Chapter 4

Carbon and its Compounds

10 min video15 MCQs8 exam questions7 practice problems

Carbon has atomic number 6, electronic configuration 2,4. It needs 4 more electrons for an octet, but losing 4 needs enormous energy and gaining 4 is impossible for a nucleus with only 6 protons — so carbon SHARES electrons and forms COVALENT bonds.

A covalent bond is formed by sharing a pair of electrons. One shared pair = single bond (H₂), two shared pairs = double bond (O₂), three shared pairs = triple bond (N₂).

ELECTRON DOT (Lewis) STRUCTURES — a frequent 2–3 mark question. Rules: show only the OUTERMOST (valence) electrons as dots; put one shared pair between the two bonded atoms for each bond; then check every atom has 8 electrons around it (hydrogen needs only 2, a duplet). Examples: H:H (H₂, one shared pair); Cl with 6 lone dots :Cl:Cl: (Cl₂); O::O (O₂, two shared pairs); N:::N (N₂, three shared pairs); methane = C at the centre with four shared pairs, one to each H. For ethanoic acid, CH₃COOH: the CH₃ carbon shares with 3 H and the second carbon, which is double bonded to one O and single bonded to an O–H group.

Properties of covalent compounds: molecules are neutral, so intermolecular forces are weak → LOW melting and boiling points; POOR conductors of electricity (no ions, no free electrons); generally insoluble in water but soluble in organic solvents.

CATENATION: the ability of carbon atoms to link with one another forming long chains, branched chains and rings. Carbon–carbon bonds are exceptionally strong, so these compounds are stable.

TETRAVALENCY: carbon has a valency of 4, so it can bond with four other atoms — carbon, hydrogen, oxygen, nitrogen, sulphur or halogens. Catenation + tetravalency together explain why carbon forms more compounds than all other elements combined.

Allotropes of carbon: DIAMOND — each carbon bonded to 4 others in a rigid 3-D structure, hardest natural substance, does NOT conduct. GRAPHITE — each carbon bonded to 3 others in flat hexagonal layers that slide over each other, so it is soft, slippery and a GOOD conductor of electricity. FULLERENE (C-60) — sixty carbon atoms in a hollow football-shaped ball.

SATURATED hydrocarbons have only single bonds (alkanes). UNSATURATED hydrocarbons have at least one double bond (alkenes) or triple bond (alkynes).

STRUCTURAL ISOMERS: compounds with the SAME molecular formula but DIFFERENT structures. Butane (C₄H₁₀) has 2 isomers — straight-chain (n-butane) and branched (iso-butane). Pentane (C₅H₁₂) has 3.

HOMOLOGOUS SERIES: a family of compounds where each member differs from the next by one –CH₂– unit (a mass difference of 14 u). Alkanes CnH2n+2, alkenes CnH2n, alkynes CnH2n-2. Members show similar chemical properties, while physical properties (melting point, boiling point) change gradually with chain length.

FUNCTIONAL GROUPS (the atom/group that gives a compound its characteristic properties): halogen –Cl / –Br; alcohol –OH; aldehyde –CHO; ketone >C=O; carboxylic acid –COOH.

NOMENCLATURE = prefix + root + suffix. Roots by carbon count: meth (1), eth (2), prop (3), but (4), pent (5), hex (6). Suffix: –ane (single), –ene (double), –yne (triple); functional-group endings –ol (alcohol), –al (aldehyde), –one (ketone, e.g. propanone), –oic acid (carboxylic acid). RULE: if the suffix begins with a vowel, DROP the final 'e' of the parent name (ethane – e + ol = ethanol).

COMBUSTION: carbon compounds burn in air to give CO₂, water, heat and light. Saturated hydrocarbons give a clean BLUE flame; unsaturated ones give a yellow SOOTY flame. A limited supply of air also makes a saturated fuel burn sooty — which is why a gas stove gives a yellow flame when its air holes are blocked.

