This chapter is the “Fuel of Civilization”—from the methane in your stove to the petrol in your car and the polymers in your clothes, it all starts here.

The Backbone of Energy: Mastering Hydrocarbons

Hydrocarbons are the simplest organic compounds, made entirely of Carbon and Hydrogen. But don’t let their simplicity fool you. By changing a single bond to a double bond or twisting a chain into a ring, the chemical properties shift entirely.

In this chapter, we explore the “Big Three”—Alkanes (the saturated), Alkenes (the unsaturated), and Alkynes (the high-energy)—along with the aromatic world of Benzene.

The Core Pillars of Hydrocarbons

1. Alkanes: The Stable Paraffins

Alkanes feature single C-C bonds. They are relatively unreactive, which is why they are used as fuels.

• Key Reaction: Free Radical Substitution. Under UV light, Chlorine can replace Hydrogen atoms one by one.
• Conformations: Because single bonds can rotate, alkanes can take different shapes like “Staggered” or “Eclipsed.” Staggered is always more stable because the atoms are further apart.

2. Alkenes & Alkynes: The Reactive Unsaturateds

Double and triple bonds are regions of high electron density. They act as “nucleophile” centers, waiting for something positive to attack.

• Markovnikov’s Rule: In an addition reaction, the negative part of the reagent goes to the carbon with fewer hydrogens.
• The Exception: In the presence of Peroxide, the rule reverses (Anti-Markovnikov).

3. Aromatic Hydrocarbons: The Benzene Ring

Benzene (C₆H₆) is the king of aromatics. It doesn’t behave like a normal alkene because its pi-electrons are “delocalized” in a ring.

• Stability: This delocalization gives it “Resonance Energy,” making it incredibly stable.
• Reactivity: Instead of addition reactions (which would break the ring), Benzene prefers Electrophilic Substitution.

The Gauntlet: 10 Challenging Aptitude Questions

Question 1: The Wurtz Reaction Limit

Why is the Wurtz Reaction (2RX + 2Na → R-R) considered a poor method for preparing “unsymmetrical” alkanes like Propane?

Question 2: Stability of Alkenes

Arrange the following in increasing order of stability: Ethene, Propene, 2-Methylpropene, and Trans-2-Butene. Use hyperconjugation to explain.

Question 3: The Baeyer’s Test

When an unknown hydrocarbon is treated with cold, dilute, alkaline KMnO₄, the purple color disappears and a brown precipitate forms. What does this tell you about the bonds in the hydrocarbon?

Question 4: Ozonolysis Mystery

An alkene on ozonolysis gives a mixture of Ethanal and Propanone. What is the structure and IUPAC name of the original alkene?

Question 5: The Acidic Alkyne

Why is Ethyne (Acetylene) more acidic than Ethene or Ethane? Hint: Think about the s-character of the hybridized carbon.

Question 6: Markovnikov Challenge

Predict the major product when 1-Methylcyclohexene reacts with HBr.

Question 7: Geometric Isomerism

Does 2-Methylbut-2-ene show cis-trans (geometrical) isomerism? Why or why not?

Question 8: Friedel-Crafts Alkylation

Benzene reacts with n-Propyl Chloride in the presence of anhydrous AlCl₃. Why is the major product Isopropylbenzene (Cumene) instead of n-Propylbenzene?

Question 9: The Hückel’s Rule

Which of the following is aromatic: Cyclobutadiene, Cyclopentadienyl anion, or Cycloheptatriene? Show your work using the 4n + 2 rule.

Question 10: Pyrolysis (Cracking)

What happens when n-Hexane is heated to 773 K under high pressure? Name the process and the possible products.

Detailed Explanations & Solutions

1. Wurtz Reaction

If you use two different halides (Methyl Chloride and Ethyl Chloride), you get a mixture of three alkanes: Ethane, Propane, and Butane. Separating this mixture is difficult because their boiling points are close.

Result: It’s only efficient for symmetrical alkanes (R-R).

2. Alkene Stability

Stability increases with the number of alkyl groups attached to the double bond (more alpha-hydrogens for hyperconjugation).

Result: Ethene < Propene < Trans-2-Butene < 2-Methylpropene.

3. Baeyer’s Test

The purple color of KMnO₄ is discharged only by unsaturated compounds.

Result: The hydrocarbon contains a C=C or C≡C bond.

4. Ozonolysis Logic

To find the alkene, “remove” the oxygens from the products and join the carbons with a double bond.

Ethanal (CH₃CHO) + Propanone (CH₃COCH₃) → CH₃-CH=C(CH₃)₂.

Result: 2-Methylbut-2-ene.

5. Alkyne Acidity

Ethyne carbon is sp hybridized (50% s-character). Ethene is sp² (33%), and Ethane is sp³ (25%). High s-character means electrons are closer to the nucleus, making the C-H bond more polar and easier to break.

Result: More s-character = Higher acidity.

6. Markovnikov Product

The H⁺ attacks first to form the more stable tertiary carbocation at the 1-position. The Br⁻ then attaches there.

Result: 1-Bromo-1-methylcyclohexane.

7. Isomerism Rule

For cis-trans isomerism, each carbon of the double bond must have two different groups. In 2-methylbut-2-ene, one carbon has two methyl groups.

Result: No, it does not show geometrical isomerism.

8. Carbocation Rearrangement

The n-propyl carbocation (primary) is formed first, but it immediately undergoes a 1,2-Hydride shift to become a more stable isopropyl carbocation (secondary).

Result: Isopropylbenzene is formed.

9. Hückel’s Rule

Cyclopentadienyl anion has 6 pi-electrons (4 from double bonds + 2 from the lone pair/charge). 4n + 2 = 6 where n=1.

Result: Cyclopentadienyl anion is aromatic.

10. Pyrolysis

This is Cracking. Higher alkanes break into smaller alkanes and alkenes.

Result: A mixture of methane, ethane, propene, and butane.

Pro-Tip: The Benzene Defense

Benzene is so stable that it will almost always choose to keep its ring intact. If you see a reaction with Benzene, look for the product where the ring is still there, but a Hydrogen has been swapped for something else!