Organic Chemistry — Basic Principles & Techniques — NEET Notes

Tetravalence of carbon & shapes of organic molecules

Carbon's electronic configuration is 1s² 2s² 2p² — four valence electrons sitting in the second shell. To form bonds it hybridises: one 2s and three 2p orbitals collapse into four equivalent sp³ orbitals (methane, 109.5°, tetrahedral); one 2s and two 2p orbitals into three sp² orbitals plus an unhybridised p (ethene, 120°, trigonal planar); one 2s and one 2p into two sp orbitals plus two unhybridised p (ethyne, 180°, linear). The number of σ bonds + lone pairs around carbon determines the hybridisation state. Four → sp³. Three → sp². Two → sp.

The hybridisation also fixes the bond length, bond strength, and acidity. An sp orbital has 50% s-character, an sp² orbital 33%, and an sp³ orbital 25%. More s-character means the orbital sits closer to the nucleus — shorter bonds, stronger bonds, more electronegative carbon, more acidic C–H. That is why ethyne (sp) is more acidic than ethene (sp²) is more acidic than ethane (sp³) — a NEET 2017 question that hinged on this exact rule.

Carbon's four bonds are not a chemical accident — they are the structural premise on which every organic molecule, from methane to DNA, is built.

Why tetravalence matters

A π bond is built from sideways overlap of two parallel p-orbitals. Its electron cloud lies above and below the bonding plane, making the electrons accessible to attacking reagents. This is why multiple bonds are the most reactive centres in a molecule and why double-bond rotation is restricted — twisting one CH₂ relative to the other destroys p-orbital overlap. That restriction is the origin of geometrical (cis-trans) isomerism.

Structural representations

The same molecule can be drawn in several conventions, each with a purpose. The complete structural formula shows every atom and every bond (dash = single, two dashes = double, three dashes = triple). The condensed structural formula compresses bonds and counts identical substituents with subscripts — CH₃CH₂CH₂CH₃ for butane. The bond-line (skeletal) formula draws only the carbon skeleton as a zig-zag line; line junctions and terminals are carbon atoms (hydrogens implied to satisfy tetravalence), and only heteroatoms (O, N, Cl, etc.) are labelled. A bond-line cyclohexane is just a hexagon.

For three-dimensional structures, the wedge–dash convention is used. A solid wedge denotes a bond projecting out of the page toward the observer; a dashed wedge denotes a bond projecting behind the page; normal lines lie in the plane. Three physical models also exist — framework (bonds only), ball-and-stick (atoms + bonds), and space-filling (van der Waals radii). NEET problems set in the bond-line and wedge-dash conventions appear in stereochemistry questions; learning to convert fluently between condensed, skeletal, and 3D forms is non-negotiable.

Classification of organic compounds

Every organic compound belongs to one of two broad families. Acyclic (open-chain or aliphatic) compounds have a straight or branched chain of carbon atoms — alkanes, alkenes, alkynes, and their derivatives. Cyclic compounds contain at least one closed ring. Cyclic compounds split further into homocyclic (carbocyclic), where the ring is made of carbon only, and heterocyclic, where one or more atoms (usually N, O, or S) replace ring carbons. Carbocyclic compounds split again into alicyclic (saturated or non-aromatic — cyclohexane, cyclopentene) and aromatic (benzene and its derivatives, with delocalised π electrons obeying Hückel's 4n+2 rule).

Within these structural families, compounds are further classified by the functional group they carry — an atom or group responsible for the characteristic chemistry of the molecule: –OH (alcohols, suffix -ol), –CHO (aldehydes, -al), >C=O (ketones, -one), –COOH (carboxylic acids, -oic acid), –NH₂ (amines, -amine), –X (haloalkanes, prefix halo-), –NO₂ (nitro, prefix nitro-), and so on. Compounds sharing the same functional group form a homologous series with a common general formula (alkanes C_nH_2n+2, alkenes C_nH_2n, alkynes C_nH_2n−2), where each successive member differs from the previous by a CH₂ unit.

