Lesson 9: Carbon Compounds

Rule 1: Organic compounds — Compounds of carbon are organic compounds. Exceptions that are inorganic: carbon dioxide, carbon monoxide, carbides, carbonates and bicarbonates. After urea was made in a laboratory from an inorganic compound, the idea of a "life force" was dropped and they became known as carbon compounds.

• Easy : Ethanol boils at 78 °C and acetic acid at 118 °C, both well below 300 °C.
• Medium : Common salt melts at 801 °C, but most carbon compounds melt and boil below 300 °C because the forces between their molecules are weak.
• Exam-type : Why are most carbon compounds poor conductors of electricity? They have covalent bonds and no ions, so there are no free charges to carry current.

Rule 2: Covalent bonds — Carbon (2, 4) has four valence electrons. It does not lose four (C⁴⁺ would be unstable) or gain four (C⁴⁻ would be unstable). It shares electrons and forms covalent bonds, so the atoms stay neutral. One shared pair is a single bond, two pairs a double bond and three pairs a triple bond.

In the figure : In the figure covalent bonds are formed by sharing electrons.

• Easy : Hydrogen H₂ has a single bond, oxygen O₂ a double bond and nitrogen N₂ a triple bond.
• Medium : Methane CH₄: carbon shares one pair with each of four hydrogens, so it has four single bonds.
• Exam-type : In carbon dioxide O=C=O, carbon forms two double bonds. Counting shared pairs: 2 pairs on each side = 4 pairs, which fills carbon's valency of 4.

Spot the difference

Rule 1: Why carbon is so versatile — About 10 million carbon compounds are known; DNA has a molecular mass up to 10¹². Reasons: catenation (शृंखलाबंधन: strong C–C bonds form open chains, branched chains and rings), single, double and triple bonds, tetravalency (चतुःसंयुजता: CH₄, CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄) and isomerism (समघटकता).

• Easy : In CH₃Cl, one hydrogen of methane is replaced by chlorine, and carbon still has four bonds.
• Medium : Carbon atoms in a ring: cyclohexane has six carbons joined in a ring, a catenation example.
• Exam-type : Silicon also has four valence electrons but its Si–Si chains are weak, so carbon forms far more compounds.

Rule 2: Saturated and unsaturated hydrocarbons — Methane CH₄ is the smallest hydrocarbon. Saturated hydrocarbons have only single bonds and are alkanes (ethane, propane). Unsaturated ones have a double bond (alkenes, ethene C₂H₄) or a triple bond (alkynes, ethyne C₂H₂). Unsaturated compounds are generally more reactive.

In the figure : In the figure single, double and triple bonds in hydrocarbons.

• Easy : Ethane C₂H₆ has only single bonds, so it is saturated.
• Medium : Ethene C₂H₄ has a C=C double bond and ethyne C₂H₂ has a C≡C triple bond, so both are unsaturated.
• Exam-type : Which has more hydrogen for the same carbon count? Ethane (6 H) has more than ethene (4 H) and ethyne (2 H): more bonds between carbons means fewer hydrogens.

Rule 3: Alkanes and structural isomers — Straight-chain alkanes: methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane. Structural isomers (संरचनात्मक समघटक) have the same molecular formula but different structures: C₄H₁₀ has a straight-chain and a branched-chain isomer. Cyclohexane (C₆H₁₂) is a ring; benzene (C₆H₆) is a ring with three alternate double bonds (aromatic).

In the figure : In the figure structural isomers have the same formula but different structures.

• Easy : Pentane C₅H₁₂ and 2-methylbutane have the same formula C₅H₁₂ but different shapes: they are isomers.
• Medium : Isomers have the same formula but different physical properties; the branched isomer usually boils at a lower temperature than the straight chain.
• Exam-type : Count the hydrogens of an alkane with 8 carbons: C_nH_2n+2 gives C₈H₁₈ (octane).

