Hydrocarbons Chapter Notes for CBSE Class 11 Science

Key Definitions

Hydrocarbons

Organic compounds composed exclusively of hydrogen and carbon atoms.

Saturated Hydrocarbons

Hydrocarbons containing only single C-C bonds (e.g., Alkanes).

Unsaturated Hydrocarbons

Hydrocarbons containing at least one C=C double bond (Alkenes) or C≡C triple bond (Alkynes).

Alkanes

Saturated acyclic hydrocarbons with the general formula CnH2n+2. They exhibit free radical substitution reactions.

Alkenes

Unsaturated acyclic hydrocarbons with at least one C=C double bond, general formula CnH2n. They undergo electrophilic addition reactions.

Alkynes

Unsaturated acyclic hydrocarbons with at least one C≡C triple bond, general formula CnH2n-2. They show electrophilic addition and acidic character.

Aromatic Hydrocarbons

Cyclic, planar, fully conjugated compounds that obey Huckel's Rule (4n+2 π electrons) and exhibit unusual stability.

Markovnikov's Rule

In electrophilic addition of an unsymmetrical reagent (HX) to an unsymmetrical alkene, the positive part of the reagent adds to the carbon atom of the double bond that has more hydrogen atoms.

Alkanes: Structure, Preparation & Reactions

Alkanes are the simplest class of hydrocarbons, characterized by single covalent bonds between carbon atoms and between carbon and hydrogen atoms. They are saturated compounds with the general formula CnH2n+2. The carbon atoms in alkanes are sp3 hybridized, resulting in a tetrahedral geometry around each carbon, with bond angles of approximately 109.5°. Alkanes are relatively unreactive due to the strong C-C and C-H sigma bonds, which are non-polar. Their stability makes them excellent fuels.

Preparation Methods:

• From Unsaturated Hydrocarbons: Hydrogenation of alkenes or alkynes in the presence of catalysts like Ni, Pd, or Pt. E.g., CH2=CH2 + H2 → CH3-CH3
• Wurtz Reaction: Alkyl halides react with sodium metal in dry ether to form higher alkanes. 2RX + 2Na → R-R + 2NaX. This reaction is good for preparing symmetrical alkanes.
• Decarboxylation of Carboxylic Acids: Sodium salts of carboxylic acids react with sodalime (NaOH + CaO) on heating to produce alkanes with one less carbon atom. RCOONa + NaOH (CaO, heat) → R-H + Na2CO3.
• Kolbe's Electrolytic Method: Electrolysis of aqueous solutions of sodium or potassium salts of carboxylic acids yields alkanes. 2RCOONa + 2H2O (electrolysis) → R-R + 2CO2 + H2 + 2NaOH.

Key Reactions:

• Halogenation (Free Radical Substitution): Alkanes react with halogens (Cl2, Br2) in the presence of UV light or high temperature. This reaction proceeds via a free radical mechanism involving initiation, propagation, and termination steps. E.g., CH4 + Cl2 (UV light) → CH3Cl + HCl. This reaction can lead to a mixture of products.
• Combustion: Alkanes burn in the presence of oxygen to produce carbon dioxide and water, releasing a large amount of heat (exothermic). This makes them good fuels. CnH2n+2 + (3n+1)/2 O2 → nCO2 + (n+1)H2O.
• Pyrolysis (Cracking): Decomposition of higher alkanes into smaller hydrocarbons when heated to high temperatures (e.g., 700-800 K) in the absence of air. This is important in the petroleum industry to produce gasoline from heavier fractions.
• Isomerization: Straight-chain alkanes convert to branched-chain isomers in the presence of anhydrous AlCl3 and HCl at moderate temperatures.

Alkenes & Alkynes: Unsaturated Hydrocarbons

Alkenes

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C). Their general formula is CnH2n. Each carbon atom involved in the double bond is sp2 hybridized, resulting in a trigonal planar geometry around it with bond angles of 120°. The presence of the pi (π) bond makes alkenes much more reactive than alkanes, typically undergoing electrophilic addition reactions.

