Carbon CompoundsClass 10 Science Part 1 Chapter 9 Notes – Maharashtra Board

Summary: Carbon Compounds

Look around your home: the dal in your plate, the cotton of your shirt, the LPG in the cylinder and the plastic bucket in the bathroom are all made of carbon compounds. Once people thought such compounds came only from living things, but after urea was made in a laboratory, they came to be called simply carbon compounds or organic compounds.

Carbon has four valence electrons. Losing or gaining four electrons would need too much energy, so carbon shares its electrons and forms covalent bonds. Because carbon can bond strongly with other carbon atoms, form single, double and triple bonds, and join with many different elements, it makes about 10 million compounds — more than all other elements put together.

The simplest carbon compounds are hydrocarbons. Alkanes have only single bonds, alkenes have a double bond and alkynes have a triple bond. Carbon chains can be straight, branched or rings. When a hetero atom or a group such as –OH or –COOH replaces a hydrogen, it decides how the compound behaves. Compounds with the same functional group form a homologous series, and IUPAC rules give each compound one clear name.

Carbon compounds burn, get oxidised, take part in addition and substitution reactions. Ethanol (the spirit in medicines) and ethanoic acid (the acid in vinegar) show many such reactions, including making sweet-smelling esters. Finally, small molecules called monomers join to form giant polymers — from starch and proteins in our body to polythene bags, PVC pipes and Teflon pans in our kitchen.

Key points to remember

• All carbon-containing compounds are organic, except CO₂, CO, carbides, carbonates and bicarbonates.
• Carbon (atomic number 6, configuration 2, 4) is tetravalent and forms covalent bonds by sharing electrons.
• A covalent bond is formed by sharing a pair of electrons; one shared pair = one bond (single), two pairs = double, three pairs = triple.
• Catenation is carbon's ability to form strong bonds with other carbon atoms, giving long chains, branches and rings.
• Saturated hydrocarbons (alkanes) have only single bonds; unsaturated hydrocarbons have C=C (alkenes) or C≡C (alkynes).
• Structural isomers have the same molecular formula but different structural formulae, e.g. n-butane and isobutane (C₄H₁₀).
• Benzene (C₆H₆) has a six-carbon ring with three alternate double bonds; such compounds are aromatic.
• Neighbouring members of a homologous series differ by one –CH₂– unit and by 14 u in molecular mass.
• General formulae: alkanes C_nH_2n+2, alkenes C_nH_2n, alkynes C_nH_2n−2.
• IUPAC name = prefix + parent + suffix; halogens are always written as a prefix (chloro, bromo, iodo).
• Saturated compounds burn with a blue flame; unsaturated compounds give a yellow, sooty flame.
• Addition of hydrogen to vegetable oil using a nickel catalyst gives Vanaspati ghee (hydrogenation).
• Ethanoic acid + ethanol (acid catalyst) → ethyl ethanoate (an ester) + water; esters smell sweet.
• A polymer is a giant molecule made by regular repetition of a small unit called a monomer.

Carbon Compounds – Notes

1. Organic compounds and the covalent bond

• Food, fuels, paper, medicines, wood and fibres all contain carbon as the common element. Such compounds are called organic or carbon compounds.
• The idea that organic compounds come only from living things ended when urea was prepared from an inorganic compound in a laboratory.
• Exceptions: CO₂, CO, carbides, carbonates and bicarbonates contain carbon but are treated as inorganic.
• Most carbon compounds melt and boil below about 300 °C, so the forces between their molecules are weak. Methane, for example, boils at −162 °C.
• Glucose or urea solution does not conduct electricity, so carbon compounds do not contain ions — their bonds are not ionic.
• Carbon (2, 4) cannot easily lose 4 electrons to form C⁴⁺ or gain 4 to form C⁴⁻: each step needs more energy, and the small, highly charged ion would be unstable.
• So carbon shares its 4 valence electrons with other atoms. Both atoms reach a noble gas configuration and stay neutral. A bond formed by sharing two electrons between two atoms is a covalent bond.