Rule 4: Crude oil — Crude oil, formed from dead organisms under the sea floor, is a mixture of hydrocarbons; natural gas is mainly methane. Fractional distillation (प्रभाजी ऊर्ध्वपातन) of crude oil gives CNG, LPG, petrol, kerosene, diesel and engine oil.

In the figure : In the figure products of crude oil.

• Easy : Petrol and diesel are separated from crude oil because they boil at different temperatures.
• Medium : Smaller molecules such as LPG boil off first, while heavy ones such as engine oil come out last.
• Exam-type : Natural gas is mainly methane, so burning it (as CNG) gives CO₂ and water.

Spot the difference

Rule 1: Functional groups — A hetero atom (halogen, oxygen, nitrogen, sulphur) or group of atoms in place of hydrogen is a functional group: halo (–Cl, –Br, –I), alcohol (–OH), aldehyde (–CHO), ketone (–CO–), carboxylic acid (–COOH), ether (–O–), ester (–COO–), amine (–NH₂).

• Easy : CH₃OH is methane with one H replaced by –OH, so it is an alcohol.
• Medium : CH₃COOH has the –COOH group, so it is a carboxylic acid.
• Exam-type : Name the group in CH₃CH₂Cl: –Cl is a halo group, so the compound is a halo-compound, chloroethane.

Rule 2: Homologous series — A homologous series (समजात श्रेणी) has the same functional group on chains of increasing length. Each member differs from the next by one –CH₂– unit (14 u). Chemical properties are alike, physical properties such as boiling point change gradually, and one general formula applies: alkanes C_nH_2n+2, alkenes C_nH_2n, alkynes C_nH_2n−2.

In the figure : In the figure each member differs by one –CH₂– unit (14 u).

• Easy : Methanol CH₃OH is 32 u and ethanol C₂H₅OH is 46 u; the difference is 14 u, one –CH₂– unit.
• Medium : The alkene with 7 carbons is C₇H₁₄ and the next member C₈H₁₆ differs by CH₂ (14 u).
• Exam-type : An alkyne has the formula C_nH_2n−2. For n = 4, it is C₄H₆: hydrogens = 2 × 4 − 2 = 6.

Rule 3: IUPAC naming — IUPAC name = prefix – parent – suffix. The parent alkane comes from the number of carbons; a double bond changes "ane" to "ene" and a triple bond to "yne". Suffixes: –ol (alcohol), –al (aldehyde), –oic acid, –one (ketone), –amine; halogens are prefixes (chloro, bromo). Examples: CH₃OH methanol, CH₃CH₂OH ethanol, CH₃COOH ethanoic acid, CH₃COCH₃ propanone, CH₃CH₂Cl chloroethane. Common names: ethylene = ethene, acetylene = ethyne, acetone = propanone.

In the figure : In the figure how an IUPAC name is built.

• Easy : CH₃CH₂CH₃ has 3 carbons and single bonds only, so the name is propane.
• Medium : CH₃CH₂COOH has 3 carbons and a –COOH group: propane, drop the "e", add "oic acid": propanoic acid.
• Exam-type : Name CH₃CHO: 2 carbons with an aldehyde group –CHO, so ethane becomes ethanal.

Spot the difference

Rule 1: Combustion and oxidation — Combustion: carbon compounds burn in oxygen to give CO₂, water, heat and light (CH₄ + 2O₂ → CO₂ + 2H₂O). Saturated compounds burn with a clean blue flame; unsaturated ones burn with a yellow flame and black soot because they contain a larger proportion of carbon. Oxidation: alkaline potassium permanganate or potassium dichromate oxidise ethanol to ethanoic acid; the pink colour of KMnO₄ vanishes first and stays when it is in excess.

In the figure : In the figure flame colour shows saturation.

• Easy : Propane burns: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
• Medium : Ethanol burns with a clean flame: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.
• Exam-type : Soot on the bottom of a cooking vessel shows that the air inlet of the stove is blocked, so the fuel burns incompletely.

