← Chemistry — Std 11
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Organic Chemistry — Some Basic Principles

Ch. 9Std 11

Easy Overview

Why does carbon form millions of compounds while other elements barely manage a few hundred? The answer lies in catenation — carbon's unique ability to bond with itself in long chains and rings — and its perfect range of bond strengths that allow molecules to be both stable and reactive. Organic chemistry studies carbon compounds. Carbon has four valence electrons and forms four covalent bonds via sp³, sp², or sp hybridization. This versatility produces straight chains, branches, rings, and multiple bonds. Add functional groups — OH, C=O, COOH, NH₂ — and you get alcohols, ketones, acids, and amines with distinct properties. IUPAC nomenclature gives every compound a unique, systematic name. Reaction mechanisms (SN1, SN2, E1, E2) explain exactly how bonds break and form — homolytic (radical) or heterolytic (ionic). This chapter builds the foundation: bonding, classification, naming, functional groups, and the electronic effects (inductive, resonance, hyperconjugation) that control reactivity.

Why Carbon Is Special — Catenation and Hybridization

Carbon sits right in Period 2, Group 14. Its four valence electrons (2s²2p²) can form four bonds via sp³, sp², or sp hybridization. The C-C bond (348 kJ/mol) is strong enough for stability but not so strong that reactions are impossible. Carbon also forms strong π bonds (C=C, 614 kJ/mol; C≡C, 839 kJ/mol) enabling multiple bonds. No other element matches carbon's balance of bond strength and versatility — silicon, the closest neighbor, forms Si-Si bonds (222 kJ/mol) that are too weak for long chains.

Tetravalence of Carbon — The Four-Armed Connector

Carbon always forms four covalent bonds (tetravalent). sp³: four σ bonds, 109.5° tetrahedral (CH₄, ethane). sp²: three σ + one π, 120° trigonal planar (ethene). sp: two σ + two π, 180° linear (ethyne, CO₂). Carbon can bond with H, O, N, S, halogens, and metals. It forms single, double, and triple bonds. It creates straight chains, branched chains, rings, and three-dimensional networks. No other element in the periodic table matches this.

Classification of Organic Compounds

Two broad categories: acyclic (open chain, aliphatic) and cyclic (closed ring). Cyclic compounds are further divided into homocyclic (only carbon in ring, e.g., cyclohexane, benzene) and heterocyclic (one or more heteroatoms N/O/S in ring, e.g., pyridine, furan). Aromatic compounds contain a benzene ring or similar conjugated planar ring system. Alicyclic compounds are saturated or unsaturated non-aromatic rings. Benzenoid aromatics contain benzene rings; non-benzenoid aromatics (like azulene, tropone) have aromatic character without a benzene ring.

Functional Groups — The Reactive Centers

A functional group is an atom or group that gives a compound its characteristic properties. Alcohols (-OH): polar, H-bonding, higher BP than hydrocarbons. Aldehydes (-CHO): easily oxidized to acids; reducing sugars test (Tollens' silver mirror, Fehling's red Cu₂O). Ketones (C=O): less easily oxidized, used in fragrances and solvents. Carboxylic acids (-COOH): acidic (pKa ~4-5), form hydrogen-bonded dimers. Amines (-NH₂): basic, fishy smell, found in amino acids and drugs. Alkyl halides (-X): polar, undergo nucleophilic substitution. Ethers (-O-): low reactivity, excellent solvents. Esters (-COO-): fruity smell, in flavors and fragrances. Nitriles (-C≡N): polar, versatile intermediates.

Homologous Series — The Family Pattern

A homologous series shares the same functional group and general formula. Each member differs by -CH₂- (14 amu). Physical properties change gradually: alkanes CₙH₂ₙ₊₂: CH₄ (gas), C₅H₁₂ (liquid), C₁₈H₃₈ (wax). Boiling points increase by ~30°C per CH₂ due to London dispersion forces. Chemical properties remain similar within a series — once you know one alcohol, you know them all. The trend allows prediction of properties for unknown members.

IUPAC Nomenclature — The Universal Language

IUPAC names have three parts: prefix (substituents), root (longest carbon chain), suffix (functional group). Steps: (1) Find longest continuous carbon chain — parent name from number of carbons (meth-1, eth-2, prop-3, but-4, pent-5, hex-6, hept-7, oct-8, non-9, dec-10). (2) Number the chain to give lowest locants to substituents. (3) Name and arrange substituents alphabetically (ignoring prefixes like di-, tri-). (4) Add functional group suffix: -ol (alcohol), -al (aldehyde), -one (ketone), -oic acid (carboxylic acid). For multiple bonds: -ane (single), -ene (double), -yne (triple). Priority order for suffixes: carboxylic acid > ester > amide > aldehyde > ketone > alcohol > amine > ether > alkene > alkyne > alkane.

