Hydrocarbons

Hydrocarbons

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Hydrocarbons are organic compounds containing only carbon and hydrogen atoms bonded by covalent bonds. They are the simplest organic compounds and form the backbone of organic chemistry.

Classification:

• Saturated hydrocarbons: Alkanes (general formula CₙH₂ₙ₊₂) — only single C–C bonds.
• Unsaturated hydrocarbons: Alkenes (CₙH₂ₙ, at least one C=C) and Alkynes (CₙH₂ₙ₋₂, at least one C≡C).

Key Formulas:

• Combustion: CₓHᵧ + (x + y/4)O₂ → xCO₂ + (y/2)H₂O (complete)
• Alkenes decolourise bromine water (orange-brown fades).
• Alkenes + cold dilute KMnO₄ → diol (purple fades).
• Markovnikov’s rule: H adds to the carbon with more H atoms already attached.

SLMC High-Yield Pointers:

1. Bromine test distinguishes alkenes from alkanes — alkanes do not decolourise.
2. Alkynes with terminal C≡C give white precipitate with silver nitrate (AgNO₃).
3. IUPAC naming: longest chain, lowest locants; prefix tells carbon count (meth- = 1, eth- = 2, prop- = 3, but- = 4).
4. Catenation — carbon’s ability to bond to itself — explains why so many hydrocarbon structures exist.
5. Combustion products: complete → CO₂ + H₂O; incomplete → CO + H₂O or C + H₂O.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Homologous Series and Classification

A homologous series is a family of organic compounds sharing the same general formula, similar chemical properties, and a regular gradation in physical properties. Each successive member differs by –CH₂– (one carbon and two hydrogens).

Series	General Formula	Bond Type	First Member
Alkanes	CₙH₂ₙ₊₂	C–C single only	Methane (CH₄)
Alkenes	CₙH₂ₙ	One C=C double	Ethene (C₂H₄)
Alkynes	CₙH₂ₙ₋₂	One C≡C triple	Ethyne (C₂H₂)

Alkanes — Substitution Reactions

Alkanes are saturated and undergo substitution reactions. The classic example is chlorination:

CH₄ + Cl₂ → CH₃Cl + HCl (under UV light)

The reaction proceeds via a free-radical mechanism — initiation (Cl₂ splits), propagation (CH₃• formed), and termination steps. Similar bromination is slower but more selective.

Alkenes — Addition Reactions

Alkenes are unsaturated and undergo addition reactions at the C=C bond. Key reactions for SLMC:

• Hydrogenation: C₂H₄ + H₂ → C₂H₆ (Ni catalyst, heat)
• Halogenation: C₂H₄ + Br₂ → C₂H₄Br₂ (bromine water test — decolourises orange-brown)
• Hydration: C₂H₄ + H₂O → C₂H₅OH (H⁺ catalyst, Markovnikov product)
• Oxidation by cold KMnO₄: forms a diol (two –OH groups on adjacent carbons)

Markovnikov’s rule applies: when HX adds to an unsymmetrical alkene, the H attaches to the carbon with more existing H atoms, and the halogen goes to the more substituted carbon.

Alkynes — Distinguishing Test

Alkynes undergo similar addition reactions. Terminal alkynes (–C≡CH) give a white precipitate with ammoniacal AgNO₃:

RC≡CH + AgNO₃ → RC≡CAg↓ + HNO₃

This test does not occur with alkenes or internal alkynes, making it a key distinction for SLMC questions.

Structural Isomerism

Hydrocarbons show structural isomerism — same molecular formula, different connectivity:

• Chain isomerism: straight vs. branched carbon skeleton (e.g., butane vs. isobutane)
• Positional isomerism: double/triple bond in different positions (e.g., but-1-ene vs. but-2-ene)

Combustion

Complete combustion: CₓHᵧ + (x + y/4)O₂ → xCO₂ + (y/2)H₂O + heat

Incomplete combustion (limited O₂): produces CO or carbon (soot) instead of CO₂.

Nomenclature Basics

1. Identify the longest carbon chain (parent name).
2. Number from the end giving the lowest locant to the multiple bond or substituent.
3. Prefix = carbon count: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6).

Example: CH₃–CH₂–CH=CH–CH₃ → Pent-2-ene (5 carbons, double bond at C2).

Cracking and Reforming

Cracking breaks large alkanes into smaller, more useful alkanes and alkenes (thermal or catalytic). Catalytic reforming converts straight-chain alkanes to branched isomers (higher octane rating) — important in petroleum refining.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Mechanism of Free-Radical Halogenation in Alkanes

The chlorination of methane proceeds via three stages:

1. Initiation: UV light causes homolytic cleavage: Cl₂ → 2Cl•
2. Propagation: Cl• abstracts H from CH₄ → CH₃• + HCl; then CH₃• reacts with Cl₂ → CH₃Cl + Cl• (chain continues)
3. Termination: Two radicals combine: Cl• + Cl• → Cl₂; CH₃• + Cl• → CH₃Cl; CH₃• + CH₃• → C₂H₆

Multiple substituted products (CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄) form as the reaction progresses — this is why controlled conditions matter in SLMC questions.

Beyond Markovnikov — Anti-Markovnikov Addition

In the presence of peroxides (R–O–O–R), HBr adds anti-Markovnikov to alkenes: Br attaches to the more substituted carbon. This is a radical mechanism contrasting with the standard electrophilic addition. peroxides acts as a “reverse” agent important for understanding why conditions change outcomes — a classic SLMC trap question.

Polymerization — Ethene to Polythene

Addition polymerization links many unsaturated monomers into a polymer chain:

n CH₂=CH₂ → [–CH₂–CH₂–]ₙ (polythene)

The double bond breaks and each monomer contributes two carbon atoms to the polymer backbone. This reaction occurs under high pressure and temperature with a peroxide initiator.

Connecting to Adjacent Topics

• Environmental organic chemistry: Hydrocarbons are precursors to BTEX compounds (benzene, toluene, ethylbenzene, xylene) found in petroleum pollution.
• Biochemistry link: The hydrocarbon backbone of fatty acids (alkane chains) and the aromatic ring structure of amino acid phenylalanine extend hydrocarbon knowledge into biology.
• Medicinal chemistry: Alkyl halides (from hydrocarbon substitution) are used as anaesthetics; aromatic hydrocarbons form steroid skeletons.

Common Mistakes in SLMC Exams

1. Writing alkane reactions as addition — remember: alkanes only undergo substitution. Writing CH₄ + Br₂ → CH₃Br + HBr as an addition reaction costs full marks.
2. Markovnikov misapplication — only valid for electrophilic addition (HCl, H₂O, HBr without peroxides). Do not apply it to H₂ or catalytic hydrogenation.
3. Confusing structural isomers vs. stereoisomers — hydrocarbons show only structural (chain, positional, functional group) isomerism at this level. No stereochemistry required for SLMC.
4. Forgetting to specify conditions — UV light for alkane substitution, catalyst + heat for alkene addition, cold dilute vs. hot conc. KMnO₄ for alkenes. Missing conditions changes the product entirely.

Practice Prompts

1. A hydrocarbon (C₄H₁₀) is subjected to catalytic cracking. Draw the possible products and identify which would decolourise bromine water. Explain your reasoning using the chemistry of C=C bonds.

2. Compound X (C₃H₆) decolourises bromine water rapidly and gives a white precipitate with ammoniacal silver nitrate. Compound Y (C₃H₈) does not decolourise bromine water. Identify X and Y with equations. Classify each by hydrocarbon type and write the IUPAC name.

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