Hydrocarbons is part of Chemistry. Below are notes on 10 sub-topics, followed by practice MCQs from State PSC and UPSC papers.

Acidity of Terminal Alkynes

  • Terminal alkynes possess acidic hydrogen atoms attached to sp-hybridized carbon.
  • This acidity arises from the high electronegativity of the sp-hybridized carbon, which has 50% s-character.
  • The sp-hybridized carbon pulls electron density from the C-H bond, facilitating the removal of hydrogen as a proton (H+).
  • The resulting carbanion (acetylide ion) is stabilized by the electronegative sp carbon.
  • Acidity order: Terminal Alkynes > Alkenes > Alkanes.
  • Terminal alkynes react with strong bases (e.g., sodium, sodium amide (NaNH2), Grignard reagents) to form metal acetylides.
  • Example: Acetylene (ethyne) contains two acidic hydrogen atoms.

Alicyclic Hydrocarbons: Preparation and Properties

  • Alicyclic Hydrocarbons: Cyclic organic compounds resembling aliphatic hydrocarbons in properties. They contain rings of carbon atoms and can be saturated (cycloalkanes) or unsaturated (cycloalkenes, cycloalkynes).
  • Cycloalkanes: Saturated alicyclic hydrocarbons with the general formula CnH2n (for monocyclic rings). Examples: Cyclopropane, cyclobutane, cyclopentane, cyclohexane.
  • Preparation of Alicyclic Hydrocarbons:
    • Hydrogenation of Aromatic Compounds: E.g., Benzene (C6H6) + 3H2 → Cyclohexane (C6H12) over Ni/Pt/ Pd catalyst.
    • Wurtz Reaction (Intramolecular): Dihaloalkanes (e.g., 1,3-dibromopropane) react with Na/Zn to form small rings (e.g., cyclopropane).
    • Diels-Alder Reaction: For cyclic alkenes (e.g., butadiene + ethene → cyclohexene derivative).
  • Properties of Alicyclic Hydrocarbons:
    • Physical Properties: Generally nonpolar, higher melting and boiling points compared to their open-chain alkane counterparts due to more efficient packing in the solid/liquid state.
    • Chemical Properties:
      • Ring Strain: Small rings (cyclopropane, cyclobutane) exhibit significant angle strain and undergo ring-opening reactions (e.g., with H2/Ni, HBr).
      • Stability: Cyclopentane and cyclohexane are relatively strainfree and stable, behaving similarly to alkanes (undergo free radical substitution reactions, e.g., halogenation).
  • Cyclohexane (C6H12):
    • A six-membered saturated alicyclic hydrocarbon (cycloalkane).
    • Structure: All carbons are sp3 hybridized, forming a puckered ring (chair, boat conformations) to minimize strain.
    • Bonds: Contains 6 Carbon-Carbon single (saturated) bonds and 12 Carbon-Hydrogen single (saturated) bonds.
    • Total Saturated Bonds: 6 (C-C) + 12 (C-H) = 18.
    • Unsaturated Bonds: 0 (zero).

Alkanes: Preparation, Physical and Chemical Properties

Alkanes are saturated hydrocarbons with the 1. general formula CnH₂n+2. They are also known as Paraffins. Methane (CH₄) is the simplest paraffin struc2. ture and its molecular formula is CH₄. Methane (CH₄) is the major component of 3. natural gas, biogas, Gobar gas, and Compressed Natural Gas (CNG). It is also known as Marsh gas. Liquefied Petroleum Gas (LPG) is primarily 4. a mixture of Propane (C₃H₈) and Butane (C₄H₁₀) or Isobutane. Isobutane is used as a refrigerant, replacing 5. HFCs. Petroleum is a mixture of various hydrocar6. bons; its grade depends on the relative proportions of these hydrocarbons. Kerosene is a mixture of hydrocarbons and is 7. absent of oxygen. Methane (CH₄) is a significant greenhouse 8. gas. Hydrocarbons are primary pollutants 9. released from automobiles. Molecular Weights (increasing order): Meth10. ane (16) < Ethane (30) < Propane (44) < Butane (58) < Pentane (72). Boiling Points (increasing order for normal 11. alkanes): Methane < Ethane < Propane < Butane < Pentane < Heptane < Octane. Boiling point generally increases with molecular weight; branching decreases boiling point. Ethane (C₂H₆) has 80% carbon by mass. 12. Hexadecane (C₁₆H₃₄) is known as Cetane. 13. 2,2,3-Trimethylbutane is known as Triptane. 14. Octane Number Order (increasing resistance 15. to knocking): Branched alkanes > short straight-chain alkanes > long straight-chain alkanes. Example: Cyclohexane > Butane > Pentane > Hexane. Combustion of Methane: 16.

    • Complete combustion: CH₄ + 2O₂ → CO₂ + 2H₂O. (1 mole CH₄ produces 2 moles H₂O).
    • Partial oxidation: CH₄(g) + ½ O₂ → CO + 2H₂. (When 2 moles CH₄ react with 2 moles O₂, 2 moles CO are obtained, as CH₄ is the limiting reactant).
    • Steam reforming: CH₄ + H₂O → CO + 3H₂. (This reaction produces the maximum quantity of H₂ gas by the decomposition of 1g of CH₄). In a propane (C₃H₈) molecule, the ratio of C17. C bonds to C-H bonds is 2:8 or 1:4. Biogas is a mixture primarily of CH₄, CO₂, 18. H₂, H₂S.

