Some Basic Principles of Organic Chemistry

1. Tetravalency, Hybridization and Shapes

• Tetravalency of carbon: carbon has 4 valence electrons and almost always forms exactly 4 covalent bonds, explained by promoting one 2s electron to the empty 2p orbital and then hybridizing.
• sp³ hybridization: 4 equivalent sp³ orbitals, tetrahedral geometry, bond angle ≈109.5°. Example: methane, ethane (each carbon).
• sp² hybridization: 3 sp² orbitals in a plane (120° apart) + 1 unhybridized p orbital forming a π bond. Trigonal planar, bond angle ≈120°. Example: ethene (each doubly-bonded carbon), the carbonyl carbon.
• sp hybridization: 2 sp orbitals (180° apart) + 2 unhybridized p orbitals forming two π bonds (or two separate π bonds to two different atoms, as in an allene/ketene). Linear, bond angle = 180°. Example: ethyne, the central carbon of an allene (C=C=C) or ketene (C=C=O).
• Shape follows hybridization directly: more π bonds at a carbon → fewer sp-hybrid orbitals needed → more s-character in each → shorter, stronger σ bonds and a more linear local geometry.

2. Classification of Organic Compounds

• By carbon skeleton: acyclic (open-chain/aliphatic) vs cyclic (ring); cyclic further splits into homocyclic (all-carbon ring — alicyclic or aromatic) and heterocyclic (ring contains at least one non-carbon atom, e.g. pyridine's nitrogen, furan's oxygen).
• By functional group: compounds are grouped by the reactive group that dominates their chemistry — halogens (haloalkanes/haloarenes), oxygen-containing (alcohols, ethers, aldehydes, ketones, carboxylic acids, esters), nitrogen-containing (amines, amides, nitriles), and sulphur-containing (thiols, sulphonic acids) groups each define a distinct homologous series.
• Homologous series: a family of compounds with the same general formula and the same functional group, each member differing from the next by a constant –CH₂– unit. Consequences: (i) all members can be prepared by similar general methods, (ii) all show similar chemical properties (since the functional group is unchanged), (iii) physical properties (b.p., m.p., density) show a steady gradation as molecular mass increases, (iv) consecutive members differ in molecular mass by 14 u (one –CH₂–).

3. Structural Isomerism

• Structural (constitutional) isomers share the same molecular formula but differ in how the atoms are CONNECTED to each other.

• Chain isomerism: only the carbon skeleton (branching pattern) differs; functional group and its position are not the variable. For an alkane, every structural isomer is automatically a chain isomer (there is no functional group to shift). Heptane, C₇H₁₆, has 9 chain isomers in total.
• Position isomerism: same carbon skeleton and same functional group, but the functional group (or a substituent) sits at a different position on that skeleton. Example: 1-propanol vs 2-propanol.
• Functional isomerism: the molecular formula is identical but the functional GROUP itself is different. Example: propanal (an aldehyde) vs propanone (a ketone), both C₃H₆O; ethanol (alcohol) vs dimethyl ether (ether), both C₂H₆O.
• Metamerism: a special case restricted to compounds built around a divalent atom (O, N, S) — the two alkyl/aryl groups attached on either side of that atom differ, while the functional group (and divalent atom) stays the same. Example: methoxypropane (CH₃–O–C₃H₇) vs ethoxyethane (C₂H₅–O–C₂H₅), both ethers.
• Ring-chain isomerism: one isomer is cyclic, the other open-chain, with the same molecular formula. Example: cyclopropane vs propene (both C₃H₆).
• Tautomerism: a special, DYNAMIC equilibrium (not a fixed pair of isomers) between two structures that interconvert rapidly via migration of a proton together with a shift of a double bond — most commonly keto-enol tautomerism. The two tautomers usually have different functional groups (C=O vs C=C–OH) and genuinely co-exist in rapid equilibrium, unlike ordinary structural isomers, which are fixed, separable compounds.

4. Stereoisomerism: Geometrical and Optical

• Stereoisomers share the same molecular formula AND the same connectivity (same structural formula) — they differ only in the spatial (3-D) arrangement of atoms.