2. Electron-dot and line structures

• In an electron-dot structure, valence electrons are shown as dots or crosses around the symbol; shared electrons sit in the overlap of the two circles (the circles may be omitted).
• In a line (structural) formula, each covalent bond is shown as a short line between two symbols.
• H₂: each H atom has 1 electron; they share one pair to reach the helium configuration — a single bond, H–H.
• O₂: two oxygen atoms share two pairs — a double bond, O=O. N₂: two nitrogen atoms share three pairs — a triple bond, N≡N.
• CH₄ (methane): carbon shares one electron pair with each of four H atoms, forming four single bonds and reaching the neon configuration.
• Always show ALL valence electrons in an electron-dot structure, including lone pairs (for example the two lone pairs on oxygen in water).
• Ball-and-stick and space-filling models help us see the 3-D shape of molecules like methane.

3. Carbon: a versatile element

• About 10 million carbon compounds are known — more than the compounds of all other elements combined. Their molecular masses range from 16 (methane) to about 10¹² (DNA).
• Catenation: carbon forms strong, stable C–C bonds, so carbon atoms join into long open chains (straight or branched) and closed chains (rings). Oxygen cannot do this well because the O–O bond is weak — that is why H₂O₂ breaks down on its own.
• Multiple bonds: two carbon atoms can be joined by a single, double or triple bond, e.g. ethane, ethene and ethyne all have two carbons.
• Tetravalency: one carbon can bond with four other atoms such as H, O, N, S, P or halogens. With just H and Cl, five compounds form: CH₄, CH₃Cl, CH₂Cl₂, CHCl₃ and CCl₄.
• Isomerism: the same set of atoms can be arranged in different ways, giving still more compounds.
• Everyday link: the LPG cylinder (propane + butane), camphor in the pooja room, sugar and starch in food, and polythene bags are all carbon compounds of very different sizes.

4. Hydrocarbons: saturated, unsaturated, chains and rings

• Compounds of only carbon and hydrogen are hydrocarbons. Methane, CH₄, is the smallest.
• To draw a structural formula: first join the carbon atoms with single bonds, then add H atoms so each carbon has four bonds. If a valency is still free, change a C–C single bond to a double or triple bond.
• Saturated hydrocarbons (alkanes) have only single bonds, e.g. ethane C₂H₆, propane C₃H₈.
• Unsaturated hydrocarbons have a C=C (alkenes, e.g. ethene C₂H₄) or a C≡C (alkynes, e.g. ethyne C₂H₂). Unsaturated compounds are more reactive.
• The bonded carbon atoms form the skeleton of the molecule; H atoms sit on the outside. Straight chain alkanes from 1 to 10 carbons: methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane.
• C₄H₁₀ can be a straight chain (n-butane) or a branched chain (isobutane). Same molecular formula, different structures = structural isomerism.
• Some compounds have rings, e.g. cyclohexane C₆H₁₂ (saturated) and cyclohexene C₆H₁₀ (unsaturated). Benzene C₆H₆ has three alternate double bonds in its ring — it is aromatic.
• Natural gas is mainly methane. Crude oil, formed from organisms buried under the sea for millions of years, is separated by fractional distillation into CNG, LPG, petrol, kerosene, diesel and lubricants.

5. Functional groups

• An atom other than C and H that replaces a hydrogen in a hydrocarbon chain is a hetero atom (e.g. Cl, O, N, S).
• A hetero atom, or a group of atoms containing it, gives the compound its typical chemical properties whatever the chain length. It is called a functional group.
• Halogen: –X (–Cl, –Br, –I), named halo (chloro, bromo, iodo).
• Oxygen groups: alcohol –OH, aldehyde –CHO, ketone –CO–, carboxylic acid –COOH, ether –O–, ester –COO–.
• Nitrogen group: amine –NH₂.
• C=C and C≡C are also treated as functional groups because they give special reactions (like addition).
• Example: ethanol (CH₃–CH₂–OH) behaves as an alcohol because of –OH; acetic acid in vinegar is acidic because of –COOH.

6. Homologous series

• A homologous series is a family of compounds having the same functional group in place of a particular H on carbon chains of steadily increasing length, e.g. CH₃OH, C₂H₅OH, C₃H₇OH…
• Each next member has one extra methylene unit (–CH₂–), one more carbon atom and 14 u more molecular mass (12 + 2 × 1).
• Members show similar chemical properties because they share the same functional group.
• Physical properties such as boiling point change gradually in one direction (gradation): methane −162 °C, ethane −88.5 °C, propane −42 °C, butane about 0 °C.
• All members fit one general formula: alkanes C_nH_2n+2 (first member n = 1), alkenes C_nH_2n and alkynes C_nH_2n−2 (first member n = 2).
• There are separate series for alcohols, aldehydes, carboxylic acids, alkenes and so on.

7. Nomenclature of carbon compounds (IUPAC)

• Early common names (methane, n-butane, iso-butane, acetic acid, acetone) had no single logic and became confusing as compounds multiplied.
• The IUPAC (International Union of Pure and Applied Chemistry) system gives every compound one unique name based on its structure: prefix – parent – suffix.
• Step 1: Count the carbons in the straight chain and write the parent alkane name. For a C=C change 'ane' to 'ene'; for a C≡C change it to 'yne'.
• Step 2: For a functional group, replace the final 'e' of the parent by its suffix: –OH 'ol', –CHO 'al', –CO– 'one', –COOH 'oic acid', –NH₂ 'amine'. Halogens go in front as prefixes: chloro, bromo, iodo.
• Step 3: Number the chain so the carbon carrying the functional group gets the smallest number. The carbon of –CHO or –COOH is always C1. Separate a number from letters by a hyphen, e.g. butan-2-ol.
• Examples: CH₃–CH₂–CH₂–OH is propan-1-ol; CH₃–CH₂–CH₂–CHO is butanal; HCOOH is methanoic acid.
• Common vs IUPAC: ethylene = ethene, acetylene = ethyne, methyl alcohol = methanol, acetaldehyde = ethanal, acetone = propanone.

8. Chemical properties: combustion, oxidation, addition, substitution

• Combustion: carbon compounds burn in oxygen giving CO₂, H₂O, heat and light, e.g. CH₄ + 2O₂ → CO₂ + 2H₂O + heat + light.
• Saturated compounds burn with a clean blue flame. Unsaturated compounds have a higher proportion of carbon, so some carbon stays unburnt; the glowing soot makes the flame yellow and smoky. Even saturated fuels give a yellow flame if air is limited — so a gas stove with sooty vessels needs its air holes cleaned.
• Oxidation: oxidising agents such as alkaline KMnO₄ or K₂Cr₂O₇ act on the functional group. Ethanol is oxidised to ethanoic acid. The pink KMnO₄ colour disappears while ethanol remains, and stays once all ethanol is used up.
• Addition reaction: an unsaturated compound joins another molecule and all atoms end up in one product, which is saturated. Bromine water or iodine is decolourised instantly by C=C — a test for unsaturation.
• Hydrogenation: H₂ adds to C=C in presence of Ni or Pt catalyst. Vegetable oils (unsaturated) become Vanaspati ghee (saturated). Unsaturated fats are healthier than saturated fats.
• Substitution reaction: one atom or group is replaced by another. Alkanes are unreactive, but in sunlight methane reacts with chlorine step by step to give CH₃Cl, CH₂Cl₂, CHCl₃ and CCl₄, with HCl each time.
• A catalyst changes the rate of a reaction without itself being changed.

