Chapter in a nutshell: Organic chemistry is the chemistry of carbon compounds. Carbon's tetravalency and catenation create millions of compounds, grouped into homologous series (alkanes, alkenes, alkynes, alcohols, acids). Each series has a general formula and a functional group, named by the IUPAC system. Saturated hydrocarbons give substitution reactions; unsaturated ones give addition reactions.
1. Why So Many Carbon Compounds?
- Tetravalency: carbon has 4 valence electrons → forms 4 covalent bonds.
- Catenation: carbon atoms link to one another forming long chains, branched chains and rings.
- Carbon also forms strong bonds with H, O, N, S, and multiple bonds (double/triple) → vast diversity.
2. Hydrocarbons
Compounds of carbon and hydrogen only.| Class | Bonding | General formula | Example |
|---|---|---|---|
| Alkanes (saturated) | single C–C | CₙH₂ₙ₊₂ | CH₄ (methane), C₂H₆ (ethane) |
| Alkenes (unsaturated) | one C=C | CₙH₂ₙ | C₂H₄ (ethene) |
| Alkynes (unsaturated) | one C≡C | CₙH₂ₙ₋₂ | C₂H₂ (ethyne) |
- Unsaturated: contain C=C or C≡C — more reactive, give addition.
3. Homologous Series
A family of organic compounds with the same general formula and same functional group, where successive members differ by a –CH₂– (14 u) unit.- Characteristics: same general formula; gradual gradation in physical properties (m.p./b.p. rise with molecular mass); similar chemical properties; same method of preparation.
4. IUPAC Nomenclature
Name = prefix (chain length) + suffix (family).| Carbons | Stem | Alkane | Alkene | Alkyne |
|---|---|---|---|---|
| 1 | meth | methane | — | — |
| 2 | eth | ethane | ethene | ethyne |
| 3 | prop | propane | propene | propyne |
| 4 | but | butane | butene | butyne |
5. Functional Groups
| Group | Formula | Family | Example |
|---|---|---|---|
| Hydroxyl | –OH | alcohol | ethanol C₂H₅OH |
| Aldehyde | –CHO | aldehyde | ethanal CH₃CHO |
| Ketone | >C=O | ketone | propanone |
| Carboxyl | –COOH | carboxylic acid | ethanoic acid CH₃COOH |
| Halo | –X (Cl/Br) | haloalkane | chloromethane CH₃Cl |
6. Isomerism
Isomers = compounds with the same molecular formula but different structural formula (hence different properties). Example: C₄H₁₀ exists as n-butane (straight chain) and iso-butane (branched). Isomerism increases with the number of carbon atoms.7. Alkanes — Methane (CH₄)
- Preparation: $\mathrm{Al_4C_3 + 12H_2O \rightarrow 4Al(OH)_3 + 3CH_4}$ ; or sodium acetate + soda lime: $\mathrm{CH_3COONa + NaOH \xrightarrow{CaO,\,\Delta} CH_4 + Na_2CO_3}$.
- Combustion: $\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}$ (much heat — fuel).
- Substitution (with Cl₂ in sunlight): $\mathrm{CH_4 + Cl_2 \xrightarrow{sunlight} CH_3Cl + HCl}$ (stepwise to CH₂Cl₂, CHCl₃, CCl₄).
8. Alkenes — Ethene (C₂H₄)
- Preparation: dehydration of ethanol: $\mathrm{C_2H_5OH \xrightarrow{conc.\,H_2SO_4,\,170^\circ C} C_2H_4 + H_2O}$.
- Addition reactions (the C=C opens up):
- Combustion gives CO₂ + H₂O (luminous, smoky flame).
9. Alkynes — Ethyne / Acetylene (C₂H₂)
- Preparation: $\mathrm{CaC_2 + 2H_2O \rightarrow Ca(OH)_2 + C_2H_2}$ (calcium carbide + water).
- Addition: with H₂ → ethene → ethane; with Br₂ → decolourises (unsaturation). Burns with a very sooty, luminous flame (oxy-acetylene torch for welding).