Rule 2: Addition and substitution — Addition: unsaturated compounds add bromine or iodine (the colour disappears, a test for multiple bonds) and add hydrogen in the presence of a nickel or platinum catalyst. Hydrogenation turns unsaturated vegetable oils into saturated vanaspati ghee. Substitution: saturated hydrocarbons are inert to most reagents, but in sunlight chlorine replaces hydrogen atoms one by one (CH₄ + Cl₂ → CH₃Cl + HCl); chlorination of methane gives four products.

In the figure : In the figure addition of hydrogen saturates the oil.

• Easy : Ethene + H₂ with a nickel catalyst gives ethane: C₂H₄ + H₂ → C₂H₆.
• Medium : Shake a sample with brown bromine water: if the colour disappears, the sample is unsaturated.
• Exam-type : Chlorination of methane gives CH₃Cl, CH₂Cl₂, CHCl₃ and CCl₄, as one hydrogen after another is replaced.

Spot the difference

Rule 1: Ethanol and ethanoic acid — Ethanol is a colourless liquid (b.p. 78 °C), mixes with water in all proportions and is neutral to litmus. It is used in tincture iodine and tonics and is harmful to health. Methanol is poisonous; ethanol mixed with methanol and a blue dye is denatured spirit. Ethanol reacts with sodium to give sodium ethoxide and hydrogen; heated with excess conc. H₂SO₄ at 170 °C it loses water and gives ethene (dehydration). Ethanol blended with petrol is gasohol. Ethanoic acid is a colourless liquid (b.p. 118 °C) that turns blue litmus red; vinegar is a 5–8% solution. Pure acid freezes at 17 °C (glacial acetic acid). It neutralises NaOH to give sodium ethanoate and reacts with carbonates and bicarbonates to give CO₂, which turns lime water milky.

• Easy : 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂: bubbles of hydrogen show the –OH group.
• Medium : Test for an acid: add baking soda NaHCO₃ to vinegar; the fizz is CO₂ and it turns lime water milky.
• Exam-type : A bottle of pure ethanoic acid kept in a cold room at 10 °C turns to ice-like crystals, because its freezing point is 17 °C: this is why it is called glacial.

In the figure : In the figure an acid and an alcohol give a sweet-smelling ester.

Rule 2: Esters and soap — Esterification: an acid and an alcohol with an acid catalyst give an ester (CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O, ethyl ethanoate). Esters smell sweet and give fruits their smell. Saponification: ester + NaOH → sodium carboxylate + alcohol; soap is made from fats this way.

• Easy : Ethanol + ethanoic acid → ethyl ethanoate (a sweet-smelling ester) + water.
• Medium : Esterification is reversible and needs a few drops of conc. H₂SO₄ as catalyst.
• Exam-type : Soap-making: fat or oil + NaOH is boiled, giving soap and glycerol; this is the reverse idea of making an ester.

Rule 3: Polymers — Macromolecules are giant molecules. Polymers (बहुवारिक) are made by regular repetition of a small unit, the monomer (एकवारिक); the process is polymerization. Natural: starch and cellulose (glucose), proteins (amino acids), DNA and RNA (nucleotides), rubber (isoprene). Man-made: polyethylene (carry bags), polystyrene (thermocol), PVC (pipes), polyacrylonitrile (winter clothing), Teflon (non-stick cookware), polypropylene (syringes).

In the figure : In the figure monomers join to make a polymer.

• Easy : Ethene (monomer) joins end to end to give polyethylene (polymer), used for carry bags.
• Medium : If one ethene unit is 28 u and 500 units join, the polymer mass is about 500 × 28 = 14,000 u.
• Exam-type : Starch and cellulose are both polymers of glucose, yet one is food for us and the other is plant fibre: the way the units join differs.

Spot the difference