Preparation Methods:

• Dehydration of Alcohols: Alcohols lose a molecule of water when heated with concentrated H2SO4 or H3PO4 to form alkenes. R-CH2-CH2-OH (conc. H2SO4, heat) → R-CH=CH2 + H2O (follows Saytzeff's Rule).
• Dehydrohalogenation of Alkyl Halides: Alkyl halides react with alcoholic KOH to form alkenes by eliminating a hydrogen halide molecule. R-CH2-CH(X)-R' (alc. KOH, heat) → R-CH=CH-R' + HX (also follows Saytzeff's Rule).
• From Alkynes: Partial hydrogenation of alkynes using Lindlar's catalyst (Pd/CaCO3 poisoned with lead acetate and quinoline) gives cis-alkenes, while reduction with sodium in liquid ammonia gives trans-alkenes.

Key Reactions (Electrophilic Addition):

• Hydrogenation: Addition of H2 in presence of Ni, Pd, or Pt to form alkanes. R-CH=CH2 + H2 → R-CH2-CH3.
• Halogenation: Addition of X2 (Cl2, Br2) to form vicinal dihalides. Br2 water test for unsaturation.
• Hydrohalogenation: Addition of HX (HCl, HBr, HI). Follows Markovnikov's Rule for unsymmetrical alkenes. Anti-Markovnikov's Rule (peroxide effect) is observed only with HBr in the presence of peroxides.
• Hydration: Addition of H2O in presence of acid catalyst (H2SO4) to form alcohols, following Markovnikov's rule.
• Ozonolysis: Reaction with ozone (O3) followed by hydrolysis (Zn/H2O) to cleave the double bond and form aldehydes/ketones. Useful for determining the position of the double bond.

Alkynes

Alkynes are unsaturated hydrocarbons containing at least one carbon-carbon triple bond (C≡C). Their general formula is CnH2n-2. Each carbon atom involved in the triple bond is sp hybridized, resulting in a linear geometry around it with bond angles of 180°. Alkynes are highly reactive, undergoing electrophilic addition reactions similar to alkenes, but can add two molecules of reagent across the triple bond. Terminal alkynes (with a hydrogen attached to the sp carbon) exhibit acidic character due to the high electronegativity of the sp hybridized carbon, which makes the C-H bond polar.

Preparation Methods:

• From Vicinal Dihalides: Dehydrohalogenation of vicinal dihalides (halogens on adjacent carbons) with strong bases like alcoholic KOH followed by sodamide (NaNH2) or Na in liquid ammonia.
• From Calcium Carbide: CaC2 + 2H2O → Ca(OH)2 + CH≡CH (ethyne).

Key Reactions:

• Acidic Character of Terminal Alkynes: Reaction with active metals (Na), sodamide (NaNH2), ammoniacal silver nitrate (Tollens' reagent), or ammoniacal cuprous chloride (Fehling's solution) to form metal acetylides. E.g., CH≡CH + Na → CH≡C-Na + 1/2 H2.
• Electrophilic Addition: Addition of H2, X2, HX, H2O (in presence of H2SO4, HgSO4). Follows Markovnikov's rule. For H2O, forms an enol intermediate, which tautomerizes to a ketone (except ethyne, which forms ethanal).
• Polymerization:
• Linear Polymerization: Ethyne undergoes linear polymerization to polyacetylene under specific conditions.
• Cyclic Polymerization: Three molecules of ethyne pass through a red-hot iron tube to form benzene (aromatization).

Aromatic Hydrocarbons: Benzene and its Reactions

Aromatic hydrocarbons are a special class of cyclic, unsaturated compounds exhibiting unusual stability due to electron delocalization. The most important example is Benzene (C6H6). Its structure is a regular hexagon with each carbon sp2 hybridized, and all C-C bond lengths are identical (139 pm, intermediate between single and double bonds) due to resonance. Benzene fulfills Huckel's Rule, having (4n+2) π electrons (for benzene, n=1, so 6 π electrons).

Preparation of Benzene:

• Cyclic Polymerization of Ethyne: As mentioned above, passing ethyne through a red-hot iron tube.
• Decarboxylation of Aromatic Acids: Heating sodium benzoate with sodalime.
• Reduction of Phenol: Heating phenol with zinc dust.

Key Reactions (Electrophilic Aromatic Substitution - EAS): Aromatic compounds primarily undergo electrophilic substitution reactions, where an electrophile replaces a hydrogen atom on the ring, maintaining aromaticity.

