Lesson 13 of 13 · 16 min
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Must-know facts
18 facts
- 1s character: sp 50%, sp² 33%, sp³ 25%; electronegativity of carbon sp > sp² > sp³.
- 2Principal group priority: –COOH > –SO₃H > –COOR > –COCl > –CONH₂ > –CN > –CHO > >C=O > –OH > –NH₂ > C=C > C≡C.
- 3Halo, nitro and alkoxy groups are always prefixes.
- 4Multiplying prefixes (di, tri) are ignored when alphabetising substituents.
- 5Carbocations: sp², planar; carbanions: sp³, pyramidal.
- 6Carbocation stability: 3° > 2° > 1° > CH₃⁺.
- 7−I effect of halogens: F > Cl > Br > I; alkyl groups are +I.
- 8Inductive effect fades rapidly and is negligible beyond about three bonds.
- 9Electromeric effect is temporary and needs an attacking reagent; inductive and resonance effects are permanent.
- 10Hyperconjugation = σ-π conjugation = no-bond resonance; more α-hydrogens, more stabilisation.
- 11Benzene C–C bond length is 139 pm, between single and double bond lengths.
- 12Steam distillation: aniline; reduced-pressure distillation: glycerol from spent-lye; fractional distillation: crude oil.
- 13Rf = distance travelled by substance / distance travelled by solvent front.
- 14Lassaigne: Prussian blue for N, violet with nitroprusside and black PbS for S, blood red for N + S.
- 15AgCl white (soluble in NH₃), AgBr pale yellow (sparingly soluble), AgI yellow (insoluble).
- 16Kjeldahl fails for nitro, azo and ring nitrogen (pyridine).
- 17Carius method estimates halogens (as AgX) and sulphur (as BaSO₄).
- 18Oxygen percentage is found by difference.
Common traps
Where marks are lost
Taking the total number of carbons as the parent chain length.
Numbering to give a halogen a lower locant than the double bond.
Calling halogens +R so they must be electron-donating overall.
Treating the electromeric effect as permanent like the inductive effect.
Assuming inductive effect is transmitted all along a long chain.
Describing a carbanion as planar like a carbocation.
Thinking resonance structures interconvert rapidly.
Using Kjeldahl's method for nitrobenzene or pyridine.
Choosing simple distillation for liquids with close boiling points.
Forgetting to boil the Lassaigne extract with HNO₃ before adding AgNO₃.
Formulas
9 to know
Retardation factor
Rf = distance moved by substance from baseline / distance moved by solvent from baseline
Dimensionless; always less than 1.
Percentage of carbon
% C = (12 × m₁ × 100) / (44 × m)
m₁ = mass of CO₂ formed, m = mass of compound.
Percentage of hydrogen
% H = (2 × m₂ × 100) / (18 × m)
m₂ = mass of water formed.
Nitrogen by Dumas method
% N = (28 × V × 100) / (22400 × m)
V = volume of N₂ in mL corrected to 273 K and 760 mm Hg, using the pressure of dry gas (atmospheric pressure minus aqueous tension); 22400 mL is the volume of 28 g of N₂ under those conditions, the value NCERT's derivation uses. m = mass of compound in g.
Nitrogen by Kjeldahl's method
% N = 1.4 × M × 2 × (V − V₁/2) / m
Both the H₂SO₄ and the NaOH have molarity M. V = mL of H₂SO₄ taken; V₁ = mL of NaOH used to titrate the leftover acid, which matches V₁/2 mL of the acid; so (V − V₁/2) mL of acid reacted with ammonia, equal to 2(V − V₁/2) mL of M ammonia. m = mass of compound in g. The 1.4 comes from 14 × 100 / 1000.
Halogen by Carius method
% X = (atomic mass of X × m₁ × 100) / (molar mass of AgX × m)
m₁ = mass of AgX formed.
Sulphur by Carius method
% S = (32 × m₁ × 100) / (233 × m)
m₁ = mass of BaSO₄ (233 g mol⁻¹).
Phosphorus
% P = (31 × m₁ × 100) / (1877 × m) or (62 × m₁ × 100) / (222 × m)
First for ammonium phosphomolybdate (1877 g mol⁻¹), second for Mg₂P₂O₇ (222 g mol⁻¹).
Oxygen
% O = 100 − (sum of percentages of all other elements)
Found by difference.
Key terms
15 terms
- Functional group
- An atom or group that decides the characteristic reactions of a compound family.
- Homologous series
- Compounds with the same functional group differing successively by CH₂.
- Bond-line formula
- A skeletal drawing where line ends and bends are carbons and C–H hydrogens are implied.
- Metamerism
- Isomerism from different alkyl groups on the two sides of a functional group.
- Heterolytic cleavage
- Bond breaking in which one fragment keeps both electrons, producing ions.
- Homolytic cleavage
- Bond breaking in which each fragment keeps one electron, producing free radicals.
- Carbocation
- A positively charged carbon species with six valence electrons.
- Nucleophile
- An electron-pair donor that attacks electron-poor centres.
- Electrophile
- An electron-seeking species that attacks electron-rich centres.
- Inductive effect
- Permanent polarisation of σ bonds by an electronegative or electropositive group.
- Electromeric effect
- Temporary complete shift of π electrons in response to an attacking reagent.
- Hyperconjugation
- Delocalisation of alpha C–H σ electrons into an adjacent empty p orbital or π system.
- Resonance energy
- Energy by which the real molecule is more stable than its most stable contributing structure.
- Sodium fusion extract
- Aqueous extract of a compound fused with sodium, used to test for N, S, halogens and P.
- Chromatography
- Separation by differential distribution between a stationary and a mobile phase.