Carbon (C, atomic number 6) is unique because it can form millions of compounds. This diversity arises from two special properties: catenation and tetravalency.
Key Concepts
Catenation: Carbon atoms can bond to other carbon atoms to form long chains, branches, and rings. No other element shows this property to such an extent.
Tetravalency: Carbon has 4 valence electrons and can form 4 covalent bonds — with other carbons or with H, O, N, S, Cl, etc.
Covalent Bond: Formed by sharing of electron pairs between atoms.
- Types of Carbon Chains:
- Straight chain: C-C-C-C (n-butane)
- Branched chain: Isobutane
- Cyclic: Cyclohexane (closed ring)
Homologous Series: A series of organic compounds with the same functional group and general formula, differing by -CH2- (14 mass units) from one member to the next.
- Functional Groups:
- Alcohol: -OH (e.g., ethanol C2H5OH)
- Aldehyde: -CHO (e.g., ethanal)
- Ketone: C=O (e.g., propanone)
- Carboxylic acid: -COOH (e.g., ethanoic acid CH3COOH)
- Halogens: -Cl, -Br
Saturated Compounds (Alkanes): Single bonds only; general formula CnH(2n+2). Example: Methane CH4, Ethane C2H6.
- Unsaturated Compounds:
- Alkenes: One double bond; CnH(2n). Example: Ethene C2H4
- Alkynes: One triple bond; CnH(2n-2). Example: Ethyne C2H2
Allotropes of Carbon: Diamond (hard, poor conductor), Graphite (soft, good conductor), Fullerene (C60).
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Write the molecular formula and structure of ethane and state whether it is saturated.
- Ethane: C2H6 (n=2, CnH(2n+2) = C2H6)
- Structure: CH3-CH3 (single bond between carbons)
- It is saturated because all carbon-carbon bonds are single bonds.
What is the structural difference between ethanol and ethanoic acid?
- Ethanol (C2H5OH): Has -OH (hydroxyl) functional group → alcohol
- Ethanoic acid (CH3COOH): Has -COOH (carboxyl) functional group → carboxylic acid
- Both have 2 carbon atoms but different functional groups give them very different properties.
How does ethanol react with sodium metal?
- 2C2H5OH + 2Na → 2C2H5ONa + H2
- Hydrogen gas is evolved (similar to acid + metal reaction).
- This reaction shows that ethanol has an active -OH group.
What happens when ethanol is oxidised?
- C2H5OH + [O] (from acidified K2Cr2O7 or alkaline KMnO4) → CH3COOH
- Ethanol is oxidised to ethanoic acid (acetic acid).
- The orange dichromate turns green as it is reduced (colour change is a test).
Explain the combustion of ethanol.
- C2H5OH + 3O2 → 2CO2 + 3H2O (complete combustion, clean blue flame)
- Ethanol burns in sufficient oxygen to give CO2 and H2O.
- In limited oxygen: incomplete combustion → CO + soot (carbon)
What is saponification?
- Fats/oils (esters) + NaOH → Soap (sodium salt of fatty acid) + Glycerol
- CH3COOC2H5 + NaOH → CH3COONa + C2H5OH (ester + base → soap-like product + alcohol)
- Soap molecule has a hydrophilic (water-loving) head and hydrophobic (water-repelling) tail.
Distinguish between addition and substitution reactions with examples.
- Addition reaction: Unsaturated compounds add molecules across double/triple bonds.
- CH2=CH2 + H2 → CH3-CH3 (Ni catalyst, hydrogenation of ethene)
- Substitution reaction: Saturated compounds; one atom/group replaces another.
- CH4 + Cl2 → CH3Cl + HCl (in sunlight, chlorination of methane)
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Key Formulas
- Alkane: CnH(2n+2)
- Alkene: CnH(2n)
- Alkyne: CnH(2n-2)
- Combustion: Hydrocarbon + O2 → CO2 + H2O
Common mistakes
- Confusing structural isomers with homologs: Isomers have the same molecular formula; homologs differ by CH2.
- Forgetting functional groups: The functional group determines chemical behaviour, not the carbon chain length.
- Soap vs. Detergent: Soaps are sodium/potassium salts of long-chain fatty acids; detergents work in hard water.
Summary
Carbon forms millions of compounds due to catenation and tetravalency. Organic compounds are classified by functional groups. Homologous series show gradual change in properties. Key reactions include combustion, oxidation, addition, substitution, esterification, and saponification. Ethanol and ethanoic acid are important practical compounds.