Carbon and its Compounds

1. Why carbon deserves its own chapter

Carbon makes up a tiny fraction of the Earth's crust (about 0.02% as minerals like carbonates, coal and petroleum) and only 0.03% of the atmosphere as CO₂. Yet every living thing — food, clothes, medicines, fuels, plastics — is built from carbon compounds. Millions of carbon compounds are known, far more than the compounds of all other elements combined. This chapter explains how carbon manages this, by first looking at the kind of bond it forms.

2. How carbon bonds — the covalent bond

Carbon has atomic number 6, so its electronic configuration is 2,4 — four electrons in the outermost shell. To reach a stable, fully-filled outer shell (the octet of the noble gas neon, 2,8), it has two options:

• Gain 4 electrons to become C⁴⁻ — but adding 4 extra electrons to 6 protons is energetically very hard.
• Lose 4 electrons to become C⁴⁺ — removing 4 electrons needs enormous energy and leaves a carbon nucleus holding just two electrons over 6 protons.

Both ionic routes are impossible. So carbon does something different: it shares electrons. When two atoms each contribute electrons to a shared pair, both effectively complete their octet. This shared-pair bond is the covalent bond.

Properties of covalent (molecular) compounds: because the molecules are held to each other by only weak forces, these compounds have low melting and boiling points; they are generally poor conductors of electricity (no free electrons or ions); and they are often insoluble in water but soluble in organic solvents.

3. Allotropes of carbon

Allotropes are different physical forms of the same element. The pure-carbon allotropes look and behave completely differently even though each is only carbon:

The contrast between hard, insulating diamond and soft, conducting graphite — both pure carbon — is a favourite exam point.

4. The versatile nature of carbon

Two special abilities let carbon form millions of compounds:

• Catenation: carbon atoms link to one another through strong, stable C–C bonds to form long chains, branched chains and rings. Carbon's catenation is unmatched because C–C bonds are very strong (carbon is small, so the shared electrons are held tightly). Silicon also catenates but its bonds are far weaker, so its chains are short.
• Tetravalency + small size: with four valencies, each carbon can bond to four other atoms — carbon, hydrogen, oxygen, nitrogen, sulphur, chlorine and more. Its small size means the bonds to the nucleus are strong and stable.

Together with the ability to form double and triple bonds, this gives an almost unlimited variety of structures. Compounds of carbon (except oxides, carbonates and hydrogencarbonates) are studied as organic chemistry.

5. Hydrocarbons — saturated and unsaturated

A hydrocarbon is a compound of only carbon and hydrogen.

• Saturated hydrocarbons (alkanes): all carbon–carbon bonds are single bonds. General formula C_nH_2n+2. Examples: methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀.
• Unsaturated hydrocarbons: contain at least one double (alkene) or triple (alkyne) carbon–carbon bond.
  • Alkenes (C=C): general formula C_nH_2n. Examples: ethene C₂H₄, propene C₃H₆.
  • Alkynes (C≡C): general formula C_nH_2n−2. Examples: ethyne C₂H₂ (acetylene), propyne C₃H₄.

Carbon chains can also close into rings — e.g. cyclohexane C₆H₁₂ and benzene C₆H₆.

6. Homologous series

A homologous series is a family of compounds with the same general formula and the same functional group, in which any two successive members differ by a –CH₂– unit (a difference of CH₂, i.e. mass 14 u).

Key features: members of a series show a gradual change in physical properties (melting/boiling point and solubility change steadily as molecular mass increases) but show similar chemical properties because they share the same functional group. This is why a whole series can be studied through one or two members.

7. Functional groups

A functional group is an atom or group of atoms that replaces one or more hydrogens of a hydrocarbon and gives the molecule its characteristic chemical properties. The hydrocarbon part is mostly inert; the functional group is the "reactive site".

8. Nomenclature (IUPAC naming)

Naming a carbon compound is a recipe:

• Step 1 — count carbons in the longest chain and pick the root: 1=meth, 2=eth, 3=prop, 4=but, 5=pent, 6=hex.
• Step 2 — add the family ending: alkane → -ane, alkene → -ene, alkyne → -yne.
• Step 3 — show the functional group as a prefix or suffix:

When adding a suffix that starts with a vowel (like -ol, -al, -one), the final "e" of the alkane name is dropped: ethane → ethanol, propane + -one → propanone.

