Organic Chemistry - Some Basic Principles and Techniques

2.1 Core concepts

• Organic chemistry deals with carbon compounds; the unique property of carbon to form long chains, branched chains and rings by C–C bonding is called catenation, and explains the millions of organic compounds known. Carbon's small size, tetravalence and ability to form strong σ-bonds with itself and with H, N, O, S, halogens make catenation maximum among all elements (NCERT §8.1, p. 256).
• Wöhler's synthesis (1828) of urea by heating ammonium cyanate (NH₄CNO → NH₂CONH₂) and Kolbe's synthesis of acetic acid (1845) disproved the vital-force theory and showed organic compounds can be made from inorganic ones — laying the basis for modern organic chemistry as a branch of structure-based science rather than "chemistry of living things" (NCERT §8.1, p. 256).
• Carbon is tetravalent; in saturated compounds it is sp³ hybridised (tetrahedral, 109.5° bond angle, equivalent C–H bonds in CH₄), in alkenes/C=C it is sp² (trigonal planar, 120°, one π-bond), and in alkynes/C≡C and C≡N it is sp (linear, 180°, two π-bonds) (NCERT §8.2.1, p. 257).
• A C–C single bond is longer (≈154 pm) and weaker than C=C (≈134 pm) which in turn is longer than C≡C (≈120 pm); s-character increases sp³ < sp² < sp, so bonds become shorter and stronger and the carbon becomes more electronegative (a vinyl/aryl/alkynyl carbon is more electronegative than alkyl) (NCERT §8.2.2, p. 257).
• Structural formulas have three forms: complete (every bond drawn, useful for showing all atoms), condensed (e.g. CH₃CH₂OH, (CH₃)₂CHCH₃, omits some C–H bonds) and bond-line (zig-zag, terminals are CH₃, vertices are carbons, H atoms implicit — quickest representation for complex skeletons) (NCERT §8.3.1, p. 258).
• Three-dimensional structure is shown by wedge-dash notation — solid wedge for a bond projecting towards the viewer, dashed/broken wedge for one going away, and ordinary line for bonds in the plane of the paper; equivalent to Newman, sawhorse and Fischer projections used elsewhere (NCERT §8.3.2, p. 260).
• Organic compounds are classified as acyclic (open-chain or aliphatic) or cyclic (closed-chain); cyclic compounds split into alicyclic (cyclopropane, cyclohexane, tetrahydrofuran — properties resemble open-chain analogues) and aromatic (benzenoid like benzene/aniline/naphthalene/anthracene and non-benzenoid like tropone — show special π-electron stability, Hückel's 4n+2 rule) (NCERT §8.4, p. 261).
• Cyclic compounds with at least one heteroatom (O, N, S) in the ring are heterocyclic (furan O, thiophene S, pyridine N); these include both alicyclic (tetrahydrofuran THF) and aromatic (pyridine, furan, thiophene — all 6-π aromatic) types (NCERT §8.4, p. 262).
• A functional group (–OH, –CHO, >C=O, –COOH, –NH₂, –NO₂, –C≡N, halogen etc.) determines the chemical properties; compounds with the same functional group differing by –CH₂– form a homologous series with regularly graded physical properties (e.g., alcohols methanol → ethanol → propanol show steady rise in bp) (NCERT §8.4.1–§8.4.2, p. 262).
• IUPAC nomenclature (1957, IUPAC) names a compound by combining a root word indicating number of carbons (meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec-) with a primary suffix for saturation/unsaturation (-ane, -ene, -yne) and a secondary suffix/prefix for functional groups (NCERT §8.5, p. 263; Table 8.2 p. 263, Table 8.4 p. 267).
• For a branched alkane: select the longest carbon chain as the parent, number from the end that gives the lowest set of locants to substituents, name substituents alphabetically with locants and join (e.g. 2,5-dimethylhexane). Apply "first point of difference" rule when comparing two locant sets (NCERT §8.5.1, p. 263–264).
• For alkenes/alkynes, give the C=C/C≡C the lowest locant; cyclic compounds use the prefix cyclo- before the alkane/alkene root (cyclopropane, cyclohexene); compounds with both C=C and C≡C are "enynes" with C=C taking precedence in numbering (NCERT §8.5.2, p. 264–265).
• Substituted benzenes are named by giving the substituent locants the lowest set; for disubstituted benzenes, the prefixes ortho- (1,2-), meta- (1,3-), para- (1,4-) are also used. Some common (trivial) names — toluene, aniline, phenol, anisole — are retained by IUPAC (NCERT §8.5.4, p. 268–269).