Alkenes: Preparation, Physical and Chemical Properties

  • Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C).
  • The general formula for open-chain alkenes is CnH2n. E.g., C4H8 belongs to the alkene series.
  • Ethene (common name: Ethylene) is the simplest alkene.
  • Unsaturated hydrocarbons (like alkenes) burn with a yellow, sooty flame and black smoke due to incomplete combustion.
  • Alkenes primarily undergo addition reactions due to the presence of the double bond.
  • Hydrogenation: Alkenes add hydrogen (H₂) in the presence of catalysts like Nickel (Ni), Platinum (Pt), or Palladium (Pd) to form saturated hydrocarbons (alkanes).
  • Hydrogenation of vegetable oils (unsaturated) to produce vanaspati ghee (saturated fats) is an industrial application using Nickel catalyst; this is an addition reaction.
  • Ethylene gas is widely used for the artificial ripening of green fruits.
  • The ethylene (C₂H₄) molecule has a coplanar triangular shape.

Alkynes: Preparation, Physical and Chemical Properties

  • Alkynes are unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon triple bond (C≡C).
  • The general formula for alkynes is CnH2n-2.
  • Ethyne (common name: Acetylene, C₂H₂) is the simplest alkyne, featuring a C≡C bond.
  • Preparation:
    • Acetylene is commonly prepared by the action of water on calcium carbide (CaC₂): CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂.
    • Note: Silicon carbide (SiC) does not produce acetylene when mixed with water.
  • Homologous Series: Ethyne (C₂H₂) is the first member. Its homologues include propyne (C₃H₄), butyne (C₄H₆), etc., each differing by a -CH₂- unit.
  • Uses of Acetylene (Ethyne):
    • Welding: When burned with oxygen, it produces an oxy-acetylene flame, reaching very high temperatures (up to 3000°C), essential for welding and cutting metals.
    • Raw Material for Plastics: Acetylene serves as a crucial raw material in the synthesis of various organic compounds, including monomers like vinyl chloride, which are then polymerized to produce plastics (e.g., PVC).

Aromatic Hydrocarbons: Concept of Aromaticity (Hückel's Rule), Resonance Structures

  • Aromatic hydrocarbons are cyclic, planar, fully conjugated compounds exhibiting unusual stability due to delocalized pi electrons.
  • Hückel's Rule for Aromaticity: An aromatic compound must possess (4n+2) pi electrons, where n = 0, 1, 2, ... (i.e., 2, 6, 10, 14, ... pi electrons).
  • Resonance Structures: Aromatic compounds exhibit resonance (electron delocalization), which contributes significantly to their stability (e.g., Benzene's two Kekulé structures).
  • Benzene (C₆H₆) is a planar, cyclic molecule with 6 sp² hybridized carbon atoms.
  • Benzene has 12 sigma bonds (6 C-C and 6 CH) and 3 delocalized pi bonds.
  • One mole of benzene requires 3 moles of H₂(g) for complete saturation (hydrogenation) to form cyclohexane.
  • Aromatic hydrocarbons (e.g., Benzene, Naphthalene, Anthracene) have a high carbon-tohydrogen (C/H) ratio.
  • Due to their high C/H ratio, aromatic hydrocarbons typically undergo incomplete combustion and burn with a yellow, sooty flame.
  • Aliphatic hydrocarbons (e.g., Hexane), having a lower C/H ratio, generally burn with a clean, non-sooty flame.
  • Gas Chromatography (GC) is a widely used analytical instrument for the detection and analysis of polyaromatic hydrocarbons (PAHs).

Conformational Analysis of Alkanes

  • Conformations are different spatial arrangements of atoms that can be interconverted by rotation around C-C single bonds.
  • Conformational Isomers (Conformers) are rapidly interconverting isomers at room temperature.
  • Newman projections are used to visualize conformations by looking down a specific CC bond.
  • Torsional Strain (Pitzer Strain): Strain due to repulsion between electron clouds of bonds on adjacent atoms, highest in eclipsed conformations.
  • Steric Strain: Repulsion when two atoms or groups are forced too close, interacting unfavorably. Significant with bulky groups.
  • Ethane (C2H6) Conformations:
    • Staggered: Hydrogens on adjacent carbons are 60° apart (most stable, lowest energy, no torsional strain).
    • Eclipsed: Hydrogens on adjacent carbons directly align (least stable, highest energy due to torsional strain).
  • n-Butane (along C2-C3 bond) Conformations:
    • Anti Conformation: Methyl groups are 180° apart. Most stable, lowest energy, minimizes torsional and steric strain.
    • Gauche Conformation: Methyl groups are 60° apart. More stable than eclipsed, but less than anti due to slight steric strain (methyl-methyl repulsion).
    • Eclipsed Conformation: Methyl group eclipses a hydrogen. Higher energy than gauche/anti due to torsional strain.
    • Fully Eclipsed Conformation: Two methyl groups directly eclipse each other. Least stable, highest energy due to maximum torsional and steric strain.
  • Stability Order (n-Butane): Anti > Gauche > Eclipsed > Fully Eclipsed.