10. Alcohols & Carboxylic Acids
- Ethanol (C₂H₅OH): with sodium → H₂; oxidation → ethanoic acid; with conc. H₂SO₄ → ethene (dehydration).
- Ethanoic acid (CH₃COOH): weak acid; with NaOH → sodium acetate + water; with carbonates → CO₂; with ethanol (esterification) → fruity-smelling ester.
11. Worked / Structured Examples (ICSE pattern)
Q1. Give the general formula of alkanes, alkenes, alkynes. Solution: CₙH₂ₙ₊₂, CₙH₂ₙ, CₙH₂ₙ₋₂. Q2. Name C₃H₈ and C₃H₆. Solution: Propane and propene. Q3. How is unsaturation tested? Solution: Unsaturated compounds decolourise bromine water (addition); alkanes do not. Q4. Write the substitution of methane with chlorine. Solution: CH₄ + Cl₂ →(sunlight) CH₃Cl + HCl. Q5. Why do members of a homologous series show similar chemical properties? Solution: They have the same functional group.12. Key Terms — Quick Glossary
| Term | One-line definition |
|---|---|
| Catenation | self-linking of carbon atoms into chains/rings. |
| Hydrocarbon | compound of carbon and hydrogen only. |
| Saturated / unsaturated | only single bonds / contains C=C or C≡C. |
| Homologous series | family with same general formula + functional group. |
| Functional group | atom/group that decides chemical properties. |
| Isomers | same molecular formula, different structure. |
| Substitution | an atom replaced by another (alkanes). |
| Addition | atoms added across a multiple bond (alkenes/alkynes). |
| Esterification | acid + alcohol → ester + water. |
13. Common Mistakes to Avoid
- Mixing up the general formulas (alkene CₙH₂ₙ, alkyne CₙH₂ₙ₋₂).
- Saying alkanes undergo addition — saturated alkanes give substitution.
- Forgetting the conditions (sunlight for chlorination; conc. H₂SO₄/170 °C for dehydration; Ni for hydrogenation).
- Confusing isomers (same formula, different structure) with members of a homologous series.
- Forgetting that the bromine-water test distinguishes unsaturated from saturated.
14. Likely Exam Questions (with crisp answers)
- Why does carbon form a large number of compounds? → Tetravalency + catenation (+ multiple bonds).
- Define a homologous series. → A family with the same general formula and functional group, members differing by –CH₂–.
- Give the general formula of an alkene. → CₙH₂ₙ.
- Name the IUPAC names of CH₄ and C₂H₂. → Methane and ethyne.
- What is the test for unsaturation? → Decolourisation of bromine water.
- Write the reaction for the laboratory preparation of methane from sodium acetate. → CH₃COONa + NaOH →(CaO, Δ) CH₄ + Na₂CO₃.
- Name the reaction type of alkanes with chlorine. → Substitution.
- How is ethene prepared from ethanol? → Dehydration with conc. H₂SO₄ at 170 °C.
- What is prepared when calcium carbide reacts with water? → Ethyne (acetylene).
- Define isomerism with an example. → Same molecular formula, different structure; e.g. n-butane and iso-butane (C₄H₁₀).
- Name the functional group in ethanoic acid. → Carboxyl (–COOH).
- Give one use of ethyne. → Oxy-acetylene flame for welding/cutting metals.
- What is esterification? → Reaction of a carboxylic acid with an alcohol to form an ester (sweet smell) + water.
- Why is methane called a saturated hydrocarbon? → All its carbon bonds are single bonds.
