salt | KI (no copper) | → ArI | Benzenediazonium chloride → iodobenzene |
| Wurtz-Fittig | Na in dry ether | ArX + RX → Ar–R | Chlorobenzene + methyl chloride → toluene |
| Fittig | Na in dry ether | 2 ArX → Ar–Ar | Chlorobenzene → biphenyl |
Watch out for (5)
- Only HBr shows the peroxide effect→ Haloalkanes from alcohols, alkenes and hydrocarbons
- Phenol does not give an aryl halide with HX→ Haloalkanes from alcohols, alkenes and hydrocarbons
- Light chlorinates the side chain, iron(III) chloride the ring→ Haloalkanes from alcohols, alkenes and hydrocarbons
- Gattermann makes aryl chlorides and bromides, not cyanides→ Named reactions that make or couple organic halides
- Finkelstein runs because the salt precipitates→ Named reactions that make or couple organic halides
Reference tables (1)
SN1 and SN2 compared: steps, rate law and conditions9 rows
| Feature | SN1 | SN2 |
|---|---|---|
| Steps | Two: slow ionisation, then fast attack | One, concerted |
| Rate law | rate = , first order | rate = , second order |
| Intermediate | Planar carbocation | None; a five-coordinate transition state |
| Stereochemistry at a chiral carbon | Racemisation (mostly) | Inversion (Walden inversion) |
| Substrate order | 3° > 2° > 1° > | > 1° > 2° > 3° |
| Nucleophile | Weak, often the solvent | Strong, at high concentration |
| Best solvent | Polar protic: water, alcohols | Polar aprotic: acetone, DMSO, DMF |
| Rearrangement | Possible, by a hydride or methyl shift | Never |
| Leaving group | I > Br > Cl > F | I > Br > Cl > F |
Watch out for (5)
- A secondary halide does not have one fixed mechanism→ SN1 and SN2 compared: steps, rate law and conditions
- Adding more nucleophile does not speed SN1→ SN1 and SN2 compared: steps, rate law and conditions
- Polar solvents do not speed every substitution→ SN1 and SN2 compared: steps, rate law and conditions
- Inversion does not always change R to S→ Stereochemistry of substitution: inversion, racemisation and retention
- Tosylation keeps the configuration→ Stereochemistry of substitution: inversion, racemisation and retention
Formulas (2)
Reference tables (1)
Halides that cannot ionise: vinylic, aryl and bridgehead8 rows
| Halide | Cation it would give | SN1 and the AgNO₃ test |
|---|---|---|
| 3° cation, stabilised by hyperconjugation | Fast; AgCl precipitates at once | |
| Benzyl cation, stabilised by resonance | Fast; AgCl precipitates | |
| Allyl cation, stabilised by resonance | Fast; AgCl precipitates | |
| 1° cation, unstable | Very slow; precipitate only on long warming | |
| Vinyl cation, charge on an sp carbon | No SN1; no precipitate | |
| Phenyl cation, empty orbital in the ring plane | No SN1; no precipitate | |
| 1-Bromobicyclo[2.2.2]octane | Bridgehead cation that cannot become planar | Extremely slow; no practical SN1 |
| 3-Bromocyclopropene | Cyclopropenyl cation, aromatic with 2 π electrons | Ionises readily; AgBr precipitates |
Watch out for (6)
- A para-chloro group slows SN1→ SN1 reactivity: ranking halides by carbocation stability
- Primary halides can still go SN1→ SN1 reactivity: ranking halides by carbocation stability
- A tertiary bridgehead halide does not ionise→ Halides that cannot ionise: vinylic, aryl and bridgehead
- Some cyclic halides ionise because the cation is aromatic→ Halides that cannot ionise: vinylic, aryl and bridgehead
- Primary does not guarantee fast SN2→ SN2 reactivity: crowding, neighbouring groups and benzylic halides
- Benzylic halides are fast by both mechanisms→ SN2 reactivity: crowding, neighbouring groups and benzylic halides
Reference tables (1)
Ambident nucleophiles and the reagent-to-product table10 rows
| Reagent with R–X | Attacking atom | Product | Class of product |
|---|---|---|---|
| Aqueous NaOH or KOH | O | Alcohol | |
| O | Ether (Williamson synthesis) | ||
| NaI in acetone | I | Alkyl iodide | |
| N | , then further alkylation | Amine | |
| KCN (alcoholic) | C | Nitrile (alkyl cyanide) | |
| AgCN | N | Isocyanide (isonitrile) | |
| O | Alkyl nitrite | ||
| N | Nitroalkane | ||
| O | Ester | ||