9. Chemical properties of carbon compounds

Four reactions are central to the chapter.

(a) Combustion. Carbon and its compounds burn in oxygen (air) to give CO₂, water and heat and light. This is why they are our chief fuels.

Clean vs sooty flame: saturated hydrocarbons burn with a clean blue flame (enough oxygen); unsaturated hydrocarbons and compounds burned in limited air burn with a yellow, sooty (smoky) flame because of unburnt carbon particles. A blackened cooking vessel means incomplete combustion / insufficient air supply.

(b) Oxidation. Alcohols can be oxidised to carboxylic acids by oxidising agents like alkaline KMnO₄ (potassium permanganate) or acidified K₂Cr₂O₇ (potassium dichromate).

(c) Addition reaction. Unsaturated hydrocarbons add hydrogen across the double/triple bond in the presence of catalysts (nickel or palladium) to become saturated. This hydrogenation is used in industry to convert liquid vegetable oils (unsaturated) into solid fats (vanaspati ghee).

Health note: animal fats and hydrogenated oils contain saturated fats, which are less healthy; unsaturated oils are better for health.

(d) Substitution reaction. Saturated hydrocarbons are fairly unreactive, but in the presence of sunlight chlorine replaces hydrogen atoms one at a time. Chlorine is said to substitute for hydrogen.

10. Ethanol (C₂H₅OH)

Ethanol is the alcohol in alcoholic drinks; it is a liquid at room temperature, soluble in water and an important industrial solvent and fuel (added to petrol). Its functional group is –OH.

Reaction with sodium — alcohols react with reactive metals (like sodium) to give a salt (sodium ethoxide) and release hydrogen gas (a test for the –OH group):

Dehydration to ethene — heating ethanol at about 443 K with excess concentrated sulphuric acid (a dehydrating agent) removes water and gives ethene:

Harmful effects: drinking ethanol slows the body and harms health; methanol (CH₃OH) is far more dangerous — even small amounts cause blindness or death because the body oxidises it to formaldehyde/formic acid. Industrial alcohol is made unfit to drink by adding poisonous substances and dyes — this is denatured alcohol.

11. Ethanoic acid (CH₃COOH)

Ethanoic acid (acetic acid) is the acid in vinegar (5–8% solution). Its functional group is –COOH. Pure ethanoic acid freezes at about 290 K (in cold weather it freezes like ice) — hence the name glacial acetic acid. It is a weak acid (does not fully ionise).

(a) Esterification. Ethanoic acid reacts with an alcohol in the presence of an acid catalyst to give a sweet-smelling ester. Esters are used in perfumes and flavourings.

Saponification: on treating an ester with a base (NaOH) it is hydrolysed back to the alcohol and the sodium salt of the acid — this reaction is the basis of soap making.

(b) Reaction with a base. Like all acids, ethanoic acid neutralises NaOH to give a salt (sodium ethanoate) and water.

(c) Reaction with carbonates and hydrogencarbonates. Ethanoic acid reacts with sodium carbonate / sodium hydrogencarbonate to give a salt, water and carbon dioxide (brisk effervescence; turns limewater milky — a test for –COOH).

12. Soaps, detergents and micelles

Soap is the sodium (or potassium) salt of a long-chain carboxylic (fatty) acid. A soap molecule has two ends:

• a long hydrocarbon tail that is hydrophobic (water-repelling) but dissolves in oil/grease;
• an ionic –COO⁻Na⁺ head that is hydrophilic (water-loving).

How cleaning works (micelle formation): dirt on clothes is held by oily films. When soap is added to water, the hydrophobic tails bury themselves into the oil droplet while the hydrophilic heads point outward into the water. This forms a spherical cluster called a micelle, with the oil/dirt trapped inside. The outward-facing ionic heads keep the micelles suspended (an emulsion), so the dirt is lifted off and rinsed away.

Problem with hard water: hard water contains Ca²⁺ and Mg²⁺ ions. Soap reacts with these to form an insoluble white scum (calcium/magnesium salt of the fatty acid). Soap is wasted forming scum, so cleaning is poor in hard water.

