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Organic chemistry is the branch of chemistry that deals with the study of compounds containing carbon. Carbon is the fundamental building block of all forms of life on Earth, due to its property of catenation, carbon forms covalent bonds with other carbon atoms as well as with atoms of other elements such as sulphur, phosphorus, and nitrogen. Below, we have provided the important notes of NCERT Class 11 Chemistry (Part II), Chapter 8: Organic Chemistry – Some Basic Principles and Techniques.
Contents
- 1 Introduction
- 2 History of Organic Chemistry
- 3 Tetravalence of Carbon: Shapes of Organic Compounds
- 4 Structural Representations of Organic Compounds
- 5 Classification of Organic Compounds
- 6 Nomenclature of Organic Compounds
- 7 IUPAC System of Nomenclature
- 8 Isomerism
- 9 Types of Organic Reactions
- 10 Methods of Purification of Organic Compounds
- 11 Qualitative Analysis of Organic Compounds
- 12 Quantitative Analysis of Organic Compounds
- 13 Important Formulas in NCERT Notes Class 11 Chemistry (Part-II) Chapter 8: Organic Chemistry- Some Basic Principles and Techniques
- 14 FAQs
Explore Notes of Class 11 Chemistry
Introduction
Organic compounds are essential for life, including DNA (genetic material) and proteins (components of blood, muscles, and skin). They are present in many materials such as clothing, fuels, polymers, dyes, and medicines. Organic chemistry is about 200 years old.
History of Organic Chemistry
In 1780, chemists distinguished between organic compounds (from plants/animals) and inorganic compounds (from minerals). Berzelius proposed the “vital force” theory for the formation of organic compounds. In 1828, Friedrich Wöhler disproved the “vital force” theory by synthesising urea from ammonium cyanate (an inorganic compound).
Pioneering syntheses:
- Acetic acid by Kolbe (1845)
- Methane by Berthelot (1856)
These experiments proved that organic compounds can be synthesised from inorganic sources in laboratories.
Tetravalence of Carbon: Shapes of Organic Compounds
Tetravalence of carbon: shapes of organic compounds are discussed below.
Shapes of Carbon Compounds
- Understanding molecular structure helps predict properties of organic compounds.
- Tetravalence of carbon and covalent bond formation are explained by its electronic configuration and hybridisation (sp³, sp², sp).
- Examples:
- sp³: methane (CH₄)
- sp²: ethene (C₂H₄)
- sp: ethyne (C₂H₂)
- Hybridisation affects bond length, bond strength (enthalpy), and electronegativity.
- S-character in hybrid orbitals:
- Sp (50% s) → shortest, strongest bonds, highest electronegativity.
- sp² (33% s) → intermediate properties.
- sp³ (25% s) → longest, weakest bonds, lowest electronegativity.
- Greater s-character → greater electronegativity of carbon.
Some Characteristic Features of π Bonds
Some of the important characteristic features of π bond are discussed below.
- π bond forms by sideways overlap of parallel p orbitals on adjacent atoms.
- Example: in ethene (H₂C=CH₂), all atoms lie in the same plane; p orbitals are perpendicular to this plane.
- Rotation around the C=C double bond is restricted because it disturbs p orbital overlap.
- The electron cloud of the π bond is above and below the plane of the atoms.
- π bonds are electron-rich and hence act as reactive centres in multiple-bond molecules.
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Structural Representations of Organic Compounds
The structural representations of organic compounds are discussed below.
Complete, Condensed and Bond-line Structural Formulas
Organic compounds can be represented in different ways:
- Lewis (dot) structure – shows all valence electrons as dots.
- Dash structure – replaces shared electron pairs with dashes:
- Single dash (–) = single bond
- Double dash (=) = double bond
- Triple dash (≡) = triple bond
Complete structural formula – shows all atoms and bonds (e.g., CH₃OH).
Condensed structural formula – omits some bonds and groups identical atoms with subscripts (e.g., CH₃(CH₂)₆CH₃).
Bond-line formula – uses zig-zag lines for C–C bonds:
- Carbon and hydrogen atoms are usually not shown.
- Terminals = CH₃ groups (unless specified otherwise).
- Line junctions = carbon atoms with implied hydrogens.
- Cyclic compounds can also be drawn using bond-line formulas (e.g., cyclopropane, cyclopentane, chlorocyclohexane).
Three-Dimensional Representation of Organic Molecules
3-D structures are represented on paper using:
- Solid wedge (▲) – bond projecting out of the plane towards the observer.
- Dashed wedge (▿) – bond projecting behind the plane away from the observer.
- Normal line (—) – bond lying in the plane of the paper.
