1 · p-Block Group 18 — Noble Gases
P2 · 1–2 Q, pure recallFamily facts worth exactly one mark each
- Members: He, Ne, Ar, Kr, Xe, Rn (Rn is radioactive, from ²²⁶Ra decay). Og is synthetic.
- Configuration ns²np⁶; helium alone is 1s².
- Argon is the most abundant noble gas in the atmosphere (≈0.934% by volume).
- Helium is the second most abundant element in the universe after hydrogen; on Earth it comes from natural gas / radioactive α-decay.
- All are monatomic gases, colourless, odourless, sparingly soluble in water.
- Highest ionisation enthalpy in each period; He has the highest IE of all elements.
- Electron gain enthalpy is large and positive — they resist gaining electrons.
- Only London dispersion forces → very low melting and boiling points, increasing down the group with atomic size.
- He has the lowest boiling point of any known substance (4.2 K); below 2.2 K it becomes a superfluid (helium-II).
- Atomic radii are the largest in each period because van der Waals radii are quoted, not covalent radii.
Uses — asked as matching questions
| Gas | Uses |
|---|---|
| He | Filling balloons/airships (non-inflammable, low density); He–O₂ mixture for deep-sea divers (low solubility in blood avoids bends); cryogenics and superconducting magnets in NMR/MRI; inert atmosphere for welding |
| Ne | Discharge tubes and neon signs, fluorescent bulbs, botanical gardens/warning signals |
| Ar | Inert atmosphere in high-temperature metallurgy and arc welding; filling electric bulbs with N₂; laboratory inert atmosphere |
| Kr, Xe | Light bulbs, flash lamps, Xe in bactericidal lamps; Kr used in the older definition of the metre |
| Rn | Radiotherapy for cancer |
Trap — abundance vs universe
"Most abundant noble gas" needs you to check whether the question says atmosphere (→ argon)
or universe (→ helium). Both statements are true in their own context; the option list will
contain the other one as a distractor.
2 · Xenon Compounds — structure & hybridisation
P1 · nearly guaranteedHistory and reactivity
Neil Bartlett (1962) noticed that O₂ and Xe have almost the same ionisation enthalpy. Having made O₂⁺[PtF₆]⁻, he reacted Xe with PtF₆ and obtained Xe⁺[PtF₆]⁻ — the first noble gas compound.
Only Xe (and to a small extent Kr, as KrF₂) forms real compounds, and only with the two most electronegative elements: fluorine and oxygen. Xe is chosen because it has the lowest ionisation enthalpy among the non-radioactive members.
Preparation of the fluorides — the ratios are the question
| Product | Xe : F₂ | Conditions |
|---|---|---|
| XeF₂ | 2 : 1 | 673 K, 1 bar |
| XeF₄ | 1 : 5 | 873 K, 7 bar |
| XeF₆ | 1 : 20 | 573 K, 60–70 bar |
Structures — the master table
| Compound | Bond pairs | Lone pairs | Hybridisation | Shape |
|---|---|---|---|---|
| XeF₂ | 2 | 3 | sp³d | Linear |
| XeF₄ | 4 | 2 | sp³d² | Square planar |
| XeF₆ | 6 | 1 | sp³d³ | Distorted octahedral |
| XeO₃ | 3 | 1 | sp³ | Pyramidal |
| XeOF₄ | 5 | 1 | sp³d² | Square pyramidal |
| XeO₂F₂ | 4 | 1 | sp³d | See-saw |
| XeO₄ | 4 | 0 | sp³ | Tetrahedral |
Hydrolysis — the reaction chain NEET asks about
Partial: XeF₆ + H₂O → XeOF₄ + 2HF
Further: XeF₆ + 2H₂O → XeO₂F₂ + 4HF
Complete: XeF₆ + 3H₂O → XeO₃ + 6HF
XeF₄ + H₂O → XeO₃ + Xe + O₂ + HF (disproportionation)
2XeF₂ + 2H₂O → 2Xe + 4HF + O₂ · XeF₆ + MF → M⁺[XeF₇]⁻ (M = Na, K, Rb, Cs)
XeF₂, XeF₄, XeF₆ are all powerful fluorinating agents and are readily hydrolysed even by traces of water.
