Hydrocarbons — NCERT Class 11 Chemistry

Hydrocarbons are compounds of carbon and hydrogen only. They are classified as: 1. Open-chain (aliphatic): (a) Alkanes (CₙH₂ₙ₊₂) — single bonds, saturated; (b) Alkenes (CₙH₂ₙ) — double bond(s), unsaturated; (c) Alkynes (CₙH₂ₙ₋₂) — triple bond(s), unsaturated 2. Closed-chain (cyclic): (a) Alicyclic — cycloalkanes, cycloalkenes; (b) Aromatic — benzene and derivatives (contain benzene ring)

In ethane (CH₃–CH₃), rotation about the C–C bond produces different conformations. The two extreme conformations are: 1. Staggered conformation: The H atoms on the front and back carbon are maximally apart (dihedral angle = 60°). This is more stable (lower energy) because the electron clouds of the C–H bonds are as far apart as possible, minimising torsional strain. 2. Eclipsed conformation: The H atoms on the front and back carbon are aligned (dihedral angle = 0°). This is less stable (higher energy) due to maximum torsional strain from overlapping electron clouds.

The energy difference between staggered and eclipsed is about 12.5 kJ mol⁻¹. At room temperature, ethane molecules rapidly interconvert between conformations, so conformers cannot be isolated.

Staggered: Front carbon has 3 H atoms at 12, 4, and 8 o'clock positions. Back carbon has 3 H atoms at 2, 6, and 10 o'clock positions — each back H sits between two front H atoms.

Eclipsed: Front carbon has 3 H atoms at 12, 4, and 8 o'clock. Back carbon has 3 H atoms at the same 12, 4, and 8 o'clock positions — each back H is directly behind a front H.

Conformational isomerism arises from rotation about C–C single bonds. Conformers interconvert rapidly at room temperature and cannot be isolated — they are not distinct compounds. Structural isomerism involves different connectivity of atoms. Structural isomers are distinct compounds with different physical and chemical properties. They cannot interconvert without breaking and reforming bonds.

Alkenes contain a carbon-carbon double bond (C=C), which consists of a strong σ-bond and a weak π-bond. The π-electron cloud is above and below the plane of the molecule, making it easily accessible to electrophiles. The π-bond can be broken relatively easily (bond dissociation energy ≈ 270 kJ mol⁻¹) to form two new σ-bonds. Since this provides a thermodynamically favourable pathway, alkenes undergo addition reactions rather than substitution.

Markovnikov's rule: When an unsymmetrical reagent adds to an unsymmetrical alkene, the hydrogen of the reagent attaches to the carbon that already has more hydrogen atoms.

Example: Addition of HBr to propene (CH₃CH=CH₂)

• H⁺ adds to C-1 (which has more H atoms) → forms secondary carbocation CH₃–C⁺H–CH₃ (more stable)
• Br⁻ adds to C-2 → product is 2-bromopropane (CH₃CHBrCH₃)

The alternative (H⁺ adding to C-2) would give a less stable primary carbocation, so Markovnikov addition is preferred.

In the presence of organic peroxides (ROOR), HBr adds to an unsymmetrical alkene in an anti-Markovnikov fashion. This is called the peroxide effect or Kharasch effect.

Example: HBr + propene (with peroxides) → 1-bromopropane (CH₃CH₂CH₂Br)

Mechanism: The peroxide undergoes homolytic cleavage to generate free radicals (RO•). The radical adds to the double bond, forming the more stable radical intermediate (2° or 3°), and then abstracts H from HBr. Since the radical addition step is the reverse of electrophilic addition, the regiochemistry is reversed.

Important: The peroxide effect works only with HBr, not with HCl or HI. HCl has a strong H–Cl bond (radical formation is endothermic), and HI adds so rapidly that the radical pathway cannot compete.

Terminal alkynes (RC≡CH) are acidic because the sp-hybridised carbon has 50% s-character, which holds the electron pair closer to the nucleus. This makes the C–H bond more polar and the hydrogen more easily lost as H⁺. The pKa of terminal alkynes is about 25 (much more acidic than alkenes, pKa ≈ 44, or alkanes, pKa ≈ 50). Terminal alkynes react with strong bases like NaNH₂ to form acetylide ions: RC≡CH + NaNH₂ → RC≡C⁻Na⁺ + NH₃

Acidity order: HC≡CH (pKa ≈ 25) > H₂C=CH₂ (pKa ≈ 44) > CH₃–CH₃ (pKa ≈ 50) The reason is the s-character of the orbital holding the H atom: sp (50% s) > sp² (33% s) > sp³ (25% s). Higher s-character means the electron pair is held closer to the nucleus, stabilising the conjugate base (carbanion) and making the proton more acidic. Only terminal alkynes react with NaNH₂ or Na/NH₃ to form acetylides; alkenes and alkanes do not.

