Hydrocarbons: Alkanes, Alkenes, Alkynes – Structure, Properties & Reactions

Hydrocarbons: The Building Blocks of Organic Chemistry

Hydrocarbons are compounds made only of carbon (C) and hydrogen (H). They are the simplest organic compounds and form the basis of fuels, plastics, and many natural products. This unit focuses on three types:

1. Alkanes (single bonds, saturated)
2. Alkenes (one double bond, unsaturated)
3. Alkynes (one triple bond, unsaturated)

We’ll learn how to:

• Name them using IUPAC rules.
• Draw their structures.
• Compare their properties and reactions.
• Understand their importance in daily life.

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1. Alkanes: The Saturated Hydrocarbons

Definition & Structure

Alkanes are saturated hydrocarbons—they contain only single bonds (C–C and C–H) between carbon atoms. The general formula for alkanes is: $${\text{C}}_n{\text{H}}_{2n+2}$$ where $n$ = number of carbon atoms.

Naming Alkanes (IUPAC Rules)

1. Find the longest carbon chain (parent chain).
2. Number the chain to give substituents (branches) the lowest possible numbers.
3. Name the substituents (e.g., –CH₃ = methyl, –C₂H₅ = ethyl).
4. Combine the names (e.g., CH₃–CH(CH₃)–CH₃ = 2-methylpropane).

Example: Draw and name the following structure:

    CH₃
     |
CH₃–CH–CH₃

Answer: 2-methylpropane (3 carbons in the main chain, a methyl group on the 2nd carbon).

Physical Properties of Alkanes

Chemical Reactions of Alkanes

Alkanes are relatively unreactive but undergo:

1. Combustion (burning in oxygen): $${\text{C}}_n{\text{H}}_{2n+2}+\left(\frac{3n+1}{2}\right){\text{O}}_2\to n{\text{CO}}_2+(n+1){\text{H}}_2\text{O}$$ Example: Burning methane (natural gas): $${\text{CH}}_4+2{\text{O}}_2\to {\text{CO}}_2+2{\text{H}}_2\text{O}+\text{Heat}$$

2. Substitution reactions (with halogens like Cl₂ under UV light): $${\text{CH}}_4+{\text{Cl}}_2\stackrel{\text{UV light}}{\to }{\text{CH}}_3\text{Cl}+\text{HCl}$$ (This is how chlorofluorocarbons (CFCs) are made.)

Uses of Alkanes

• Fuels: Methane (natural gas), propane (LPG), octane (petrol).
• Solvents: Hexane (in nail polish remover).
• Plastics: Polyethylene (from ethene, but derived from alkanes).

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2. Alkenes: The Unsaturated Hydrocarbons with Double Bonds

Definition & Structure

Alkenes contain at least one carbon-carbon double bond (C=C). Their general formula is: $${\text{C}}_n{\text{H}}_{2n}$$ The double bond makes them unsaturated (can add more atoms).

Naming Alkenes

1. Find the longest chain containing the double bond.
2. Number the chain to give the double bond the lowest possible number.
3. Add the suffix "–ene" to the parent name.
4. Indicate the position of the double bond (e.g., CH₂=CH–CH₃ = propene).

Example: Name this structure:

CH₃–CH=CH–CH₃

Answer: But-2-ene (4 carbons, double bond between C₂ and C₃).

Physical Properties of Alkenes

Chemical Reactions of Alkenes

The double bond is reactive! Alkenes undergo:

1. Addition reactions (breaking the double bond to add atoms):
   • Hydrogenation (adds H₂): $${\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\to {\text{CH}}_3{\text{CH}}_3$$
   • Halogenation (adds Br₂/Cl₂): $${\text{CH}}_2={\text{CH}}_2+{\text{Br}}_2\to {\text{CH}}_2{\text{BrCH}}_2\text{Br}$$
   • Hydration (adds H₂O to form alcohols): $${\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\text{O}\to {\text{CH}}_3{\text{CH}}_2\text{OH}$$

2. Polymerization (many alkenes join to form polymers): $$n({\text{CH}}_2={\text{CH}}_2)\to (-{\text{CH}}_2{\text{CH}}_2-{)}_n\text{ (Polyethylene)}$$

Uses of Alkenes

• Plastics: Polyethylene (from ethene), PVC (from chloroethene).
• Rubber: Synthetic rubber (from butadiene).
• Food industry: Margarine (hydrogenated oils).