Memory hook — count the waters
1 water → XeOF₄, 2 waters → XeO₂F₂, 3 waters → XeO₃. Each water swaps two F for one O.
3 · Electrochemistry — Cells & EMF
P1 · 3–4 Q| Galvanic (voltaic) cell | Electrolytic cell | |
|---|---|---|
| Energy | chemical → electrical | electrical → chemical |
| Reaction | spontaneous, ΔG < 0, Ecell > 0 | non-spontaneous, ΔG > 0 |
| Anode | negative, oxidation | positive, oxidation |
| Cathode | positive, reduction | negative, reduction |
| Example | Daniell cell, dry cell | electrolysis, electroplating, recharging |
Trap — the sign of the electrodes flips, oxidation does not
In both cells: anode = oxidation, cathode = reduction. Only the +/− labels swap.
Never memorise "anode is negative" alone.
Cell notation & salt bridge
Zn(s) | Zn²⁺(1 M) ‖ Cu²⁺(1 M) | Cu(s) E°cell = +1.10 V
Anode on the left (oxidation), cathode on the right, single bar = phase boundary, double bar = salt bridge.
Salt bridge (KCl, KNO₃, NH₄NO₃ in agar-agar): completes the circuit and maintains electrical neutrality in both half-cells, preventing accumulation of charge. The chosen salt has cation and anion of nearly equal transport numbers. It also minimises the liquid junction potential.
Standard electrode potential & SHE
- SHE: Pt(s) | H₂(1 bar) | H⁺(1 M), 298 K, E° = 0.00 V by definition.
- All E° values in NCERT are reduction potentials. More positive E° → stronger oxidising agent, greater tendency to be reduced.
- F₂/F⁻ = +2.87 V (strongest oxidising agent in the table); Li⁺/Li = −3.05 V (strongest reducing agent).
- E° is an intensive property — do not multiply it when you multiply the half-equation.
E°cell = E°cathode − E°anode = E°right − E°left (both as reduction potentials)
ΔG° = −nFE°cell · ΔG° = −2.303RT log Kc · E°cell = (0.059/n) log Kc at 298 K
Trap — n in ΔG° = −nFE°
n is the number of electrons actually transferred in the balanced cell reaction, not the charge on
one ion. For Zn + Cu²⁺ → Zn²⁺ + Cu, n = 2. For Cr₂O₇²⁻ → 2Cr³⁺, n = 6.
Aamirah — this n-factor slip has cost marks on more than one paper. Balance the electrons first, then substitute.
4 · Nernst Equation — live calculator
P1
Ecell = E°cell − (2.303RT/nF) log Q → at 298 K: E = E° − (0.059/n) log Q
For Zn + Cu²⁺ ⇌ Zn²⁺ + Cu: E = E° − (0.059/2) log([Zn²⁺]/[Cu²⁺])
At equilibrium Ecell = 0 and Q = Kc → the cell is dead.
Solids and pure liquids do not appear in Q. Only the products of the cell reaction over the reactants.
Daniell cell — change the concentrations
E°cell = +1.10 V, n = 2. Watch how weakly E depends on concentration — a ten-fold change moves it by only 0.0295 V.
log Q
—
E cell
—
ΔG
—
Spontaneous?
—
Memory hook — which way does E move?
Increase the reactant concentration (Cu²⁺) → reaction is pushed forward → E rises.
Increase the product (Zn²⁺) → E falls. Le Chatelier, translated into volts.
5 · Conductance of Electrolytic Solutions
P1
Conductance G = 1/R (siemens, S = ohm⁻¹ = mho)
Conductivity κ = G × (l/A) = G × cell constant (S m⁻¹ or S cm⁻¹)
Cell constant = l/A (m⁻¹) — found using a standard KCl solution
Molar conductivity Λm = κ/c = κ × 1000 / M (S cm² mol⁻¹, with c in mol L⁻¹)
Degree of dissociation α = Λm/Λ°m · Ka = cα²/(1 − α)
Kohlrausch: Λ°m = ν₊λ°₊ + ν₋λ°₋
| On dilution | Conductivity κ | Molar conductivity Λm |
|---|---|---|
| Trend | decreases | increases |
| Reason | fewer ions per unit volume | more ions per mole (greater dissociation / less interionic attraction) |
Λm versus √c — the shape that identifies the electrolyte
Strong electrolytes give a straight line you can extrapolate. Weak electrolytes shoot up near infinite dilution and cannot be extrapolated.