9. Important carbon compounds: ethanol and ethanoic acid

• Ethanol (alcohol, spirit): colourless liquid, boiling point 78 °C, mixes with water in all proportions, neutral to litmus. Used as a solvent in tincture iodine, cough syrups and tonics. Drinking alcohol harms the nervous system and health; pure (absolute) alcohol can kill.
• Denatured spirit is ethanol mixed with poisonous methanol (plus a blue dye) so it cannot be misused as a drink. Methanol can cause blindness or death.
• Ethanol + sodium: 2Na + 2CH₃CH₂OH → 2CH₃CH₂ONa (sodium ethoxide) + H₂↑. The hydrogen burns with a pop. Less reactive Mg does not react.
• Dehydration: ethanol heated to 170 °C with excess concentrated H₂SO₄ loses water to give ethene: CH₃CH₂OH → CH₂=CH₂ + H₂O.
• Ethanol from fermenting sugarcane molasses burns cleanly; petrol blended with it is called gasohol.
• Ethanoic acid (acetic acid): colourless liquid, boiling point 118 °C, melting point 17 °C — it freezes in cold weather like ice, hence glacial acetic acid. Vinegar is 5–8 % acetic acid in water, used in pickles. It is a weak acid compared with HCl (shown better with pH paper than litmus).
• Reactions of ethanoic acid: with NaOH → CH₃COONa + H₂O; with Na₂CO₃ or NaHCO₃ → sodium ethanoate + H₂O + CO₂↑ (turns lime water milky).
• Esterification: CH₃COOH + CH₃CH₂OH → CH₃COOCH₂CH₃ (ethyl ethanoate) + H₂O, with an acid catalyst. Esters smell fruity and are used in perfumes and flavours. Ester + NaOH → sodium salt of acid + alcohol is saponification (soap making).

10. Macromolecules and polymers

• Giant carbon molecules made of hundreds of thousands of atoms are macromolecules; they are polymers.
• Natural macromolecules: polysaccharides (starch, cellulose), proteins, nucleic acids (DNA, RNA) and rubber. They give us food, clothing and shelter, build our bodies and control heredity.
• Polymer: a big molecule formed by regular repetition of a small unit, the monomer. The process is polymerisation; many alkene monomers join this way, e.g. ethylene → polyethylene.
• Man-made polymers: polyethylene (ethylene) – carry bags; polystyrene (styrene) – thermocol; PVC (vinyl chloride) – pipes, mats; polyacrylonitrile (acrylonitrile) – woollen-type winter clothes, blankets; Teflon (tetrafluoroethylene) – non-stick tawa; polypropylene (propylene) – syringes, plastic chairs.
• Homopolymers are made from one type of monomer; copolymers from two or more, e.g. PET (polyethylene terephthalate) used in water bottles.
• Polymer chains can be linear, branched or cross-linked; the monomer and structure decide the properties.
• Natural polymers and monomers: starch and cellulose – glucose; proteins – α-amino acids; DNA and RNA – nucleotides; rubber – isoprene.

Formulas at a glance

Key terms

Carbon Compounds – Flashcards

Read the question, answer it in your head, then tap the card to check. 20 cards.

Carbon Compounds – MCQ Quiz

15 questions. Tap an option — you will see at once if it is right and why.

How to study Carbon Compounds

⏱ About 6 hours over 4 days

1. Day 1 — Bonds and the versatile carbon
   Read why carbon shares electrons. Draw electron-dot and line structures of H₂, O₂, N₂, CH₄, H₂O, NH₃, CO₂ until you can do them without looking. Write the four reasons for the huge number of carbon compounds with one example each.
2. Day 2 — Hydrocarbons, isomers, functional groups
   Write names and formulae of alkanes C1 to C10. Draw structures of ethane, ethene, ethyne, propane, isomers of C₄H₁₀ and C₅H₁₂, cyclohexane and benzene. Make a chart of functional groups with name, formula and IUPAC suffix/prefix.
3. Day 3 — Homologous series and IUPAC names
   Practise general formulae by writing members for different n and calculating their masses. Then name 15 compounds using the three IUPAC steps and convert common names to IUPAC names. Do our flashcards for quick revision.
4. Day 4 — Reactions and polymers
   Write every reaction from the chapter (combustion, oxidation, addition, hydrogenation, substitution, ethanol and ethanoic acid reactions, esterification, saponification) three times with conditions. Learn the polymer–monomer–use table. Finish with our 15-question quiz and revise what you got wrong.

✍️ Diagrams to practise

• Electron-dot and line structures of H₂, O₂, N₂, CH₄, C₂H₆, C₂H₄, C₂H₂, H₂O, NH₃, CO₂, CH₃OH
• Structural formulae of the isomers of butane and pentane
• Ring structures of cyclohexane and benzene
• Apparatus for reaction of acetic acid with sodium carbonate and testing CO₂ with lime water
• Hot water bath set-up for esterification
• Polymerisation of ethylene to polyethylene (repeating unit)