CBSE Class 10 Chemistry Carbon and its Compounds Notes Set 28

Bonding in Carbon Compounds

1. Carbon has four valence electrons. It does not lose or gain four electrons, so it forms covalent bonds by sharing. A covalent bond is a shared pair of electrons; single, double and triple bonds share one, two and three pairs.
2. Covalent compounds have low melting and boiling points and are generally poor conductors because they contain no ions. Ionic compounds (made of metal and non-metal) have high melting points and conduct when molten or dissolved.
3. Carbon forms a vast number of compounds because of catenation (self-linking into chains, branched chains and rings) and tetravalency, together with isomerism.
4. Allotropes: diamond (each C bonded to four others in a rigid network; hardest, non-conductor), graphite (hexagonal layers held by weak forces; soft, slippery, conducts electricity) and fullerenes (C₆₀, football shape).

Hydrocarbons and Homologous Series

A homologous series is a family with the same functional group and general formula, in which successive members differ by CH₂ (14 u). Physical properties such as boiling point change gradually with size, and chemical properties are similar. Methane, ethane, propane and butane are gases at room temperature; pentane to heptadecane are liquids.

Functional Groups and IUPAC Naming

Naming steps: choose the longest carbon chain containing the functional group; number it so the functional group gets the lowest number, then substituents; write substituents alphabetically with locants before the parent name. Examples: 2-methylpropan-2-ol (t-butyl alcohol), 3-methylbutan-2-one. Isomers have the same molecular formula but different structures: butane has 2, pentane 3 and hexane 5 isomers; propanal and propanone are functional isomers (C₃H₆O).

Chemical Properties

1. Combustion: carbon compounds burn in oxygen to give CO₂, water, heat and light: CH₄ + 2O₂ → CO₂ + 2H₂O. Saturated hydrocarbons burn with a clean blue flame; unsaturated ones give a sooty yellow flame. Incomplete combustion blackens vessels.
2. Oxidation: ethanol is oxidised to ethanoic acid by alkaline KMnO₄ or acidified K₂Cr₂O₇.
3. Addition: unsaturated compounds add hydrogen (with Ni catalyst), halogens and water: CH₂=CH₂ + H₂ → CH₃CH₃. Hydrogenation turns vegetable oils into vanaspati ghee. Bromine water or alkaline KMnO₄ is decolourised by unsaturated compounds.
4. Substitution: alkanes react with chlorine in sunlight, replacing hydrogen: CH₄ + Cl₂ → CH₃Cl + HCl.

Ethanol and Ethanoic Acid

1. Ethanol: colourless liquid, soluble in water in all proportions; made by fermentation of sugars with yeast (zymase, invertase) at 25–35 °C: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Rectified spirit is about 95% ethanol; denatured alcohol has poisons added.
2. Reactions of ethanol: with sodium, 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂; dehydration with conc. H₂SO₄ at 443 K gives ethene; oxidation gives ethanoic acid; burning gives CO₂ and water.
3. Ethanoic acid (acetic acid): vinegar is a 5–8% solution; pure acid freezes at 17 °C (glacial acetic acid). It is a weak acid: turns blue litmus red, reacts with NaHCO₃ to give CO₂ (test for –COOH), reacts with NaOH to give sodium ethanoate.
4. Esterification: carboxylic acid + alcohol in the presence of conc. H₂SO₄ gives a sweet-smelling ester and water: CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O. Alkaline hydrolysis of an ester (saponification) gives the sodium salt of the acid and the alcohol.

Soaps and Detergents

1. Soap is the sodium or potassium salt of a long-chain fatty acid (for example C₁₇H₃₅COONa), made by saponification: fat or oil + NaOH gives soap + glycerol.
2. Cleaning action: the hydrocarbon tail dissolves in oil and the ionic head dissolves in water, so oily dirt is enclosed in spherical micelles that are washed away. Agitation helps lift the dirt.
3. Soaps form scum with the Ca and Mg ions of hard water, but detergents (sodium salts of sulphonic acids or alkyl sulphates) do not. Branched-chain detergents are not biodegradable and cause water pollution.