Hydrocarbons and Polymers

Why Hydrocarbons and Polymers Matter in CDS

A hydrocarbon is a compound made of only two elements — hydrogen and carbon. From the methane in your kitchen cylinder to the petrol in a truck and the polythene in a shopping bag, hydrocarbons and their products surround daily life. That is exactly why the CDS General Science paper keeps returning to them: the questions are practical, fact-based and reward memory over mathematics.

Carbon is special because of two properties. First, catenation — carbon atoms link to one another in long chains, branches and rings. Second, tetravalency — each carbon forms exactly four bonds. Together these let carbon build millions of compounds, far more than any other element. Almost every organic-chemistry question in CDS traces back to these two ideas.

Treat this chapter as pattern recognition, not rote learning. Once you know the general formulae and the “add one CH₂” rule of a homologous series, you can name members, count hydrogens and spot isomers in seconds. The polymer half is even simpler — it is mostly “which everyday plastic comes from which monomer”. Over the years, CDS papers have consistently carried at least one organic-chemistry question per paper, and they rarely demand any calculation, so the marks-per-minute return on mastering these patterns is among the highest in the entire General Science syllabus.

Tetravalency and Catenation: Carbon's Two Superpowers

Carbon has 6 electrons, arranged 2, 4. To complete its outer shell of 8 it needs four more electrons, so it forms four covalent bonds by sharing. This is its tetravalency. Because the bonds are covalent and the carbon atom is small, these bonds are strong and stable.

Catenation is carbon's ability to bond with other carbon atoms, forming chains (straight or branched) and rings. No other element does this on such a scale. Silicon catenates a little, but its chains are short and reactive. Carbon's long, stable skeletons are the reason organic chemistry exists. Carbon almost always forms covalent bonds by sharing electrons rather than transferring them, so most carbon compounds are poor conductors of electricity, have low melting and boiling points, and are usually insoluble in water.

• Straight chain — carbons in a single line, e.g. n-butane.
• Branched chain — side groups attached, e.g. isobutane.
• Ring (cyclic) — carbons joined end to end, e.g. cyclohexane, benzene.

Saturated vs Unsaturated Hydrocarbons

Hydrocarbons split into two big groups depending on the type of carbon-carbon bonds they contain.

Saturated hydrocarbons (alkanes)

These contain only single bonds (C−C). Each carbon holds the maximum possible hydrogens, so the molecule is “saturated” — it cannot take up any more hydrogen. They are relatively unreactive towards ordinary reagents and burn cleanly, which is why they make good fuels. Their main reaction is substitution (in sunlight, hydrogen atoms are replaced one by one by chlorine or bromine). General formula: C_nH_2n+2. Because their bonds are already full, alkanes are sometimes called paraffins, meaning “little affinity”.

Unsaturated hydrocarbons (alkenes and alkynes)

These contain at least one double bond (alkenes) or triple bond (alkynes). The extra bonds make them more reactive; they readily undergo addition reactions, where the multiple bond breaks open and new atoms add on — for example, adding hydrogen (hydrogenation) converts an alkene to an alkane, and this is exactly how liquid vegetable oils are hardened into vanaspati ghee. They also add bromine and hydrogen halides. Alkenes: C_nH_2n. Alkynes: C_nH_2n−2.

The Three Families: Alkanes, Alkenes, Alkynes

These three series are the backbone of every CDS organic question. Learn the first few members and their formulae.

Alkanes (single bond, C_nH_2n+2)

• Methane — CH₄ (n=1)
• Ethane — C₂H₆
• Propane — C₃H₈ (used in LPG)
• Butane — C₄H₁₀ (used in LPG)

Alkenes (one double bond, C_nH_2n)

• Ethene (ethylene) — C₂H₄, ripens fruit, makes polythene
• Propene — C₃H₆

Alkynes (one triple bond, C_nH_2n−2)

• Ethyne (acetylene) — C₂H₂, used in welding (oxy-acetylene flame)
• Propyne — C₃H₄

The Homologous Series Shortcut

A homologous series is a family of compounds with the same general formula and similar chemical properties, where each member differs from the next by a fixed unit of −CH₂− (a mass of 14 u).