OXIDATION: alcohols are oxidised to carboxylic acids by alkaline potassium permanganate (KMnO₄) or acidified potassium dichromate (K₂Cr₂O₇). Substances that add oxygen to others are called OXIDISING AGENTS.

ADDITION REACTION: an unsaturated hydrocarbon adds hydrogen in the presence of a NICKEL catalyst to become saturated. Used to harden vegetable oils (unsaturated) into vanaspati ghee (saturated). Note: animal fats are saturated and are considered harmful for health, while vegetable oils with long unsaturated chains are healthier.

SUBSTITUTION REACTION: saturated hydrocarbons react in SUNLIGHT with chlorine, which replaces hydrogen atoms one at a time — CH₄ + Cl₂ → CH₃Cl + HCl.

ETHANOL (C₂H₅OH): a liquid at room temperature, commonly called alcohol. With sodium it gives sodium ethoxide and hydrogen. Heated with excess concentrated H₂SO₄ at 443 K it is DEHYDRATED to ethene (the acid acts as a dehydrating agent). Used in medicines such as tinctures and cough syrups. Drinking ethanol harms health; methanol is far more dangerous and can cause blindness or death.

ETHANOIC ACID (CH₃COOH), commonly acetic acid: vinegar is a 5–8% solution of it in water. Pure ethanoic acid freezes in cold weather to an ice-like solid at 290 K, hence the name GLACIAL acetic acid.

Reactions of ethanoic acid — with a BASE: CH₃COOH + NaOH → CH₃COONa + H₂O. With a CARBONATE / hydrogencarbonate: gives salt + water + CO₂ (brisk effervescence, turns lime water milky — a test for the –COOH group).

ESTERIFICATION: carboxylic acid + alcohol, with an acid catalyst, gives an ESTER — sweet-smelling compounds used in perfumes and flavourings. CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O.

SAPONIFICATION: an ester heated with sodium hydroxide gives back the alcohol and the sodium salt of the carboxylic acid. When the ester is that of a LONG-CHAIN fatty acid (as in oils and fats), that sodium salt is SOAP — hence the name. (Saponifying ethyl ethanoate gives sodium ethanoate, which is NOT soap: its chain is far too short to form micelles.)

SOAP is the sodium or potassium salt of a long-chain carboxylic acid. Each molecule has a hydrophobic hydrocarbon TAIL (dissolves in oil/grease) and a hydrophilic ionic HEAD (attracted to water). In water the tails point inwards into the oil and the heads outwards, forming a spherical cluster called a MICELLE, which traps the dirt and is rinsed away.

HARD WATER contains calcium and magnesium salts. Soap reacts with these ions to form an insoluble precipitate called SCUM, so it does not lather. DETERGENTS do not form scum with Ca²⁺ and Mg²⁺ ions, so they clean effectively even in hard water.

Try answering on paper first — then reveal the model answer. 📄

Solve in your notebook. Stuck? Take the hint before the solution. ✏️

1. Write the molecular formula and the name of the alkane, the alkene and the alkyne that each contain three carbon atoms.

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2. Name the following compounds: (a) CH₃–CH₂–OH (b) CH₃–COOH (c) CH₃–CHO

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3. An organic compound X with the molecular formula C₂H₆O reacts with sodium to liberate hydrogen gas. On oxidation with alkaline KMnO₄ it gives compound Y, which turns blue litmus red. Identify X and Y and write both equations.

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4. Explain why the conversion of ethanol into ethanoic acid is called an oxidation reaction.

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5. A gas jar contains an unsaturated hydrocarbon. Describe one reaction that would convert it into a saturated hydrocarbon, and state one industrial use of this reaction.

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6. Write the electron dot structure of (a) ethane, C₂H₆ and (b) ethene, C₂H₄, and state how many shared pairs lie between the two carbon atoms in each.

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7. You are given ethanol and ethanoic acid in two unlabelled test tubes. Describe a simple chemical test to identify each, stating what you observe.

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The electronic configuration of carbon (atomic number 6) is: