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ORGANIC CHEMISTRY
Basis | Alkanes | Alkenes | Alkynes |
|---|---|---|---|
Definition | Hydrocarbons with only C–C single bonds | Hydrocarbons with at least one C=C double bond | Hydrocarbons with at least one C≡C triple bond |
Saturation | Saturated (contain maximum H atoms possible) | Unsaturated (contain fewer H atoms than alkanes) | Unsaturated (contain fewer H atoms than alkenes) |
Other Name | Paraffins (less reactive) | Olefins | Acetylenes |
General Formula | CₙH₂ₙ₊₂ | CₙH₂ₙ | CₙH₂ₙ₋₂ |
Bond Type | All sigma (σ) bonds | One sigma (σ) + one pi (π) in double bond | One sigma (σ) + two pi (π) in triple bond |
Hybridization | sp³ | sp² (at double bond carbons) | sp (at triple bond carbons) |
Bond Angle | 109°28′ (tetrahedral) | 120° (trigonal planar) | 180° (linear) |
Structure | Tetrahedral | Planar around double bond | Linear around triple bond |
Bond Length (approx.) | C–C: 1.54 Å C–H: 1.112 Å | C=C: 1.34 Å C–H: ~1.09 Å | C≡C: 1.20 Å C–H: ~1.06 Å |
Bond Energy (Kcal/mole) | C–C: 82.67 C–H: 98.67 | C=C: ~146 C–H: ~99 | C≡C: ~200 C–H: ~99 |
Reactivity | Least reactive (due to nonpolar C–H bonds & strong σ bonds) | More reactive than alkanes | Most reactive (triple bond easily participates in reactions) |
Example | CH₄ (Methane), C₃H₈ (Propane) | C₂H₄ (Ethene), C₃H₆ (Propene) | C₂H₂ (Ethyne), C₃H₄ (Propyne) |
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ALKANES
▢ Introduction:
❖ Definition:
- Alkanes are hydrocarbons containing only carbon–carbon single bonds.
- They are saturated hydrocarbons since they have the maximum number of hydrogens per carbon.
❖ Comparison:
◉ Alkenes Alkynes: Unsaturated hydrocarbons containing fewer hydrogens than alkanes.
❖ Paraffins:
◉ Reason for inertness:
- Non-polar nature of C−H bonds
- Strong C−H and C−C sigma bonds
❖ Bonding:
◉ Hybridization: sp³
◉ Bond angle: 109°28′ (tetrahedral)
◉ Bond length:
◈ C−C: 1.54 Å
◈ C−H: 1.112 Å
◉ Bond energy:
◈ C−C: 82.67 kcal/mol
◈ C−H: 98.67 kcal/mol
❖ Formula: CₙH₂ₙ₊₂ (n = 1, 2, 3 …)
❖ Isomerism:
◉ Types:
- Chain isomerism
- Position isomerism
- Functional isomerism
- Geometrical isomerism
- Optical isomerism
- Ring-chain isomerism
- Conformational isomerism
◉ Details:
◈ Chain: Starts from C4 in alkanes, C4 in alkenes, C5 in alkynes
◈ Position: Starts from C6 in alkanes, C4 in alkenes, C4 in alkynes
◈ Functional: Not in alkanes; from C3 in alkenes and alkynes
◈ Geometrical: From C4 in alkenes, C6 in alkynes
◈ Optical: From C7 in alkanes, C6 in alkenes and alkynes
◈ Ring chain: From C3 in alkenes and alkynes
◈ Conformational: From C2 in alkanes
❖ Sources:
◉ Petroleum: Contains liquid hydrocarbons (up to 40 carbons) + solid paraffin wax.
◉ Natural gas: Contains ~80% methane, 10% ethane, and 10% higher alkanes, plus small gases (H₂, N₂, CO₂).
◉ Methane: Also called marsh gas (fire damp), found in marshy areas.
◉ Coal gas: Contains ~32% methane.
▢ Methods of Preparation:
❖ Reduction:
◉ Unsaturated Hydrocarbons:
◈ Catalytic reduction:
- Hydrogenation with Ni, Pd, or Pt (Sabatier-Senderens reaction at 200–300°C).
- Hydrogenation with Raney Ni at room temperature.