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3. Alkynes: The Unsaturated Hydrocarbons with Triple Bonds

Definition & Structure

Alkynes contain at least one carbon-carbon triple bond (C≡C). Their general formula is: $${\text{C}}_n{\text{H}}_{2n-2}$$ The triple bond makes them highly unsaturated.

Naming Alkynes

1. Find the longest chain containing the triple bond.
2. Number the chain to give the triple bond the lowest possible number.
3. Add the suffix "–yne" to the parent name.
4. Indicate the position of the triple bond (e.g., CH≡C–CH₃ = propyne).

Example: Name this structure:

CH≡C–CH₂–CH₃

Answer: But-1-yne (4 carbons, triple bond between C₁ and C₂).

Physical Properties of Alkynes

Chemical Reactions of Alkynes

The triple bond is very reactive! Alkynes undergo:

1. Addition reactions (two steps for terminal alkynes):

   • Hydrogenation (can add 2 H₂ molecules): $$\text{CH≡CH}+2{\text{H}}_2\to {\text{CH}}_3{\text{CH}}_3$$
   • Halogenation (adds 2 Br₂/Cl₂ molecules): $$\text{CH≡CH}+2{\text{Br}}_2\to {\text{CHBr}}_2{\text{CHBr}}_2$$

2. Reaction with metals (terminal alkynes react with Ag⁺/Cu²⁺): $$\text{CH≡CH}+2{\text{Ag(NH}}_3{)}_2^{+}+2{\text{OH}}^{-}\to \text{AgC≡CAg}+4{\text{NH}}_3+2{\text{H}}_2\text{O}$$ (Used to detect terminal alkynes—forms a white precipitate.)

Uses of Alkynes

• Welding fuel: Acetylene (ethyne, C₂H₂) in oxy-acetylene torches.
• Synthetic fibers: Polyacrylonitrile (from propyne).
• Pharmaceuticals: Some alkynes are used in drug synthesis.

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Comparison of Alkanes, Alkenes, and Alkynes

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4. Isomerism in Hydrocarbons

Isomers are compounds with the same molecular formula but different structures.

Types of Isomerism in Hydrocarbons

1. Structural ( Constitutional ) Isomerism

   • Chain isomerism: Different carbon chain arrangements. Example: Butane (CH₃CH₂CH₂CH₃) and 2-methylpropane (CH₃–CH(CH₃)–CH₃) both have C₄H₁₀.
   • Position isomerism: Double/triple bond or substituent in different positions. Example: But-1-ene (CH₂=CH–CH₂–CH₃) vs. But-2-ene (CH₃–CH=CH–CH₃).
   • Functional group isomerism: Different types of hydrocarbons (e.g., alkene vs. cycloalkane). Example: C₄H₈ can be butene (CH₂=CH–CH₂–CH₃) or cyclobutane (a ring structure).

2. Stereoisomerism (Geometric Isomerism in Alkenes)

   • Occurs due to restricted rotation around C=C double bonds.
   • Cis-trans isomerism:
     • Cis: Same groups on the same side of the double bond.
     • Trans: Same groups on opposite sides.
   • Example: But-2-ene has cis and trans forms.

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5. Industrial Preparation of Hydrocarbons

From Petroleum (Fractional Distillation)

Petroleum is separated into fractions (alkanes) based on boiling points:

• Refinery gases: Methane, ethane (C₁–C₂).
• Petrol (gasoline): C₅–C₁₀ (alkanes + alkenes).
• Diesel: C₁₁–C₁₄.
• Lubricating oil: C₁₅–C₂₀.
• Bitumen: C₂₀+ (used in roads).

From Coal (Coal Gasification)

Coal is heated in the absence of air to produce coal gas (a mixture of H₂, CH₄, and CO).

From Natural Gas (Cracking)

Large alkane molecules are broken down into smaller, more useful hydrocarbons (e.g., converting octane into ethene and propene for plastics).