• Isomerism — same molecular formula, different structure. Structural isomerism = chain, position, functional, metamerism, tautomerism; stereoisomerism = geometrical (cis-trans/E-Z) and optical (NCERT §8.6, p. 270; flowchart p. 270). Tautomerism is a dynamic interconversion (e.g., keto ⇌ enol).
• Fission of a covalent bond: homolytic (each fragment takes one electron, gives free radicals, shown by single-headed "fish-hook" arrow) vs heterolytic (one fragment takes both electrons, gives a cation + anion / carbocation + nucleophile, shown by double-headed arrow). Homolytic fission predominates in non-polar conditions (gas phase, hν, peroxides); heterolytic in polar conditions (NCERT §8.7.1, p. 271).
• Carbocations are sp² hybridised, planar, electron-deficient with an empty p-orbital; carbanions are sp³, pyramidal with a lone pair; stability of carbocations follows 3° > 2° > 1° > methyl, opposite for carbanions (alkyl groups destabilise carbanion via +I effect) (NCERT §8.7.1, p. 271–272).
• Nucleophiles are electron-rich species (lone pair or –ve charge: OH⁻, CN⁻, RO⁻, H₂O, NH₃, R₂NH) that attack electron-poor carbons; electrophiles are electron-deficient (carbocations, AlCl₃, BF₃, ⁺NO₂, ⁺SO₃H, ⁺Cl) and attack electron-rich sites (alkenes, aromatics) (NCERT §8.7.2, p. 272–273).
• Electron displacement effects (NCERT §8.7.3):
• Inductive (I) effect — permanent, transmitted through σ-bonds, dies out beyond 3rd carbon. −I groups (–NO₂, –CN, –COOH, halogens, –OH, –NH₂…) withdraw electrons; +I groups (alkyl, –COO⁻) push electrons (p. 274).
• Resonance (M/R) effect — π/lone-pair delocalisation; +R groups (–OH, –OR, –NH₂, –Cl) donate electrons by resonance, −R groups (–NO₂, –CN, –CHO, –COOH, –COR) withdraw (p. 275–276). On benzene, +R groups are o/p-directing; −R groups are m-directing.
• Electromeric (E) effect — temporary, occurs in presence of attacking reagent at a multiple bond; +E (alkenes towards H⁺) and –E (towards CN⁻ etc.) (p. 277).
• Hyperconjugation — delocalisation of σ-electrons of a C–H bond α to an sp² carbon (alkene or carbocation) into the empty p / π* orbital; explains stability order of carbocations (3° > 2° > 1° > methyl), Saytzeff orientation of alkene products, and the order of alkene stability (more α-H = more stable alkene) (p. 277–278).
• Types of organic reactions — substitution (SN1/SN2 ; SE Ar in aromatics), addition (Markovnikov, anti-Markovnikov), elimination (E1/E2), rearrangement (Wagner-Meerwein); detailed treatment in Class XII (NCERT §8.7.10, p. 278).
• Methods of purification (NCERT §8.8, p. 278) match the property used: sublimation (solid ⇌ vapour, e.g. naphthalene, camphor, NH₄Cl, anthracene), crystallisation (different solubility in a solvent or solvent pair), distillation (different b.p.), differential extraction (different solubility in two immiscible solvents), chromatography (different adsorption or partition between phases).
• Distillation variants — simple distillation for liquids with appreciable b.p. difference (>25 K); fractional distillation (fractionating column with glass beads/Vigreux) for liquids with close b.p. (industrial separation of petroleum); steam distillation for water-insoluble, steam-volatile compounds (e.g. aniline, o-nitrophenol, essential oils), works because P(total) = P(steam) + P(compound) reaches atmospheric below 100 °C; distillation under reduced pressure for liquids that decompose below their b.p. (e.g. glycerol, b.p. 290 °C) (NCERT §8.8.3, p. 279–281).
• Chromatography — separates by differential adsorption (adsorption chromatography: column on alumina/silica, TLC plate of glass coated with adsorbent); or differential partition (paper chromatography uses moisture as stationary phase). In TLC, retardation factor R_f = (distance moved by substance)/(distance moved by solvent front) is always ≤ 1 and characteristic of a compound under given conditions (NCERT §8.8.5, p. 282–284).