• Geometrical (cis-trans / E-Z) isomerism: requires restricted rotation (a C=C double bond, or a ring) AND two DIFFERENT groups on EACH of the two restricted carbons. A carbon bearing two identical groups (pattern a₂C=Cbd) has no geometrical isomerism at all.
  • cis/trans naming compares whether the two "similar-looking" groups are on the same side (cis) or opposite sides (trans) — informal and only unambiguous for simple disubstituted cases.
  • E/Z (CIP) naming is the rigorous system: assign priority (higher atomic number wins) to the two groups on EACH carbon; if the two higher-priority groups are on the same side, it is Z (zusammen); opposite sides, E (entgegen).
  • Stability: the trans (or E) isomer is usually more stable than cis (or Z) because the bulkier groups are farther apart, reducing steric strain — reflected in a lower heat of hydrogenation and (for symmetric cases) often a smaller/zero dipole moment for trans.
• Optical isomerism (chirality): arises when a molecule has NO internal element of symmetry (no plane of symmetry, no centre of symmetry) — most commonly because it has a chiral centre, a carbon bonded to 4 different groups.
  • Enantiomers: a pair of non-superimposable mirror images; identical physical properties except they rotate plane-polarised light by equal amounts in OPPOSITE directions (dextro +, laevo −). Specific rotation: [α] = θ / (l × C), where l = path length (dm), C = concentration (g/mL).
  • Diastereomers: stereoisomers that are NOT mirror images of each other (relevant when a molecule has more than one stereocentre) — they have genuinely different physical properties (unlike enantiomers).
  • Racemic mixture: an exactly 50:50 mixture of two enantiomers; the rotations cancel exactly, so it is optically INACTIVE even though each individual enantiomer is optically active. Optical purity (% ee) = (observed specific rotation / specific rotation of the pure enantiomer) × 100.
  • Meso compound: contains stereocentres, but also has an INTERNAL plane (or centre) of symmetry relating them — its own mirror image is superimposable on itself, so it is optically INACTIVE despite having chiral-looking centres. A meso form reduces the total count of optically active stereoisomers for that molecule.
  • Counting formula (n stereocentres): unsymmetrical molecule → N = 2ⁿ stereoisomers; a symmetrical molecule has fewer due to meso forms being counted once.
  • Chirality without a classical stereocentre: allenes with an odd number of cumulated double bonds, and ortho-tetrasubstituted biphenyls (restricted rotation, atropisomerism) can be chiral with no sp³ stereocentre at all.

5. Conformational Isomerism

• Conformers are NOT separate, isolable isomers — they are different spatial arrangements of the SAME molecule generated purely by rotation about a C–C single bond, rapidly interconverting at room temperature.

• n-Butane (dihedral angle vs energy, Newman projection along C2–C3): anti (dihedral 180°) is the MOST stable (0 kJ/mol, bulky methyl groups farthest apart); fully eclipsed with the two methyls overlapping (dihedral 0°/360°) is the LEAST stable (≈19 kJ/mol, maximum steric + torsional strain); gauche (60°/300°, ≈3.8 kJ/mol) and the other partially-eclipsed form (120°/240°, ≈16 kJ/mol) are intermediate.
• Cyclohexane conformers: chair (lowest energy, >99% of molecules at any instant, all bond angles ≈109.5°, no eclipsing strain) ≪ twist-boat ≪ boat (flagpole-H steric repulsion) ≪ half-chair (the highest-energy, transition-state form during ring-flipping). Ring-flipping interconverts axial and equatorial positions; a bulky substituent strongly prefers the equatorial position (e.g. methylcyclohexane is ~95% equatorial) to minimise 1,3-diaxial repulsion.

6. Nomenclature (IUPAC Basics)

• General procedure: (i) identify the principal characteristic group (the one with highest seniority — acids > esters > amides > nitriles > aldehydes > ketones > alcohols > amines > ethers, roughly); (ii) select the longest continuous carbon chain that includes the principal group's carbon, if applicable; (iii) number the chain to give the principal group (or, if none, the first point of difference among substituents) the LOWEST possible locant; (iv) name substituents alphabetically with their locants, and cite the principal group as a suffix (or the whole molecule as a substituent prefix, for a non-principal group).
• Common ester-naming trap: name the alcohol-derived part first (as an alkyl group) then the acid-derived part as "-oate" — e.g. n-propyl 2-bromo-5-methylheptanoate names a propyl ester of a substituted heptanoic acid; always re-derive the LONGEST chain through the carbonyl carbon on the acid side, since a branch can masquerade as part of a shorter chain if read carelessly.
• Trivial names still expected at JEE level: formic/acetic/oxalic acid, formaldehyde/acetaldehyde, acetone, catechol (benzene-1,2-diol), resorcinol (benzene-1,3-diol), quinol/hydroquinone (benzene-1,4-diol) — know both the trivial and IUPAC name for these common compounds.
• Classifying a carbon atom (1°/2°/3°/4°): based on how many OTHER CARBON atoms it is directly bonded to — 1° (one carbon neighbour), 2° (two), 3° (three), 4° (four, no H left) — NOT based on how many hydrogens it carries by itself.