- Wedges are drawn with the broad end toward the observer for proper perspective.
- Example: Methane can be depicted in 3-D using wedge-and-dash notation to show tetrahedral geometry.
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Classification of Organic Compounds
Organic compounds are classified based on structure:
Acyclic or Open Chain Compounds
Also called aliphatic compounds. It consists of straight chains or branched chains.
Cyclic or Closed Chain or Ring Compounds
It consists of alicyclic and aromatic compounds.
Alicyclic Compounds
- Cyclic compounds with carbon atoms joined in a ring.
- It may also contain atoms other than carbon (heterocyclic).
- Example: Tetrahydrofuran (contains oxygen in the ring).
- Share some properties with aliphatic compounds.
Aromatic Compounds
- Special type of cyclic compounds.
- Include benzene and related compounds (benzenoid).
- Non-benzenoid aromatic compounds exist as well.
- Can also be heterocyclic aromatic compounds (contain atoms like O, N, S in the ring).
Examples: Furan, Thiophene, Pyridine.
Functional Group
An atom or group of atoms attached to a carbon chain that gives the compound its characteristic chemical properties. Examples:
- –OH (hydroxyl group)
- –CHO (aldehyde group)
- –COOH (carboxylic acid group)
Homologous Series
The homologous series is discussed below.
- A group/series of compounds with the same functional group.
- Members (homologs) have the same general formula.
- Successive members differ by a –CH₂– unit.
- Examples of series:
- Alkanes
- Alkenes
- Alkynes
- Haloalkanes
- Alkanols
- Alkanals
- Alkanones
- Alkanoic acids
- Amines
- Compounds with two or more functional groups are called polyfunctional compounds.
Nomenclature of Organic Compounds
Organic chemistry contains millions of compounds. A systematic naming method ensures each compound has a unique name that reveals its structure. This method is called the IUPAC system (International Union of Pure and Applied Chemistry). Before IUPAC, compounds were named based on origin or properties → trivial/common names.
- Examples: Citric acid (from citrus fruits), Formic acid (from ants, formica in Latin).
- Common names are still in use for convenience, e.g., Buckminsterfullerene (C₆₀).
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IUPAC System of Nomenclature
Derived from identifying the parent hydrocarbon and functional group(s). Prefixes and suffixes are added to indicate substituents and functional groups.
Hydrocarbons:
- Saturated → only single bonds → Alkanes (suffix: -ane).
- Unsaturated → contain at least one double or triple bond.
IUPAC Nomenclature of Alkanes
The IUPAC Nomenclature of Alkanes is discussed below.
Straight Chain Hydrocarbons
- Names end with -ane.
- Prefix indicates the number of carbon atoms (from CH₄ to C₄H₁₀ have special prefixes from trivial names).
- Members differ by a –CH₂– unit → homologous series
Branched Chain Hydrocarbons
- Alkyl group: formed by removing a hydrogen atom from an alkane (replace -ane with -yl).
- Example: CH₄ → -CH₃ (methyl group).
- Common alkyl groups: n-propyl, isopropyl, sec-butyl, tert-butyl, neopentyl.
Rules for Naming Branched Alkanes
The rules for naming branched alkanes are pointed out below.
- Select the longest chain as the parent chain (must include the maximum carbon atoms).
- Number the chain from the end nearest to the first substituent.
- Name and number substituents:
- List substituents as prefixes in alphabetical order.
- Separate numbers with commas and use hyphens between numbers and names.
- Identical substituents: use di-, tri, tetra, etc. (not counted for alphabetical order).
- If two chains of equal length, choose the one with more substituents.
- Cyclic compounds: Prefix “cyclo” to the alkane name.
- Numbering follows the same rules.
Examples:
- 6-ethyl-2-methylnonane
- 2,4-dimethylpentane
- 3-ethyl-4,4-dimethylheptane
- 3-ethyl-6-methyloctane
Nomenclature of Compounds with Functional Group(s)
The nomenclature of compounds with functional groups is discussed below.
- Functional group: Atom/group of atoms giving characteristic chemical properties.
- Compounds with the same functional group → similar reactions.
- Steps:
- Identify the principal functional group → decide suffix.
- Choose the longest chain containing the functional group.
- Number chain to give the functional group the lowest number.
- Priority order (highest to lowest):
-COOH > –SO₃H > –COOR > –COCl > –CONH₂ > –CN > –CHO > >C=O > –OH > –NH₂ > >C=C< > –C≡C–- R, C₆H₅-, halogens, –NO₂, –OR are always prefix substituents.