• Detection of elements (qualitative) — C and H by heating with dry CuO (gives CO₂ that turns lime water milky; H₂O that turns anhydrous CuSO₄ blue). N, S, halogens by Lassaigne's test (fuse organic with Na metal → covalent N, S, X become ionic NaCN, Na₂S, NaX, easy to detect) (NCERT §8.9, p. 284–285).
• N: Na fusion extract + FeSO₄ + dil H₂SO₄ → Prussian blue (Fe₄[Fe(CN)₆]₃).
• S: extract + sodium nitroprusside → violet (Na₂[Fe(CN)₅NOS]); or + lead acetate → black PbS.
• Halogens: extract + AgNO₃ → white AgCl (soluble in NH₃), pale-yellow AgBr (sparingly soluble in NH₃), yellow AgI (insoluble in NH₃) — the colour and solubility of AgX distinguishes Cl/Br/I.
• N + S together: violet/blood-red colouration from NaSCN + FeCl₃ → Fe(SCN)₃.
• Quantitative estimation (NCERT §8.10, p. 285):
• C and H – Liebig's method: known mass burnt in excess O₂ over hot CuO → CO₂ absorbed in KOH (mass gain measured), H₂O in anhydrous CaCl₂; % C = (12/44)×(mass CO₂/mass compound)×100, % H = (2/18)×(mass H₂O/mass compound)×100.
• N – Dumas' method: combustion with CuO in CO₂ atmosphere → N₂ collected over KOH (other gases absorbed); % N = (28/22400)×(V at STP/m)×100.
• N – Kjeldahl's method: digest with conc H₂SO₄ + K₂SO₄ + CuSO₄ catalyst → (NH₄)₂SO₄, distil with NaOH → NH₃ absorbed in known volume of standard H₂SO₄; the unused acid is back-titrated with NaOH; % N = (1.4 × N(acid) × V(acid used by NH₃))/mass; not applicable to N in ring (pyridine), –NO₂ or azo compounds.
• Halogen – Carius method: heat with fuming HNO₃ and AgNO₃ in a sealed Carius tube → AgX is weighed; % X = (atomic mass X / mol mass AgX) × (mass AgX / mass compound) × 100.
• Sulphur – Carius method: oxidise with fuming HNO₃ → H₂SO₄, precipitate as BaSO₄; % S = (32/233) × (mass BaSO₄ / mass compound) × 100.
• Phosphorus: oxidise → H₃PO₄, precipitate as MgNH₄PO₄, ignite to Mg₂P₂O₇; % P = (62/222) × (mass Mg₂P₂O₇ / mass compound) × 100.
• Oxygen: usually obtained by difference, % O = 100 − Σ(% of all other elements).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Fig. 8.1 — wedge/dash 3-D representation of CH₄ (p. 260): central carbon, four sp³ orbitals, two bonds in plane (lines), one solid wedge (toward viewer), one dashed wedge (away); explains tetrahedral geometry at a glance.
• Fig. 8.2 — framework, ball-and-stick, space-filling models of methane (p. 260): three complementary 3-D depictions showing increasing levels of detail; emphasises that all four H atoms are equivalent.
• Classification flowchart of organic compounds (p. 261): branching tree from "organic compounds" → acyclic/cyclic; cyclic → alicyclic/aromatic; aromatic → benzenoid/non-benzenoid; with examples at each leaf.
• Functional-group table (Table 8.4, p. 267): for every common class — alcohol, ether, aldehyde, ketone, carboxylic acid, ester, amide, amine, nitrile, nitro, halide — gives IUPAC prefix, suffix and example. Use this as the master nomenclature key.
• Flowchart of isomerism: structural vs stereo (p. 270): structural (chain, position, functional, metamerism, tautomerism) and stereo (geometrical, optical). Quick reference for MCQ identification.
• Carbocation pyramid (sp², empty p) and carbanion (sp³, lone pair) shapes (Figs. 8.3a/8.3b, p. 272): carbocation drawn as planar trigonal with empty p-orbital perpendicular; carbanion as pyramidal sp³ with lone pair in the fourth orbital — visually highlights why nucleophiles attack carbocation perpendicular to plane.
• Orbital picture of hyperconjugation in CH₃CH₂⁺ (Fig. 8.4(a), p. 277) and propene (Fig. 8.4(b), p. 277): σ C–H orbital overlapping the empty p-orbital of the adjacent C⁺ (or the π* of C=C); shows the "no-bond resonance" canonical form (H⁺ ... C=C–C–H...).