Isomerism — Same Formula, Different Molecules

Structural isomers differ in connectivity. Chain isomers: butane (C₄H₁₀) n-butane vs isobutane. Position isomers: propanol 1-propanol vs 2-propanol. Functional group isomers: C₂H₆O ethanol vs dimethyl ether. Metamerism: different alkyl groups around same functional group (C₄H₁₀O diethyl ether vs methyl propyl ether). Tautomerism: rapid equilibrium between keto and enol forms (acetoacetic ester). Stereoisomers have same connectivity but different spatial arrangement — covered in the next concept.

Stereoisomerism — 3D Arrangements

Geometric (cis-trans) isomerism: restricted rotation around double bond or ring. cis-1,2-dichloroethene (same side, dipole) vs trans (opposite sides, no dipole). Different physical properties. Optical isomerism: chiral molecules (non-superimposable mirror images). Chiral carbon: four different substituents. Enantiomers: pair of optical isomers, rotate plane-polarized light in opposite directions. Racemic mixture (d,l): 1:1 mixture, no net rotation. Lactic acid (CH₃CHOHCOOH) has one chiral carbon — two enantiomers. Biological systems are stereospecific: L-amino acids, D-sugars.

Inductive Effect — The Sigma Electron Shift

Electronegativity differences cause permanent polarization along σ bonds. -I groups (withdraw electrons): F, Cl, Br, I, OH, NH₂, NO₂, CN, C=O, COOH. +I groups (donate electrons): alkyl groups (CH₃ > C₂H₅ > CH(CH₃)₂ > C(CH₃)₃). -I effect increases acidity (electron withdrawal stabilizes conjugate base): ClCH₂COOH (pKa 2.86) > CH₃COOH (4.76). +I effect decreases acidity: CH₃CH₂COOH (4.87) > CH₃COOH (4.76). Inductive effect decreases rapidly with distance — significant up to 2-3 carbons.

Resonance Effect — Electron Delocalization

Delocalization of π electrons or lone pairs through conjugated systems. +R effect: groups that donate electrons by resonance (OH, NH₂, OR, halogens) — they have lone pairs that can conjugate with π system. -R effect: groups that withdraw electrons by resonance (NO₂, CN, C=O, COOH, SO₃H) — they have π bonds or empty orbitals. Resonance explains: stability of benzene (all C-C 139 pm), acidity of phenol (C₆H₅OH, pKa 10) vs cyclohexanol (pKa 16) — phenoxide ion stabilized by resonance, and the direction of electrophilic aromatic substitution (activating groups donate, deactivating withdraw). The effect is stronger than the inductive effect.

Hyperconjugation — No Bond Resonance

Delocalization of σ electrons (C-H or C-C) into adjacent empty or partially filled p orbital or π* orbital. Alkyl groups attached to sp² carbon show hyperconjugation. Stability order of alkyl carbocations: 3° > 2° > 1° > CH₃⁺. Each α C-H bond contributes ~3-4 kcal/mol stabilization. More α H atoms = more hyperconjugative structures = more stable. Explains: stability of alkenes (more substituted = more stable — Saytzeff rule), orientation in E1 elimination, and Markovnikov addition in electrophilic addition to alkenes.

Reaction Mechanisms — How Bonds Break and Form

Bond cleavage: homolytic (each atom gets one electron, forms free radicals) — common in substitution reactions (Cl₂ + hν → 2Cl•) and combustion. Heterolytic (one atom gets both electrons, forms ions) — common in polar organic reactions. Carbocations (R₃C⁺): sp² hybridized, planar, electron-deficient, stabilized by alkyl groups. Carbanions (R₃C⁻): sp³ hybridized, pyramidal, electron-rich. Free radicals: unpaired electron, neutral, highly reactive. Understanding the intermediate determines the reaction pathway.

Types of Organic Reactions

Substitution: replacement of one atom/group by another. SN1 (unimolecular, carbocation intermediate, racemization, rate ∝ [RX]) vs SN2 (bimolecular, concerted, inversion, rate ∝ [RX][Nu⁻]). Addition: π bond breaks, two new σ bonds form. Electrophilic addition to alkenes (Markovnikov or anti-Markovnikov). Elimination: removal of atoms to form π bond. Zaitsev (Saytzeff) rule: more substituted alkene is major product. Rearrangement: carbon skeleton rearrangement (carbocation rearrangements via hydride or methyl shift). Oxidation-reduction: change in oxidation state of carbon.

