Class 10 › Science › Chapter 4

Carbon and its Compounds

Covalent bonding, versatile nature of carbon, hydrocarbons, functional groups, homologous series, nomenclature, chemical properties, ethanol, ethanoic acid, and soaps and detergents.

1. Bonding in Carbon: The Covalent Bond
  • Carbon makes up only ~0.02% of the earth's crust (minerals, coal, petroleum) and 0.03% of the atmosphere (as CO2), yet all living things, food, clothes and medicines are carbon-based.
  • Why not ionic? Carbon (atomic number 6, configuration 2,4) has 4 valence electrons. Gaining 4 would make C4− (6 protons holding 10 electrons is hard); losing 4 would make C4+ (needs a large energy). So carbon shares electrons.
Covalent bond: a bond formed by the sharing of an electron pair between two atoms so that both attain a completely filled outer shell (noble gas configuration).
MoleculeElectrons neededBond type
H2 (H–H)1 each (K shell needs 2)Single bond (1 shared pair)
Cl2 (Cl–Cl)1 each (valence 7)Single
O2 (O=O)2 each (valence 6)Double bond (2 pairs)
N2 (N≡N)3 each (valence 5)Triple bond (3 pairs)
H2O, NH3, CH4O, N, C share with 2, 3, 4 hydrogensAll single bonds
Covalent compounds

Properties

Strong bonds within molecules but weak intermolecular forces, so low melting and boiling points. No ions form, so they are generally poor conductors of electricity.

Examples

Data (K)

Methane CH4: m.p. 90, b.p. 111
Ethanol: 156, 351
Chloroform: 209, 334
Acetic acid: 290, 391. All low compared with ionic compounds.

  • Methane is a major component of bio-gas and CNG.
2. Allotropes of Carbon
Diamond

3-D rigid structure

Each carbon bonded to 4 others. Hardest known substance. Can be made synthetically at very high pressure and temperature.

Graphite

Hexagonal layers

Each carbon bonded to 3 others in a plane (one double bond), layers stacked. Smooth, slippery and a good conductor of electricity.

Fullerene

C-60 football shape

First fullerene found, C-60 (Buckminsterfullerene), named after architect Buckminster Fuller's geodesic dome.

  • Diamond and graphite differ in physical properties (different bonding arrangement) but have the same chemical properties.
3. Versatile Nature of Carbon
Catenation

Carbon-carbon bonding

The unique ability to form bonds with other carbon atoms, giving long chains, branched chains and rings with single, double or triple bonds. The C–C bond is very strong. Silicon hydrides form chains of only 7 to 8 atoms and are very reactive.

Tetravalency

Valency of 4

Carbon bonds with four other atoms (C, H, O, N, S, Cl, etc.). Its small size lets the nucleus hold shared electrons tightly, so bonds with most elements are strong and compounds stable.

  • Millions of carbon compounds are known, far more than those of all other elements combined.
  • Organic compounds: once thought to need a "vital force". Wöhler (1828) disproved this by making urea from ammonium cyanate. Carbides, oxides of carbon, carbonates and hydrogencarbonates are not studied under organic chemistry.
4. Saturated and Unsaturated Carbon Compounds
Saturated

Only single C–C bonds

Alkanes: general formula CnH2n+2. Not very reactive. Example: ethane C2H6.

Unsaturated

Double bond: Alkenes

General formula CnH2n. Ethene C2H4 (H2C=CH2). More reactive.

Unsaturated

Triple bond: Alkynes

General formula CnH2n−2. Ethyne C2H2 (HC≡CH). More reactive.