- Alkynes to alkenes by Lindlar's catalyst (Pd/BaSO₄ poisoned with quinoline).
◈ Nascent hydrogen: LiAlH₄ reduces alkyl halides, not alkenes/alkynes.
◉ Alkyl halides: Reduced to alkanes with HI + Red P.
◉ Alcohols Aldehydes Ketones Acids AcidChlorides: Reduced with HI + Red P to alkanes.
◉ Aldehydes Ketones:
◈ Clemmensen reduction: Zn–Hg in conc. HCl
◈ Wolff Kishner reduction: NH₂–NH₂ + KOH
❖ Wurtz reaction:
◉ Equation: 2 R−X + 2 Na → R−R + 2 NaX (in dry ether)
◉ Notes:
- Methane cannot be obtained.
- Mixtures of alkyl halides give 3 alkanes (difficult separation).
- Tertiary alkyl halides undergo elimination (alkenes).
- Bromo and iodoalkanes preferred.
◉ Mechanism:
- Free radical (most accepted)
- Carbanion (ionic)
❖ Frankland reaction: Similar to Wurtz but with Zn instead of Na.
❖ Grignard reagent: RMgX hydrolyzed to give alkanes.
❖ Corey House synthesis:
◉ Equation: R−X → R−Li → R₂CuLi (Gilman’s reagent) + R′−X → R−R′
◉ Advantage: Prepares both symmetrical and unsymmetrical alkanes.
❖ Decarboxylation:
◉ Soda lime method: R−COONa + NaOH → R−H + Na₂CO₃ (with CaO).
◉ Notes:
- NaOH is hygroscopic, so CaO is used.
- Rate ∝ stability of carbanion: CH₃CH₂COOH > (CH₃)₂CHCOOH > (CH₃)₃CCOOH.
- Yields good for lower members.
❖ Kolbe electrolysis:
◉ Reaction: 2 RCOONa → R−R + 2 CO₂ + H₂
◉ Notes:
- Methane cannot be obtained.
- Only symmetrical alkanes formed.
- Solution becomes basic due to NaOH.
❖ Hydrolysis of Carbides:
- Be₂C + H₂O → CH₄ + Be(OH)₂
- Al₄C₃ + H₂O → CH₄ + Al(OH)₃
- CaC₂ + H₂O → C₂H₂ (Wöhler’s method)
▢ Properties:
❖ Physical:
◉ State:
◈ Alkanes:
■ C1–C4: Gases
■ C5–C17: Liquids
■ C18+: Waxy solids
◈ Alkenes:
■ C2–C4: Gases
■ C5–C18: Liquids
■ C19+: Solids
◈ Alkynes:
■ C2–C4: Gases
■ C5–C12: Liquids
■ C13+: Solids
◉ Solubility: Alkanes are non-polar; soluble in non-polar solvents (CCl₄), insoluble in water.
◉ Boiling point:
◈ Factors:
- Proportional to mol. weight and surface area
- Decreases with branching
◈ Order:
- n-pentane > isopentane > neopentane
◈ Example question: Minimum B.P.: isooctane < 2,3-dimethylhexane < 2-methylheptane < n-octane
◉ Melting point:
◈ Variation: Irregular, shows alternation effect.
◈ Reason: Even C atoms pack better in crystal lattice → higher m.p.
❖ Chemical:
◉ General reactivity:
- Alkanes are chemically inert due to strong non-polar C–H and C–C bonds.
- Alkenes and alkynes more reactive due to π bonds.
Q1.
On reaction with Baeyer's reagent, ethylene and acetylene form [IOM 2006]
📅2006
Q2.
The compound B formed in the reaction in the following sequence of reaction is [IOM 2005]
CH₃CH₂CH₂OH →(PCl₅) A →(Alk. NaOH) B
📅2005
Q3.
Identify Z in the following reaction [IOM 2004]
C₂H₅Cl →(Alc. KOH) X →(Br₂) Y →(KCN) Z
📅2004
Q4.
Ethylene and acetylene can be separated by [IOM 2003]
📅2003
Q5.
Cracking of hydrocarbon is the process in which [IOM 2003]
📅2003
Q6.
The gas obtained by adding water on aluminum carbide is [MOE - Curriculum/IOE]
Q7.