IUPAC nomenclature

The International Union of Pure and Applied Chemistry's 1958 system replaced the chaos of trivial names (formic acid from formica, marsh gas for methane) with a rule-based protocol. Every IUPAC name has three parts — a word root (carbon-count: meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec-), a suffix (saturation: -ane, -ene, -yne; functional group: -ol, -al, -oic acid…), and one or more prefixes (substituents in alphabetical order, with their position locants). The protocol is mechanical — apply the rules in order and the name writes itself.

For molecules with more than one functional group, IUPAC assigns priority — only the highest-priority group becomes the suffix; the rest become prefixes. The simplified priority order (descending): –COOH > –COOR > –SO₃H > –COX > –CONH₂ > –CHO > –CO– (ketone) > –CN > –OH > –NH₂ > –X (halogen) > –NO₂ > –C=C– > –C≡C–. Thus CH₃CH(OH)CH₂CH(Br)COOH is named 2-bromo-4-hydroxypentanoic acid, with COOH as suffix and the rest as prefixes. NEET 2022 set exactly such a multi-substituent IUPAC problem (1-bromo-5-chloro-4-methylhexan-3-ol) and 2017 tested a polyfunctional name (3-keto-2-methylhex-4-enal).

Isomerism

Two compounds with the same molecular formula but different structures are isomers. NCERT and NIOS together recognise two broad families: structural (constitutional) isomerism, where the connectivity differs, and stereoisomerism, where the connectivity is the same but the spatial arrangement differs. Structural isomerism further breaks into four sub-types; stereoisomerism into two main configurational sub-types.

The R/S system assigns absolute configuration at a chiral carbon. Rank the four substituents by Cahn-Ingold-Prelog priority (1 = highest, 4 = lowest). Orient the molecule so the lowest-priority group points away from you. Trace 1→2→3: clockwise is R (rectus), anticlockwise is S (sinister). The older D/L system (Fischer projections, used for sugars and amino acids) and R/S can both be applied to the same molecule, but they describe different conventions — and the d/l labels for rotation direction are entirely separate from D/L configuration.

Tautomerism is a special dynamic functional isomerism — keto and enol forms of a carbonyl compound interconvert via migration of a proton, e.g. acetone (keto, CH₃COCH₃) ⇌ propen-2-ol (enol, CH₃C(OH)=CH₂). The keto form is usually overwhelmingly more stable. Tautomers are a sub-class of structural isomers, not stereoisomers.

Bond fission — homolytic vs heterolytic

Every organic reaction begins with a bond breaking. There are exactly two ways. In homolytic fission, the two bonding electrons separate equally — each atom keeps one. The result is two neutral species with unpaired electrons: free radicals. In heterolytic fission, one atom takes both electrons; the result is a cation (electron-deficient, often a carbocation) and an anion (electron-rich, often a carbanion). The two pathways generate different intermediates, demand different conditions, and follow different rules.

The charged species produced by heterolysis are classified by what they want. An electrophile ("electron-loving") is electron-deficient — positively charged (H⁺, NO₂⁺, R⁺, Cl⁺) or neutral with an empty orbital (BF₃, AlCl₃). It attacks regions of high electron density. A nucleophile ("nucleus-loving") is electron-rich — negatively charged (OH⁻, CN⁻, Cl⁻, RO⁻) or neutral with a lone pair (H₂O, NH₃, ROH). It attacks regions of low electron density. NEET 2017 asked precisely this definition: an electrophile can be neutral or positively charged and forms a bond by accepting a pair of electrons from a nucleophile — option (1) was the only correct statement.

Electronic effects — inductive, electromeric, resonance, hyperconjugation

Reactions require polarised bonds. The polarisation comes from four well-defined electron-displacement effects. Two are permanent (always present in the ground state): inductive and resonance. Two are temporary (induced only when a reagent attacks): electromeric, and to a partial extent hyperconjugation. Mastering these four effects explains almost every reactivity trend in organic chemistry — acid strength, basicity, electrophilic aromatic substitution patterns, carbocation stability, alkene stability.

