ICSE Class 10 Chemistry: Complete Conceptual Guide to Organic Chemistry

Organic chemistry is the study of carbon-based compounds whose unique bonding capabilities create immense molecular diversity. For ICSE Class 10 students, mastering this topic requires moving beyond memorization to understand the fundamental logic of carbon bonding, systematic IUPAC nomenclature, isomerism, and the characteristic reaction pathways of hydrocarbons, alcohols, and carboxylic acids.

The Unique Nature of Carbon and Homologous Series

Carbon sits in Group 14 with an atomic number of 6, possessing four valence electrons. To achieve a stable octet, it exclusively forms covalent bonds, a trait known as tetravalency. Coupled with its exceptionally small atomic radius, carbon bonds strongly to other carbon atoms in chains, branched skeletons, and rings. This self-linking capability is termed catenation. The small size of the carbon atom allows its nucleus to hold shared electron pairs firmly, giving carbon-carbon bonds remarkable thermal and chemical stability compared to heavier Group 14 elements like silicon.

A homologous series is a family of organic compounds containing the same functional group and displaying similar chemical properties, wherein successive members differ by a methylene group (-CH₂-) corresponding to a molecular mass difference of 14 atomic mass units (u). All members conform to a general molecular formula, such as CₙH₂ₙ₊₂ for alkanes, CₙH₂ₙ for alkenes, and CₙH₂ₙ₋₂ for alkynes.

As molecular mass increases along a homologous series, physical properties show a predictable gradation: boiling points, melting points, and densities increase due to stronger intermolecular van der Waals forces, whereas solubility in polar solvents like water typically decreases.

IUPAC Nomenclature and Structural Isomerism

The International Union of Pure and Applied Chemistry (IUPAC) system provides an unambiguous name for every organic molecule based on three core components: the root word (longest continuous carbon chain, e.g., meth-, eth-, prop-, but-), the primary suffix (degree of saturation: -ane, -ene, -yne), and the functional group prefix or suffix (such as -ol for alcohols, -al for aldehydes, -oic acid for carboxylic acids).

• Longest Chain Rule: Identify the longest continuous chain of carbon atoms containing the principal functional group or multiple bond.
• Lowest Locant Rule: Number the chain from the end that assigns the lowest possible locant numbers to double/triple bonds, functional groups, and substituents.
• Alphabetical Priority: When multiple alkyl substituents are present, list them alphabetically (e.g., ethyl before methyl), ignoring numerical prefixes like di- or tri-.

Isomerism occurs when compounds share the same molecular formula but possess different structural arrangements. ICSE Class 10 focuses on two primary types of structural isomerism: chain isomerism (differing in the arrangement of the carbon skeleton, such as butane and 2-methylpropane) and position isomerism (differing in the location of a double bond, triple bond, or functional group on the same carbon skeleton, such as but-1-ene and but-2-ene, or propan-1-ol and propan-2-ol).

Alkanes: Methane and Ethane

Alkanes are saturated hydrocarbons containing only carbon-carbon single sigma bonds, making them relatively unreactive under standard conditions (historically called paraffins). In the laboratory, methane (CH₄) is prepared by heating anhydrous sodium ethanoate (sodium acetate) with soda lime (a dry mixture of NaOH and CaO in an approximate 3:1 ratio). The role of calcium oxide is crucial: it is deliquescent, keeping the sodium hydroxide dry, and prevents the glass test tube from fusing with corrosive molten NaOH. The reaction is a decarboxylation: CH₃COONa + NaOH (with CaO, heat) → CH₄ + Na₂CO₃. Ethane is prepared analogously from sodium propanoate (C₂H₅COONa).

