Hydrocarbons — Saturated (Alkanes & Cycloalkanes) and Unsaturated (Alkenes & Alkynes)

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Hydrocarbons are compounds containing only carbon and hydrogen atoms. They are classified as saturated (single bonds only: alkanes) or unsaturated (double/triple bonds: alkenes, alkynes). Cycloalkanes are saturated hydrocarbons in ring form. For SLMC, know the general formulas, key reactions, and distinguishing tests between saturated and unsaturated hydrocarbons.

High-Yield Facts for SLMC:

• Alkanes: CₙH₂ₙ₊₂ (saturated, single bonds only)
• Alkenes: CₙH₂ₙ (one double bond); cycloalkanes: CₙH₂ₙ (ring, no double bond)
• Alkynes: CₙH₂ₙ₋₂ (one triple bond)
• Unsaturation test: Bromine water (brown) or KMnO₄ (purple) decolorizes with alkenes/alkynes but NOT with alkanes
• ⚡ Exam tip: Burning alkane flame — blue (complete) vs yellow luminous (incomplete). Methane + 2O₂ → CO₂ + 2H₂O

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🟡 Standard — Regular Study (2d–2mo)

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Hydrocarbons — SLMC Medical (Sri Lanka) Study Guide

Classification of Hydrocarbons

Hydrocarbons
├── Saturated (Alkanes: CₙH₂ₙ₊₂, only σ bonds)
│   ├── Acyclic: straight-chain and branched alkanes
│   └── Cyclic: cycloalkanes (CₙH₂ₙ, ring of CH₂ units)
└── Unsaturated
    ├── Alkenes (CₙH₂ₙ, at least one C=C double bond)
    └── Alkynes (CₙH₂ₙ₋₂, at least one C≡C triple bond)

Acyclic Alkanes (Saturated Open-Chain Hydrocarbons)

Nomenclature of Straight-Chain Alkanes

Carbon Atoms	Name	Formula	Boiling Point (°C)
1	Methane	CH₄	–162
2	Ethane	C₂H₆	–89
3	Propane	C₃H₈	–42
4	Butane	C₄H₁₀	–0.5
5	Pentane	C₅H₁₂	36
6	Hexane	C₆H₁₄	69
7	Heptane	C₇H₁₆	98
8	Octane	C₈H₁₈	126
9	Nonane	C₉H₂₀	151
10	Decane	C₁₀H₂₂	174

Boiling point trend: Increases with molecular size (more electrons → stronger London dispersion forces). Branching decreases boiling point (more spherical shape → less surface area for intermolecular forces).

Properties of Alkanes

• Physical state: C₁–C₄: gases; C₅–C₁₇: liquids; C₁₈+: solids
• Insoluble in water (non-polar; water is polar)
• Less dense than water (density ~0.8 g/cm³ for liquids)
• Chemical reactivity: Alkanes are relatively inert — strong C–C and C–H bonds
• Combustion: Complete: CO₂ + H₂O; Incomplete (limited O₂): CO + H₂O or C + H₂O
• Halogenation: CH₄ + Cl₂ → CH₃Cl + HCl (homolytic fission, UV light required)

Cycloalkanes

Cycloalkanes have the formula CₙH₂ₙ (same as alkenes — both have one degree of unsaturation).

Key points:

• Simplest cycloalkane: cyclopropane (C₃H₆) — highly strained bond angle (60° vs normal 109.5°)
• Cyclobutane (C₄H₈), cyclopentane (C₅H₁₀), cyclohexane (C₆H₁₂)
• Cyclohexane exists in chair and boat conformations; chair is more stable
• Substitution reactions: Like alkanes, cycloalkanes undergo free radical halogenation
• Addition reactions: NOT like alkenes — cycloalkanes do NOT decolorize bromine water

Alkenes (Unsaturated Hydrocarbons with C=C)

General Formula and Structure

• CₙH₂ₙ — one degree of unsaturation (one double bond)
• sp² hybridized carbons; trigonal planar geometry (120°)
• The double bond consists of one σ bond (sp²–sp² overlap) + one π bond (p-orbital overlap)