Alkynes undergo addition reactions similar to alkenes, but in two stages (since there are two π-bonds): 1. Addition of H₂ (hydrogenation): With Lindlar's catalyst (Pd/BaSO₄, quinoline) → cis-alkene; With Na/NH₃ → trans-alkene; With excess H₂/Ni → alkane 2. Addition of halogens: Br₂ → trans-dibromoalkene → tetrabromoalkane (with excess Br₂) 3. Addition of HX: HBr → vinyl bromide → gem-dibromoalkane (with excess HBr) 4. Addition of water (in presence of HgSO₄/H₂SO₄): → enol → keto form (Markovnikov addition)

Aromaticity is a special stability associated with planar, cyclic, conjugated systems. A compound is aromatic if it satisfies Hückel's rule: (4n + 2) π-electrons in a conjugated, planar, cyclic system, where n = 0, 1, 2, 3...

Requirements for aromaticity: 1. Cyclic (ring) structure 2. Planar geometry (all atoms in one plane) 3. Fully conjugated (p-orbital on every ring atom) 4. (4n + 2) π-electrons (Hückel's rule)

Examples: Benzene (6 π-electrons, n=1) is aromatic; cyclobutadiene (4 π-electrons, n=0 gives 2 but actual count is 4) is anti-aromatic; cyclooctatetraene (8 π-electrons) is non-aromatic (tub-shaped, not planar).

Benzene undergoes electrophilic substitution (SE) reactions where an electrophile replaces a hydrogen atom on the ring, preserving the aromatic system:

1. Nitration: C₆H₆ + HNO₃ →⁽ᴴ²ᔠᴼ⁴⁾ C₆H₅NO₂ + H₂O (electrophile: NO₂⁺) 2. Sulfonation: C₆H₆ + H₂SO₄ → C₆H₅SO₃H + H₂O (electrophile: SO₃) 3. Halogenation: C₆H₆ + Cl₂ →⁽ᶠᵉᶜˡ³⁾ C₆H₅Cl + HCl (electrophile: Cl⁺) 4. Friedel-Crafts alkylation: C₆H₆ + RCl →⁽ᴬˡᶜˡ³⁾ C₆H₅R + HCl (electrophile: R⁺) 5. Friedel-Crafts acylation: C₆H₆ + RCOCl →⁽ᴬˡᶜˡ³⁾ C₆H₅COR + HCl (electrophile: RCO⁺)

A substituent on the benzene ring directs the incoming electrophile to specific positions:

Ortho/para directors (activate the ring, positions 2,4,6):

• Strongly activating: –OH, –NH₂, –NHR, –NR₂, –OR
• Moderately activating: –CH₃, –C₂H₅ (alkyl groups)
• Weakly activating: –C₆H₅ (phenyl)
• Weakly deactivating: –Cl, –Br, –I (deactivate but still ortho/para directing due to +R effect)

Meta directors (deactivate the ring, position 3):

• Strongly deactivating: –NO₂, –CN, –SO₃H, –CHO, –COOH, –COOR
• Moderately deactivating: –COR (acyl groups)

(a) Pentane (b) 2-Methylbutane (c) 2,3-Dimethylbutane (or 2,3-dimethylbutane) (d) 3,3-Dimethylpentane

(a) C₃H₈ + 5O₂ → 3CO₂ + 4H₂O (b) 2CH₄ + O₂ →⁽ᶜᵃᵗ/ᶜᵘ⁾ 2CO + 4H₂ (incomplete combustion) or CH₄ + O₂ →⁽ᴺᵃ⁾ HCHO + H₂O (controlled oxidation) (c) CH₄ →⁽¹²⁷³ ᴷ⁾ C + 2H₂ (thermal decomposition/pyrolysis)

Step 1 — Initiation: Cl₂ →ˣʰᵛ 2Cl• (homolytic cleavage of Cl₂ by UV light or heat)

Step 2 — Propagation:

• Cl• + CH₄ → CH₃• + HCl (chlorine radical abstracts H from methane)
• CH₃• + Cl₂ → CH₃Cl + Cl• (methyl radical attacks Cl₂, forming methyl chloride and regenerating Cl•)

Step 3 — Termination (radicals combine):

• Cl• + Cl• → Cl₂
• CH₃• + CH₃• → C₂H₆ (ethane)
• CH₃• + Cl• → CH₃Cl

The chain reaction can produce CH₂Cl₂, CHCl₃, and CCl₄ with excess Cl₂.