How to draw a structure (ethane C2H6)
Step 1: join the carbon atoms with a single bond: C–C.
Step 2: fill the remaining valencies with hydrogen: each C gets 3 H, giving H3C–CH3.
If a valency is left (ethene): each C has one unsatisfied valency after 2 H each, so make a double bond between the carbons.
  • Hydrocarbons: compounds of only carbon and hydrogen.
  • Ethane has 7 covalent bonds (1 C–C + 6 C–H).
5. Chains, Branches and Rings
C atomsNameFormula
1MethaneCH4
2EthaneC2H6
3PropaneC3H8
4ButaneC4H10
5PentaneC5H12
6HexaneC6H14
  • Structural isomers: compounds with the same molecular formula but different structures. Butane (C4H10) has 2: straight chain (n-butane, C–C–C–C) and branched (a C–C–C chain with a CH3 on the middle carbon). Pentane has 3.
  • Rings: cyclohexane C6H12 (saturated ring); benzene C6H6 (unsaturated ring with alternate double bonds). Cyclopentane is C5H10.
  • Straight, branched and cyclic compounds may all be saturated or unsaturated.
6. Functional Groups
Heteroatom: an atom (Cl, Br, O, N, S) that replaces hydrogen in a hydrocarbon chain. Functional group: a heteroatom or group of atoms that gives the compound its specific properties, regardless of chain length.
ClassFunctional groupExample
Haloalkane−Cl, −Br (replaces H)CH3CH2Cl
Alcohol−OHCH3CH2OH
Aldehyde−CHO (C=O with one H)CH3CHO
Ketone>C=O (inside the chain)CH3COCH3
Carboxylic acid−COOHCH3COOH
7. Homologous Series
Homologous series: a series of compounds in which the same functional group replaces hydrogen in a carbon chain of increasing length.
  • Successive members differ by a −CH2− unit (14 u in mass: 12 + 2).
  • Alcohols: CH3OH, C2H5OH, C3H7OH, C4H9OH. Alkenes: C2H4, C3H6, C4H8, C5H10.
  • Physical properties (m.p., b.p., solubility) show a gradation as molecular mass increases.
  • Chemical properties stay similar, since they depend on the functional group.
8. Nomenclature of Carbon Compounds
Naming steps
1. Count the carbon atoms for the base name (3 C: propane).
2. Show the functional group as a prefix or suffix.
3. If the suffix starts with a vowel (a, e, i, o, u), drop the final 'e' of the chain name: propane − e + one = propanone.
4. For unsaturation, change 'ane' to 'ene' (double bond) or 'yne' (triple bond).
ClassPrefix / suffixExample (3 C)
HaloalkanePrefix chloro-, bromo-Chloropropane, bromopropane
AlcoholSuffix -olPropanol
AldehydeSuffix -alPropanal
KetoneSuffix -onePropanone
Carboxylic acidSuffix -oic acidPropanoic acid
AlkeneSuffix -enePropene
AlkyneSuffix -ynePropyne
  • CH3CH2Br is bromoethane; HCHO is methanal.
9. Chemical Properties of Carbon Compounds
Combustion

Burns to CO2 + H2O + heat and light

C + O2 → CO2 + heat + light
CH4 + 2O2 → CO2 + 2H2O
CH3CH2OH + 3O2 → 2CO2 + 3H2O

Oxidation

Alcohol to carboxylic acid

CH3CH2OH → CH3COOH using alkaline KMnO4 + heat, or acidified K2Cr2O7 + heat. These are oxidising agents (they add oxygen).

Addition

Unsaturated + H2

Alkene + H2 → alkane, with a Ni or Pd catalyst. Used in hydrogenation of vegetable oils (unsaturated) to make saturated fats.

Substitution

Cl replaces H

CH4 + Cl2 → CH3Cl + HCl (in sunlight). Saturated hydrocarbons are otherwise unreactive.

Combustion: flames and fuels
Saturated hydrocarbons give a clean (blue) flame; unsaturated ones give a yellow sooty flame with black smoke (carbon).
Incomplete combustion (limited air) gives soot even with saturated fuels. A blackened vessel bottom means the fuel is not burning completely (blocked air holes) and fuel is wasted.
Coal and petroleum contain N and S, so burning them releases oxides of sulphur and nitrogen, major pollutants. They are fossil fuels, formed from ancient plants and sea organisms.
A flame forms only when gaseous substances burn; charcoal just glows red. Ethyne burnt with oxygen (not air) gives a very hot flame for welding.
  • Test for unsaturation: unsaturated compounds burn with a sooty yellow flame and add H2 (hydrogenation); saturated compounds burn with a clean flame.
  • Catalyst: a substance that makes a reaction occur or change its rate without being affected itself.
10. Ethanol (Alcohol)
  • A liquid at room temperature, soluble in water in all proportions; a good solvent (tincture iodine, cough syrups, tonics); the active ingredient of alcoholic drinks.
  • Pure ethanol is called absolute alcohol.
2Na + 2CH3CH2OH → 2CH3CH2O−Na+ + H2With sodium: H2 gas (pop test) and sodium ethoxide.
CH3CH2OH → CH2=CH2 + H2O hot conc. H2SO4, 443 KDehydration; conc. H2SO4 is a dehydrating agent.
Effects

Harm

Large quantities depress the central nervous system (loss of coordination, confusion, stupor). Long-term use causes health problems. Methanol is oxidised to methanal in the liver; even a small amount can cause death and blindness (optic nerve).