• Nitration: Reaction with nitrating mixture (conc. HNO3 + conc. H2SO4) to form nitrobenzene. Electrophile: NO2+ (nitronium ion).
• Halogenation: Reaction with X2 (Cl2, Br2) in the presence of Lewis acid (FeCl3, FeBr3) to form halobenzenes. Electrophile: X+ (halogenium ion).
• Sulphonation: Reaction with fuming H2SO4 or conc. H2SO4 to form benzenesulphonic acid. Electrophile: SO3 (sulfur trioxide).
• Friedel-Crafts Alkylation: Reaction with an alkyl halide (R-X) in the presence of anhydrous AlCl3 to form alkylbenzenes. Electrophile: R+ (carbocation).
• Friedel-Crafts Acylation: Reaction with an acyl halide (RCO-X) or acid anhydride in the presence of anhydrous AlCl3 to form acylbenzenes (ketones). Electrophile: RCO+ (acylium ion).

Comparison: Alkanes, Alkenes & Alkynes

Worked Examples

• {"title":"1. Nomenclature","bodyMarkdown":"Q: Name the following compound: CH3-CH(CH3)-CH2-CH3\n\nA:\n1. Longest Chain: Identify the longest continuous carbon chain, which is 4 carbons (butane).\n2. Numbering: Number the chain to give the substituent the lowest possible number. Numbering from left gives methyl at C2. Numbering from right gives methyl at C3. So, numbering from left is correct.\n3. Name: 2-Methylbutane."}
• {"title":"2. Reaction Completion (Markovnikov's Rule)","bodyMarkdown":"Q: Predict the major product of the reaction: CH3-CH=CH2 + HBr → ?\n\nA: This is an electrophilic addition reaction, and HBr is an unsymmetrical reagent adding to an unsymmetrical alkene. According to Markovnikov's Rule, the H (positive part) adds to the carbon with more hydrogens (C1), and Br (negative part) adds to the carbon with fewer hydrogens (C2).\n\nCH3-CH=CH2 + HBr → CH3-CH(Br)-CH3 (2-Bromopropane is the major product)."}

Key Points to Remember

• Alkanes are saturated hydrocarbons; alkenes and alkynes are unsaturated.
• Alkanes undergo free radical substitution, while alkenes and alkynes primarily undergo electrophilic addition.
• Markovnikov's rule governs the addition of unsymmetrical reagents to unsymmetrical alkenes/alkynes.
• Anti-Markovnikov addition (peroxide effect) occurs only with HBr in the presence of peroxides to alkenes.
• Terminal alkynes are acidic due to the sp hybridized carbon's high electronegativity.
• Benzene exhibits extraordinary stability due to aromaticity (delocalization of π electrons) and follows Huckel's rule (4n+2 π electrons).
• Aromatic compounds primarily undergo electrophilic aromatic substitution (EAS) reactions, not addition.
• Learn the mechanisms for free radical halogenation and electrophilic addition/substitution in conceptual steps, not just rote memorization.

Exam Tip: Differentiating Reactions

A common exam trap involves distinguishing between reactions that seem similar. For instance, Markovnikov's vs. Anti-Markovnikov's Rule. Remember, Anti-Markovnikov's addition only applies to HBr in the presence of peroxides; for HCl or HI, or in the absence of peroxides, Markovnikov's rule always applies. Also, pay close attention to catalysts (e.g., Lindlar's catalyst for cis-alkene formation from alkynes, or Na/liq. NH3 for trans-alkene). Practicing reaction conditions and products is crucial for scoring well in organic chemistry questions.

Practice Questions with Solutions

• Q: Why are terminal alkynes acidic in nature? A: Terminal alkynes have a hydrogen atom attached to an sp hybridized carbon. The sp carbon is more electronegative than sp2 or sp3 carbons, pulling electron density towards itself and making the C-H bond more polar. This allows the hydrogen to be easily removed as a proton (H+).
• Q: What is the main difference in reactivity between alkanes and alkenes? A: Alkanes are saturated and primarily undergo free radical substitution reactions. Alkenes are unsaturated (due to the pi bond) and are much more reactive, mainly undergoing electrophilic addition reactions.
• Q: State Huckel's Rule and its significance for aromaticity. A: Huckel's Rule states that a cyclic, planar, fully conjugated system with (4n+2) π electrons (where n is an integer 0, 1, 2...) will be aromatic. Its significance is that it predicts the exceptional stability and unique chemical properties of aromatic compounds like benzene.
• Q: Name the major product when propene reacts with HBr in the presence of peroxide. A: When propene reacts with HBr in the presence of peroxide, it follows the Anti-Markovnikov's Rule. The major product formed is 1-Bromopropane (CH3-CH2-CH2Br).

Frequently Asked Questions