Key features that examiners test:

• All members share the same general formula (e.g. alkanes C_nH_2n+2).
• Consecutive members differ by CH₂ and a molar mass of 14 u.
• They have similar chemical properties because they share the same functional behaviour.
• Physical properties (boiling point, density) change gradually as chain length grows.

Isomerism: Same Formula, Different Structure

Isomers are compounds with the same molecular formula but different structural arrangements, and therefore different properties. Isomerism begins from butane (C₄H₁₀) in the alkane series — the first three members (methane, ethane, propane) have no isomers.

Butane C₄H₁₀ exists in two forms:

• n-butane — a straight chain of four carbons.
• iso-butane — a three-carbon chain with one carbon branching off the middle.

Both have the formula C₄H₁₀, but their boiling points and structures differ — the branched iso-butane boils lower than the straight n-butane because branching reduces the surface contact between molecules. As the number of carbons rises, the number of possible isomers rises sharply: pentane (C₅H₁₂) has three isomers, hexane has five, and the count keeps climbing. This explosion of isomers is another major reason carbon forms so many distinct compounds.

Important Hydrocarbons and Everyday Uses

CDS frequently asks “which gas/fuel is which”. Fix these everyday facts.

• Methane (CH₄) — main component of CNG and biogas; also marsh gas.
• LPG — liquefied petroleum gas, mainly butane and propane. The bad-smell warning agent added is ethyl mercaptan.
• CNG — compressed natural gas, mainly methane; cleaner vehicle fuel.
• Ethyne / acetylene (C₂H₂) — the oxy-acetylene flame is used for welding and cutting metals.
• Ethene / ethylene (C₂H₄) — used to artificially ripen fruits and to make polythene.
• Benzene (C₆H₆) — a ring (aromatic) hydrocarbon, parent of many dyes and plastics.
• Petrol, diesel and kerosene — mixtures of many hydrocarbons obtained by the fractional distillation of crude petroleum; petrol contains shorter chains than diesel.
• Biogas — produced by the anaerobic decay of organic waste; about 60–65% methane, making it a clean rural fuel.

Polymers: Building Big from Small

A polymer is a very large molecule built by joining together many small repeating units called monomers. The process of joining is polymerisation. The word comes from Greek: poly (many) + mer (unit).

Polymers may be natural or synthetic:

• Natural polymers — cellulose, starch, proteins, natural rubber, silk, wool.
• Synthetic (man-made) polymers — polythene, PVC, nylon, terylene, bakelite, teflon.

Polymers are broadly classed by how the monomers join, which gives the two types most tested in CDS: addition and condensation polymers. A second classification, by heat behaviour, splits them into thermoplastics (soften on heating, can be remoulded) and thermosetting plastics (set permanently and char rather than melt). Both classifications appear regularly in the exam, so keep them clearly separate in your mind.

Addition vs Condensation Polymers

Addition polymers

Formed when many unsaturated monomers (containing double bonds) add together with no by-product lost. The repeating unit has the same composition as the monomer.

• Polythene — from ethene; bags, sheets, bottles.
• PVC (polyvinyl chloride) — from vinyl chloride; pipes, insulation.
• Teflon (PTFE) — from tetrafluoroethene; non-stick coatings.
• Polystyrene — from styrene; thermocol, packaging.

Condensation polymers

Formed when monomers join with the loss of a small molecule (usually water). Two different monomers usually combine.

• Nylon — a polyamide; ropes, fabrics, parachutes.
• Terylene (Dacron) — a polyester; fabrics.
• Bakelite — phenol + formaldehyde; switches, handles (a thermosetting plastic).

Worked Example: Naming and Counting Atoms

Let us apply the general formulae to a typical reasoning question.

Previous-Year Style Question

Test yourself with a question in the exact CDS objective style.

Quick Revision