Kohlrausch's law of independent migration — what it is used for
- Λ°m of weak electrolytes, which cannot be measured directly:
Λ°(CH₃COOH) = Λ°(CH₃COONa) + Λ°(HCl) − Λ°(NaCl) - Degree of dissociation α = Λm/Λ°m
- Dissociation constant Ka = cα²/(1 − α)
- Solubility of sparingly soluble salts (AgCl, BaSO₄): S = κ × 1000 / Λ°m
Trap — unit conversion
Λm = κ × 1000/c only when κ is in S cm⁻¹ and c in mol L⁻¹, giving S cm² mol⁻¹.
If κ is in S m⁻¹ and c in mol m⁻³, then Λm = κ/c in S m² mol⁻¹.
1 S m² mol⁻¹ = 10⁴ S cm² mol⁻¹. Half the marks lost in this topic are unit slips, not concept slips.
6 · Electrolysis & Faraday's Laws
P1 · numerical
First law: m ∝ Q → m = Z I t (Z = electrochemical equivalent)
Second law: for the same Q, m ∝ equivalent mass
1 faraday F = 96487 ≈ 96500 C mol⁻¹ = charge of 1 mole of electrons
moles deposited = It / (n × F) · mass = (It/96500) × (M/n)
n here is the number of electrons per ion in the electrode half-reaction.
| Half reaction | n | 1 F deposits / liberates |
|---|---|---|
| Ag⁺ + e⁻ → Ag | 1 | 1 mol Ag = 108 g |
| Cu²⁺ + 2e⁻ → Cu | 2 | ½ mol Cu = 31.75 g |
| Al³⁺ + 3e⁻ → Al | 3 | ⅓ mol Al = 9 g |
| 2H⁺ + 2e⁻ → H₂ | 2 | ½ mol H₂ = 11.2 L at STP |
| 2H₂O → O₂ + 4H⁺ + 4e⁻ | 4 | ¼ mol O₂ = 5.6 L at STP |
Products of electrolysis — preferential discharge
- The species with the higher reduction potential is reduced at the cathode; the one with the lower reduction potential (easier to oxidise) is oxidised at the anode.
- Molten NaCl → Na at cathode, Cl₂ at anode.
- Aqueous NaCl → H₂ at cathode (water is reduced in preference to Na⁺), Cl₂ at anode (because of the overpotential of oxygen, chlorine wins even though water has the more favourable E°).
- Aqueous CuSO₄ with Pt electrodes → Cu at cathode, O₂ at anode.
- Aqueous CuSO₄ with Cu electrodes → Cu deposited at cathode, Cu dissolved at anode (electro-refining).
- Dilute H₂SO₄ → H₂ and O₂ in a 2:1 volume ratio.
7 · Batteries, Fuel Cells & Corrosion
P2 · recall| Cell | Anode | Cathode | Electrolyte | EMF / note |
|---|---|---|---|---|
| Dry (Leclanché) cell primary | Zn container | graphite rod surrounded by MnO₂ + C | moist paste of NH₄Cl + ZnCl₂ | ≈1.5 V, falls with use; not rechargeable |
| Mercury cell primary | Zn–Hg amalgam | HgO + carbon | paste of KOH + ZnO | 1.35 V, constant — hearing aids, watches |
| Lead storage battery secondary | Pb | PbO₂ on a lead grid | 38% H₂SO₄ | 2 V per cell; PbSO₄ forms at both electrodes on discharge; recharged by reversing the current |
| Ni–Cd cell secondary | Cd | NiO₂ / Ni(OH)₂ | KOH | longer life than lead storage, costlier |
| H₂–O₂ fuel cell | H₂ at porous carbon | O₂ at porous carbon | concentrated aqueous KOH | catalysts Pt / Pd / finely divided metal; efficiency ~70%; product is water; used in Apollo space programme |
Lead battery discharge: Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O
Fuel cell: anode 2H₂ + 4OH⁻ → 4H₂O + 4e⁻ · cathode O₂ + 2H₂O + 4e⁻ → 4OH⁻
Corrosion of iron — an electrochemical cell on the metal surface
- Anode (impure/strained spot): Fe → Fe²⁺ + 2e⁻, E° = −0.44 V
- Cathode: O₂ + 4H⁺ + 4e⁻ → 2H₂O, E° = +1.23 V (H⁺ from dissolved CO₂/acid)
- Fe²⁺ is further oxidised by atmospheric O₂ to Fe₂O₃·xH₂O — hydrated ferric oxide, rust
- Requires both moisture and oxygen; accelerated by salt and acidity
- Prevention: painting/greasing, galvanising with zinc, cathodic protection with a sacrificial Mg or Zn block (a metal with more negative E°), alloying to stainless steel
8 · Coordination Compounds — Werner & terminology
P1 · 3–4 QWerner's theory
- Primary valence = oxidation number; satisfied by negative ions; ionisable; non-directional.
