Introduction to Hydrocarbons
Organic compounds composed of only carbon and hydrogen.
Types: (1) Aliphatic (Alkanes, Alkenes, Alkynes) | (2) Aromatic (Arenes)
Sources of Aliphatic Hydrocarbons
Petroleum (Crude Oil)
Dark-colored oily liquid with offensive odor, found below earth’s crust, floats over brine.
Composition of Petroleum
(i) Alkanes: 30–70%, up to C₄₀, straight & branched chain
(ii) Cycloalkanes: 16–64%, e.g., cyclohexane, methylcyclopentane
(iii) Aromatic: 8–15%, e.g., Benzene, Toluene, Xylene, Naphthalene
(iv) S, N, O compounds: Mercaptans (R–SH), Sulphides (R–S–R), Pyridines, Phenols
(v) Natural Gas: CH₄ (80%), C₂H₆ (13%), C₃H₈ (3%), C₄H₁₀ (1%)
(vi) CNG: Compressed natural gas, mainly CH₄, octane rating 130
Theories of Origin
- Mendeleeff’s Carbide Theory: Inorganic origin
- Engler’s Theory: Organic origin
- Modern Theory: Combines both
Petroleum Processing
Mining
Drilled from 500–15,000 ft depth.
Refining: Fractional Distillation
| Fraction | Boiling Range (°C) | Composition | Uses |
|---|---|---|---|
| Uncondensed gas | Up to RT | C₁–C₄ | Fuel, refrigerant, carbon black |
| Petroleum ether | 30–70 | C₅–C₆ | Solvent |
| Petrol/Gasoline | 70–120 | C₆–C₈ | Motor fuel |
| Kerosene | 150–250 | C₁₁–C₁₆ | Fuel, illuminant |
| Diesel oil | 250–350 | C₁₅–C₁₈ | Diesel engines |
| Lubricating oil | >400 | C₁₇–C₂₀ | Lubrication |
| Paraffin wax | >400 | C₂₀–C₃₀ | Candles, polish |
| Pitch | >400 | C₃₀–C₄₀ | Road surfacing |
Purification
- H₂SO₄ treatment: Removes aromatics, thiophene
- Doctor sweetening: 2RSH + Na₂PbS₂ → RSSR + PbS + 2NaOH
- Adsorbents: Alumina, silica, clay
Synthetic Petrol
Cracking
Liquid Phase: 475–530°C, 7–70 atm, 70% yield, octane 65–70
Catalysts: SiO₂, ZnO, TiO₂, Fe₂O₃, Al₂O₃
Vapor Phase: 600–800°C, 3.5–10.5 atm, 70% yield
Synthesis
Coal + H₂ 450–500°C/250 atm → Crude oil
CO + H₂ 1200°C/Co catalyst → Hydrocarbons
Catalyst: Co(100) + ThO₂(5) + MgO(8) + Kieselguhr(200)
Fuel Quality Parameters
Knocking
Metallic sound in engine due to premature ignition.
Order: Straight chain > branched > olefins > cycloalkanes > aromatics
Octane Number
% of iso-octane in iso-octane + n-heptane mixture with same knocking.
2,2,4-Trimethylpentane (iso-octane): Octane = 100
Factors increasing octane:
- Branching ↑
- Double/triple bond ↑
- Cyclic > aliphatic
- Aromatics highest
- TEL (Pb(C₂H₅)₄)
Antiknock Compounds
TEL (63%) + Ethylene bromide (26%) + Ethylene chloride (9%) + Dye (2%)
Modern: AK-33-X (Mn-based)
Octane Improvement
- Isomerisation: n-Pentane → iso-Pentane (AlCl₃, 200°C)
- Alkylation: Isobutane + Isobutylene → Iso-octane
- Aromatisation: Heptane → Toluene + 4H₂
Cetane Number
For diesel: % cetane in cetane + α-methylnaphthalene mixture.
α-Methylnaphthalene: 0
Flash Point
Lowest temp for explosive vapor-air mixture. India: 44°C
Alkanes (Paraffins)
Saturated hydrocarbons with C–C single bonds. General formula: CₙH₂ₙ₊₂
General Methods of Preparation
Alkene + H₂ Ni, heat → Alkane
2RX + 2Na dry ether → R–R + 2NaX
RCOONa + NaOH CaO, heat → RH + Na₂CO₃
2RCOO⁻ → R–R + 2CO₂
Physical Properties
| Alkane | State | M.P. (K) |
|---|---|---|
| C₁–C₄ | Gas | — |
| C₅–C₁₇ | Liquid | — |
| C₁₈+ | Solid | — |
M.P. even > odd carbon atoms due to symmetry.
Chemical Properties
RH + X₂ hv/light → RX + HX
Reactivity: F₂ > Cl₂ > Br₂ > I₂
CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → nCO₂ + (n+1)H₂O
High MW → Low MW hydrocarbons
JEE Main Weightage
4–5 questions expected from petroleum, octane, knocking, cracking, alkane reactions.