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Organic Chemistry – Some Basic Principles and Techniques – Class 11 Chemistry notes, important questions and MCQs

NCERT Class 11 Chemistry, chapter 8. Notes, key terms, important questions with answers and MCQs.

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Chapter overview

Organic chemistry studies carbon compounds. Carbon's tetravalency and catenation give many compounds, named by IUPAC rules; electronic effects explain reactivity, and special methods purify and analyse them.

Structure and naming

  • Hybridisation of carbon: sp³ (single), sp² (double), sp (triple bonds).
  • IUPAC: longest chain, lowest locants, suffix for the principal functional group, prefixes alphabetically.
  • Isomerism: structural (chain, position, functional, metamerism, tautomerism) and stereoisomerism.

Reactivity and techniques

  • Inductive effect (−I: NO₂, Cl; +I: alkyl), resonance (±R), hyperconjugation and electromeric effects.
  • Carbocation stability: 3° > 2° > 1° > methyl; free radicals follow the same order.
  • Purification: crystallisation, distillation (fractional, steam, vacuum), chromatography; Lassaigne's test detects N, S, halogens.

Key terms

Homologous series
compounds differing by –CH₂– with similar properties
Hyperconjugation
delocalisation of σ electrons of C–H bonds with an adjacent π system or empty p orbital
Nucleophile
electron-rich species that attacks electron-poor centres

Important questions with answers

What is the difference between homolytic and heterolytic cleavage?2 marks

In homolytic cleavage each atom keeps one electron of the bond, giving free radicals. In heterolytic cleavage one atom takes both electrons, giving a cation and an anion.

Calculate the percentage of carbon and hydrogen if 0.2 g of an organic compound gives 0.44 g of CO₂ and 0.18 g of H₂O on combustion.3 marks

C = (12/44) × 0.44 = 0.12 g, so %C = 0.12/0.2 × 100 = 60%. H = (2/18) × 0.18 = 0.02 g, so %H = 10%. (The rest, 30%, could be oxygen.)

Explain the inductive effect, resonance effect and hyperconjugation with one example each, and use them to arrange the acidity of CH₃COOH, ClCH₂COOH and FCH₂COOH.5 marks

Inductive effect: permanent shift of σ-electrons along a chain due to an electronegative atom; –Cl pulls electrons (−I), –CH₃ pushes (+I). Resonance effect: delocalisation of π or lone-pair electrons in conjugated systems, e.g. benzene, or –OH donating (+R) into a ring. Hyperconjugation: delocalisation of C–H σ-electrons into an adjacent empty p orbital or π system; explains why tert-butyl carbocation is more stable than methyl. Acidity: −I groups stabilise the carboxylate anion, and F has a stronger −I effect than Cl, so FCH₂COOH > ClCH₂COOH > CH₃COOH.

Quick revision

  • Carbocation stability 3° > 2° > 1°.
  • −I groups pull electrons.
  • Chromatography separates by differential adsorption.

Exam tips

  • Steam distillation suits volatile, water-immiscible compounds like aniline.
  • For IUPAC names, number the chain to give the principal functional group the lowest locant, even if substituents then get higher numbers.

Practice MCQs

  1. Hybridisation of carbon in methane is

    1. sp³
    2. sp²
    3. sp
    4. sp³d
    Show answer

    Answer: (a) sp³. Four single bonds.

  2. Which is a nucleophile

    1. NH₃
    2. H⁺
    3. BF₃
    4. NO₂⁺
    Show answer

    Answer: (a) NH₃. Lone pair.

  3. IUPAC name of CH₃CH₂CH₂OH is

    1. propan-2-ol
    2. propanal
    3. propan-1-ol
    4. ethanol
    Show answer

    Answer: (c) propan-1-ol. OH on C1.

  4. Most stable carbocation is

    1. methyl
    2. primary
    3. secondary
    4. tertiary
    Show answer

    Answer: (d) tertiary. Hyperconjugation and +I.

  5. IUPAC name of CH₃–CH(CH₃)–CH₂–CH₃ is

    1. 2-methylbutane
    2. 3-methylbutane
    3. pentane
    4. isobutane
    Show answer

    Answer: (a) 2-methylbutane. Lowest locant.

  6. Lassaigne's test for nitrogen gives a colour of

    1. blood red
    2. violet
    3. Prussian blue
    4. white ppt
    Show answer

    Answer: (c) Prussian blue. Ferric ferrocyanide.

  7. Which group shows −I effect

    1. –NO₂
    2. –CH₃
    3. –C₂H₅
    4. –C(CH₃)₃
    Show answer

    Answer: (a) –NO₂. Electron withdrawing.

  8. Which is the most stable carbanion

    1. CH₃⁻
    2. (CH₃)₃C⁻
    3. CH₃CH₂⁻
    4. (CH₃)₂CH⁻
    Show answer

    Answer: (a) CH₃⁻. Alkyl groups (+I) destabilise carbanions.

All chapters of Class 11 Chemistry

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.