- Secondary valence = coordination number; satisfied by ligands; non-ionisable; directional — it fixes the geometry.
- Ligands inside the coordination sphere are written in [ ] and do not give a precipitate with AgNO₃.
| Complex | Ionisable Cl⁻ | Mol of AgCl with excess AgNO₃ | Ions in solution |
|---|---|---|---|
| [Co(NH₃)₆]Cl₃ | 3 | 3 | 4 |
| [Co(NH₃)₅Cl]Cl₂ | 2 | 2 | 3 |
| [Co(NH₃)₄Cl₂]Cl | 1 | 1 | 2 |
| [Co(NH₃)₃Cl₃] | 0 | 0 | 0 (non-electrolyte) |
Ligand vocabulary
| Type | Meaning | Examples |
|---|---|---|
| Monodentate | one donor atom | NH₃, H₂O, Cl⁻, CN⁻, CO, NO₂⁻ |
| Bidentate | two donor atoms | en (ethane-1,2-diamine), ox (oxalate C₂O₄²⁻), gly |
| Hexadentate | six donor atoms | EDTA⁴⁻ (2 N + 4 O) |
| Ambidentate | two possible donor atoms, only one binds at a time | NO₂⁻ (via N = nitrito-N, via O = nitrito-O), SCN⁻ (thiocyanato-S / isothiocyanato-N), CN⁻ |
| Chelate | a ring formed by a polydentate ligand — extra stability (chelate effect) | [Co(en)₃]³⁺, EDTA complexes |
Trap — ambidentate ≠ bidentate
An ambidentate ligand has two possible donor atoms but uses only one — it is monodentate in
practice and gives linkage isomerism. A bidentate ligand uses both and forms a chelate ring.
Applications — the named-complex list
| Complex / compound | Metal | Role |
|---|---|---|
| Chlorophyll | Mg | photosynthesis |
| Haemoglobin | Fe | oxygen transport |
| Vitamin B₁₂ (cyanocobalamin) | Co | anti-pernicious anaemia |
| cis-[Pt(NH₃)₂Cl₂] — cisplatin | Pt | anti-cancer (trans isomer is inactive) |
| EDTA complexes | Ca, Mg | water hardness estimation, lead poisoning treatment |
| Wilkinson's catalyst [(Ph₃P)₃RhCl] | Rh | hydrogenation of alkenes |
| [Ag(CN)₂]⁻, [Au(CN)₂]⁻ | Ag, Au | extraction (Mac Arthur–Forrest cyanide process) |
| [Ni(CO)₄] | Ni | Mond process for pure nickel |
| Na₂[Fe(CN)₅NO], Fe₄[Fe(CN)₆]₃ | Fe | qualitative analysis — Prussian blue |
9 · IUPAC Nomenclature of Complexes
P1The rules, in order
- Cation first, anion second — whether or not the complex is the cation.
- Inside the sphere: ligands first (alphabetical order), then the metal.
- Prefixes di, tri, tetra for simple ligands; bis, tris, tetrakis for ligands whose names already contain a numeral (en, ox, and anything in brackets). Prefixes are ignored when alphabetising.