Examples:
- HOCH₂(CH₂)₃CH₂COCH₃ → 7-hydroxyheptan-2-one
- BrCH₂CH=CH₂ → 3-bromoprop-1-ene
- CH₂(OH)CH₂(OH) → ethane-1,2-diol
- CH₂=CH–CH=CH₂ → buta-1,3-diene
Nomenclature of Substituted Benzene Compounds
The nomenclature of substituted benzene compounds is discussed below.
- Substituent name + benzene as parent.
- Examples: methylbenzene (toluene), methoxybenzene (anisole), aminobenzene (aniline), nitrobenzene, bromobenzene.
- Disubstituted benzenes:
- Number to give the lowest possible locants.
- Substituents listed alphabetically.
- Example: 1,3-dibromobenzene (not 1,5-).
- Trivial prefixes:
- o- (ortho) → positions 1,2
- m- (meta) → positions 1,3
- p- (para) → positions 1,4
- Tri- or higher substituted compounds: use locants instead of o, m-, p-.
- If benzene acts as a substituent → phenyl group (C₆H₅-, Ph).
Isomerism
Same molecular formula but different properties → compounds called isomers.
Types:
- Structural isomerism – different connectivity of atoms.
- Chain isomerism – different carbon skeletons (e.g., C₅H₁₂).
- Position isomerism – functional group at different positions (e.g., C₃H₈O).
- Functional group isomerism – different functional groups (e.g., C₃H₆O → aldehyde & ketone).
- Metamerism – different alkyl chains on either side of a functional group (e.g., ethers C₄H₁₀O).
- Stereoisomerism – same connectivity, different spatial arrangement.
- Geometrical isomerism
- Optical isomerism
Substrate: Reactant supplying carbon to the new bond.
Reagent: Species attacking the substrate.
Fission of Covalent Bonds
Fission of covalent bonds is discussed below.
- Heterolytic cleavage → unequal breaking; produces:
- Carbocation (C⁺) – electron-deficient, sp² hybridised, trigonal planar.
- Carbanion (C⁻) – electron-rich, sp³ hybridised.
- Polar (ionic) reactions proceed via heterolysis.
- Homolytic cleavage → equal breaking; produces:
- Free radicals – unpaired electrons, very reactive.
- Free radical reactions proceed via homolysis.
Substrate & Reagent Types
There are two substrate and reagent types:
- Nucleophile (Nu:) – electron pair donor, attacks electron-deficient sites.
- Electrophile (E⁺) – electron pair acceptor, attacks electron-rich sites.
Electron Displacement Effects
Different types of electron displacement effects are provided below.
- Inductive Effect (I-effect) – permanent shift of σ-electrons due to electronegativity difference; decreases with distance.
- –I groups: withdraw electrons (e.g., –NO₂, –CN, –COOH).
- +I groups: donate electrons (e.g., –CH₃, –C₂H₅).
- Resonance – delocalisation of π-electrons; the actual molecule is a resonance hybrid.
- +R effect: Electron donation by resonance (e.g., –OH, –NH₂).
- –R effect: Electron withdrawal by resonance (e.g., –NO₂, –CN).
- Electromeric Effect (E-effect) – temporary complete transfer of π-electrons in the presence of a reagent.
- +E: electrons move toward the atom, bonding with the reagent.
- –E: electrons move away from the atom, bonding with the reagent.
- Hyperconjugation – delocalisation of σ C–H electrons to adjacent π-system or empty p-orbital; stabilises carbocations.
Types of Organic Reactions
Categories:
- Substitution reactions
- Addition reactions
- Elimination reactions
- Rearrangement reactions
Methods of Purification of Organic Compounds
Purpose: Remove impurities from extracted or synthesised compounds.
Common techniques:
- Sublimation–separates sublimable solids from non-sublimable impurities.
- Crystallisation – uses solubility differences; a pure solid crystallises on cooling.
- Distillation – separates volatile liquids from non-volatile impurities or liquids with large boiling point differences.
- Fractional distillation – for liquids with close boiling points; uses a fractionating column.
- Differential extraction – separates an organic compound from an aqueous medium using an immiscible organic solvent.
- Chromatography – separates mixtures into components, purifies compounds, and tests purity.
Qualitative Analysis of Organic Compounds
In this section, we have discussed the qualitative analysis of organic compounds.
Elements in Organic Compounds
Always present: Carbon (C) and Hydrogen (H)
May also contain: Oxygen (O), Nitrogen (N), Sulphur (S), Halogens (Cl, Br, I), Phosphorus (P)
Detection of Carbon & Hydrogen
The detection of carbon and hydrogen can be accomplished below
- Compound heated with CuO →
- C → CO₂ (lime water → turbidity)
- H → H₂O (anhydrous CuSO₄ → blue)
- Key reactions:
C + 2CuO → 2Cu + CO₂
2H + CuO → Cu + H₂O
Detection of Other Elements (Lassaigne’s Test)
Principle: Convert covalent elements to ionic form by fusing with sodium metal.