| H (hydride) | Alkane |
Watch out for (6)
- The solvent reverses the halide order→ Ranking nucleophiles: charge, basicity, size and solvent
- Basicity ranks only donors of the same atom→ Ranking nucleophiles: charge, basicity, size and solvent
- A strong base can be a poor nucleophile→ Ranking nucleophiles: charge, basicity, size and solvent
- Potassium nitrite gives the nitrite, silver nitrite the nitro compound→ Ambident nucleophiles and the reagent-to-product table
- AgCN is not ionic→ Ambident nucleophiles and the reagent-to-product table
- Aryl halogens survive while the side chain reacts→ Ambident nucleophiles and the reagent-to-product table
Formulas (2)
Watch out for (5)
- A bulky alkoxide gives the alkene, not the ether→ Substitution or elimination: reading the reagent, solvent and halide
- No β-hydrogen, no elimination→ Substitution or elimination: reading the reagent, solvent and halide
- Count cis and trans separately→ Zaitsev rule and counting the alkenes from dehydrohalogenation
- Conjugation beats the Zaitsev count→ Zaitsev rule and counting the alkenes from dehydrohalogenation
- A dihalide with excess base gives a diene→ Zaitsev rule and counting the alkenes from dehydrohalogenation
Formulas (1)
Reference tables (1)
Reactions of organic halides with magnesium and sodium8 rows
| Reactants | Conditions | Product | Name |
|---|---|---|---|
| R–X + Mg | Dry ether | R–MgX | Grignard reagent |
| R–MgX + | Any trace of water | R–H + Mg(OH)X | Hydrolysis: the reason the ether must be dry |
| R–MgX + | Heavy water | R–D | Deuterium labelling at the old C–X carbon |
| A dibromide + excess Mg | Dry ether | Both C–Br become C–MgBr | Di-Grignard reagent |
| 2 R–X + 2 Na | Dry ether | R–R | Wurtz reaction |
| + Zn or Na | Heat | Cyclopropane | Ring closure (intramolecular Wurtz) |
| ArX + RX + 2 Na | Dry ether | Ar–R | Wurtz-Fittig reaction |
| 2 ArX + 2 Na | Dry ether | Ar–Ar | Fittig reaction |
Watch out for (4)
- Halogens deactivate yet direct ortho and para→ Substitution on the haloarene ring: nucleophilic and electrophilic
- Aryl substitution is neither SN1 nor SN2→ Substitution on the haloarene ring: nucleophilic and electrophilic
- Two different halides give a mixture in the Wurtz reaction→ Reactions of organic halides with magnesium and sodium
- Water is not the only thing that destroys a Grignard reagent→ Reactions of organic halides with magnesium and sodium
PYQ weightage by concept
16 concepts · 92 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
16 concepts · 92 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| Types of organic halides and the aryl C–X bond | 5 | 5% |
| Boiling point, melting point, density and polarity of halides | 4 | 4% |
| Polyhalogen compounds: formulas and uses | 4 | 4% |
| Concept | PYQs | Share |
|---|---|---|
| Haloalkanes from alcohols, alkenes and hydrocarbons | 7 | 8% |
| Named reactions that make or couple organic halides | 4 | 4% |
| Concept | PYQs | Share |
|---|---|---|
| SN1 and SN2 compared: steps, rate law and conditions | 7 | 8% |
| Stereochemistry of substitution: inversion, racemisation and retention | 7 | 8% |
| Concept | PYQs | Share |
|---|---|---|
| SN1 reactivity: ranking halides by carbocation stability | 7 | 8% |
| SN2 reactivity: crowding, neighbouring groups and benzylic halides | 5 | 5% |
| Halides that cannot ionise: vinylic, aryl and bridgehead | 4 | 4% |
| Concept | PYQs | Share |
|---|---|---|
| Ambident nucleophiles and the reagent-to-product table | 8 | 9% |
| Ranking nucleophiles: charge, basicity, size and solvent | 5 | 5% |
| Concept | PYQs | Share |
|---|---|---|
| Substitution or elimination: reading the reagent, solvent and halide | 10 | 11% |
| Zaitsev rule and counting the alkenes from dehydrohalogenation | 5 | 5% |
| Concept | PYQs | Share |
|---|---|---|
| Substitution on the haloarene ring: nucleophilic and electrophilic | 7 | 8% |
| Reactions of organic halides with magnesium and sodium | 3 | 3% |
Test yourself on Haloalkanes and Haloarenes
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.