Electrophilic Addition Reactions of Alkenes (Markownikoff's and Anti-Markownikoff's Rules, Ozonolysis)

  • Electrophilic Addition Reactions (EAR) are characteristic reactions of alkenes, where an electrophile attacks the electron-rich C=C bond.
  • Markownikoff's Rule: In the addition of an unsymmetrical reagent (e.g., HX, H₂O) to an unsymmetrical alkene, the negative part of the reagent adds to the carbon atom of the double bond that has fewer hydrogen atoms.
  • Markownikoff's rule proceeds via the formation of a more stable carbocation intermediate (tertiary > secondary > primary).
  • Anti-Markownikoff's Rule (Peroxide Effect / Kharasch Effect): Applies specifically to the addition of HBr to unsymmetrical alkenes in the presence of peroxides.
  • In Anti-Markownikoff addition, the bromine atom adds to the carbon atom of the double bond that has more hydrogen atoms.
  • Anti-Markownikoff's rule follows a free radical mechanism and is observed only with HBr, not HCl or HI.
  • Ozonolysis: Reaction of alkenes with ozone (O₃) followed by reductive workup (e.g., Zn/ H₂O or (CH₃)₂S).
  • Ozonolysis cleaves the C=C double bond, forming carbonyl compounds (aldehydes and/or ketones).
  • If a carbon of the C=C bond is H₂C=, it yields formaldehyde (HCHO).
  • If a carbon of the C=C bond is R-CH=, it yields an aldehyde (RCHO).
  • If a carbon of the C=C bond is R₂C=, it yields a ketone (R₂C=O).
  • For the alkene (CH₃)₂C=CH₂, ozonolysis yields (CH₃)₂C=O (propan-2-one) and H₂C=O (formaldehyde).

Isomerism in Hydrocarbons (Structural, Geometrical, Conformational)

  • Isomerism: Compounds with same molecular formula but different structural or spatial arrangement of atoms.
  • Structural (Constitutional) Isomerism: Isomers differ in the connectivity of atoms.
    • Chain (Skeletal) Isomerism: Different carbon chain arrangements.
      • Example: Pentane (C5H12) has 3 structural (chain) isomers: npentane, isopentane (2-methylbutane), and neopentane (2,2dimethylpropane).
    • Position Isomerism: Different position of the functional group, substituent, or multiple bond on the same carbon skeleton.
      • Example: But-1-ene and But-2ene.
    • Functional Group Isomerism: Different functional groups.
      • Example: Ethanol (alcohol) and Dimethyl ether (ether) for C2H6O.
    • Metamerism: Different alkyl groups attached to the same polyvalent functional group.
      • Example: Diethyl ether and Methyl propyl ether.
    • Tautomerism: Rapid interconversion of two structural isomers in dynamic equilibrium, usually involving proton migration.
      • Example: Keto-enol tautomerism.
  • Stereoisomerism: Isomers have the same connectivity but different spatial arrangements of atoms.
    • Conformational Isomerism (Conformers/Rotamers): Interconverted by rotation around single (sigma) bonds without breaking any bonds.
      • Characterized by rapid interconversion at room temperature; conformers generally cannot be isolated.
      • Examples: Ethane (staggered, eclipsed), Butane (anti, gauche, eclipsed).
    • Configurational Isomerism: Interconversion requires breaking and reforming bonds.
      • Geometrical Isomerism (cistrans or E/Z Isomerism): Arises from restricted rotation around a double bond or in a cyclic structure.
      • Condition: Each sp2 hybridized carbon atom of the double bond must be attached to two different groups.
      • Cis-isomer: Similar groups on the same side of the double bond/ring.
      • Trans-isomer: Similar groups on opposite sides of the double bond/ring.
      • E/Z Nomenclature: E (entgegen - opposite) and Z (zusammen - together) system, based on CIP priority rules, for more complex alkenes.
      • Example: But-2-ene exists as cis-but-2-ene and trans-but-2-ene.

Nomenclature of Hydrocarbons (IUPAC and common systems)

IUPAC Nomenclature - General Rules: Sys1. tematic naming based on identifying the parent chain, functional groups, and substituents. Parent Chain Selection: Identify the longest 2. continuous carbon chain. For unsaturated hydrocarbons (alkenes, alkynes), the parent chain MUST include the multiple bond(s). Numbering the Parent Chain: 3.

    • Number the chain from the end that gives the lowest possible locant (number) to the highest priority functional group (e.g., double/triple bond, then substituents).
    • If multiple bonds are equidistant from both ends, number to give the substituents the lowest possible locants. Naming Substituents: 4.
    • Alkyl groups (carbon branches) are named by replacing the '-ane' suffix of the corresponding alkane with '-yl' (e.g., methane → methyl, ethane → ethyl).
    • Locate substituents by their position number on the parent chain.
    • If identical substituents occur multiple times, use prefixes like di-, tri-, tetra(e.g., dimethyl, triethyl). These prefixes are ignored when alphabetizing substituents for the full name. Prefixes for Carbon Chain Length (Parent 5. Chain):
    • Meth- (1 Carbon)
    • Eth- (2 Carbons)
    • Prop- (3 Carbons)
    • But- (4 Carbons)
    • Pent- (5 Carbons)
    • Hex- (6 Carbons)
    • Hept- (7 Carbons)
    • Oct- (8 Carbons)
    • Non- (9 Carbons)
    • Dec- (10 Carbons) Suffixes for Hydrocarbons: 6.
    • Alkanes (only single C-C bonds): -ane (e.g., Ethane for 2 carbons, Butane for 4 carbons).
    • Alkenes (at least one C=C double bond): -ene. The double bond locant precedes -ene (e.g., But-2-ene).
    • Alkynes (at least one C≡C triple bond): -yne. The triple bond locant precedes -yne (e.g., But-1-yne). Constructing the IUPAC Name: [Locant(s) of 7. substituent(s)]-[Name(s) of substituent(s)][Parent chain prefix]-[Locant(s) of multiple bond(s)]-[Suffix]. Example for Alkenes (MCQ 1): For CH38. CH(C2H5)-CH=CH-CH3:
    • Expand C2H5 to CH2CH3. The structure becomes CH3-CH(CH2CH3)CH=CH-CH3.
    • The longest carbon chain containing the double bond is a 6-carbon chain (hexene derivative).
    • Numbering from the right end gives the double bond at C2 (C2-C3) and the methyl substituent at C4.
    • Therefore, the IUPAC name is 4methylhex-2-ene. Shortest Straight Chain with Two Carbons 9. (MCQ 2): Based on prefixes (Eth- for 2 carbons) and alkane suffix (-ane), the hydrocarbon is Ethane.