Markovnikov addition: When HBr adds to propene in the absence of peroxides: CH₃–CH=CH₂ + HBr → CH₃–CHBr–CH₃ (2-bromopropane) The H adds to C-1 (more H atoms), Br adds to C-2, giving the more stable 2° carbocation intermediate.

Anti-Markovnikov addition (peroxide effect): When HBr adds to propene in the presence of peroxides: CH₃–CH=CH₂ + HBr →⁽ᴿᴼᴼᴿ⁾ CH₃–CH₂–CH₂Br (1-bromopropane) The radical mechanism reverses the regiochemistry: the RO• radical adds to the less substituted carbon first, and then H is abstracted from HBr. This effect is observed only with HBr.

Ozonolysis cleaves the C=C double bond using ozone (O₃) to form carbonyl compounds. The reaction proceeds through an ozonide intermediate: 1. Alkene + O₃ → molozonide → ozonide (1,2,4-trioxolane) 2. Ozonide + Zn/H₂O (reductive workup) → two carbonyl compounds

Example: Ozonolysis of 2-butene: CH₃–CH=CH–CH₃ →⁽ᴼ³⁾ CH₃CHO + CH₃CHO (two molecules of ethanal)

Ozonolysis of ethene: CH₂=CH₂ →⁽ᴼ³⁾ HCHO + HCHO (two molecules of formaldehyde)

Ozonolysis helps identify the position of the double bond in an unknown alkene.

Polymerisation is the process of combining many small molecules (monomers) into a large molecule (polymer). In addition polymerisation, alkene monomers add to each other:

• Ethene → polythene: nCH₂=CH₂ → (–CH₂–CH₂–)ₙ
• Propene → polypropylene: nCH₃CH=CH₂ → (–CH(CH₃)–CH₂–)ₙ
• Vinyl chloride → PVC: nCH₂=CHCl → (–CH₂–CHCl–)ₙ
• Tetrafluoroethene → Teflon: nCF₂=CF₂ → (–CF₂–CF₂–)ₙ

(a) From alkenes — catalytic hydrogenation: RCH=CHR' + H₂ →⁽ᴺⁱ/ᴾᵈ⁾ RCH₂–CH₂R' (alkane) (b) From alkyl halides — reduction with LiAlH₄: R–X + LiAlH₄ → R–H (alkane); or with Zn/HCl: R–X + Zn + HCl → R–H + ZnCl₂ (c) From carboxylic acids — decarboxylation: RCOONa + NaOH →⁽ᶜᴀ/ᴺᵃ ᴼᴴ, ᴰʳʸ⁾ RH + Na₂CO₃ (soda lime decarboxylation)

(a) From alkyl halides — elimination (dehydrohalogenation): RCH₂CH₂X + KOH (alcoholic) → RCH=CH₂ + KX + H₂O (β-elimination, follows Saytzeff rule: more substituted alkene is major product) (b) From alcohols — dehydration: RCH₂CH₂OH →⁽ᴴ²ˢᴼ⁴/⁴⁷³ ᴷ⁾ RCH=CH₂ + H₂O (E1 mechanism, Saytzeff rule applies)

Terminal alkynes (RC≡CH) have acidic hydrogen because the sp-hybridised carbon (50% s-character) strongly holds the electron pair, polarising the C–H bond. The resulting carbanion (RC≡C⁻) is stabilised by the high s-character.