- Anionic ligands end in -o (chlorido, cyanido, hydroxido, oxalato, sulphato, nitrito-N).
- Neutral ligands keep their names, with four exceptions: aqua (H₂O), ammine (NH₃), carbonyl (CO), nitrosyl (NO).
- Oxidation state of the metal in Roman numerals in parentheses.
- If the complex is an anion, the metal name takes -ate — and often the Latin stem: ferrate (Fe), cuprate (Cu), argentate (Ag), aurate (Au), plumbate (Pb), stannate (Sn), nickelate (Ni), cobaltate (Co).
- The name of the whole complex ion or molecule is written as one word.
| Formula | IUPAC name |
|---|---|
| [Co(NH₃)₆]Cl₃ | Hexaamminecobalt(III) chloride |
| [CoCl₂(en)₂]Cl | Dichloridobis(ethane-1,2-diamine)cobalt(III) chloride |
| K₃[Fe(CN)₆] | Potassium hexacyanidoferrate(III) |
| K₄[Fe(CN)₆] | Potassium hexacyanidoferrate(II) |
| [Ni(CO)₄] | Tetracarbonylnickel(0) |
| [Pt(NH₃)₂Cl(NO₂)] | Diamminechlorido(nitrito-N)platinum(II) |
| [Cr(H₂O)₄Cl₂]⁺ | Tetraaquadichloridochromium(III) ion |
| [Ag(NH₃)₂][Ag(CN)₂] | Diamminesilver(I) dicyanidoargentate(I) |
| Fe₄[Fe(CN)₆]₃ | Iron(III) hexacyanidoferrate(II) |
Memory hook — oxidation state in three seconds
Overall charge = (metal O.S.) + (sum of ligand charges). In K₃[Fe(CN)₆]: the complex ion is 3−,
six CN⁻ contribute 6−, so Fe = +3. Neutral ligands (NH₃, H₂O, CO, en) contribute zero.
10 · Isomerism in Coordination Compounds
P1A · Structural isomerism
| Type | What differs | Example pair |
|---|---|---|
| Ionisation | which ion is inside vs outside the sphere | [Co(NH₃)₅SO₄]Br / [Co(NH₃)₅Br]SO₄ |
| Hydrate (solvate) | water inside vs as water of crystallisation | [Cr(H₂O)₆]Cl₃ (violet) / [Cr(H₂O)₅Cl]Cl₂·H₂O / [Cr(H₂O)₄Cl₂]Cl·2H₂O |
| Linkage | which atom of an ambidentate ligand binds | [Co(NH₃)₅(NO₂)]²⁺ / [Co(NH₃)₅(ONO)]²⁺ |
| Coordination | ligands swapped between a complex cation and a complex anion | [Co(NH₃)₆][Cr(CN)₆] / [Cr(NH₃)₆][Co(CN)₆] |
B · Stereoisomerism
| Formula type | Geometry | Geometrical isomers | Optical activity |
|---|---|---|---|
| MA₂B₂ | square planar | 2 — cis and trans | none (has a plane of symmetry) |
| MA₂B₂ | tetrahedral | 0 — all positions adjacent | none |
| MA₄B₂ | octahedral | 2 — cis and trans | none |
| MA₃B₃ | octahedral | 2 — fac and mer | none |
| [M(AA)₂X₂] | octahedral | 2 — cis and trans | cis is optically active (d and l); trans is inactive |
| [M(AA)₃] | octahedral | — | optically active — d and l forms, e.g. [Co(en)₃]³⁺ |
| MABCD | tetrahedral | — | optically active (no symmetry) |
11 · VBT, CFT & Magnetic Moment — live splitting diagram
P1 · the money question
Spin-only magnetic moment μ = √[n(n + 2)] BM, n = number of unpaired electrons
n = 1 → 1.73 n = 2 → 2.83 n = 3 → 3.87 n = 4 → 4.90 n = 5 → 5.92 BM
Δt = (4/9) Δo → tetrahedral complexes are always high spin
CFSE (octahedral) = (−0.4 nt2g + 0.6 neg) Δo
Crystal field splitting — build the configuration yourself
Choose the d-electron count, the geometry, and the ligand strength. The boxes fill by Hund's rule when the field is weak, and pair up when the field is strong.
