Hydrocarbons

Hydrocarbons are organic compounds composed exclusively of carbon and hydrogen. They are the simplest organic molecules and the primary constituents of petroleum and natural gas. Hydrocarbons are classified into alkanes, alkenes, alkynes, and arenes (aromatic hydrocarbons).

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Alkanes (Saturated Hydrocarbons)

Alkanes have only single bonds. General formula: C_nH_2n+2 (acyclic alkanes).
Physical properties: Low boiling points; increase with molar mass. Insoluble in water, soluble in organic solvents.
Conformations: Different spatial arrangements from rotation about C-C bond — staggered (more stable, lower energy) and eclipsed (less stable, higher energy). Represented using Newman projections or sawhorse structures.
Reactions of Alkanes:
Combustion: CH₄ + 2O₂ → CO₂ + 2H_2O (complete combustion, highly exothermic).
Halogenation (Free radical substitution): CH₄ + Cl₂ → CH_3Cl + HCl (in UV light). Mechanism: Initiation → Propagation → Termination.
Pyrolysis (Cracking): Breaking C-C bonds at high temperature to form smaller molecules.

Alkenes (Unsaturated — one double bond)

General formula: C_nH_2n.
Structure: The two carbons of the double bond are sp² hybridised; the double bond consists of one sigma and one pi bond. The molecule is planar around the double bond.
Geometrical (cis-trans) Isomerism: Arises due to restricted rotation around the double bond. Cis: same groups on the same side; Trans: same groups on opposite sides.
Reactions of Alkenes:
Electrophilic Addition: The pi bond acts as a nucleophile and attracts electrophiles.
Addition of HBr: CH₂=CH₂ + HBr → CH_3CH_2Br.
Markovnikov's Rule: In the addition of HX to an unsymmetrical alkene, the H adds to the carbon with more hydrogen atoms (forming the more stable carbocation intermediate).
Anti-Markovnikov (Peroxide effect / Kharasch effect): In presence of peroxides, HBr adds against Markovnikov's rule via free radical mechanism.
Hydrogenation: Addition of H₂ in presence of Ni/Pd/Pt catalyst → alkane.
Addition of H_2O (acidic): Forms alcohol.
Ozonolysis: O₃ followed by Zn/H_2O cleaves the double bond to give aldehydes/ketones.
Polymerisation: Many alkene molecules join to form a polymer (e.g., polyethylene from ethene).

Alkynes (Unsaturated — one triple bond)

General formula: C_nH_2n-2.
Structure: Triple bond carbon atoms are sp hybridised — linear geometry (180° bond angle).
Acidity of Terminal Alkynes: The sp-hybridised C-H bond is more acidic than alkene or alkane C-H bonds; terminal alkynes react with strong bases like NaNH₂ to form sodium acetylide (NaC≡CH).
Reactions: Similar electrophilic additions as alkenes, but proceed in two steps (one pi bond at a time). Also undergo addition of H₂, HX, H_2O, halogens.

Arenes (Aromatic Hydrocarbons)

Benzene (C_6H₆): Six-membered ring with alternating single and double bonds (Kekulé structure), but all C-C bond lengths are equal due to resonance/delocalisation of pi electrons.
All carbons are sp² hybridised; the molecule is planar.
Aromaticity (Hückel's rule): A cyclic, planar, fully conjugated system with (4n+2) pi electrons (n = 0,1,2…) is aromatic. Benzene has 6 pi electrons (n=1): aromatic.
Reactions of Benzene — Electrophilic Aromatic Substitution (EAS):
Nitration: C_6H₆ + HNO₃ → C_6H_5NO₂ + H_2O (conc. H_2SO₄ catalyst). Electrophile: NO₂+ (nitronium ion).
Halogenation: C_6H₆ + Cl₂ → C_6H_5Cl + HCl (anhydrous FeCl₃ catalyst).
Sulphonation: C_6H₆ + H_2SO₄ (fuming) → C_6H_5SO_3H + H_2O. Reversible.
Friedel-Crafts Alkylation: C_6H₆ + RCl → C_6H_5R + HCl (AlCl₃ catalyst).
Friedel-Crafts Acylation: C_6H₆ + RCOCl → C_6H_5COR + HCl (AlCl₃ catalyst).
Directive effects of substituents: Electron-donating groups (-OH, -NH₂, -CH₃) are ortho/para directors; electron-withdrawing groups (-NO₂, -COOH, -CHO) are meta directors.
Carcinogenicity: Polynuclear aromatic hydrocarbons (e.g., benzo[a]pyrene) are carcinogenic.

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Worked Examples

Example 1

Write the products of free radical chlorination of methane.
CH₄ + Cl₂ (UV light) → CH_3Cl (chloromethane) + HCl. Further chlorination can give CH_2Cl₂, CHCl₃, CCl₄.

Example 2

Apply Markovnikov's rule: CH_3CH=CH₂ + HBr → ?
H adds to CH₂ (more H-bearing C), Br to CH (less H-bearing C). Product: CH_3CHBrCH₃ (2-bromopropane).

Example 3

Identify cis and trans isomers of but-2-ene.
Cis-but-2-ene: both CH₃ groups on same side of double bond.
Trans-but-2-ene: CH₃ groups on opposite sides. Cis has lower melting point but higher boiling point.

Example 4

Give the mechanism of EAS (nitration of benzene).
Step 1: NO₂+ (electrophile) generated from HNO₃ + H_2SO₄.
Step 2: NO₂+ attacks benzene ring to form an arenium ion (sigma complex).
Step 3: Proton lost to restore aromaticity → nitrobenzene.

Example 5

Which position will the incoming group attack in toluene (methylbenzene)?
-CH₃ is an electron-donating ortho/para director. The incoming electrophile attacks ortho and para positions predominantly.

Example 6

Explain why alkynes are more acidic than alkenes.
In alkynes, carbon is sp hybridised (50% s-character), making the C-H bond electrons closer to the nucleus — the H is more easily released as H+. Alkene C is sp² (33% s-character), less acidic.

Example 7

Ozonolysis of CH_3CH=CHCH₃ gives?
Cleavage at the double bond: CH_3CH=CHCH₃ + O₃ then Zn/H_2O → CH_3CHO + CH_3CHO (two molecules of acetaldehyde/ethanal).

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Key Formulas

Alkanes: C_nH_2n+2 ; Alkenes: C_nH_2n ; Alkynes: C_nH_2n-2
Markovnikov's rule: H goes to C with more H atoms
Hückel's rule: (4n+2) pi electrons for aromaticity
Degree of unsaturation: (2C + 2 - H + N - X) / 2

Common mistakes

Applying Markovnikov's rule when peroxides are present — the peroxide effect reverses the regiochemistry for HBr addition only (not HCl or HI).
Thinking benzene undergoes addition reactions easily — benzene prefers substitution to preserve aromaticity.
Confusing cis/trans isomerism with optical isomerism — cis-trans arises from restricted rotation around a double bond.

Summary

Hydrocarbons span from saturated alkanes (free radical substitution) to unsaturated alkenes (electrophilic addition, Markovnikov's rule) and alkynes (more acidic, linear geometry) to aromatic benzene (EAS, directive effects). Understanding hybridisation, reaction mechanisms, and isomerism is central to this chapter.