Organic Chemistry
Introduction
Organic chemistry is the study of carbon compounds. Carbon forms covalent bonds and can catenate (form chains), leading to millions of organic compounds. In ICSE Class 10 Chemistry, you study hydrocarbons, functional groups, and the basics of organic reactions.
Homologous Series
A homologous series is a group of organic compounds with the same general formula, similar chemical properties, and a regular gradation in physical properties.
| Series | General formula | Functional group | Example |
|---|---|---|---|
| Alkanes | CₙH₂ₙ₊₂ | C−C (single bonds) | CH₄, C₂H₆ |
| Alkenes | CₙH₂ₙ | C=C (double bond) | C₂H₄, C₃H₆ |
| Alkynes | CₙH₂ₙ₋₂ | C≡C (triple bond) | C₂H₂, C₃H₄ |
| Alcohols | CₙH₂ₙ₊₁OH | −OH (hydroxyl) | CH₃OH, C₂H₅OH |
| Carboxylic acids | CₙH₂ₙ₊₁COOH | −COOH (carboxyl) | HCOOH, CH₃COOH |
Characteristics of Homologous Series
- Same general formula.
- Each successive member differs by −CH₂− (14 atomic mass units).
- Gradual change in physical properties (melting point, boiling point).
- Similar chemical properties.
Alkanes (Saturated Hydrocarbons)
Single bonds only — CₙH₂ₙ₊₂.
| Name | Formula | Structure | State at RT |
|---|---|---|---|
| Methane | CH₄ | CH₄ | Gas |
| Ethane | C₂H₆ | CH₃−CH₃ | Gas |
| Propane | C₃H₈ | CH₃−CH₂−CH₃ | Gas |
| Butane | C₄H₁₀ | CH₃−(CH₂)₂−CH₃ | Gas |
| Pentane | C₅H₁₂ | CH₃−(CH₂)₃−CH₃ | Liquid |
Reactions of Alkanes
Substitution reaction (with Cl₂ in sunlight):
CH₄ + Cl₂ → CH₃Cl + HCl (chloromethane) CH₃Cl + Cl₂ → CH₂Cl₂ + HCl (dichloromethane) Further substitution yields CHCl₃ (chloroform) and CCl₄ (carbon tetrachloride).
Combustion:
CH₄ + 2O₂ → CO₂ + 2H₂O + Heat
Alkenes (Unsaturated Hydrocarbons)
Contain C=C double bond — CₙH₂ₙ.
| Name | Formula | Structure |
|---|---|---|
| Ethene | C₂H₄ | CH₂=CH₂ |
| Propene | C₃H₆ | CH₃−CH=CH₂ |
| Butene | C₄H₈ | CH₃−CH₂−CH=CH₂ |
Reactions of Alkenes
Addition reaction (with Br₂):
C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane, colour of Br₂ disappears)
Addition of H₂ (hydrogenation):
C₂H₄ + H₂ → C₂H₆ (using Ni catalyst, heat)
Addition of H₂O (hydration):
C₂H₄ + H₂O → C₂H₅OH (ethanol, using H₃PO₄ catalyst)
Alkynes (Unsaturated Hydrocarbons)
Contain C≡C triple bond — CₙH₂ₙ₋₂.
| Name | Formula | Structure |
|---|---|---|
| Ethyne (Acetylene) | C₂H₂ | HC≡CH |
| Propyne | C₃H₄ | CH₃−C≡CH |
Reaction of Alkynes
Addition reaction:
HC≡CH + Br₂ → CHBr=CHBr (first addition) CHBr=CHBr + Br₂ → CHBr₂−CHBr₂ (second addition)
Alcohols
Functional group: −OH (hydroxyl).
| Name | Formula | Common name | Uses |
|---|---|---|---|
| Methanol | CH₃OH | Wood spirit | Fuel, solvent |
| Ethanol | C₂H₅OH | Alcohol | Beverages, fuel, solvent |
| Propanol | C₃H₇OH | Propyl alcohol | Solvent |
Properties of Ethanol
- Colourless liquid, miscible with water.
- Burns with a blue flame: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
- Reaction with sodium: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑
- Oxidation: CH₃CH₂OH → CH₃CHO → CH₃COOH (with K₂Cr₂O₇/H⁺)
Carboxylic Acids
Functional group: −COOH (carboxyl).
| Name | Formula | Common name | Source |
|---|---|---|---|
| Methanoic acid | HCOOH | Formic acid | Ant sting, nettles |
| Ethanoic acid | CH₃COOH | Acetic acid | Vinegar (5−8%) |
| Propanoic acid | C₂H₅COOH | Propionic acid | Dairy products |
Properties of Ethanoic Acid
- Colourless liquid with pungent smell.
- Freezes at 17°C (glacial acetic acid).
- Weak acid: CH₃COOH ⇌ CH₃COO⁻ + H⁺
- Esterification: CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O (fruity smell)
IUPAC Nomenclature
Rules
- Identify the longest continuous carbon chain (parent chain).
- Number the chain so that the functional group/substituent gets the lowest number.
- Name substituents alphabetically with their positions.
- Use suffixes: -ane (alkane), -ene (alkene), -yne (alkyne), -ol (alcohol), -oic acid (carboxylic acid).
Examples
| Structure | IUPAC name |
|---|---|
| CH₄ | Methane |
| C₂H₅OH | Ethanol |
| CH₃CH₂CH₂OH | Propan-1-ol |
| CH₃CH(OH)CH₃ | Propan-2-ol |
| CH₃COOH | Ethanoic acid |
| CH₃CH₂CH₂COOH | Butanoic acid |
| CH₂=CH₂ | Ethene |
| HC≡CH | Ethyne |
Substitution vs Addition Reactions
| Feature | Substitution | Addition |
|---|---|---|
| Occurs in | Saturated compounds (alkanes) | Unsaturated compounds (alkenes, alkynes) |
| What happens | H atom is replaced by another atom/group | Double/triple bond breaks, atoms are added |
| Example | CH₄ + Cl₂ → CH₃Cl + HCl | C₂H₄ + Br₂ → C₂H₄Br₂ |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Writing C₂H₅ instead of C₂H₅OH for ethanol | The −OH group must be explicitly shown |
| Confusing -ene and -yne suffixes | -ene = double bond; -yne = triple bond |
| Forgetting that alkanes only undergo substitution | Alkanes do NOT undergo addition reactions |
| Incorrect numbering in IUPAC | Number from the end nearest the functional group |
| Writing molecular formula instead of structural | Know the difference: C₂H₆ (molecular) vs CH₃−CH₃ (structural) |
ICSE Exam Focus
This chapter carries 6–8 marks. Key topics: homologous series, IUPAC naming, alkane/alkene/alkyne reactions, ethanol and ethanoic acid properties, substitution vs addition.
Marks Blueprint: Homologous series — 2 marks, IUPAC nomenclature — 2 marks, Reactions — 2 marks, Ethanol/Ethanoic acid — 2 marks.
Self-Test Questions
-
What is a homologous series? List three characteristics.
-
Write the IUPAC names for: (a) CH₃CH₂CH₂OH, (b) CH₃COOH, (c) CH₂=CHCH₃, (d) CH₃CH₂CH₃.
-
Distinguish between substitution and addition reactions with examples.
-
Describe the properties of ethanol. Write its reaction with sodium.
-
What is esterification? Write the equation for the formation of ethyl ethanoate.
-
Write the reaction of ethene with (a) bromine, (b) hydrogen, (c) water.
