Learning Objectives
- Understand classification of hydrocarbons: alkanes, alkenes, alkynes, arenes
- Learn preparation methods and chemical reactions of each class
- Understand Markovnikov's and anti-Markovnikov's rules
- Study aromatic hydrocarbons and electrophilic aromatic substitution
- Understand conformational and geometrical isomerism in hydrocarbons
Key Concepts
Alkanes (CₙH₂ₙ₊₂) - Paraffins
Saturated hydrocarbons with single bonds only. sp³ hybridised.
Preparation: Wurtz reaction (2RX + 2Na → R-R + 2NaX), decarboxylation (RCOONa + NaOH/CaO → RH), hydrogenation of alkenes.
Reactions:
- Halogenation (free radical substitution): CH₄ + Cl₂ → CH₃Cl + HCl (UV light). Reactivity: F₂ > Cl₂ > Br₂ > I₂. H abstraction: 3° > 2° > 1°.
- Combustion: CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → nCO₂ + (n+1)H₂O
- Pyrolysis (cracking): Thermal decomposition at high temperature.
Conformations of ethane: Staggered (most stable, 0 kJ/mol) and Eclipsed (least stable, 12.5 kJ/mol torsional strain).
Alkenes (CₙH₂ₙ) - Olefins
Unsaturated with one C=C double bond. sp² hybridised. Show geometrical (cis-trans) isomerism.
Preparation: Dehydration of alcohols (ROH → R=R' + H₂O, with H₂SO₄/Al₂O₃), dehydrohalogenation (RX + alc. KOH → alkene, Saytzeff rule: more substituted alkene preferred).
Reactions (Electrophilic Addition):
- HX addition (Markovnikov's rule): H adds to carbon with more H's. CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (2-bromopropane).
- Anti-Markovnikov (Kharasch/Peroxide effect): With peroxide, HBr adds anti-Markovnikov. Only for HBr (not HCl, HI). CH₃CH=CH₂ + HBr → CH₃CH₂CH₂Br.
- Halogenation: Br₂/CCl₄ → decolourises (test for unsaturation).
- Hydrogenation: R₂C=CR₂ + H₂ → R₂CH-CHR₂ (Ni/Pt/Pd catalyst).
- Ozonolysis: O₃ then Zn/H₂O → aldehydes/ketones (identifies position of double bond).
- KMnO₄ test (Baeyer's test): Decolourises pink KMnO₄ → test for unsaturation.
Alkynes (CₙH₂ₙ₋₂)
Unsaturated with one C≡C triple bond. sp hybridised.
Preparation: CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂. Dehydrohalogenation (2 mol HX removal).
Reactions:
- Addition of H₂, X₂, HX (Markovnikov's rule applies).
- Acidic nature of terminal alkynes: RC≡CH + NaNH₂ → RC≡CNa + NH₃ (sp C-H is most acidic: sp > sp² > sp³).
- Metal acetylide formation: RC≡CH + [Ag(NH₃)₂]⁺ → RC≡CAg (white ppt, test for terminal alkynes).
Aromatic Hydrocarbons (Arenes)
Benzene (C₆H₆): planar, hexagonal, sp² hybridised, delocalised π electrons.
Huckel's rule: Aromatic if planar, cyclic, conjugated, and has (4n+2) π electrons (n = 0, 1, 2, ...).
Electrophilic Aromatic Substitution (EAS):
- Halogenation: C₆H₆ + Cl₂ → C₆H₅Cl + HCl (with Lewis acid catalyst: AlCl₃ or FeCl₃).
- Nitration: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O (with H₂SO₄).
- Friedel-Crafts Alkylation: C₆H₆ + RCl → C₆H₅R + HCl (with AlCl₃).
- Friedel-Crafts Acylation: C₆H₆ + RCOCl → C₆H₅COR + HCl (with AlCl₃).
- Sulphonation: C₆H₆ + H₂SO₄(fuming) → C₆H₅SO₃H + H₂O.
Directive effects:
Ortho/para directors (activating): -OH, -NH₂, -OR, -R, -NHCOR (except halogens which are o/p directing but deactivating).
Meta directors (deactivating): -NO₂, -CN, -CHO, -COOH, -SO₃H, -COR.
Summary
Hydrocarbons are classified by saturation and structure. Alkanes undergo free radical substitution. Alkenes and alkynes undergo electrophilic addition following Markovnikov's rule (anti-Markovnikov with peroxide for HBr). Aromatic compounds undergo electrophilic aromatic substitution with directing effects from substituents.
Important Terms
- Markovnikov's Rule: In HX addition, H goes to C with more H's
- Anti-Markovnikov: Opposite addition with peroxide (HBr only)
- Huckel's Rule: (4n+2) π electrons → aromatic
- Electrophilic Substitution: Characteristic reaction of aromatic compounds
- Ozonolysis: Cleavage of double bond using ozone
- Conformation: Different arrangements from rotation around single bonds
Quick Revision
- Alkanes: CₙH₂ₙ₊₂, sp³, free radical substitution
- Alkenes: CₙH₂ₙ, sp², electrophilic addition, Markovnikov's rule
- Alkynes: CₙH₂ₙ₋₂, sp, terminal alkynes are acidic
- Benzene: (4n+2)π electrons, EAS reactions
- Directing: -OH, -NH₂ (o,p activating); -NO₂ (m, deactivating); -X (o,p deactivating)
- Acidity: sp > sp² > sp³ (alkyne > alkene > alkane)