• Fig. 8.5 — simple distillation apparatus (p. 279): distillation flask, thermometer, condenser, receiver; for liquids with >25 K bp difference.
• Fig. 8.6 — fractional distillation column (p. 280): packed column above the flask provides many "plates" of repeated condensation/evaporation; needed for close-boiling liquids like petroleum fractions.
• Fig. 8.7 — fractionating columns of different designs (p. 280): Vigreux, glass-bead, plate columns.
• Fig. 8.8 — distillation under reduced pressure (p. 281): vacuum pump lowers boiling temperature, prevents decomposition of heat-sensitive liquids like glycerol.
• Fig. 8.9 — steam distillation (p. 282): steam generator + flask + condenser; principle: P(total) = P(steam) + P(compound) reaches atmospheric below 100 °C.
• Fig. 8.10 — differential extraction in a separating funnel (p. 282): aqueous + organic layers, repeated extractions move solute to organic phase.
• Figs. 8.11–8.13 — column chromatography, TLC plate and developed chromatogram (p. 283–284): column (gravity), TLC (capillary action), developed plate with separated spots and R_f calculation.
• Fig. 8.14 — Liebig's combustion apparatus for C and H (p. 286): O₂ flow, CuO furnace, anhydrous CaCl₂ tube (H₂O absorber), KOH tube (CO₂ absorber); mass gain measures each element.
• Fig. 8.15 — Dumas' apparatus for N (p. 287): combustion in CO₂ atmosphere; N₂ collected over KOH solution (KOH absorbs CO₂); volume of N₂ at STP gives % N.
• Fig. 8.16 — Kjeldahl's apparatus (p. 288): digestion flask + distillation set-up; NH₃ is steam-distilled into known H₂SO₄ and the excess back-titrated with NaOH.
• Fig. 8.17 — Carius tube for halogens/sulphur (p. 289): sealed glass tube heated in a Carius furnace with fuming HNO₃ + AgNO₃ (for X) or HNO₃ (for S); allows quantitative recovery as AgX or BaSO₄.

2.4 Common confusions / NTA trap points

• "Lowest locant" rule confused with "lowest first locant": apply the first point of difference rule on the whole set, not just the smallest number (NCERT §8.5.1, p. 263–264). For two possible sets {2,5,6} vs {3,4,5}, choose {2,5,6} because 2 < 3.
• Carbocation vs carbanion shape: carbocation is sp² planar; carbanion is sp³ pyramidal with a lone pair (p. 272). NTA often swaps these.
• Order of carbocation stability (3° > 2° > 1° > methyl) is the opposite of carbanion stability (methyl > 1° > 2° > 3°) — students invert.
• Inductive (–I) vs resonance (–M): –NH₂, –OH are –I but +M on benzene ring; NTA likes to ask the net effect (typically activating, ortho/para-directing on benzene despite −I).
• Halogens are −I AND +M on the ring, but they are weakly deactivating overall while still ortho/para-directing — a unique combination that catches students.
• Steam distillation works because vapour pressure of the mixture reaches atmospheric below 100 °C; common error is to say the compound's b.p. is below 100 °C (p. 281). What is true is that the combined pressure reaches atmospheric below 100 °C.
• Kjeldahl does not work for N in pyridine ring, nitro groups (–NO₂) or azo compounds (–N=N–) because these are not converted to (NH₄)₂SO₄ under digestion (p. 288).
• Sodium fusion extract must be made alkaline before testing for halogens with AgNO₃ — and any cyanide/sulphide must be removed by boiling with dil HNO₃, else they precipitate with Ag⁺ and give false positives (p. 285).
• R_f value is always ≤ 1; it is the ratio of distance moved by substance to distance moved by solvent front, not by the spot from the bottom of the plate (p. 284). R_f depends on the solvent and adsorbent used.
• Hyperconjugation is also called no-bond resonance and requires α-C–H bonds — vinyl/aryl cations show no hyperconjugation because the p-orbital is on a C that has no sp³ C–H next to it (p. 277).
• Lassaigne's test does NOT detect oxygen — there is no specific qualitative test for O; it is found by difference in mass.
• Resonance ≠ tautomerism: resonance is delocalisation in a SINGLE structure (canonical forms have only π/lone-pair shifts, atoms don't move); tautomerism is two different structures in equilibrium (atoms move, usually H).
• Functional isomers can interconvert (e.g., propanone ⇌ prop-1-en-2-ol via keto-enol tautomerism); chain and position isomers cannot interconvert without C–C bond breaking.