Nomenclature

• Suffix: -ene
• Number the chain to give the double bond the lowest possible number
• Examples: ethene (C₂H₄), propene (C₃H₆), but-1-ene, but-2-ene

Isomerism in Alkenes

• Structural isomerism: Position of double bond (but-1-ene vs but-2-ene)
• Geometric (cis-trans) isomerism: In alkenes where each carbon of the double bond has two different groups attached (e.g., but-2-ene: CH₃–CH=CH–CH₃)
  • Cis: Same groups on same side (e.g., both methyl groups above the double bond)
  • Trans: Same groups on opposite sides

Reactions of Alkenes

1. Addition reactions (most characteristic):

• Hydrogenation: CH₂=CH₂ + H₂ → CH₃–CH₃ (Ni catalyst, heat)
• Halogenation: CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br (bromine water DECOLORIZES — key test for unsaturation)
• Hydrogen halide (HCl, HBr): Markovnikov’s addition — H adds to carbon with MORE hydrogens; X/Hal adds to carbon with FEWER hydrogens
  • Example: CH₃–CH=CH₂ + HBr → CH₃–CH(Br)–CH₃ (2-bromopropane)
• Hydration (H₂O + H⁺): Follows Markovnikov’s rule — OH group goes to the MORE substituted carbon

2. Oxidation reactions:

• With cold KMnO₄ (Baeyer reagent): Purple color disappears → diol (HO–CH₂–CH₂–OH from ethene)
• With hot KMnO₄: Complete cleavage → CO₂ + H₂O (for ethene) or carboxylic acids/ketones
• ** ozonolysis:** Breaks the double bond at the C=C; products identified by O₃ + Zn/H₂O workup

3. Polymerization:

• n(CH₂=CH₂) → [–CH₂–CH₂–]ₙ (polyethene/polyethylene)

Distinguishing Alkenes from Alkanes

Test	Alkane	Alkene
Bromine water (brown)	No decolorization	Decolorizes (addition reaction)
Acidified KMnO₄ (purple)	No decolorization	Decolorizes (oxidation)
Combustion flame	Blue/yellow	Luminous yellow flame

Alkynes (Unsaturated Hydrocarbons with C≡C)

General Formula and Structure

• CₙH₂ₙ₋₂ — two degrees of unsaturation (one triple bond OR two double bonds)
• sp hybridized carbons; linear geometry (180°)
• The triple bond = one σ bond + two π bonds

Nomenclature

• Suffix: -yne
• Examples: ethyne (C₂H₂), propyne (C₃H₄), but-1-yne, but-2-yne

Special Reactivity: Terminal vs Internal Alkynes

• Terminal alkyne: C≡C–H (acetylene H–C≡C–H)
  • Acidic hydrogen — reacts with Na, NaNH₂, AgNO₃ to form salt
  • Forms silver acetylide (white) or copper(I) acetylide (reddish-brown)
  • Test: AgNO₃ + terminal alkyne → precipitate (alcoholic NH₃ needed)
• Internal alkyne: No acidic hydrogen; no silver镜 precipitate

Reactions of Alkynes

1. Addition reactions (proceeds in two stages):

• H₂ + Ni or Pt: First adds to give alkene, then alkane
• H₂ + Lindlar’s catalyst (Pd/CaCO₃ + quinoline): Cis-alkene (syn addition)
• Na + NH₃ (liquid): Trans-alkene (anti addition)
• Halogenation: Br₂ adds across triple bond → dibromoalkene → tetra-bromoalkane

2. Oxidation:

• Hot KMnO₄ → carboxylic acids (or CO₂ from terminal alkyne)

3. Reaction with AgNO₃/CuCl:

• Terminal alkynes ONLY: RC≡CH + AgNO₃ → silver acetylide precipitate (white)
• Internal alkynes: NO reaction