13. Alcohol

Practice MCQs: Hydrocarbons

Acidity of Terminal Alkynes

  1. The hydrogen atoms present in acetylene molecule are :

    Asked in: UPSC CAPF 2015

    1. acidic
    2. basic
    3. both acidic and basic
    4. neutral
    Show answer

    Answer: A. acidic

    The acidity of terminal alkynes (like acetylene) is due to the high electronegativity of the sp-hybridized carbon atom to which the hydrogen is attached. This polarizes the C-H bond, making the proton relatively easy to remove.

Alicyclic Hydrocarbons: Preparation and Properties

  1. The number of saturated and unsaturated bonds in cyclohexane are :

    Asked in: UPSC CDS 1 2022

    1. 9 and 0 respectively.
    2. 18 and 3 respectively.
    3. 18 and 0 respectively.
    4. 9 and 3 respectively.
    Show answer

    Answer: C. 18 and 0 respectively.

    In organic chemistry, molecules containing only single bonds between carbon atoms and single bonds to hydrogen atoms are considered saturated. The term "saturated bond" commonly refers to a single bond, while "unsaturated bond" refers to double or triple bonds.

Alkanes: Preparation, Physical and Chemical Properties

  1. The correct order of octane number of butane, pentane, hexane and cyclohexane is

    Asked in: UPSC CDS 1 2024

    1. butane > pentane > cyclohexane > hexane
    2. butane > pentane > hexane > cyclohexane
    3. butane > cyclohexane > pentane > hexane
    4. cyclohexane > butane > pentane > hexane
    Show answer

    Answer: C. butane > cyclohexane > pentane > hexane

    Octane number is inversely related to chain length for n-alkanes and higher for branched/cyclic structures compared to straight chains.

  2. Which one of the following is known as cetane?

    Asked in: UPSC CDS 1 2024

    1. Hexadecane
    2. Heptadecane
    3. Octadecane
    4. Nonadecane
    Show answer

    Answer: A. Hexadecane

    Cetane is the common name for the alkane molecule hexadecane (C₁₆H₃₄). It is used as a reference standard with a high cetane number (defined as 100) to evaluate the ignition quality of diesel fuel.

  3. How many moles of CO can be obtained by reacting 2·0 mole of CH₄ with 2·0 mole of O₂ according to the equation given below? CH₄(g) + 1/2 O₂ → CO + 2H₂

    Asked in: UPSC CDS 2 2018

    1. 2·0
    2. 0·5
    3. 2·5
    4. 4·0
    Show answer

    Answer: A. 2·0

    The balanced chemical equation is CH₄(g) + 1/2 O₂ → CO + 2H₂. We need to determine the limiting reactant based on the initial amounts (2.0 mol CH₄ and 2.0 mol O₂).

  4. Which of the following is the general formula for saturated hydrocarbons?

    Asked in: UPSC CDS 2 2018

    1. CnH₂n+2
    2. CnH₂n-2
    3. CnH₂n+1
    4. CnH₂n-1
    Show answer

    Answer: A. CnH₂n+2

    - Saturated hydrocarbons are alkanes. - They contain only single bonds between carbon atoms. - The general formula represents the relationship between the number of carbon and hydrogen atoms in such molecules.

  5. In which one of the following reactions, the maximum quantity of H₂ gas is produced by the decomposition of 1 g of compound by H₂O/O₂?

    Asked in: UPSC CDS 2 2018

    1. CH₄ + H₂O → CO + 3H₂
    2. CO + H₂O → CO₂ + H₂
    3. CH₄ + ½O₂ → CO + 2H₂
    4. C₁₂H₂₄ + 6O₂ → 12CO + 12H₂
    Show answer