Unpaired e⁻
—
μ (spin only)
—
Magnetism
—
CFSE
—
Spectrochemical series — learn the two ends
I⁻ < Br⁻ < SCN⁻ < Cl⁻ < S²⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < edta⁴⁻ < NH₃ < en < NO₂⁻ < CN⁻ < CO
Left = weak field, small Δ, high spin. Right = strong field, large Δ, low spin.
CO and CN⁻ are the strongest; I⁻ is the weakest.
VBT — inner vs outer orbital complexes
| Inner orbital (low spin) | Outer orbital (high spin) | |
|---|---|---|
| Hybridisation | d²sp³ — uses (n−1)d | sp³d² — uses nd |
| Ligand | strong field (CN⁻, NH₃, NO₂⁻, CO) | weak field (F⁻, Cl⁻, H₂O) |
| Magnetism | usually diamagnetic / fewer unpaired e⁻ | paramagnetic, maximum unpaired e⁻ |
| Example | [Co(NH₃)₆]³⁺, [Fe(CN)₆]³⁻ | [CoF₆]³⁻, [Fe(H₂O)₆]³⁺ |
Tetrahedral → sp³ (4 coordination). Square planar → dsp², always low spin; d⁸ metals like Ni²⁺ with strong ligands, and Pt²⁺/Pd²⁺ always. [Ni(CN)₄]²⁻ is square planar and diamagnetic; [NiCl₄]²⁻ is tetrahedral and paramagnetic — a favourite comparison.
Trap — colour
Colour arises from d–d transitions, so a complex is coloured only when the d-subshell is
partially filled. d⁰ (Sc³⁺, Ti⁴⁺) and d¹⁰ (Zn²⁺, Cu⁺, Ag⁺, Cd²⁺) complexes are colourless.
The colour you see is the complementary colour of the light absorbed.
Limitations you should be able to state
VBT does not explain colour, does not predict the magnitude of magnetic data quantitatively,
and gives no reason for the existence of strong and weak ligands.
CFT treats the bond as purely ionic, ignoring the covalent character shown by the spectrochemical
series, and cannot explain why CO is such a strong-field ligand (that needs synergic π back-bonding).
12 · Haloalkanes & Haloarenes — portion subtopics only
P1 · 14 Q sectionA · Classification
| Basis | Classes |
|---|---|
| Number of halogens | mono, di (gem-dihalide = same carbon, e.g. ethylidene chloride; vic-dihalide = adjacent carbons, e.g. ethylene dichloride), tri (CHCl₃), poly |
| Hybridisation of C bearing X | sp³: alkyl (1°, 2°, 3°), allylic, benzylic · sp²: vinylic, aryl · acyl (RCOX) |
| Type | Structure | Definition |
|---|---|---|
| Allylic | CH₂=CH–CH₂–X | X on the sp³ carbon next to a C=C |
| Vinylic | CH₂=CH–X | X directly on the sp² carbon of C=C |
| Benzylic | C₆H₅–CH₂–X | X on the sp³ carbon attached to the ring |
| Aryl | C₆H₅–X | X directly on the sp² ring carbon |
Memory hook — one carbon decides everything
If X sits on the sp² carbon (vinyl, aryl) it is sluggish. If X sits one carbon away
from the unsaturation (allyl, benzyl) it is super-reactive. Same atoms, opposite behaviour.
B · Nomenclature
| Structure | Common name | IUPAC name |
|---|---|---|
| CH₃Cl | Methyl chloride | Chloromethane |
| (CH₃)₂CHBr | Isopropyl bromide | 2-Bromopropane |
| (CH₃)₃CCl | tert-Butyl chloride | 2-Chloro-2-methylpropane |
| CH₂Cl₂ | Methylene chloride | Dichloromethane |
| CHCl₃ | Chloroform | Trichloromethane |
| CCl₄ | Carbon tetrachloride | Tetrachloromethane |
| CHI₃ | Iodoform | Triiodomethane |
| CH₃CHCl–CH₃ | — | 2-Chloropropane |
| CH₂Cl–CH₂Cl | Ethylene dichloride | 1,2-Dichloroethane |
| CH₃CHCl₂ | Ethylidene chloride | 1,1-Dichloroethane |
| C₆H₅CH₂Br | Benzyl bromide | 1-(Bromomethyl)benzene |
Numbering rules: halogens are treated as substituents — they never get priority over a principal functional group. Lowest locant goes to the substituent set as a whole; when there is a tie, alphabetical order decides (bromo before chloro before iodo before methyl).