A metallic carbide on reaction with water gives a hydrocarbon which readily burns in oxygen and it gives Tollen's test, the hydrocarbon is [MOE 2061]
📅2061
Q8.
The reaction CH₃Br + C₆H₅Br →(Na/ether) CH₃–C₆H₅ + 2NaBr is called [MOE 2062]
📅2062
Q9.
A mixture of methane, ethylene and acetylene is passed through ammoniacal Cu₂Cl₂ solution in a tube. Then the mixture of gases coming out of the tube is [MOE 2056]
📅2056
Q10.
The catalytic oxidation of ethylene using silver catalyst produces [IOE]
Q11.
Ethyne with Baeyer's reagent gives out
Q12.
Markownikoff's rule governs the addition of [IOE]
Q13.
Which of the following has more acidic character [IOM]
Q14.
Formaldehyde is obtained from ethylene by [IOM]
Q15.
Major constituent of Marsh gas is [IOE]
Q16.
A compound on ozonolysis forms two molecules of HCHO, the compound is [IOM]
Q17.
Alkanes are represented by general formula [MOE/IOE]
Q18.
The number of isomers of C₆H₁₄ is [IOE, MOE]
Q19.
Propane on strong heating will yield [IOE]
Q20.
Ethyl chloride is heated in presence of alcoholic KOH. The product is [IOE]
Q21.
Which of the following is formed when acetylene is passed through hot Fe tube [MOE]
Q22.
Hydrolysis of calcium carbide yields [BPKIHS/IOE]
Q23.
The compound which will give acetaldehyde on ozonolysis is [MOE 2065]
📅2065
Q24.
Alkene and alkyne can be distinguished from each other by [MOE 2008]
📅2008
Q25.
Acetylene contains [IOM 2008]
📅2008
Q26.
In greenhouse effect methane causes [IOM 2009]
📅2009
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ADDITIONAL QUESTINS
Q1.
Successive alkanes differ by
Q2.
The shape of methane molecule is
Q3.
Pure methane can be produced by
Q4.
Both methane and ethane may be obtained in one step reaction from
Q5.
During preparation of ethane by Kolbe's electrolytic method using inert electrodes, the pH of the electrolyte
Q6.
Most appropriate method for manufacture of methane
Q7.
Which liberates methane gas on treatment with water?
Q8.
Soda lime is used extensively in decarboxylation reaction to obtain alkanes. Soda lime is
Q9.
Which cannot be prepared by Wurtz reaction?
Q10.
Methyl bromide when heated with zinc in closed tube produces
Q11.
Wurtz reaction using bromoethane yields
Q12.
Among the following hydrocarbons, the one having lowest boiling point is
Q13.
Photochemical chlorination of alkane is initiated by
Q14.
Halogenation of alkanes is an example of
Q15.
Carbon black, used in printer's ink, is obtained by decomposition of
Q16.
Marsh gas mainly contains
Q17.
Household gaseous fuel (LPG) mainly contains
Q18.
General formula for alkenes is
Q19.
Which has the smallest heat of hydrogenation per mole?
Q20.
Ethylene is formed by dehydration of
Q21.
Ethyl bromide on treatment with aqueous KOH gives
Q22.
Bromoethane on treatment with alcoholic KOH gives
Q23.
Chlorobutane on reaction with alcoholic potash gives
Q24.
Alcoholic solution of KOH is used for
Q25.
When ethyl alcohol is heated with conc. H₂SO₄ at 443K, ethylene is formed by
Q26.
Ethylene readily undergoes
Q27.
1,3-Butadiene when treated with Br₂ gives
Q28.
CH₃CHCH₃ + HBr -> CH₃CHBrCH₃ is a type of
Q29.
CH₃CH=CH₂ + HBr gives
Q30.
I-butene subjected to HBr in presence of peroxide gives
Q31.
To CH₂=CH-CH₃, hydrogen bromide is added in presence of peroxides, the resultant is
Q32.
The addition of HBr is easiest with
Q33.
Arrange the following compounds in increasing order of reactivity towards the addition of HBr: RCH=CHR, CH₂=CH₂, R₂C=CHR, R₂C=CR₂
Q34.
When HCl is passed through propene in presence of benzoyl peroxide, it gives
Q35.
Position of double bond in alkenes can be identified by
Q36.
Ozonolysis of 1,3-butadiene gives
Q37.