7. Bond Fission: Carbocations, Carbanions, Free Radicals

• Homolytic fission: the shared electron pair splits EQUALLY, one electron to each fragment, generating two free radicals. Needs energy input (heat, light, peroxides) and typically happens in the gas phase or non-polar solvents.
• Heterolytic fission: the shared electron pair goes ENTIRELY to one fragment, generating a cation and an anion together. Favoured in polar solvents, and is the usual mode of bond-breaking in ionic/nucleophilic-electrophilic organic reactions.

• Carbocation (sp², planar, electron-deficient, 6 valence electrons): stabilised by +I (electron-donating) alkyl groups and by hyperconjugation and resonance. Stability: 3° > 2° > 1° for simple alkyl cations; benzylic and allylic cations (resonance-delocalised into a ring or an adjacent π bond) are even more stable than a simple 3° alkyl cation. Vinyl and phenyl cations (positive charge on an sp² carbon that is ITSELF part of the double bond/ring, with no adjacent stabilisation) are extremely unstable and essentially never form under ordinary conditions. Carbocations are prone to 1,2-hydride or 1,2-alkyl shifts (rearrangement) toward a more stable cation.
• Carbanion (sp³ typically, pyramidal, electron-rich, lone pair): stabilised by −I (electron-withdrawing) groups and by MORE s-character in the orbital holding the lone pair — the opposite trend to cations. Stability order: 1° > 2° > 3° for simple alkyl carbanions (alkyl groups destabilise by intensifying negative charge via +I); by hybridization, sp (50% s) > sp² (33% s) > sp³ (25% s). Among simple cycloalkyl carbanions, the non-monotonic order cyclopropyl > cyclopentyl > cyclobutyl is a specifically tested exception (ring-strain effects on s-character).
• Free radical (sp² or pyramidal, one unpaired electron): follows the SAME stability order as carbocations (both are electron-deficient at the radical/cationic centre) — 3° > 2° > 1° > methyl, with benzylic/allylic radicals (resonance-delocalised) most stable of all.
• Carbenes (:CR₂, divalent carbon, 6 valence electrons): singlet carbene is sp², diamagnetic (paired electrons), and reacts stereospecifically (syn addition); triplet carbene is effectively sp, linear, paramagnetic (a diradical), and reacts non-stereospecifically.
• Nitrenes (:NR, nitrogen analogue of a carbene) and benzyne (a strained aromatic "triple bond" with an extra in-plane π bond that cannot conjugate with the aromatic ring π system) are the other two classic reactive intermediates seen at this level.

8. Electronic Displacement Effects

• Inductive effect (I): a PERMANENT displacement of σ-bond electrons toward a more electronegative atom, transmitted through the chain but weakening rapidly (effectively negligible after ~3 bonds). −I groups (halogens, −NO₂, −CN, −COOH) withdraw electron density; +I groups (alkyl groups) release electron density.
• Electromeric effect (E): a TEMPORARY effect, triggered only at the actual moment an attacking reagent approaches a multiple bond — the entire π electron pair shifts completely to one atom. +E effect: π electrons shift toward the attacking electrophile's side (shown with a nucleophile attacking); −E effect: π electrons shift away, toward the attacking electrophile itself. The electromeric effect, when operative, DOMINATES over the inductive effect.
• Resonance (mesomeric) effect (R/M): a PERMANENT delocalisation of π electrons (or a lone pair adjacent to a π system) over a conjugated system, described by drawing multiple contributing (canonical) structures; the true structure is a hybrid, generally more stable than any single contributor. +R groups (−OH, −NH₂, −OR, halogens on a ring) donate electron density into the system; −R groups (−NO₂, −CHO, −COOH, −CN) withdraw it.
• Hyperconjugation (no-bond resonance): PERMANENT delocalisation arising from overlap between a σ(C–H) [or σ(C–C)] bond on a carbon ADJACENT to an electron-deficient centre (cation, radical) or a π system, and that adjacent empty/π orbital. Strictly requires at least one α-hydrogen; more α-hydrogens means more hyperconjugative (no-bond) structures and greater stabilisation — this is exactly why (CH₃)₃C⁺ (9 α-H, 9 hyperconjugative structures) is far more stable than CH₃⁺ (0 α-H, 0 hyperconjugative structures, no adjacent carbon at all).
• Resonance-structure stability rules: (i) more covalent bonds and less charge separation is more stable; (ii) a structure where every atom has a complete octet is more stable; (iii) negative charge is best placed on a more electronegative atom, positive charge on a more electropositive one; (iv) like charges on ADJACENT atoms strongly destabilise a structure (electrostatic repulsion) — this is often the deciding factor in a "least stable resonance structure" question.