Because alkanes are saturated, they do not undergo addition reactions. Instead, their signature transformation is free-radical substitution. In diffused sunlight, methane reacts with chlorine in a stepwise displacement of hydrogen atoms: CH₄ + Cl₂ → CH₃Cl + HCl, progressing through dichloromethane (CH₂Cl₂), trichloromethane (chloroform, CHCl₃), and ultimately tetrachloromethane (carbon tetrachloride, CCl₄). In direct sunlight, however, the reaction is violent and explosive, yielding carbon soot and hydrogen chloride gas: CH₄ + 2Cl₂ → C + 4HCl.

Alkenes and Alkynes: Ethene and Ethyne

Alkenes (CₙH₂ₙ) contain a carbon-carbon double bond, whereas alkynes (CₙH₂ₙ₋₂) possess a carbon-carbon triple bond. Multiple bonds consist of a strong sigma bond and one or two weaker pi bonds, making these unsaturated hydrocarbons far more reactive than alkanes.

• Preparation of Ethene: Prepared via the dehydration of ethanol using excess concentrated sulphuric acid at 170°C (C₂H₅OH → C₂H₄ + H₂O). At a lower temperature of 140°C, diethyl ether is the predominant product. Alternatively, it is prepared by dehydrohalogenation of bromoethane using hot, concentrated alcoholic potassium hydroxide (C₂H₅Br + alc. KOH → C₂H₄ + KBr + H₂O).
• Preparation of Ethyne: Synthesized by the room-temperature hydrolysis of calcium carbide (CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂), or by dehydrohalogenation of 1,2-dibromoethane using hot alcoholic KOH.

The hallmark of unsaturated hydrocarbons is the addition reaction across the pi bond. For instance, both ethene and ethyne decolourize reddish-brown bromine dissolved in carbon tetrachloride (Br₂/CCl₄) without evolving acidic hydrogen bromide gas, converting into 1,2-dibromoethane and 1,1,2,2-tetrabromoethane respectively. They also decolourize cold, dilute, alkaline potassium permanganate (Baeyer's reagent), providing a definitive chemical test to distinguish unsaturated hydrocarbons from saturated alkanes.

Functional Derivatives: Alcohols and Carboxylic Acids

Alcohols contain the hydroxyl (-OH) group, while carboxylic acids possess the carboxyl (-COOH) group. Ethanol (C₂H₅OH) is a neutral liquid that reacts with active metals like sodium to release hydrogen gas (2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂), confirming the weakly acidic nature of the hydroxylic hydrogen. Controlled oxidation of ethanol using acidified potassium dichromate (K₂Cr₂O₇) yields ethanal (CH₃CHO) and subsequently ethanoic acid (acetic acid, CH₃COOH), accompanied by an orange-to-green colour transition as dichromate is reduced to Cr³⁺.

Esterification occurs when ethanol reacts with ethanoic acid in the presence of concentrated sulphuric acid (which acts both as an acid catalyst and a dehydrating agent to shift equilibrium forward), producing ethyl ethanoate, an ester characterized by a sweet, fruity odor: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O.

Unlike ethanol, ethanoic acid behaves as a typical weak monoprotic acid: it turns blue litmus paper red, liberates carbon dioxide effervescence with sodium carbonate or sodium hydrogencarbonate (Na₂CO₃ / NaHCO₃), and neutralizes bases to form metal ethanoates and water.

Key takeaways

• Carbon achieves structural diversity through high-energy catenation and tetravalency, forming stable single, double, and triple covalent bonds.
• Every successive member of a homologous series differs by a -CH₂- unit (14 u), exhibiting identical functional groups, similar chemical traits, and a gradual trend in physical properties.
• Alkanes are saturated and primarily undergo substitution reactions in diffused sunlight; unsaturated alkenes and alkynes rapidly undergo addition reactions across their pi bonds.
• The decolourization of bromine water (or Br₂ in CCl₄) and alkaline KMnO₄ (Baeyer's reagent) without gas evolution serves as the primary diagnostic test for unsaturation.
• Ethanol and ethanoic acid condense in the presence of concentrated sulphuric acid to form ethyl ethanoate, an ester identified by its fruity fragrance.

Test yourself