Acetylide formation: RC≡CH + NaNH₂ → RC≡C⁻Na⁺ + NH₃ (sodium acetylide) 2RC≡CH + 2Na → 2RC≡C⁻Na⁺ + H₂ (with sodium metal)

Acetylides are used to lengthen the carbon chain: RC≡C⁻Na⁺ + R'X → RC≡CR' + NaX

Friedel-Crafts Alkylation: C₆H₆ + RCl →⁽ᴬˡᶜˡ³⁾ C₆H₅R + HCl Mechanism: AlCl₃ polarises R–Cl → R⁺ (carbocation) is the electrophile. R⁺ attacks benzene ring → arenium ion → loss of H⁺ → alkylbenzene. Limitation: Polyalkylation can occur (first alkyl group activates the ring for further substitution).

Friedel-Crafts Acylation: C₆H₆ + RCOCl →⁽ᴬˡᶜˡ³⁾ C₆H₅COR + HCl Mechanism: AlCl₃ polarises RCOCl → RCO⁺ (acylium ion) is the electrophile. RCO⁺ attacks benzene → arenium ion → loss of H⁺ → acylbenzene (ketone). Advantage over alkylation: The acyl group is a meta director that deactivates the ring, so polyacylation does not occur. The product can be reduced (Clemmensen or Wolff-Kishner) to give the alkylbenzene without rearrangement.

Saytzeff's rule (Zaitsev's rule): In elimination reactions, the more substituted alkene (the one with more alkyl groups on the double-bond carbons) is the major product, because it is more stable.

Example: Dehydrohalogenation of 2-bromobutane with alcoholic KOH: CH₃–CHBr–CH₂–CH₃ + KOH (alc.) → CH₃–CH=CH–CH₃ (but-2-ene, 80%, more substituted) + CH₂=CH–CH₂–CH₃ (but-1-ene, 20%, less substituted)

But-2-ene has two alkyl groups on the double bond carbons (more stable), while but-1-ene has only one.

(a) CH₂=CH₂ →⁽ᴼ³/ᴼⁿ/ᴴ²ᴼ⁾ HCHO + HCHO (formaldehyde) (b) CH₃–CH=CH₂ →⁽ᴼ³/ᴼⁿ/ᴴ²ᴼ⁾ CH₃CHO + HCHO (ethanal + formaldehyde) (c) (CH₃)₂C=CHCH₃ →⁽ᴼ³/ᴼⁿ/ᴴ²ᴼ⁾ (CH₃)₂C=O + CH₃CHO (acetone + ethanal)

The Kharasch effect (peroxide effect) is the anti-Markovnikov addition of HBr to unsymmetrical alkenes in the presence of peroxides.

Without peroxide (Markovnikov): CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (2-bromopropane) With peroxide (anti-Markovnikov): CH₃CH=CH₂ + HBr →⁽ᴿᴼᴼᴿ⁾ CH₃CH₂CH₂Br (1-bromopropane)

Mechanism (radical addition): 1. ROOR → 2RO• (initiation) 2. RO• + HBr → ROH + Br• 3. Br• + CH₃CH=CH₂ → CH₃CH•CH₂Br (more stable 2° radical) 4. CH₃CH•CH₂Br + HBr → CH₃CH₂CH₂Br + Br• (chain propagation)

Wurtz reaction: 2RX + 2Na → R–R + 2NaX (dry ether) Used to prepare alkanes with an even number of carbon atoms by coupling alkyl halides with sodium metal.

Limitations: 1. Only symmetrical alkanes (even number of C atoms) are best prepared. If two different alkyl halides are used, a mixture of three products (R–R, R'–R', R–R') is formed, difficult to separate. 2. Cannot prepare methane (lowest alkane). 3. Not suitable for tertiary alkyl halides (elimination competes). 4. The reaction involves free radical intermediates, so side reactions (elimination) can occur.

Benzene (C₆H₆) has a planar hexagonal structure with all C–C bond lengths equal (139 pm, between typical C–C single bond 154 pm and C=C double bond 134 pm). Each carbon is sp² hybridised with one unhybridised p-orbital. The six p-orbitals overlap to form a delocalised π-electron cloud above and below the ring plane (π-sextet). The resonance energy (delocalisation energy) of benzene is about 150 kJ mol⁻¹, making it about 150 kJ mol⁻¹ more stable than the hypothetical cyclohexatriene with three localised double bonds. This extra stability explains why benzene prefers substitution over addition — addition would destroy the aromatic stabilisation.

