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Hydrocarbons: Aliphatic & Aromatic

Chemistry Unit 11
25 min read
IAT Advanced
Hot Topic

1. Core Concept

Hydrocarbons are the building blocks of organic chemistry, containing only Carbon and Hydrogen. They are classified by the nature of their C-C bonds (saturated vs. unsaturated) and their structural arrangement (aliphatic vs. aromatic).

2. Classification

  • Saturated: Alkanes (CnH2n+2).
  • Unsaturated: Alkenes (CnH2n) and Alkynes (CnH2n-2).
  • Aromatic: Must follow Hückel's Rule (4n+2 π-electrons, planar, fully conjugated; e.g., Benzene).

3. Alkanes (Saturated)

Preparation:

Wurtz Reaction: 2RX + 2Na → R-R + 2NaX. (Works best for symmetrical alkanes).

Properties:

Generally inert; undergo Free Radical Substitution (Halogenation). Mechanism: Initiation (hν), Propagation, Termination.

4. Alkenes (Unsaturated)

Preparation:

Elimination reactions: Dehydration of alcohols (conc. H2SO4) or Dehydrohalogenation of haloalkanes (alc. KOH).

Saytzeff’s Rule: Major product is the most highly substituted alkene.

Reactivity:

  • Electrophilic Addition: e.g., Br2 water test (for unsaturation).
  • Markovnikov’s Rule: Electrophile (H+) adds to the carbon with more H.
  • Anti-Markovnikov: Only for HBr + Peroxides (R2O2).
  • Ozonolysis: R-CH=CH-R' → RCHO + R'CHO (Breaks double bond and adds O to each side).

5. Alkynes

Terminal alkynes (R-C≡CH) have an acidic H. The sp hybridization (50% s-character) makes the C-H bond acidic; it reacts with strong bases like NaNH2.

6. Aliphatic Hydrocarbons Comparison

Property Alkanes Alkenes Alkynes
General Formula CnH2n+2 CnH2n CnH2n-2
Hybridization (C-C) sp3 sp2 sp
Bond Angle 109.5° 120° 180°
Reactivity Low (Substitution) High (Addition) High (Addition, Acidic H)

7. Aromatic Hydrocarbons (Benzene)

Directing Influence in Electrophilic Substitution:

  • o/p-directors: Activating (-OH, -NH2, -R).
  • m-directors: Deactivating (-NO2, -COOH, -CHO).
  • Exception: Halogens (Cl, Br) are deactivating but still o/p-directors.

Nitration

Reagents: Conc. HNO3 + Conc. H2SO4 (Nitrating mixture).

Electrophile: Nitronium ion (NO2+).

Product: Nitrobenzene.

Halogenation

Reagents: X2 (Cl2 or Br2) + Anhydrous FeCl3/FeBr3 or AlCl3.

Electrophile: Halonium ion (X+).

Product: Halobenzene (e.g., Chlorobenzene).

Sulphonation

Reagents: Fuming H2SO4 (Oleum, H2S2O7).

Electrophile: Sulfur trioxide (SO3) - Note it's a neutral electrophile.

Product: Benzene sulphonic acid.

Friedel-Crafts Reactions

Alkylation: R-X + Anhydrous AlCl3 gives Alkylbenzene. Electrophile: Carbocation R+ (prone to rearrangement).

Acylation: R-COCl + Anhydrous AlCl3 gives Acylbenzene (Ketone). Electrophile: Acylium ion R-C≡O+ (no rearrangement).

8. Conceptual Insights

Wurtz Reaction Trap:

Only make symmetrical alkanes easily. Mixing two different alkyl halides (e.g., CH3Cl + C2H5Cl) produces a messy mixture of three different alkanes (C2H6, C3H8, C4H10).

Stability Analysis (But-2-ene):

But-2-ene is more stable than But-1-ene because it has 6 α-hydrogens, allowing for more hyperconjugation structures.

9. Common Mistakes

  • Anti-Markovnikov: ONLY works for HBr + Peroxide. It does not work for HCl or HI.
  • Friedel-Crafts Limit: Nitrobenzene does NOT undergo Friedel-Crafts reaction because it is too deactivated.
  • Functional Group Carbons: Don't forget to count the carbon in groups like -CN or -COOH when determining longest chains.

10. IAT Exam Focus Points

Key Exam Focus:

  • Degree of Unsaturation (DoU): Formula DoU = C + 1 - (H + X - N)/2. Use this to deduce valid structures quickly.
  • Kjeldahl Calculation: Understand that N → NH3 → Titrated against standard acid.
  • Hückel's Rule: Identify aromatic molecules (planar, conjugated, 4n+2 π-electrons).
  • Substitution vs Addition: Alkanes do substitution; alkenes/alkynes do addition.

11. Carcinogenicity and Toxicity

Polynuclear Hydrocarbons:

Certain polynuclear (multiring) aromatic hydrocarbons are highly toxic and carcinogenic (cancer-causing).

  • Source: Formed by the incomplete combustion of organic materials like tobacco, coal, and petroleum.
  • Mechanism: They enter the human body, undergo biochemical reactions, and damage DNA, leading to cancer.
  • Examples: 1,2-Benzanthracene, 3-Methylcholanthrene, 1,2-Benzpyrene.

12. Practice Mock Test

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Take a quick 15-question assessment specifically designed for Hydrocarbons. Challenge yourself with IAT-level questions.

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End of Chapter

Hydrocarbons

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