Detergents are the sodium salts of long-chain sulphonic acids (or ammonium/sulphate salts). Their charged ends do not form insoluble scum with Ca²⁺/Mg²⁺, so detergents clean even in hard water. They are used in shampoos and washing powders.

13. NCERT in-text questions — answered

Q. What is the number of covalent bonds in a molecule of ethane (C₂H₆)? Six C–H bonds + one C–C bond = 7 covalent bonds.

Q. Carbon mostly forms compounds that are covalent — why? / Why are carbon compounds poor conductors? Because carbon shares electrons rather than transferring them, forming covalent molecules with no free ions or electrons; hence they do not conduct electricity.

Q. Why does carbon form covalent bonds (not ionic)? Having 4 outer electrons, gaining or losing 4 electrons needs too much energy and gives an unstable ion, so carbon completes its octet by sharing electrons.

Q. Draw the electron-dot structure of (a) ethanoic acid, (b) H₂S, (c) propanone, (d) F₂. Each follows the octet/duplet rule: CH₃COOH (C–H bonds, C–C, C=O double bond, C–O–H); H₂S has two S–H single bonds with two lone pairs on S; propanone CH₃–CO–CH₃ has a central C=O; F₂ is F–F with three lone pairs on each F.

Q. What is an allotrope? Name allotropes of carbon. Different physical forms of the same element. Carbon: diamond, graphite, fullerene.

Q. What is a homologous series? Explain with an example. A family of compounds with the same general formula and functional group whose members differ by –CH₂; e.g. the alkanes CH₄, C₂H₆, C₃H₈.

Q. How can ethanol and ethanoic acid be distinguished? Add sodium hydrogencarbonate (or Na₂CO₃): ethanoic acid gives brisk effervescence of CO₂ (limewater test) while ethanol does not. (Also: litmus turns red with the acid only.)

Q. What are oxidising agents? Substances that supply oxygen (or remove hydrogen) to other substances, e.g. alkaline KMnO₄, acidified K₂Cr₂O₇; they oxidise ethanol to ethanoic acid.

Q. Why does micelle formation take place when soap is added to water but not in other solvents like ethanol? Micelles form because the hydrocarbon tails avoid water and cluster together, leaving ionic heads in water. In ethanol the tails are soluble, so there is no driving force to cluster, and soap simply dissolves — no micelle.

Q. Why are carbon and its compounds used as fuels? They combust readily in air releasing a large amount of heat and light.

Q. Explain the formation of scum when hard water is treated with soap. Ca²⁺/Mg²⁺ ions in hard water react with soap to form an insoluble precipitate (scum), wasting soap.

Q. What change is seen on adding a few drops of universal indicator to (i) soap, (ii) acetic acid? Soap solution is basic (turns indicator greenish-blue); acetic acid is acidic (turns indicator orange/red).

Q. What is hydrogenation? Its industrial application? Addition of hydrogen to an unsaturated compound over Ni catalyst; used to convert vegetable oils into solid vanaspati ghee.

Q. Which of the following will give addition reactions: C₂H₆, C₃H₈, C₃H₆, C₂H₂, CH₄? Only the unsaturated ones: C₃H₆ and C₂H₂.

Q. Distinguish butter and cooking oil chemically. Add a few drops of bromine water / dilute KMnO₄: cooking oil (unsaturated) decolourises it (addition at C=C); butter (mostly saturated) does not.

Q. Explain the cleaning action of soap. See §12 — the hydrophobic tail dissolves the grease while the hydrophilic head keeps it suspended in water as micelles, which are washed away.

14. NCERT Activities — what they show

• Burning candle/camphor over a metal plate: a black deposit (soot) shows carbon and a sooty flame = incomplete combustion.
• Saturated vs unsaturated combustion: alkane burns blue and clean; oil/unsaturated burns yellow and smoky.
• Bromine water / KMnO₄ test: unsaturated compounds decolourise these reagents by addition; saturated ones do not.
• Ethanoic acid with NaHCO₃ and limewater: brisk CO₂ turns limewater milky — test for –COOH.
• Esterification: warming ethanol + ethanoic acid + conc. H₂SO₄ gives a sweet fruity smell (ester).
• Soap with hard and soft water: less lather and white scum in hard water; detergent lathers in both.