    Answer: A. CH₄ + H₂O → CO + 3H₂

    To find the maximum quantity of H₂ produced per gram of reactant, we need to calculate the moles of H₂ produced per gram of the principal reactant in each reaction. Let's calculate the moles of H₂ per gram of the compound undergoing decomposition/ reaction: A) CH₄ + H₂O → CO + 3H₂: Molar mass of CH₄ is 16 g/mol. 1 mol CH₄ produces 3 mol H₂. Moles of H₂ per gram of CH₄ = (3 mol H₂) / (16 g CH₄) = 3/16 mol/g. B) CO + H₂O → CO₂ + H₂: Molar mass of CO is 28 g/mol. 1 mol CO produces 1 mol H₂. Moles of H₂ per gram of CO = (1 mol H₂) / (28 g CO) = 1/28 mol/g. C) CH₄ + ½O₂ → CO + 2H₂: Molar mass of CH₄ is 16 g/mol. 1 mol CH₄ produces 2 mol H₂. Moles of H₂ per gram of CH₄ = (2 mol H₂) / (16 g CH₄) = 2/16 = 1/8 mol/g. D) C₁₂H₂₄ + 6O₂ → 12CO + 12H₂: Molar mass of C₁₂H₂₄ is 168 g/mol. 1 mol C₁₂H₂₄ produces 12 mol H₂. Moles of H₂ per gram of C₁₂H₂₄ = (12 mol H₂) / (168 g C₁₂H₂₄) = 12/168 = 1/14 mol/g. Comparing mol/g, corresponding to reaction A.

  6. Consider the following hydrocarbons : A. Methane B. Ethane C. Propane D. Butane E. Pentane What is the correct sequence of increasing order of their molecular weight ? Select the code for the correct answer from the options given below :

    Asked in: KPSC KAS Prelims 2015

    1. A, B, D, C, E
    2. B, A, D, C, E
    3. A, B, C, D, E
    4. C, B, A, D, E
    Show answer

    Answer: C. A, B, C, D, E

    These are simple saturated hydrocarbons (alkanes) with the general formula CnH2n+2. The molecular formulas are: A. Methane: CH4 (n=1) B. Ethane: C2H6 (n=2) C. Propane: C3H8 (n=3) D. Butane: C4H10 (n=4) E. Pentane: C5H12 (n=5) The atomic weight of Carbon (C) is approximately 12.01 g/mol, and Hydrogen (H) is approximately 1.008 g/mol. The molecular weights are: A. CH4: 1 12.01 + 4 molecular weight increases directly with the number of carbon atoms (and consequently, hydrogen atoms). Therefore, the order A (Methane)

  7. Which of the following hydrocarbon has 80% carbon? (H = 1, C = 12)

    Asked in: MPSC CS Prelims 2023

    1. Benzene
    2. Methane
    3. Ethane
    4. Cyclohexane
    Show answer

    Answer: C. Ethane

    The percentage by mass of an element in a compound is calculated by dividing the total mass of the element in one mole of the compound by the molar mass of the compound and multiplying by 100%.

  8. Directions : The following 05 (Five) items consist of two statements, Statement I and Statement II. Examine these two statements carefully and select the answers to these items using the code given below : Code : Both the Statements are individually true and Statement II is the correct explanation of Statement I. Both the Statements are individually true but Statement II is not the correct explanation of Statement I. Statement I is true but Statement II is false. Statement I is false but Statement II is true. 51. Statement I: Petroleum is a mixture of many different hydrocarbons of different densities. Statement II: The grade of petroleum depends mainly on the relative proportion of the different hydrocarbons.

    Asked in: UPSC NDA 1 2016

    1. Both the Statements are individually true and Statement II is the correct explanation of Statement I.
    2. Both the Statements are individually true but Statement II is not the correct explanation of Statement I.
    3. Statement I is true but Statement II is false.
    4. Statement I is false but Statement II is true.
    Show answer

    Answer: A. Both the Statements are individually true and Statement II is the correct explanation of Statement I.

    Statement I is true. Petroleum is a complex mixture primarily composed of hydrocarbons, which vary in size, structure, and density. Statement II is also true. The physical and chemical properties, and thus the 'grade' (e.g., light/heavy crude, different viscosities), of petroleum depend heavily on the specific blend and relative proportions of the various hydrocarbons present. These proportions directly influence density, boiling point range, viscosity, etc., which define different petroleum grades. Statement II explains the significance of the composition mentioned in Statement I.

  9. Which of the hydrocarbons are arranged as per the increasing order of their boiling points ?

    Asked in: UPSC NDA 1 2023

    1. Methane, Butane, Propane, Heptane
    2. Propane, Butane, Pentane, Octane
    3. Propane, Butane, Heptane, Methane
    4. Octane, Ethane, Methane, Propane
    Show answer

    Answer: B. Propane, Butane, Pentane, Octane

    For a homologous series of hydrocarbons like alkanes, the boiling point generally increases with increasing molecular weight, which corresponds to an increasing number of carbon atoms. This is because larger molecules have stronger van der Waals forces (London dispersion forces), requiring more energy (higher temperature) to overcome them and transition from liquid to gas phase.

  10. Methane is the carbon compound. It's Molecular Formula is

    Asked in: TNPSC CS Prelims 2019

    1. CH₄
    2. CH₃Cl
    3. C₂H₆
    4. CCl₄
    Show answer

    Answer: A. CH₄

    - Alkanes are saturated hydrocarbons with the general formula CnH₂n+2, where 'n' is the number of carbon atoms. - Methane is the alkane with one carbon atom (n=1). - Substituting n=1 into the general formula CnH₂n+2 gives C₁H₂(1)+2 = CH₄.