Sodium fusion extract is prepared by boiling the fused mass with distilled water.
(A) Nitrogen
- Sodium fusion extract + FeSO₄ + H₂SO₄ → Prussian blue confirms N.
(B) Sulphur
- Acidify with acetic acid + Pb(CH₃COO)₂ → Black PbS.
- Na₂S + sodium nitroprusside → Violet colour.
(C) Halogens
- Acidify with HNO₃ + AgNO₃:
- White ppt (soluble in NH₄OH) → Cl
- Pale yellow ppt (partially soluble) → Br
- Yellow ppt (insoluble) → I
- If N/S present, pre-treat with conc. HNO₃ to remove interference.
(D) Phosphorus
- Oxidize with Na₂O₂ → phosphate.
- Boil with HNO₃ + (NH₄)₂MoO₄ → Yellow ammonium phosphomolybdate.
Quantitative Analysis of Organic Compounds
To determine the mass percentage of elements, → helps find the empirical & molecular formula.
Carbon & Hydrogen
The quantitative analysis of carbon and hydrogen can be accomplished below.
- Burn known mass with excess O₂ + CuO → C → CO₂, H → H₂O.
- Mass of CO₂ → %C; Mass of H₂O → %H.
Nitrogen
The quantitative analysis of nitrogen can be accomplished below.
- Dumas Method – Burn with CuO in CO₂ atmosphere → N₂ collected over KOH solution.
- Kjeldahl’s Method – Digest with H₂SO₄ → (NH₄)₂SO₄ → release NH₃ with NaOH → absorb in H₂SO₄ → back titrate.
Halogens (Carius Method)
Heat with fuming HNO₃ + AgNO₃ in Carius tube → AgX ppt weighed → %Halogen.
Sulphur
Oxidise with Na₂O₂ or HNO₃ → H₂SO₄ → ppt as BaSO₄ → weigh → %S.
Phosphorus
Oxidise to H₃PO₄ → ppt as ammonium phosphomolybdate or MgNH₄PO₄ → ignite → Mg₂P₂O₇ → weigh → %P.
Oxygen
The quantitative analysis of oxygen can be accomplished below.
- Usually found by difference method: 100 – (%C + %H + %N + %X + %S + %P).
- Direct estimation: Heat in N₂ stream → convert O → CO → oxidise with I₂O₅ → CO₂ → relate to O content.
Important Formulas in NCERT Notes Class 11 Chemistry (Part-II) Chapter 8: Organic Chemistry- Some Basic Principles and Techniques
Here are the important formulas in normal copy-paste format from NCERT Class 11 Chemistry (Part-II) Chapter 8: Organic Chemistry- Some Basic Principles and Techniques.
- Rf = Distance moved by the substance from baseline (x) / Distance moved by the solvent from baseline (y)
- %S = (32 × m1 × 100) / (233 × m)
- %P (from ammonium phosphomolybdate) = (31 × m1 × 100) / (1877 × m)
- %P (from Mg2P2O7) = (62 × m1 × 100) / (222 × m)
- %O = (32 × m1 × 100) / (88 × m)
- Successive members of a homologous series differ by a –CH2 unit
- C + 2CuO → 2Cu + CO2
2H + CuO → Cu + H2O - Kjeldahl’s method: N in compound →(H2SO4) (NH4)2SO4 →(NaOH) NH3↑
- Dumas method: CxHyNz + (2x + y/2) CuO → xCO2 + (y/2) H2O + (z/2) N2
- 6CN– + Fe2+ → [Fe(CN)6]4–
3[Fe(CN)6]4– + 4Fe3+ → Fe4[Fe(CN)6]3 (Prussian Blue)
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FAQs
Ans: Chromatography works on the principle of differential migration of the components of a mixture through a stationary phase under the influence of a mobile phase. The separation is based on the different affinities of components towards the stationary and mobile phases.
Ans: Nitrogen can be estimated by the Dumas method or Kjeldahl’s method. In Dumas’ method, the nitrogen gas formed is collected and measured. In Kjeldahl’s method, nitrogen is converted to ammonia, which is then absorbed in a known amount of acid and back-titrated with a standard alkali.
Ans: The Rf value (retardation factor) is used to identify compounds in chromatography. It is calculated as:
Rf = Distance moved by the substance from baseline / Distance moved by the solvent from baseline.
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