The presence of unsaturation in an organic compound can be tested with
Q38.
Baeyer's reagent is used in the laboratory for
Q39.
Baeyer's reagent is
Q40.
Ethylene reacts with alkaline KMnO₄ to form
Q41.
The compound B formed in the following sequence is: CH₃CH₂CH₂OH → PCl₃ → aq. KOH
Q42.
General formula of alkynes is
Q43.
Which will react with sodium metal?
Q44.
When acetylene is passed through dil. H₂SO₄ in presence of HgSO₄, the compound formed is
Q45.
When 2-pentyne is treated with dil. H₂SO₄ and HgSO₄, the product formed is
Q46.
Propyne and propene can be distinguished by
Q47.
Which of the following reagents can be used to distinguish 1-butyne and 2-butyne?
Q48.
Acetylene reacts with ammonical silver nitrate to form
Q49.
In its reaction with silver nitrate, acetylene shows
Q50.
Alkaline KMnO₄ will oxidise acetylene to
Q51.
When acetylene reacts with arsenic trichloride in the presence of anhydrous AlCl₃, it produces
Q52.
Propyne on polymerization gives
Q53.
Which of the following is a hydrocarbon?
Q54.
Coal tar is main source of
Q55.
Which is not obtained by fractionation of coal tar?
Q56.
The number of sigma and pi-bonds in a molecule of benzene is
Q57.
Heating a mixture of sodium benzoate and soda lime gives
Q58.
Most common reactions of benzene and its derivatives are
Q59.
Nitration of benzene by nitric and sulphuric acid is
Q60.
Benzene is converted to toluene by
Q61.
Benzene reacts with Cl₂ in sunlight to give
Q62.
Gammexene is
Q63.
The reaction of benzene with chlorine in presence of FeCl₃ gives
Q64.
Attacking species in nitration of benzene is
Q65.
In Friedel-Crafts alkylation besides AlCl₃, the other reactants are
Q66.
In Friedel-Crafts reaction, electrophilic reagent is
Q67.
The compound most reactive towards electrophilic nitration is
Q68.
Which of the following is not a m-directing group?
Q69.
Nitration of toluene takes place at
Q70.
Petroleum consists mainly of
Q71.
Petroleum is a mixture mainly of
Q72.
The first product obtained during fractional distillation of petroleum is
Q73.
Natural gas is composed primarily of
Q74.
The process in which higher hydrocarbons are broken down into lower hydrocarbons by controlled pyrolysis is called
Q75.
Fischer-Tropsch process is used in manufacture of
Q76.
Which has highest knocking?
Q77.
The octane number has 0 value for
Q78.
Lead tetraethyl is used as
Q79.
Which of the following substances is used as an antiknock compound?
Q80.
The attacking species in aromatic sulphonation is
Q81.
The reactive species in nitration of benzene is
Q82.
In chlorination of benzene, the real chlorinating agent is
Q83.
Benzene reacts with n-propyl chloride in anhydrous AlCl₃ to give predominantly
Q84.
m-Butylbenzene on oxidation will give
Q85.
Toluene can be oxidised to benzoic acid by
Q86.
The product of the following reaction is C₆H₅Cl + Cl₂ → ?
Q87.
Constituent of light oil is
Q88.
Benzene is obtained by fractional distillation of
Q89.
Identify the correct order of reactivity in electrophilic substitution reactions of the following compounds
Q90.
Which of the following yields both alkane and alkene?
Q91.
Complete combustion of CH₄ gives
Q92.
Which of the compounds with molecular formula C₅H₁₀ yields acetone on ozonolysis?
Q93.
Natural gas mainly consists of
Q94.
Presence of a nitro group in a benzene ring
Q95.
Reduction of 2-butyne with Na in liquid NH₃ gives predominantly
Q96.
Propene reacts with HI in presence of peroxide to give
Q97.
When 2-butyne is treated with H₂SO₄/HgSO₄, the product is
Q98.
CH₃C≡CCH₂CH₃ → O₃ → Hydrolysis; X is
Q99.
Beryllium carbide on hydrolysis yields
Q100.
Methyl magnesium bromide reacts with ethyl alcohol to form
Q101.
Which is produced from salt? (Note: CaC₂ is a salt but Al₄C₃/Be₂C are not salts)