CC07 — Carbon & Its Compounds

📖 CC07 · CDS General Science — Chemistry ★ High Priority

Carbon is unique — it forms millions of compounds due to catenation (self-bonding) and tetravalency. Organic chemistry begins here. CDS tests allotropes, hydrocarbons, functional groups and important organic compounds directly and frequently.

📌 CDS Focus: Diamond vs graphite vs fullerene (properties and uses); general formulae of alkanes (CₙH₂ₙ₊₂), alkenes (CₙH₂ₙ), alkynes (CₙH₂ₙ₋₂); functional groups (—OH alcohol, —COOH acid, —CHO aldehyde); ethanol and ethanoic acid (common names and uses). One or two questions come directly from this chapter each time.

PART 1 — ALLOTROPES OF CARBON

1. Three Allotropes of Carbon

Allotropes are different structural forms of the same element. Carbon has three major allotropes — diamond, graphite and fullerene — each with completely different properties despite being made of only carbon atoms.

Fig. 1 — Three Carbon Allotropes: Structure, Key Property and Uses

⚠ CDS Trap — Graphite: Graphite is a non-metal that conducts electricity. Students often assume all non-metals are insulators — graphite is the exception. Diamond is a non-metal that does NOT conduct (no free electrons). This contrast is directly tested in CDS.

PART 2 — HYDROCARBONS

2. Alkanes, Alkenes and Alkynes

Fig. 2 — Three Homologous Series: General Formula, Bond Type and First Members

PART 3 — FUNCTIONAL GROUPS

3. Important Functional Groups

Functional GroupStructureName SuffixExample & Use Hydroxyl (Alcohol)—OH-olEthanol C₂H₅OH — drinks, fuel, antiseptic Carboxylic Acid—COOH-oic acidEthanoic acid CH₃COOH — vinegar (acetic acid) Aldehyde—CHO-alMethanal HCHO — formaldehyde (preservative) Ketone>C=O (mid-chain)-onePropanone CH₃COCH₃ — acetone (nail polish remover) EtherR—O—R-oxy-Diethyl ether — anaesthetic Ester—COO—-oateEthyl ethanoate — fruit flavours, solvents

💡 Ethanol vs Ethanoic Acid — CDS Favourite Pair:
Ethanol (C₂H₅OH): drinkable alcohol; used as fuel, solvent, antiseptic. Produced by fermentation of glucose.
Ethanoic acid (CH₃COOH): acetic acid; 5–8% solution = vinegar. Weak acid. Reacts with ethanol (esterification) to form ethyl acetate + water.

📝 CDS PYQs — Carbon & Its Compounds

Q1. Which allotrope of carbon is the hardest natural substance? CDS PYQ

(a) Graphite(b) Fullerene(c) Diamond(d) Carbon black

✔ Answer: (c) Diamond

Diamond is the hardest natural substance (Mohs hardness = 10). Each carbon atom is covalently bonded to 4 others in a rigid 3D tetrahedral network — no free electrons means it cannot conduct electricity. Used in cutting, drilling, and jewellery. Graphite is soft (layers slide over each other) and conducts electricity due to delocalised electrons. This diamond-graphite comparison is the most tested allotrope question.

Q2. The general formula of alkanes is: CDS PYQ

(a) CₙH₂ₙ₊₂(b) CₙH₂ₙ(c) CₙH₂ₙ₋₂(d) CₙHₙ

✔ Answer: (a) CₙH₂ₙ₊₂

Alkanes are saturated hydrocarbons (all single bonds) with general formula CₙH₂ₙ₊₂. Example: methane CH₄ (n=1: 2×1+2=4H); ethane C₂H₆ (n=2: 2×2+2=6H). Alkenes = CₙH₂ₙ (one double bond); Alkynes = CₙH₂ₙ₋₂ (one triple bond). The formula comparison is a direct CDS question.

Q3. Vinegar is a dilute solution of: CDS PYQ

(a) Hydrochloric acid(b) Citric acid(c) Acetic acid (ethanoic acid)(d) Lactic acid

✔ Answer: (c) Acetic acid (ethanoic acid)

Vinegar is a 5–8% aqueous solution of acetic acid (ethanoic acid, CH₃COOH). It is a weak carboxylic acid produced by oxidation of ethanol. Its distinctive sour taste is due to the —COOH functional group. Used in food preservation and cooking. This is one of the most directly repeated CDS chemistry facts.

Q4. Graphite conducts electricity because: ⚡ Tricky

(a) It is a metal(b) It has free electrons between layers(c) It has a 3D network(d) It contains impurities

✔ Answer: (b) It has free (delocalised) electrons between layers

In graphite, each carbon atom forms 3 covalent bonds in a hexagonal layer (sp² hybridised). The 4th valence electron remains delocalised between the layers, free to move and carry charge — making graphite a good conductor. Diamond has no free electrons (all 4 bonds are used) so it is an insulator. This is exactly why graphite is used as an electrode material in batteries and electrolysis.

Q5. Ethanol is produced by fermentation of glucose using: CDS PYQ

(a) Bacteria(b) Yeast(c) Mould(d) Fungi

✔ Answer: (b) Yeast

Ethanol is produced by anaerobic fermentation of glucose by yeast (Saccharomyces cerevisiae): C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. This is carried out in the absence of oxygen. Yeast contains the enzyme zymase which catalyses the reaction. The CO₂ released causes bread to rise (same process). This is a regularly repeated CDS question.

🧠 Quick Memory Chart — CC07

◈ Allotropes of C

Diamond: hardest; 3D; insulator
Graphite: soft; conducts; lubricant
Fullerene: C₆₀; soccer-ball cage
Diamond = 4 bonds; Graphite = 3
Catenation = C bonds to itself

⚛ Hydrocarbons

Alkane: CₙH₂ₙ₊₂ (single bonds)
Alkene: CₙH₂ₙ (double bond)
Alkyne: CₙH₂ₙ₋₂ (triple bond)
Methane CH₄; Ethene C₂H₄
Ethyne C₂H₂ = welding gas

⚡ Functional Groups

—OH: alcohol (ethanol = drinks)
—COOH: acid (vinegar = ethanoic)
—CHO: aldehyde (formaldehyde)
>C=O: ketone (acetone)
—COO—: ester (fruit flavour)

📝 Practice Exercise

E1. Which carbon allotrope is used as a lubricant?

(a) Diamond(b) Fullerene(c) Graphite(d) Carbon black

E2. The molecular formula of propane (alkane, n=3) is:

(a) C₃H₆(b) C₃H₈(c) C₃H₄(d) C₃H₂

E3. Acetone (nail polish remover) belongs to which functional group?

(a) Alcohol(b) Aldehyde(c) Ketone(d) Carboxylic acid

E4. Fullerene (C₆₀) was discovered in which year?

Answers:
E1 → (c) Graphite [layers slide; also used in pencils, electrodes] | E2 → (b) C₃H₈ [CₙH₂ₙ₊₂ = C₃H(2×3+2) = C₃H₈] | E3 → (c) Ketone [propanone CH₃COCH₃; carbonyl C=O in mid-chain] | E4 → (c) 1985 [discovered by Kroto, Curl and Smalley; Nobel Prize 1996]

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