Question 7(2013)
Give reasons for:
(i) Methane does not undergo addition reactions, but ethene does.
(ii) Ethyne is more reactive than ethane.
(iii) Hydrocarbons are excellent fuels.
(i) Methane is a saturated hydrocarbon whereas Ethene is an unsaturated hydrocarbon. In saturated hydrocarbons, all the four valencies of each carbon atom are satisfied by the hydrogen atoms, forming single covalent bond. Thus, due to non-availability of electrons methane is less reactive and hence does not undergo addition reactions.
On the other hand, ethene has two carbon atoms forming a double covalent bond as their valencies are not fully satisfied by hydrogen atoms. The availability of electrons in the double bond makes ethene more reactive and it undergoes addition reactions.
(ii) Ethyne is an unsaturated hydrocarbon having two carbon atoms forming a triple covalent bond as their valencies are not fully satisfied by hydrogen atoms whereas ethane is a saturated hydrocarbon as all the four valencies of its two carbon atoms are satisfied by the hydrogen atoms. The availability of electrons in the triple bond in case of ethyne makes it more reactive than ethane which has does not have electrons available in the single covalent bond.
(iii) Hydrocarbons have high calorific value. They are easily combustible and the reaction is exothermic releasing heat energy. Hence, they are excellent fuels.
Chapter Overview: Organic Chemistry
Organic Chemistry is the study of carbon compounds. Carbon's unique ability to form four covalent bonds and catenate (form long chains) makes organic chemistry vast and diverse. The ICSE syllabus covers hydrocarbons (alkanes, alkenes, alkynes), their nomenclature (IUPAC), structural formulae, isomerism, and characteristic reactions. Alkanes (CnH2n+2) are saturated hydrocarbons that undergo substitution reactions. Alkenes (CnH2n) and alkynes (CnH2n−2) are unsaturated and undergo addition reactions. Students learn homologous series, functional groups, and the distinction between saturated and unsaturated compounds. The chapter introduces alcohols (with −OH group) and carboxylic acids (with −COOH group) as basic functional group chemistry. Students must write structural formulae, name compounds using IUPAC rules, and understand reactions like combustion, substitution, and addition. Practical tests like decolourising bromine water or acidified KMnO4 to distinguish between saturated and unsaturated compounds are important.
Key Concepts & Homologous Series
| Term / Series | Details |
|---|---|
| Catenation | Ability of carbon to form bonds with other carbon atoms, creating chains and rings |
| Homologous Series | Family of compounds with same general formula and functional group, differing by CH2 |
| Alkanes | CnH2n+2; single bonds only; saturated (e.g., CH4, C2H6) |
| Alkenes | CnH2n; one C=C double bond; unsaturated (e.g., C2H4, C3H6) |
| Alkynes | CnH2n−2; one C≡C triple bond; unsaturated (e.g., C2H2, C3H4) |
| Isomerism | Compounds with same molecular formula but different structural arrangements |
| Functional Group | Atom or group responsible for characteristic chemical properties (−OH, −COOH, C=C) |
| IUPAC Naming | Prefix (substituent) + Root (chain length) + Suffix (functional group) |
Must-Know Concepts
- Carbon prefixes: Meth- (1C), Eth- (2C), Prop- (3C), But- (4C), Pent- (5C)
- Combustion: CH4 + 2O2 → CO2 + 2H2O (complete); 2CH4 + 3O2 → 2CO + 4H2O (incomplete)
- Substitution: CH4 + Cl2 → CH3Cl + HCl (in presence of UV light)
- Addition: C2H4 + Br2 → C2H4Br2 (ethene decolourises bromine water)
- Unsaturated compounds decolourise bromine water and acidified KMnO4; saturated compounds do not
- Ethanol: C2H5OH; Ethanoic acid: CH3COOH (vinegar); Ester: CH3COOC2H5 (fruity smell)
- Isomers of butane (C4H10): n-butane and isobutane (2-methylpropane)
Saturated vs Unsaturated Hydrocarbons
| Feature | Saturated (Alkanes) | Unsaturated (Alkenes/Alkynes) |
|---|---|---|
| Bonds | Only single bonds (C−C) | Double (C=C) or triple (C≡C) bonds |
| Typical Reaction | Substitution | Addition |
| Bromine Water | No decolourisation | Decolourised |
| Combustion | Clean blue flame | Smoky/luminous flame (higher C%) |
Important Diagrams to Practice
- Structural formulae of first five members of alkanes, alkenes, and alkynes
- Isomers of butane and pentane with structural formulae
- Laboratory preparation of ethylene from ethanol (dehydration)
Common Mistakes
- Writing wrong general formulae (alkenes are CnH2n, NOT CnH2n+2)
- Confusing substitution (alkanes) with addition (alkenes) reactions
- Not showing all bonds in structural formulae
- IUPAC naming errors: not selecting the longest carbon chain or wrong numbering
- Forgetting conditions (UV light for substitution, Ni catalyst for hydrogenation)
Scoring Tips
- Draw clear structural formulae showing every C-H and C-C bond
- For IUPAC naming: identify longest chain, number from the end nearest to substituent/functional group
- Always mention conditions (catalyst, temperature, UV) in reaction equations
- Practice writing isomers for C4H10, C5H12, and C4H8
Frequently Asked Questions
Why is carbon's chemistry so vast?
Carbon has four valence electrons and can form four strong covalent bonds. Its small size allows strong C-C bonds, enabling catenation (long chains, branches, rings). This versatility leads to millions of organic compounds.
How do you test whether a hydrocarbon is saturated or unsaturated?
Add bromine water to the hydrocarbon. If the orange-brown colour of bromine water is decolourised, the compound is unsaturated (alkene or alkyne). If the colour persists, it is saturated (alkane).
What is the difference between structural isomers?
Structural isomers have the same molecular formula but different structural arrangements of atoms. For example, n-butane has a straight chain while isobutane (2-methylpropane) has a branched chain. They have different physical properties despite the same formula.