C · Nature of the C–X bond
Halogen is more electronegative than carbon → the bond is polar covalent, with C carrying δ+ and X carrying δ−. The δ+ carbon is the site attacked by nucleophiles — this single fact explains the whole chapter.
| Property | Order | Why |
|---|---|---|
| C–X bond length | C–F < C–Cl < C–Br < C–I | halogen size increases down the group |
| C–X bond enthalpy | C–F > C–Cl > C–Br > C–I | longer bond = weaker bond |
| Reactivity towards nucleophiles | R–I > R–Br > R–Cl > R–F | weakest bond breaks first — opposite to electronegativity |
| Dipole moment | CH₃Cl > CH₃F > CH₃Br > CH₃I | μ = charge × distance — F's high electronegativity is offset by its very short bond |
| Boiling point | RI > RBr > RCl > RF | larger molecule = stronger dispersion forces |
| Boiling point among isomers | straight chain > branched | branching reduces surface contact |
| Density | RI > RBr > RCl | all haloalkanes are denser than water except some chlorides/fluorides |
Trap — the dipole moment order
CH₃Cl > CH₃F, not the other way round. Fluorine is the most electronegative, but the C–F bond is
so short that the charge × distance product is smaller. This exact ordering appears every couple of years.
D · Electrophile and nucleophile
| Nucleophile | Electrophile | |
|---|---|---|
| Meaning | "nucleus loving" — electron rich, donates an electron pair | "electron loving" — electron deficient, accepts an electron pair |
| Charge | anion or neutral with a lone pair | cation or neutral with an incomplete octet |
| Lewis classification | Lewis base | Lewis acid |
| Attacks | the δ+ carbon | the δ− / π-electron-rich site |
| Examples | OH⁻, CN⁻, NH₃, H₂O, RO⁻, RS⁻, X⁻, R⁻ | H⁺, NO₂⁺, Cl⁺, BF₃, AlCl₃, R₃C⁺, carbonyl carbon |
In a haloalkane, the carbon is the electrophilic centre and the halogen leaves as the nucleophile X⁻. Ambident nucleophiles like CN⁻ can attack through C (giving nitriles with KCN, which is ionic) or through N (giving isocyanides with AgCN, which is covalent).
E · Nucleophilicity and leaving group ability
- Basicity = affinity for H⁺ (thermodynamic). Nucleophilicity = affinity for carbon (kinetic). They are not the same thing.
- Down a group in a protic solvent (water, alcohol): I⁻ > Br⁻ > Cl⁻ > F⁻ — the small F⁻ is heavily solvated by hydrogen bonding and is caged.
- In a polar aprotic solvent (DMSO, DMF, acetone) the order reverses to F⁻ > Cl⁻ > Br⁻ > I⁻, because there is no H-bond cage and basicity takes over.
- For the same attacking atom, the conjugate base is the better nucleophile: OH⁻ > H₂O, RO⁻ > ROH, NH₂⁻ > NH₃.
- Leaving group ability: the weaker the base, the better the leaving group. I⁻ > Br⁻ > Cl⁻ ≫ F⁻. OH⁻, NH₂⁻ and OR⁻ are strong bases and therefore very poor leaving groups — which is why alcohols must be protonated (to leave as H₂O) before substitution.
Trap — a good nucleophile is not always a good leaving group
F⁻ is a strong base: a poor leaving group but a good nucleophile in aprotic media.
I⁻ is a weak base: an excellent leaving group and also a good nucleophile in protic media
(which is why iodide catalyses many substitutions).
F · Why vinyl and aryl halides barely react
Four reasons vinyl and aryl halides resist nucleophilic substitution
- Resonance. A lone pair on the halogen is delocalised into the π system, giving the C–X bond partial double-bond character. It is shorter and stronger, so it will not break easily.