Alkenes: Preparation, Physical and Chemical Properties

  1. A hydrocarbon in which two carbon atoms are joined by a double bond is called as an

    Asked in: BPSC Prelims Exam 63rd

    1. alkane
    2. alkene
    3. alkyne
    4. ionic bond
    Show answer

    Answer: B. alkene

    - Hydrocarbons are classified by carbon-carbon bond types. - Alkanes: Single bonds. - Alkenes: Double bonds. - Alkynes: Triple bonds.

  2. While burning hydrocarbon fuels, if we see a yellow flame with lots of black smoke, it means that the fuel is :

    Asked in: UPSC CDS 1 2022

    1. made of saturated hydrocarbons.
    2. made of unsaturated hydrocarbons.
    3. burning completely.
    4. wet.
    Show answer

    Answer: B. made of unsaturated hydrocarbons.

    Complete combustion produces a clean, often blue, flame and primarily carbon dioxide and water. Incomplete combustion produces a yellow, luminous, and sooty flame, along with carbon monoxide and unburnt carbon particles (soot), which appear as black smoke. The likelihood of incomplete combustion depends on the fuel type and the oxygen supply.

  3. The gas used for artificial ripening of fruits is

    Asked in: JPSC CS Prelims 2011 · KPSC KAS Prelims 2014

    1. ether
    2. ammonia
    3. acetylene
    4. ethylene
    Show answer

    Answer: D. ethylene

    Ethylene is a plant hormone that promotes fruit ripening and is widely used for artificial ripening.

  4. C4H8 belongs to the homologous series of

    Asked in: UPSC NDA 2 2022

    1. alkanes
    2. alkenes
    3. alkynes
    4. cycloalkanes
    Show answer

    Answer: B. alkenes

    Homologous series are a series of compounds with the same general formula, usually differing by a single methylene (CH₂) unit. The general formula for alkanes is CnH2n+2, for alkenes is CnH2n (with one double bond), and for alkynes is CnH2n-2 (with one triple bond). For n=4, C₄H₈ fits the general formula CnH2n, which corresponds to alkenes. While cycloalkanes also have the formula CnH2n, alkenes are the primary open-chain homologous series with this formula.

  5. In ethylene which of the following shape is acquired by the molecule ?

    Asked in: OPSC CS Prelims 2021

    1. Linear
    2. Tetrahedral
    3. Coplanar triangular
    4. Hectohedral
    Show answer

    Answer: C. Coplanar triangular

    Ethylene (C2H4) has a structure where two carbon atoms are joined by a double bond, and each carbon atom is also bonded to two hydrogen atoms (H2C=CH2). - The geometry around each carbon atom can be predicted using VSEPR theory. Each carbon atom is bonded to three other atoms (one carbon and two hydrogens), and there are no lone pairs on the carbon atoms involved in sigma bonding to other atoms. - The three electron domains around each carbon (one double bond, two single bonds) arrange themselves as far apart as possible, resulting in a trigonal planar geometry around each carbon. The ideal bond angles are approximately 120 degrees. - Because of the rigidity of the double bond and the trigonal planar arrangement around each carbon, all six atoms in the ethylene molecule lie in the same plane. - Thus, the molecule is planar (coplanar), and the bond angles around each carbon are roughly 120 degrees, resembling a triangle. "Coplanar triangular" accurately describes this shape.

  6. Hydrogenation of alkenes can be carried out in the presence of

    Asked in: UPSC CAPF 2022

    1. copper
    2. zinc
    3. aluminium
    4. nickel
    Show answer

    Answer: D. nickel

    - Hydrogenation of alkenes is the addition of H₂ to the double bond. - This reaction requires a catalyst. - Transition metals like Ni, Pt, Pd, and Rh are common hydrogenation catalysts. - Nickel is a common and cost-effective catalyst for alkene hydrogenation.

  7. The gas used for artificial ripening of green fruits is

    Asked in: UPPSC Upper Mains 2007

    1. Ethane
    2. Carbon-dioxide
    3. Acetylene
    4. Ethylene
    Show answer

    Answer: D. Ethylene

    Ethylene accelerates the physiological processes associated with ripening, such as color change, softening, and development of aroma and flavor. It is the most commonly used and widely accepted substance for artificial ripening.

  8. Match List I (Industrial processes) with List II (Industry with which associated) and select the correct answer using the codes given below the Lists: List I List II I. Cracking A) Rubber II. Smelting B) Petroleum III. Hydrogenation C) Copper IV. Vulcanization D) Edible fats

    Asked in: UPPCS Prelims 2000

    1. I-C, II-B, III-A, IV-D
    2. I-B, II-C, III-D, IV-A
    3. I-B, II-C, III-A, IV-D
    4. I-C, II-B, III-D, IV-A
    Show answer

    Answer: B. I-B, II-C, III-D, IV-A

    - Cracking (I) is a process in the Petroleum (B) industry used to break down large hydrocarbon molecules into smaller, more useful ones like gasoline. - Smelting (II) is a process of applying heat to ore in order to extract a base metal, commonly used in the Copper (C) industry. - Hydrogenation (III) is the addition of hydrogen to a substance. It is used in the Edible fats (D) industry to convert liquid vegetable oils into solid or semi- solid fats like margarine. - Vulcanization (IV) is a chemical process for converting natural rubber or related polymers into more durable materials by adding sulfur or other equivalent curatives, associated with the Rubber (A) industry.

