Ch 4Carbon and its Compounds

Carbon and its Compounds

Carbon is a versatile element. The Earth's crust has only 0.02% carbon (as minerals like carbonates, coal, petroleum) and the atmosphere has 0.03% CO₂, yet carbon forms an enormous number of compounds. Its atomic number is 6, with electronic configuration 2,4 — it has 4 valence electrons.

Covalent Bonding

Carbon has 4 electrons in its outermost shell and needs 4 more to attain the noble-gas configuration. It cannot lose 4 electrons (would need huge energy to form C⁴⁺) or gain 4 electrons (hard to hold 4 extra electrons with 6 protons). So carbon shares its valence electrons to form covalent bonds.

A covalent bond is formed by the sharing of a pair of electrons between two atoms, each contributing one electron. Examples: H₂ (single bond), O₂ (double bond), N₂ (triple bond), CH₄, water. Covalent compounds generally have low melting/boiling points (weak intermolecular forces), are poor conductors of electricity (no free electrons or ions).

Allotropes of Carbon

Allotropes are different physical forms of the same element. Carbon's allotropes:

• Diamond — each carbon bonded to 4 other carbons in a rigid 3-D structure; hardest natural substance.
• Graphite — each carbon bonded to 3 others in hexagonal layers; soft, slippery, good conductor of electricity (one free electron per atom).
• Fullerene (C-60, Buckminsterfullerene) — carbon atoms arranged like a football (spherical).

Versatile Nature of Carbon

Two properties give carbon its huge variety of compounds:

• Catenation — the ability of carbon atoms to link with one another forming long chains, branched chains and rings. Carbon–carbon bonds are very strong and stable.
• Tetravalency — having a valency of 4, carbon can bond with four other atoms of carbon or other monovalent elements (like H, O, N, S, Cl).

Hydrocarbons: Saturated and Unsaturated

Hydrocarbons are compounds of only carbon and hydrogen.

• Saturated hydrocarbons (Alkanes) — carbon atoms joined by single bonds only. General formula C_nH_2n+2. Example: methane CH₄, ethane C₂H₆. Burn with a clean blue flame.
• Unsaturated hydrocarbons — contain double or triple bonds. Alkenes (one C=C double bond) general formula C_nH_2n, e.g. ethene C₂H₄. Alkynes (one C≡C triple bond) general formula C_nH_2n-2, e.g. ethyne C₂H₂. Burn with a yellow sooty flame due to incomplete combustion.

Structural formula example: methane has C at centre with 4 H atoms each joined by a single line (bond). Ethane is H₃C–CH₃. Ethene is H₂C=CH₂.

Homologous Series

A homologous series is a group of organic compounds having the same general formula and similar chemical properties in which successive members differ by a –CH₂ unit (mass difference 14 u). Physical properties (melting/boiling point) change gradually with increasing molecular mass. Example: CH₄, C₂H₆, C₃H₈, C₄H₁₀.

Functional Groups

A functional group is an atom or group of atoms that gives characteristic properties to a compound, replacing one or more hydrogen atoms.

Nomenclature (IUPAC basics)

1. Identify the number of carbon atoms → root word (meth-1, eth-2, prop-3, but-4, pent-5).
2. Add suffix: -ane (saturated), -ene (double bond), -yne (triple bond).
3. Add functional group as prefix or suffix. If suffix begins with a vowel, remove the final 'e' of the root (e.g. propane → propanol).

Chemical Properties

Oxidation: Alcohols are oxidised to carboxylic acids by oxidising agents like alkaline KMnO₄ or acidified K₂Cr₂O₇ (these are called oxidising agents because they add oxygen).

Addition reaction: Unsaturated hydrocarbons add hydrogen in presence of catalysts (Ni/Pd) — hydrogenation. Used to convert vegetable oils (unsaturated) into vegetable ghee (saturated).

Substitution reaction: Saturated hydrocarbons react with chlorine in the presence of sunlight; chlorine replaces hydrogen atoms one by one.

Ethanol (C₂H₅OH)

A liquid at room temperature, commonly called alcohol; used in medicines, tinctures, as a solvent. Consumption of unmixed ethanol in large quantities is harmful. Reactions:

Ethanoic Acid (CH₃COOH)

Commonly called acetic acid; 5–8% solution in water is called vinegar. Its freezing point is 290 K, so in cold climates it freezes to an ice-like solid — hence glacial acetic acid. It is a weak acid.

Saponification: On treating an ester with NaOH, it hydrolyses back to alcohol and the sodium salt of the acid (soap). This reaction is used in soap preparation.

Soaps and Detergents

Soaps are sodium or potassium salts of long-chain carboxylic (fatty) acids. A soap molecule has two ends: a hydrophilic (water-loving, ionic –COO^–Na⁺) head and a hydrophobic (water-repelling, hydrocarbon) tail.

Micelle Action (Cleansing)

In water, the hydrophobic tails attach to oil/dirt while the hydrophilic heads point outward into water, forming a spherical cluster called a micelle. Dirt is trapped at the centre; the micelles remain suspended and are washed away with water. Soaps form insoluble scum with hard water (Ca/Mg salts).

Detergents are sodium salts of long-chain sulphonic acids (or ammonium salts). Their charged ends do not form insoluble precipitates with Ca/Mg ions, so detergents work even in hard water.