15. NCERT Exercise — answered

Q. Ethane, with the molecular formula C₂H₆, has how many covalent bonds? (a) 6 (b) 7 (c) 8 (d) 9 → (b) 7 (six C–H + one C–C).

Q. Butanone is a four-carbon compound with the functional group: (a) carboxylic acid (b) aldehyde (c) ketone (d) alcohol → (c) ketone (>C=O).

Q. While cooking, if the bottom of the vessel is getting blackened, it means: (a) food not cooked (b) fuel not burning completely (c) fuel is wet (d) fuel burning completely → (b) the fuel is not burning completely (insufficient air supply).

Q. Explain the nature of the covalent bond using the formation of methane. Carbon (2,4) shares one electron with each of four hydrogen atoms; four shared pairs form four C–H single covalent bonds, completing carbon's octet and each hydrogen's duplet.

Q. Draw electron-dot structures for (a) ethanoic acid (b) H₂S (c) propanone (d) F₂. As in §13 — show shared pairs as bonds and remaining electrons as lone pairs, obeying the octet rule.

Q. What is a homologous series? Explain with an example. See §6 — alkanes CH₄, C₂H₆, C₃H₈, each differing by CH₂.

Q. How can ethanol and ethanoic acid be differentiated on the basis of physical and chemical properties? Physical: ethanol has a pleasant smell and burning taste; ethanoic acid (vinegar) has a sour smell and turns blue litmus red. Chemical: ethanoic acid gives CO₂ effervescence with NaHCO₃; ethanol does not.

Q. Why does micelle formation take place when soap is added to water? See §12 / §13.

Q. Why are carbon and its compounds used as fuels for most applications? They burn readily in air giving large amounts of heat and light, leaving little residue.

Q. Explain the formation of scum when hard water is treated with soap. Soap forms insoluble calcium/magnesium salts (scum) with the Ca²⁺/Mg²⁺ ions of hard water.

Q. What change will you observe if you test soap with litmus (red and blue)? Soap is mildly basic, so it turns red litmus blue; blue litmus stays blue.

Q. What is hydrogenation? What is its industrial application? Addition of H₂ across C=C over Ni catalyst; used to harden vegetable oils into vanaspati.

Q. Which compounds undergo addition reactions: C₂H₆, C₃H₈, C₃H₆, C₂H₂, CH₄? The unsaturated C₃H₆ and C₂H₂.

Q. How would you distinguish experimentally between butter and cooking oil? Bromine-water test (see §13): cooking oil decolourises it, butter does not.

Q. Explain the mechanism of the cleaning action of soaps. See §12 — micelle formation traps grease and lifts it into water.

16. Common mistakes to avoid

• Confusing the general formulas: alkane C_nH_2n+2, alkene C_nH_2n, alkyne C_nH_2n−2.
• Saying saturated hydrocarbons give addition reactions — they give substitution; only unsaturated ones give addition.
• Forgetting to drop the final "e" before -ol/-al/-one (ethane → ethanol).
• Mixing up the micelle: the tails point inward (into the oil), the ionic heads point outward (into water).
• Saying soap works in hard water — it forms scum; detergents work in hard water.
• Calling ethanoic acid a strong acid — it is a weak acid.
• Writing combustion without "heat & light" — those are the marks-earning point for fuels.

17. Quick revision checklist

• Carbon = 2,4, tetravalent, bonds by sharing → covalent (low m.p./b.p., poor conductors).
• Versatility = catenation + tetravalency + multiple bonds; allotropes = diamond, graphite, fullerene.
• Saturated = single bonds (alkanes); unsaturated = double (alkene) / triple (alkyne).
• Homologous series differ by –CH₂; functional group decides chemistry.
• Reactions: combustion, oxidation (KMnO₄/K₂Cr₂O₇), addition (H₂/Ni), substitution (Cl₂/sunlight).
• Ethanol + Na → H₂; ethanol → ethene (conc. H₂SO₄); ethanoic acid + alcohol → ester; + NaHCO₃ → CO₂.
• Soap = Na salt of fatty acid; cleans by micelle formation; fails in hard water (scum); detergents don't.