15. Chemical Equations & Formulas (quick reference)
General formulas: alkane CₙH₂ₙ₊₂ · alkene CₙH₂ₙ · alkyne CₙH₂ₙ₋₂ · alcohol CₙH₂ₙ₊₁OH · carboxylic acid CₙH₂ₙ₊₁COOH.- Methane prep: Al₄C₃ + 12H₂O → 4Al(OH)₃ + 3CH₄ ; CH₃COONa + NaOH →(CaO,Δ) CH₄ + Na₂CO₃
- Methane combustion: CH₄ + 2O₂ → CO₂ + 2H₂O
- Methane substitution: CH₄ + Cl₂ →(sunlight) CH₃Cl + HCl → CH₂Cl₂ → CHCl₃ → CCl₄
- Ethene prep: C₂H₅OH →(conc. H₂SO₄, 170°C) C₂H₄ + H₂O
- Ethene addition: C₂H₄ + H₂ →(Ni) C₂H₆ ; C₂H₄ + Br₂ → C₂H₄Br₂ ; C₂H₄ + H₂O → C₂H₅OH
- Ethyne prep: CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂
- Ethanol: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂
- Ethanoic acid: CH₃COOH + NaOH → CH₃COONa + H₂O ; CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (esterification)
16. Physical Properties & Uses of Key Compounds
| Compound | Key physical properties | Uses |
|---|---|---|
| Methane (CH₄) | colourless, odourless gas; main component of natural gas/biogas | fuel (CNG), making hydrogen, carbon black |
| Ethene (C₂H₄) | colourless gas, faint sweet smell | ripening fruit, making polythene, ethanol |
| Ethyne (C₂H₂) | colourless gas (garlic smell from impurities) | oxy-acetylene welding, making PVC/plastics |
| Ethanol (C₂H₅OH) | colourless volatile liquid | solvent, fuel, alcoholic drinks, antiseptic |
| Ethanoic acid (CH₃COOH) | pungent liquid; 5–8% = vinegar | food preservative, making esters |
17. Saturated vs Unsaturated — comparison
| Feature | Saturated (alkanes) | Unsaturated (alkenes/alkynes) |
|---|---|---|
| Bonds | only single C–C | one or more C=C / C≡C |
| Reactivity | less reactive | more reactive |
| Typical reaction | substitution | addition |
| Bromine water | no change | decolourised |
| Flame | clean, blue (more complete) | luminous/sooty |
18. More Worked Examples
Q6. Write the structural difference between ethane, ethene and ethyne. Solution: Ethane C₂H₆ (C–C single), ethene C₂H₄ (C=C double), ethyne C₂H₂ (C≡C triple). Q7. Why does ethyne burn with a sooty flame while methane burns cleanly? Solution: Ethyne has a higher %carbon; incomplete combustion releases unburnt carbon (soot). Q8. Name the products of complete combustion of any hydrocarbon. Solution: Carbon dioxide and water. Q9. Write the IUPAC names of the first four alkanes. Solution: Methane, ethane, propane, butane. Q10. What happens when ethanol is oxidised? Solution: It forms ethanoic acid (CH₃COOH).19. More Exam Questions (with crisp answers)
- Why are alkanes called paraffins? → They are relatively unreactive ("little affinity").
- State two characteristics of a homologous series. → Same general formula + functional group; gradual change in physical properties.
- Give one use each of ethene and ethyne. → Ethene: ripening fruits/making polythene; ethyne: welding.
- What is the role of conc. H₂SO₄ in preparing ethene from ethanol? → It is a dehydrating agent (removes water).
- Which gas decolourises bromine water — ethane or ethene? → Ethene (unsaturated).
- Define a hydrocarbon. → A compound containing only carbon and hydrogen.
- Name the ester formed from ethanoic acid and ethanol. → Ethyl ethanoate (ethyl acetate).
- Why does diamond not conduct but graphite does (both carbon)? → Graphite has free delocalised electrons; diamond has none.
- What is the general name for –OH compounds? → Alcohols.
- Name the process of adding hydrogen across a double bond. → Hydrogenation (catalyst: nickel).
- Give the molecular formula of the alkane with 4 carbons. → C₄H₁₀ (butane).
- Why is catenation strongest in carbon? → Carbon–carbon bonds are strong and stable, allowing long chains and rings.
- Name a fuel gas obtained from carbide. → Ethyne (acetylene) from calcium carbide.