Steric hindrance is a non-electronic factor that limits reactivity. When bulky groups crowd the reaction site, the attacking reagent simply cannot approach. Esterification rates fall in the order CH₃COOH > RCH₂COOH > R₂CHCOOH > R₃CCOOH — Meyer (1874) observed exactly this, and it explains why pivalic acid (R = CH₃) is far less reactive than acetic acid.

Reactive intermediates — carbocation, carbanion, radical

Heterolytic and homolytic fission generate three short-lived species that drive most organic mechanisms. Carbocations (positively charged carbon, sp² hybridised, planar, six electrons) are stabilised by anything that donates electron density — alkyl groups (+I and hyperconjugation), adjacent lone pairs, and π-systems (resonance). Carbanions (negatively charged carbon, sp³ pyramidal, eight electrons) are stabilised by electron-withdrawing groups (–I and –M). Free radicals (neutral, one unpaired electron, sp² planar) follow the same stability order as carbocations because hyperconjugation stabilises both.

NEET 2022 framed exactly this contrast: in S_N2 the product shows inversion of configuration; in S_N1 the planar carbocation intermediate produces a 1:1 racemic mixture of enantiomers. A racemic mixture is optically inactive because the equal d and l rotations cancel. The deeper truth is that the stability of the reactive intermediate — carbocation, carbanion, or radical — controls which mechanism the molecule prefers.

Methods of purification

Once a compound has been synthesised, it must be separated from impurities and obtained pure. Five techniques cover almost every laboratory case, each exploiting a different physical property.

Qualitative analysis — detecting the elements

Before any structure can be assigned, the elements present must be identified. Carbon and hydrogen are detected by heating the compound with CuO in a dry test tube: C → CO₂ (turns limewater milky), H → H₂O (turns anhydrous CuSO₄ blue). For everything else, the compound is first fused with sodium metal to convert covalent N, S, X (Cl, Br, I), and P into ionic form — NaCN, Na₂S, NaX, Na₃PO₄. The fused mass is boiled with distilled water to give Lassaigne's extract (L.E.), on which the element tests are performed.

Test for nitrogen: L.E. + FeSO₄, boil, acidify with H₂SO₄ — Prussian blue colour confirms N (ferriferrocyanide formation, Fe₄[Fe(CN)₆]₃). Test for sulphur: L.E. + sodium nitroprusside → violet colour; or L.E. + lead acetate (after acidification) → black PbS precipitate. Test for halogens: L.E. + HNO₃ + AgNO₃ — white precipitate (soluble in NH₄OH) = Cl; pale-yellow (partly soluble) = Br; yellow (insoluble) = I. If N or S is also present, L.E. is first boiled with concentrated HNO₃ to expel CN⁻ and S²⁻ ions that would otherwise interfere. Test for phosphorus: fuse with Na₂O₂, boil with HNO₃, add ammonium molybdate — canary-yellow precipitate of ammonium phosphomolybdate confirms P.

Quantitative analysis — measuring the elements

Once the elements are identified, their percentage composition is determined by classical combustion/precipitation methods, each named after the chemist who developed it. The percentages are then converted to mole ratios to give the empirical formula, from which (with molar mass) the molecular formula follows.

The Kjeldahl percentage of nitrogen is computed as:

% N = (1.4 × Normality of acid × Volume of acid used to neutralise NH₃) / mass of compound

Kjeldahl's formula — back-titration of liberated ammonia

Oxygen is usually obtained by difference: %O = 100 − (%C + %H + %N + %S + ...). A direct method (Aluise) decomposes the compound in N₂ atmosphere, passes evolved O over hot coke (→ CO) and then I₂O₅ (→ I₂ + CO₂), weighing the products. Modern laboratories use the CHN elemental analyser — a fully automated combustion instrument that estimates C, H, and N simultaneously from 1–3 mg of sample in minutes. It is the descendant of Liebig's combustion train and Dumas's nitrogen collection apparatus.