  9. Unsaturated hydrocarbons add hydrogen in presence of a catalyst to give saturated hydrocarbons. The catalyst may be

    Asked in: Clerk GR II Comb. Comp. Exam 2013

    1. Iron metal
    2. Iron oxide
    3. Nickel metal
    4. Active carbon
    Show answer

    Answer: C. Nickel metal

    Hydrogenation of unsaturated hydrocarbons to saturated hydrocarbons requires a catalyst, typically a transition metal. Nickel, platinum, and palladium are common hydrogenation catalysts.

  10. Hydrogenation of vegetable oils in presence of nickel catalyst is an example of -

    Asked in: School Lecturer (Sanskrit Edu.) 2022 1 2022

    1. substitution reaction
    2. addition reaction
    3. oxidation reaction
    4. elimination reaction
    Show answer

    Answer: B. addition reaction

    The question tests understanding of common chemical reactions, specifically hydrogenation, and its classification.

Alkynes: Preparation, Physical and Chemical Properties

  1. To weld metals together, high temperature is required. Such a high temperature is obtained by burning:

    Asked in: UPSC NDA 2 2015

    1. Acetylene in oxygen
    2. LPG in oxygen
    3. Methane in oxygen
    4. Acetylene in nitrogen
    Show answer

    Answer: A. Acetylene in oxygen

    Welding requires a high-temperature flame to melt and fuse metals. The combustion of acetylene (C₂H₂) with oxygen (O₂) produces an oxy-acetylene flame, which can reach temperatures of around 3500 °C, making it suitable for welding. Other fuel gases like LPG and methane produce lower temperatures when burned with oxygen. Burning acetylene in nitrogen would not sustain combustion as nitrogen is inert and would not provide the necessary oxidant.

  2. Which among the following is correct with respect to bond formation in an ethyne molecule ?

    Asked in: UPSC CAPF 2024

    1. Carbon-Carbon single bond
    2. Carbon-Carbon double bond
    3. Carbon-Carbon triple bond
    4. Carbon-Hydrogen double bond
    Show answer

    Answer: C. Carbon-Carbon triple bond

    Hydrocarbons are classified based on the types of bonds between carbon atoms. Alkanes have only single bonds, alkenes have at least one double bond, and alkynes have at least one triple bond. Ethyne is the simplest alkyne.

  3. Consider the following statements about acetylene: I. It is used in welding industry. II. It is raw material for preparing plastics. III. It is easily obtained by mixing silicon carbide and water. Of these statements:

    Asked in: UPPCS Prelims 1998

    1. I and II are correct
    2. I and III are correct
    3. II and III are correct
    4. I, II and III are correct
    Show answer

    Answer: A. I and II are correct

    Statement I: Acetylene (C₂H₂) is used in the oxy- acetylene flame for welding and cutting metals because it produces a very high temperature flame when burned with oxygen. This statement is correct. Statement II: Acetylene is an important organic chemical used as a building block (monomer) in the synthesis of various plastics, including polyvinyl chloride (PVC), neoprene rubber, and vinyl acetate. This statement is correct. Statement III: Acetylene is typically obtained industrially by the incomplete combustion of hydrocarbons like methane or by the reaction of calcium carbide (CaC₂) with water. The reaction with silicon carbide (SiC) and water does not produce acetylene. This statement is incorrect.

  4. The homologue of ethyne is

    Asked in: WBCS Main Paper 4 2019

    1. C$_2$H$_2$
    2. C$_2$H$_6$
    3. C$_3$H$_4$
    4. C$_3$H$_8$
    Show answer

    Answer: C. C$_3$H$_4$

    Ethyne (C₂H₂) is an alkyne with the general formula CₙH₂ₙ₋₂. Homologues in a homologous series differ by a CH₂ group. For ethyne (n=2, C₂H₂), the next homologue is found by setting n=3, which gives C₃H₂(₃)-₂ = C₃H₄ (propyne). This differs from C₂H₂ by one CH₂ group.

Aromatic Hydrocarbons: Concept of Aromaticity (Hückel's Rule), Resonance Structures

  1. Naphthalene burns with a yellow sooty flame. This is because

    Asked in: UPSC CDS 1 2023

    1. carbon to hydrogen ratio is low
    2. there is incomplete combustion
    3. there is excess supply of air
    4. of presence of impurities of nitrogen and sul‐ fur
    Show answer

    Answer: B. there is incomplete combustion

    - A yellow sooty flame is a characteristic sign of incomplete combustion. - Incomplete combustion produces soot (unburnt carbon particles) which glow yellow when hot. - Fuels with a high carbon- to-hydrogen ratio are more likely to produce sooty flames during incomplete combustion.

  2. For benzene which of the following statement is true?

    Asked in: CGPSC Prelims Exam 2014

    1. It has six sigma and six pi bonds
    2. It has twelve sigma and six pi bonds
    3. It has twelve sigma and three pi bonds
    4. It has six sigma and three pi bonds
    Show answer

    Answer: C. It has twelve sigma and three pi bonds

    Benzene (C₆H₆) has a cyclic structure with six carbon atoms arranged in a hexagon. Each carbon atom is bonded to one hydrogen atom. In the resonance structure representation, there are alternating single and double bonds between the carbon atoms. Each single bond (C-C and C-H) consists of one sigma (σ) bond. Each double bond (C=C) consists of one sigma (σ) bond and one pi (π) bond. In benzene, there are 6 C-H single bonds, 3 C-C single bonds, and 3 C-C double bonds (in one resonance form). Total sigma bonds = 6 (C-H) + 3 (C-C single) + 3 (C-C in double) = 12. Total pi bonds = 3 (from the three double bonds). Thus, benzene has 12 sigma bonds and 3 pi bonds.

