Carbon and Its Compounds — Telangana (SSC) Class 10 Science Solutions (Free)

Why carbon forms covalent bonds: • Carbon has 4 valence electrons (electron configuration 2,4) • Requires 4 more electrons to complete octet • Cannot lose/gain 4 electrons (would require high energy) • Forms 4 covalent bonds by sharing electrons Electron dot structure of methane (CH₄): H | H—C—H | H Each line represents shared pair (covalent bond) Carbon shares one electron with each hydrogen All atoms achieve noble gas configuration (C gets 8e⁻, H gets 2e⁻)

Saturated hydrocarbon: Hydrocarbon with only single bonds (C-C and C-H). Examples: Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈) General formula: CₙH₂ₙ₊₂ Properties: Less reactive, lower density, stable Unsaturated hydrocarbon: Hydrocarbon with one or more double/triple bonds (C=C or C≡C). Examples: Ethene (C₂H₄), Ethyne (C₂H₂), Benzene (C₆H₆) Formula: CₙH₂ₙ (alkenes), CₙH₂ₙ₋₂ (alkynes) Properties: More reactive, can undergo addition reactions, decolorizes bromine/KMnO₄ Key difference: Saturation with hydrogen determines reactivity

Functional group: Atom or group of atoms responsible for chemical properties of organic compound. Five functional groups: 1. Hydroxyl (-OH): Class = Alcohol; Example = C₂H₅OH (ethanol) 2. Carbonyl (C=O): Class = Aldehyde/Ketone; Example = CH₃CHO (acetaldehyde) 3. Carboxyl (-COOH): Class = Carboxylic acid; Example = CH₃COOH (acetic acid) 4. Amino (-NH₂): Class = Amine; Example = C₂H₅NH₂ (ethylamine) 5. Ether (-O-): Class = Ether; Example = C₂H₅OC₂H₅ (diethyl ether) Same functional group → similar chemical reactions in different compounds

Ethane (C₂H₆) - Saturated: Structure: H-C-C-H (with 3 H on each C) | | Properties: Single C-C bond, least reactive, burns in air, no color Ethene (C₂H₄) - Unsaturated: Structure: H H | = | C C | | (double bond between carbons) Properties: Double C=C bond, more reactive, burns with black smoke, decolorizes bromine solution Why ethene is more reactive: 1. Double bond (C=C) is stronger than single bond (C-C) → easier to break 2. π-electrons in double bond are exposed → more available for reaction 3. Addition reactions: C=C + X₂ → C-X bonds form 4. Example: C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane)

Homologous series: Group of compounds with same general formula, differing by CH₂ unit, showing gradation in properties. Properties of homologous series: 1. Each member differs by CH₂ unit 2. Same general formula (e.g., CₙH₂ₙ₊₂ for alkanes) 3. Similar chemical properties 4. Gradation in physical properties (melting point, boiling point, density) 5. Successive members differ by 14 in molecular weight Alkane homologous series (up to C₄): 1. Methane: CH₄ (M = 16) 2. Ethane: C₂H₆ (M = 30) 3. Propane: C₃H₈ (M = 44) 4. Butane: C₄H₁₀ (M = 58) Boiling points increase: CH₄ (-161°C) → C₂H₆ (-89°C) → C₃H₈ (-42°C) → C₄H₁₀ (-0.5°C)

Addition reaction: Reaction where atoms or groups add across double/triple bonds in unsaturated compounds. Conditions: Double/triple bonds present; no atoms removed from molecule Example 1 - Hydrogenation of ethene: C₂H₄ + H₂ → C₂H₆ (ethane) Condition: Nickel catalyst, heat (~150°C) Product: Saturated compound formed Use: Converting vegetable oils (unsaturated) to ghee/butter (saturated) Example 2 - Halogenation of ethene: C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane) Condition: Room temperature, dark Observation: Reddish-brown bromine solution decolorizes Mechanism: Br-Br bond breaks, each Br adds to carbons of C=C bond

Structure of benzene (C₆H₆): • Hexagonal ring of 6 carbons • Each carbon bonded to one hydrogen • Alternating single and double C-C bonds (actually resonance hybrid) • Can be represented as two structures resonating: Structure 1: C=C-C=C-C=C (alternate double bonds) Structure 2: C-C=C-C=C-C (shifted double bonds) • Actual structure: Hybrid with equal C-C bond order (1.5) Why benzene is less reactive than alkenes: 1. π-electrons in benzene are delocalized over entire ring (resonance stabilization) 2. Delocalization provides extra stability (~resonance energy 150 kJ/mol) 3. Breaking double bonds in benzene requires more energy than in alkenes 4. Benzene prefers substitution reactions (maintains ring) over addition reactions 5. Example: Benzene + Br₂ → Bromobenzene + HBr (substitution, not addition)