Purification of Organic Compounds

Crystallization: dissolve in hot solvent, cool slowly, pure crystals form. Impurities remain in solution. Sublimation: for compounds that go solid→vapor directly (camphor, naphthalene, benzoic acid, NH₄Cl). Distillation: simple (BP diff >25°C), fractional (close BP), steam distillation (volatile compounds immiscible with water — aniline, essential oils), vacuum distillation (high BP, decomposes at atmospheric). Chromatography: paper, TLC, column — separates based on differential adsorption. Determining purity: melting point (sharp range = pure), boiling point, TLC (single spot).

Detection of Elements in Organic Compounds

Lassaigne's test: fuse organic compound with Na metal. Na + C/N/S/X → NaCN, Na₂S, NaX. Extract with water (Lassaigne's extract). N test: add FeSO₄, heat, add FeCl₃, acidify → Prussian blue (Fe₄[Fe(CN)₆]₃). S test: add sodium nitroprusside → violet color. Halogen test: add AgNO₃ → precipitate (AgCl white, AgBr pale yellow, AgI yellow). P test: fuse with Na₂O₂, acidify, add ammonium molybdate → canary yellow. Detection by acidifying extract: S²⁻ → H₂S (lead acetate), CN⁻ → HCN (Prussian blue). Limitations: N test fails for certain heterocycles and azo compounds; halogen test requires removing N and S interference.

Quantitative Analysis — Percentage Composition

C and H: Liebig's combustion method. Burn sample in O₂, absorb CO₂ in KOH solution and H₂O in CaCl₂ or Mg(ClO₄)₂. %C = (12/44)×(mass CO₂/mass sample)×100. %H = (2/18)×(mass H₂O/mass sample)×100. N: Dumas method (N₂ gas measured) or Kjeldahl method (NH₃ distilled, titrated). %N = (1.4×V×N)/W (Kjeldahl: V = volume of acid, N = normality, W = sample mass). Halogen: Carius method (combust with HNO₃+AgNO₃, weigh AgX). S: Carius method (BaSO₄ precipitate weighed). %S = (32/233)×(mass BaSO₄/mass sample)×100. Oxygen: calculated by difference from 100%.

Empirical and Molecular Formulas from Analytical Data

From % composition: assume 100 g sample, convert masses to moles (mass/atomic mass), divide by smallest number of moles, round to nearest whole number (if .5 → multiply all by 2). Empirical formula = simplest ratio. Molecular formula = (empirical formula)n, where n = molecular mass (from Vapor density × 2 or mass spectrometry) / empirical formula mass. Example: 40% C, 6.67% H, rest O, M=60: C=40/12=3.33, H=6.67/1=6.67, O=53.33/16=3.33. Ratio C:H:O = 1:2:1 → CH₂O (emp formula mass 30). n = 60/30 = 2 → C₂H₄O₂.

Key Points

  • •Carbon is tetravalent (sp³, sp², sp hybridization) with catenation ability
  • •IUPAC: prefix-root-suffix; longest chain, lowest locants, alphabetical order
  • •Functional groups determine chemical properties: OH, C=O, COOH, NH₂, etc.
  • •Homologous series: same general formula, CH₂ difference, gradual property change
  • •Structural isomers: chain, position, functional group, metamerism
  • •Stereoisomers: geometric (cis/trans) and optical (chiral/enantiomers)
  • •Inductive effect: -I (electron withdrawing) increases acidity; +I decreases
  • •Resonance: delocalization of π electrons; stronger than inductive effect
  • •Hyperconjugation: σ electrons delocalize into empty p orbital; stabilizes carbocations
  • •SN1 (unimolecular, carbocation, racemization) vs SN2 (bimolecular, inversion)
  • •Zaitsev rule: more substituted alkene is major product in elimination
  • •Lassaigne's test: Na fusion for N (Prussian blue), S (violet), halogens (AgX)
  • •Liebig's method: C and H from CO₂ and H₂O absorption
  • •Dumas/Kjeldahl for N; Carius method for halogens and S
  • •Empirical formula: mole ratio; Molecular = n × empirical

Practice Questions

  • Write IUPAC names: (CH₃)₂CHCH₂OH, CH₃CH₂CH=CH₂, CH₃CH₂COOH, CH₃COCH₂CH₃.
  • Distinguish: SN1 vs SN2 mechanism with examples and stereochemistry.
  • Explain hyperconjugation and its effect on stability of carbocations: 1°, 2°, 3°.
  • An organic compound has C=39.9%, H=6.7%, O=53.4%. Molecular mass = 60. Find empirical and molecular formula.
  • Describe Lassaigne's test for N, S, and halogens with reactions.
  • Explain inductive effect. Why is ClCH₂COOH stronger acid than CH₃COOH?
  • Calculate %C, %H, %N from combustion data: 0.2 g compound gives 0.44 g CO₂, 0.18 g H₂O, and 22.4 mL N₂ at STP.
  • What is resonance? Draw resonance structures of aniline and nitrobenzene.