  3. Which one of the following carbon compounds will not give a sooty flame ?

    Asked in: UPSC NDA 1 2016

    1. Benzene
    2. Hexane
    3. Naphthalene
    4. Anthracene
    Show answer

    Answer: B. Hexane

    Sooty flames are a result of incomplete combustion, where there is insufficient oxygen to fully oxidize the fuel. This often occurs with hydrocarbons that have a relatively high carbon content compared to hydrogen. Aromatic hydrocarbons (like Benzene, Naphthalene, Anthracene) and unsaturated aliphatic hydrocarbons have a higher carbon-to-hydrogen ratio and tend to burn with sooty flames due to incomplete combustion and the formation of carbon particles (soot). Saturated aliphatic hydrocarbons (alkanes like Hexane, C6H14) have a lower carbon- to-hydrogen ratio and typically undergo more complete combustion, burning with a clean, blue flame, assuming sufficient oxygen is available.

  4. What is the number of mole(s) of H₂(g) required to saturate one mole benzene ?

    Asked in: UPSC NDA 1 2016

    1. 1
    2. 2
    3. 3
    4. 4
    Show answer

    Answer: C. 3

    - Benzene (C₆H₆) is a cyclic organic compound with a hexagonal ring structure containing alternating single and double bonds (represented by resonance). Effectively, there are three double bonds in the benzene ring. - Saturation of an unsaturated hydrocarbon involves adding hydrogen (H₂) to break the double or triple bonds, converting them into single bonds. - The reaction for the complete hydrogenation of benzene to cyclohexane (C₆H₁₂) is: C₆H₆ + 3 H₂ → C₆H₁₂ - Each mole of H₂ reacts with one mole of a double bond. Since benzene has three double bonds, 3 moles of H₂ are required to saturate one mole of benzene.

  5. For the detection of polyaromatic hydrocarbons which of the following instruments is used?

    Asked in: WBCS Main Paper 4 2021

    1. Atomic absorption spectrophotometer
    2. X-ray Diffractometer
    3. Gas Chromatography
    4. Flame photometer
    Show answer

    Answer: C. Gas Chromatography

    GC is effective for separating and analyzing complex mixtures of organic compounds, including PAHs, which are often found in environmental samples, combustion products, etc. It is typically coupled with detectors like Mass Spectrometry (GC-MS) for specific identification and quantification.

Conformational Analysis of Alkanes

  1. The most stable conformation of n-butane is - (1) Anti form (2) Eclipsed form (3) Gauche form (4) Fully Eclipsed form (5) Question not attempted

    Asked in: Department Recruitment 2024 (Chemist) 2024

    1. (1) Anti form
    2. (2) Eclipsed form
    3. (3) Gauche form
    4. (4) Fully Eclipsed form
    Show answer

    Answer: A. (1) Anti form

    The stability of conformational isomers is primarily determined by steric hindrance (repulsion between bulky groups) and torsional strain (repulsion between electron clouds of adjacent bonds). The anti conformation minimizes both.

Electrophilic Addition Reactions of Alkenes (Markownikoff's and Anti-Markownikoff's Rules, Ozonolysis)

  1. The ozonolysis of which one of the following alkenes yields a mixture of propan-2-one and formaldehyde?

    Asked in: UPSC Geoscientist 2023

    1. Propene
    2. Butene
    3. 2-Methylpropene
    4. 2-Methylbutene
    Show answer

    Answer: C. 2-Methylpropene

    Ozonolysis of an alkene involves the cleavage of the carbon-carbon double bond, yielding carbonyl compounds (aldehydes or ketones). The structure of the products dictates the structure of the original alkene. Propan-2-one has the structure (CH₃)₂C=O, and formaldehyde has the structure H₂C=O. For ozonolysis to yield these two products, the original alkene must have been (CH₃)₂C=CH₂.

Isomerism in Hydrocarbons (Structural, Geometrical, Conformational)

  1. The number of structural isomers of pentane is

    Asked in: UPSC NDA 2 2022

    1. 5
    2. 4
    3. 2
    4. 3
    Show answer

    Answer: D. 3

    Structural isomers are compounds with the same molecular formula but different arrangements of atoms. Pentane has the molecular formula C₅H₁₂. The three structural isomers of pentane are: 1. n- pentane (a straight chain of five carbon atoms). 2. Isopentane (2-methylbutane, a four-carbon chain with a methyl group attached to the second carbon).

Nomenclature of Hydrocarbons (IUPAC and common systems)

  1. Correct IUPAC name of the following hydrocarbon is – CH3-CH(C2H5)-CH=CH-CH3 (1) 4-methyl hex-2-ene (2) 3-methyl hex-3-ene (3) 4-ethyl pent-2-ene (4) 2-ethyl pent-3-ene (5) Question not attempted

    Asked in: Department Recruitment 2024 (Chemist) 2024

    1. 4-methyl hex-2-ene
    2. 3-methyl hex-3-ene
    3. 4-ethyl pent-2-ene
    4. 2-ethyl pent-3-ene
    Show answer

    Answer: A. 4-methyl hex-2-ene

    The IUPAC nomenclature of alkenes involves identifying the longest continuous carbon chain