9. Common Types of Organic Reactions

• Substitution: one atom or group in a molecule is replaced by another, with no change in the degree of unsaturation. Typical of saturated carbons (alkanes, haloalkanes) and aromatic rings.
• Addition: two atoms or groups add across a multiple bond (C=C, C≡C, C=O), converting it to a single bond — no atoms are lost. Typical of alkenes, alkynes, and carbonyl compounds.
• Elimination: atoms or groups are removed from two adjacent (or occasionally more distant) carbons, CREATING a new multiple bond — essentially the reverse of addition. Typical of haloalkanes and alcohols under basic/acidic dehydrohalogenation/dehydration conditions.
• Rearrangement: the carbon skeleton itself reorganises, usually via a 1,2-hydride or 1,2-alkyl (methyl) shift in a carbocation intermediate, moving toward a more stable cation before the reaction completes.
• These four categories are a classification of WHAT HAPPENS to the bonds overall, independent of the detailed step-by-step mechanism (SN1 vs SN2, E1 vs E2) — that mechanistic detail is built on this foundation in the haloalkanes, alcohols and hydrocarbons units that follow.

10. How JEE Frames These Questions

• Isomer-type matching (List I vs List II): for each pair, first check if the molecular formula is identical (rules out homologues); if identical, check if connectivity is identical (rules out structural isomers, leaves stereoisomers) or different (then classify as chain/position/functional/metamer by what specifically differs).
• Hyperconjugation/carbocation statement pairs: count α-hydrogens directly on a quick skeletal sketch rather than recalling a memorised number — this avoids being misled by a cleverly-drawn but ordinary cation.
• Carbanion/carbocation/radical stability-ordering questions: identify resonance-stabilised (benzylic/allylic) species first and rank them above simple alkyl ones, then apply the 3°>2°>1° (cation/radical) or 1°>2°>3° (carbanion) rule among the remaining alkyl species.
• Resonance-structure "least/most stable" questions: apply the four stability rules in Section 8 in order — octet completeness and charge separation usually decide it before you need the electronegativity-placement rule.
• Geometrical/optical isomer counting: always check for an internal symmetry element (meso test) before applying the 2ⁿ formula blindly, and always re-verify that BOTH carbons of a C=C genuinely carry two different groups before claiming geometrical isomerism exists at all.
• Conformer energy questions (n-butane/cyclohexane): anchor every answer to the SAME two facts — anti/chair is lowest energy, fully-eclipsed/boat-family is higher energy — and reason about intermediate conformers (gauche, twist-boat) relative to those two fixed points.

11. Quick Sheet and Checklist

Before the exam, check you can:

• Assign hybridization and shape at any carbon from the number of pi bonds it forms, including in a cumulated system like an allene or ketene.
• Classify a pair of compounds correctly as chain, position, functional, metamer, ring-chain, or tautomers — and distinguish all of these from plain homologues.
• State the condition for geometrical isomerism (two different groups on each restricted carbon) and for optical isomerism (no internal symmetry element), and apply the meso test before counting optical isomers.
• Rank the n-butane conformers (and cyclohexane's chair/boat family) by relative energy and explain why in terms of torsional/steric strain.
• Correctly classify any carbon as 1°/2°/3°/4° by counting its carbon neighbours, not its hydrogens.
• State the general stability order for carbocations, carbanions and free radicals, and explain WHY the carbanion order inverts relative to the other two.
• Distinguish all four electronic effects (inductive, electromeric, resonance, hyperconjugation) by permanence and by what structural feature each one requires.
• Classify any organic reaction as substitution, addition, elimination, or rearrangement from what happens to the bonds, independent of its detailed mechanism.