Denatured alcohol

Industrial ethanol

Made unfit to drink by adding poisonous substances such as methanol, and coloured blue with a dye.

Fuel

Alcohol as fuel

Ethanol is made by fermenting sugarcane molasses; used as a petrol additive. It burns cleanly to CO2 and H2O in sufficient air.

11. Ethanoic (Acetic) Acid
  • A carboxylic acid. 5 to 8% solution in water is vinegar (preservative in pickles). Pure acid freezes at 290 K in winter, hence glacial acetic acid.
  • A weak acid: unlike HCl it is not completely ionised, so it shows a higher pH than HCl of equal concentration (Act. 4.7).
Reactions of ethanoic acid
Esterification (acid + alcohol, conc. H2SO4 catalyst): CH3COOH + CH3CH2OH ⇌ CH3COOCH2CH3 + H2O. Esters are sweet-smelling, used in perfumes and flavouring.
Saponification (ester + NaOH): CH3COOC2H5 + NaOH → C2H5OH + CH3COONa. Used to make soap.
With a base: NaOH + CH3COOH → CH3COONa + H2O (sodium ethanoate or acetate).
With carbonates / hydrogencarbonates: 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2; CH3COOH + NaHCO3 → CH3COONa + H2O + CO2. CO2 turns lime water milky.
TestEthanolEthanoic acid
LitmusNo change (neutral)Blue litmus turns red
NaHCO3No reactionBrisk effervescence (CO2)
State, m.p.Liquid; m.p. 156 KFreezes easily; m.p. 290 K
12. Soaps and Detergents
Soap: sodium or potassium salt of a long-chain carboxylic acid. One end is hydrophilic (ionic head, water-loving); the other is hydrophobic (hydrocarbon tail, oil-loving).
Cleaning action: micelle formation
Dirt is mostly oily and does not dissolve in water.
Soap molecules cluster into a micelle: hydrophobic tails point inward, trapping the oil droplet; ionic heads face the water outside.
This forms an emulsion; ion-ion repulsion keeps micelles from joining, so dirt is rinsed away. Micelles scatter light, so soap solution looks cloudy.
In a non-aqueous solvent (a hydrocarbon or ethanol) the ionic heads would not face out, so the same kind of micelle does not form. Agitation (beating, scrubbing) helps lift and disperse the dirt.
Hard water

Soap

Reacts with Ca2+ and Mg2+ salts to form insoluble scum (curdy white precipitate) and little foam, so more soap is needed.

Detergents

Work in hard water

Sodium salts of sulphonic acids or ammonium salts with chloride or bromide ions; long hydrocarbon chain. Their charged ends do not form insoluble precipitates with Ca2+/Mg2+. Used in shampoos and clothes cleaners.

  • Soap is basic: it turns red litmus blue and leaves blue litmus unchanged.
13. Quick Revision
ConceptRemember
Covalent bondSharing of electron pairs; single, double, triple
Carbon's versatilityTetravalency + catenation (+ small size)
General formulasAlkane CnH2n+2; alkene CnH2n; alkyne CnH2n−2
Homologous seriesSame functional group; differ by −CH2−
Addition reactionUnsaturated + H2 (Ni/Pd) → saturated
SubstitutionAlkane + Cl2 (sunlight) → chloroalkane + HCl
Esterification / saponificationAcid + alcohol → ester; ester + NaOH → alcohol + soap salt
Common mistakes:
• Calling covalent compounds good conductors or high-melting; they are poor conductors with low m.p.
• Forgetting to drop the final 'e' (propan + one) when the suffix starts with a vowel.
• Mixing up aldehyde (−CHO at chain end) and ketone (C=O inside the chain).
• Saying ethanoic acid is a strong acid; it is weak.
• Saying diamond and graphite differ chemically; they differ only physically.
• Writing an unbalanced combustion equation: CH3CH2OH + 3O2 → 2CO2 + 3H2O.
• Thinking soap works in hard water; it forms scum, detergents do not.