- Hybridisation. The carbon is sp², which has 33% s-character (vs 25% for sp³). It holds the bonding electrons more tightly, so the bond is shorter and stronger and the carbon is less δ+.
- Unstable intermediate. An SN1 route would need a vinyl or phenyl carbocation, which is very high in energy because the empty orbital cannot be stabilised by the π system.
- Steric / electronic repulsion. The π electron cloud repels the incoming nucleophile, blocking backside attack, so SN2 is also shut down.
Aryl halides can be substituted, but only under forcing conditions (e.g. NaOH at 623 K and 300 atm) or when strong electron-withdrawing groups sit ortho/para to the halogen — as in 2,4-dinitrochlorobenzene, which reacts under mild conditions. A meta-nitro group does not help, because the negative charge of the intermediate cannot reach it.
G · Why allyl and benzyl halides react so fast
Both form resonance-stabilised carbocations on ionisation, so the SN1 route is unusually easy. The allyl cation is stabilised over two carbons; the benzyl cation is delocalised into the whole ring (and further stabilised in di- and triphenylmethyl systems).
SN1 rate: benzyl ≈ allyl > 3° > 2° > 1° > CH₃ ≫ vinyl, aryl
SN2 rate: CH₃ > 1° > 2° > 3° (steric crowding blocks backside attack)
Carbocation stability: benzyl (3°>2°>1°) > allyl > 3° alkyl > 2° > 1° > methyl > vinyl
| Halide | C bearing X | SN reactivity | Reason in one line |
|---|---|---|---|
| Benzyl / allyl | sp³ | Very high | resonance-stabilised carbocation → fast SN1 |
| 3° alkyl | sp³ | High (SN1) | hyperconjugation + inductive stabilisation |
| 1° alkyl / methyl | sp³ | High (SN2) | least steric hindrance to backside attack |
| Vinyl / aryl | sp² | Very low | partial double-bond character + no stable cation |
Trap — testing with AgNO₃
Benzyl and allyl halides give an immediate precipitate of AgX with alcoholic AgNO₃ at room
temperature; 3° alkyl halides react quickly, 1° halides only on warming, and vinyl/aryl halides give no
precipitate at all. This is the standard distinguishing test in assertion–reason questions.
Scope reminder for this paper
Only the seven listed subtopics are in the 14-question Haloalkanes block. Full SN1/SN2 mechanism
stereochemistry, elimination (Saytzeff), Wurtz/Fittig/Sandmeyer reactions and polyhalogen compounds are
not listed — read them only if time remains after the four chapters above.
60-Second Recall — read this in the car
Most abundant noble gasatmosphere Ar · universe He
First noble gas compoundXe⁺[PtF₆]⁻, Bartlett 1962
XeF₂ / XeF₄ / XeF₆linear · square planar · distorted oct.
XeO₃ / XeOF₄ / XeO₂F₂pyramidal · square pyramidal · see-saw
Cell EMFE° = E°cathode − E°anode
Free energyΔG° = −nFE° = −2.303RT log K
Nernst at 298 KE = E° − (0.059/n) log Q
Molar conductivityΛm = κ×1000/M; dilution ↑Λm, ↓κ
Faraday1 F = 96500 C = 1 mol e⁻
Lead batteryPb | PbO₂ | 38% H₂SO₄, PbSO₄ both sides
Fuel cellKOH, Pt/Pd, ~70% efficient
RustFe₂O₃·xH₂O; needs O₂ + moisture
Wernerprimary = ionisable · secondary = directional
Magnetic momentμ = √n(n+2) BM
Tetrahedral splittingΔt = 4/9 Δo → always high spin
Strongest / weakest ligandCO, CN⁻ / I⁻
Colourless complexesd⁰ and d¹⁰
Optically activecis-[M(AA)₂X₂], [M(AA)₃]
C–X reactivityR–I > R–Br > R–Cl > R–F
Dipole momentCH₃Cl > CH₃F > CH₃Br > CH₃I
Leaving groupweaker base = better LG · I⁻ best
Unreactivevinyl, aryl (sp², resonance)
Hyper-reactiveallyl, benzyl (stable carbocation)