(a) C₆H₆ + Cl₂ →⁽ᶠᵉᶜˡ³⁾ C₆H₅Cl + HCl (chlorobenzene, halogenation) (b) C₆H₆ + HNO₃ →⁽ᴴ²ˢᴼ⁴⁾ C₆H₅NO₂ + H₂O (nitrobenzene, nitration) (c) C₆H₆ + CH₃Cl →⁽ᴬˡᶜˡ³⁾ C₆H₅CH₃ + HCl (toluene, Friedel-Crafts alkylation) (d) C₆H₆ + CH₃COCl →⁽ᴬˡᶜˡ³⁾ C₆H₅COCH₃ + HCl (acetophenone, Friedel-Crafts acylation)

–NO₂ group (meta director, deactivating): The –NO₂ group is a strong electron-withdrawing group (–I and –R effects). It withdraws electron density from the ortho and para positions more than the meta position, making the ring less reactive. The ortho and para positions bear partial positive charges, so the electrophile attacks at the meta position. Nitrobenzene is less reactive than benzene.

–CH₃ group (ortho/para director, activating): The –CH₃ group is electron-donating (+I effect and hyperconjugation). It increases electron density at the ortho and para positions, making these positions more nucleophilic. The electrophile preferentially attacks at ortho and para positions. Toluene is more reactive than benzene.

Benzene has a special stability due to the delocalised π-electron cloud (resonance stabilisation of ≈150 kJ mol⁻¹). Addition reactions would destroy this aromatic stabilisation by breaking the conjugated π-system. Instead, benzene undergoes electrophilic substitution, which preserves the aromatic ring. Addition products are less stable than the aromatic starting material. However, under forcing conditions (e.g., H₂/Ni at high pressure and temperature, or Cl₂/hν), benzene can undergo addition.

(a) CH₂=CH₂ + H₂ →⁽ᴺⁱ⁾ CH₃–CH₃ (catalytic hydrogenation) (b) HC≡CH + H₂ →⁽ᴸⁱⁿᵈˡᵃʳ'ˢ ᶜᵃᵗ/ᴮᵃˢᴼ⁴/qᵘⁱⁿᵒˡⁱⁿᵉ⁾ CH₂=CH₂ (cis-ethene, partial hydrogenation) (c) CH₃CH=CH₂ + H₂O →⁽ᴴ³ᴾᴼ⁴⁾ CH₃CH(OH)CH₃ (2-propanol via Markovnikov addition) — For 1-propanol, use anti-Markovnikov: CH₃CH=CH₂ + HBr →⁽ᴾᵉʳᴼˣ⁾ CH₃CH₂CH₂Br → CH₃CH₂CH₂OH (1-propanol) (d) C₆H₆ + HNO₃ →⁽ᴴ²ˢᴼ⁴⁾ C₆H₅NO₂ + H₂O (nitration)

(a) Nitrobenzene cannot undergo Friedel-Crafts alkylation because the –NO₂ group is a strong deactivating group. It withdraws electron density from the ring, making it less nucleophilic. The ring becomes so deactivated that it cannot be attacked by even a strong electrophile. Additionally, AlCl₃ (the Lewis acid catalyst) complexes with the –NO₂ group itself.

(b) Vinyl chloride (CH₂=CHCl) cannot undergo Friedel-Crafts alkylation because the vinyl carbocation (CH₂=CH⁺) is extremely unstable. It is a primary carbocation with no hyperconjugative stabilisation from sp-hybridised carbons. AlCl₃ cannot generate a stable electrophile from vinyl chloride.

Polynuclear aromatic hydrocarbons (PAHs) with four or more fused benzene rings are carcinogenic (cancer-causing). Examples include benzo[a]pyrene (found in tobacco smoke, charred meat, and automobile exhaust), benzanthracene, and dibenzanthracene. These compounds are formed during incomplete combustion of organic matter. They are metabolically activated in the body to form epoxides that bind to DNA, causing mutations that can lead to cancer. Benzene itself is also carcinogenic, linked to leukaemia with long-term exposure.

Petrochemicals are chemicals derived from petroleum (crude oil) and natural gas. Important hydrocarbon petrochemicals include:

• Methane → formaldehyde, methanol, chloromethane
• Ethene → polythene, ethylene glycol, ethanol, PVC
• Propene → polypropylene, isopropanol, acetone, acrylonitrile
• Butadiene → synthetic rubber (SBR, nitrile rubber)
• Benzene → styrene, nylon, phenol, aniline, detergents
• Toluene → TNT, benzaldehyde, polyurethane
• Xylene → PET, Dacron, terephthalic acid