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Exam Tip: How to Score Full Marks in NEB Exams

1. Drawing Structures Correctly

• Always show all bonds (single, double, triple).
• Label carbons if asked (e.g., "Draw but-2-ene with carbon numbering").
• Use wedge and dash bonds for 3D structures (if required).

2. Naming Hydrocarbons

• Practice IUPAC naming daily. Common mistakes:
  • Forgetting to number the chain (e.g., writing "pentene" instead of "pent-2-ene").
  • Misplacing the double/triple bond number.
• Check for longest chain first!

3. Reaction Mechanisms

• Memorize key reactions:
  • Alkanes: Combustion + substitution.
  • Alkenes: Addition (H₂, Br₂, H₂O) + polymerization.
  • Alkynes: Addition (2 steps) + reaction with Ag⁺/Cu²⁺.
• Write balanced equations for all reactions.

4. Isomerism Questions

• Draw all possible isomers for a given formula (e.g., C₅H₁₂).
• Explain why they are isomers (same formula, different structure).
• Identify cis-trans isomers in alkenes.

5. Short Answer vs. Long Answer

6. Common Mistakes to Avoid

• ❌ Writing "ethylene" instead of "ethene" (IUPAC name).
• ❌ Forgetting to balance chemical equations.
• ❌ Drawing incorrect structures (e.g., putting a double bond on the wrong carbon).
• ❌ Mixing up structural and geometric isomerism.

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Solved Examples (NEB-Style)

Example 1: Naming

Question: Name the following compound:

CH₃–CH₂–CH=CH–CH₃

Solution:

1. Longest chain: 5 carbons (pent).
2. Double bond between C₂ and C₃.
3. Name: Pent-2-ene.

Example 2: Reaction

Question: Write the equation for the complete combustion of propane (C₃H₈). Solution: $${\text{C}}_3{\text{H}}_8+5{\text{O}}_2\to 3{\text{CO}}_2+4{\text{H}}_2\text{O}$$

Example 3: Isomerism

Question: Draw all possible structural isomers of C₄H₈. Solution:

1. But-1-ene: CH₂=CH–CH₂–CH₃
2. But-2-ene: CH₃–CH=CH–CH₃ (cis and trans forms)
3. 2-Methylpropene: CH₂=C(CH₃)–CH₃

Example 4: Addition Reaction

Question: What is the product when propene (CH₃–CH=CH₂) reacts with HBr? Solution: Follow Markovnikov’s rule (H⁺ adds to the carbon with more H atoms): $${\text{CH}}_3{\text{–CH=CH}}_2+\text{HBr}\to {\text{CH}}_3{\text{–CHBr–CH}}_3$$

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NEB Board-Style Questions (Practice)

Section A: Short Answer (2 marks each)

1. Define homologous series. Give an example with two members.
2. Why are alkenes more reactive than alkanes?
3. Write the IUPAC name of:

   CH≡C–CH₂–CH₃
   

4. What is cracking? How is it useful in the petroleum industry?
5. Draw the cis and trans forms of 2-butene.

Section B: Long Answer (5–7 marks)

6. Explain the preparation, properties, and uses of ethene.
7. Describe the structural and geometric isomerism in C₅H₁₀. Draw all possible isomers.
8. Write the reactions of propyne (C₃H₄) with:
   • Hydrogen (H₂)
   • Bromine (Br₂)
   • Ammoniacal silver nitrate (Ag(NH₃)₂⁺)
9. How would you distinguish between:
   • Hexane and hex-1-ene using bromine water?
   • But-1-yne and but-2-yne using Tollens’ reagent?

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Summary Checklist

Before the exam, ensure you can: ✅ Name alkanes, alkenes, and alkynes using IUPAC rules. ✅ Draw structures for given names and vice versa. ✅ Explain the difference between saturated and unsaturated hydrocarbons. ✅ Write combustion, addition, and substitution reactions. ✅ Identify structural and geometric isomers. ✅ Describe industrial preparation (fractional distillation, cracking). ✅ Solve NEB-style numerical and descriptive questions.

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Final Tip: Hydrocarbons are the backbone of organic chemistry. Master this unit, and you’ll ace questions on polymers, fuels, and even biomolecules in Class 12! Practice drawing structures daily—they’re worth half the marks in NEB exams. 🚀