Degree of Unsaturation (DOU) — Critical Concept

The degree of unsaturation tells you how many rings or multiple bonds are in a molecule:

DOU formula: DOU = (2C + 2 – H – X + N) / 2

Where C = carbons, H = hydrogens, X = halogens, N = nitrogens

Interpreting DOU:

• DOU = 0: Fully saturated alkane
• DOU = 1: One double bond OR one ring
• DOU = 2: Two double bonds OR one triple bond OR two rings OR one double bond + one ring
• DOU ≥ 3: Three or more π bonds/rings combined

Example: C₆H₁₂: DOU = (2×6 + 2 – 12)/2 = 1 → cyclohexane OR hexene

How to Approach Hydrocarbon Questions in SLMC

1. Find the molecular formula → calculate DOU to determine type of unsaturation
2. Check saturation status → use bromine water or KMnO₄ test
3. For alkenes → check for cis-trans isomerism when both carbons of C=C have two different groups
4. For Markovnikov addition → H goes to the carbon with MORE hydrogens already
5. For alkynes → check if terminal (has acidic H, gives Ag/Cu precipitate) or internal

⚡ Exam tip: Cycloalkanes (CₙH₂ₙ) and alkenes (CₙH₂ₙ) have the same molecular formula pattern but very different chemistry. Both have DOU = 1, but cycloalkanes undergo SUBSTITUTION reactions (like alkanes) while alkenes undergo ADDITION reactions. This is the most commonly confused point on the SLMC exam.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Hydrocarbons — Comprehensive SLMC Medical (Sri Lanka) Notes

Reaction Mechanisms — Advanced

Electrophilic Addition to Alkenes

Mechanism: The π bond is a source of electrons → it attacks electrophiles.

1. The π electrons form a bond with the electrophile (E⁺) → a carbocation forms on the more substituted carbon (carbocation stability: tertiary > secondary > primary)
2. The nucleophile (Nu⁻) attacks the carbocation → the addition product is formed

Markovnikov’s Rule explained: The electrophile adds to the carbon that can form the more stable carbocation intermediate. More substituted carbocations are stabilized by hyperconjugation and inductive effects.

Carbocation rearrangements: If a more stable carbocation can form via hydride or methyl shift, rearrangement occurs — leading to unexpected products.

Free Radical Halogenation of Alkanes

Mechanism (homolytic fission):

Initiation: Cl₂ → 2Cl• (UV light) Propagation:

• CH₄ + Cl• → •CH₃ + HCl
• •CH₃ + Cl₂ → CH₃Cl + Cl• Termination:
• Cl• + Cl• → Cl₂
• •CH₃ + Cl• → CH₃Cl
• •CH₃ + •CH₃ → C₂H₆

Selectivity: Chlorination of propane gives 2-chloropropane (secondary) preferentially over 1-chloropropane (primary) because the secondary radical is more stable.

Polymerization

Addition polymerization: Monomers with C=C double bonds link together without loss of atoms. Examples: polyethene (PE), polyvinyl chloride (PVC), polystyrene (PS).

Condensation polymerization: Monomers link with loss of small molecules (e.g., water, HCl). Examples: nylon, terylene, Bakelite.

Conformations of Cyclohexane

Chair conformation: Most stable; all bond angles ~109.5°; alternating axial (up/down) and equatorial (angled) positions. Substituents prefer equatorial positions (less steric strain).

Boat conformation: Less stable; flagpole interactions between two “flagpole” hydrogens on the bow and stern.

Substituent effects on cyclohexane:

• Axial substituents experience 1,3-diaxial steric strain (three large interactions)
• Large substituents strongly prefer equatorial position

Aromatic Hydrocarbons (Arenes) — Brief Introduction

Benzene (C₆H₆) is the simplest aromatic hydrocarbon:

• Aromaticity criteria: Cyclic, planar, conjugated, (4n+2) π electrons (Hückel’s rule)
• Benzene: 6 π electrons (n=1) → highly stable
• Electrophilic substitution reactions: nitration, halogenation, sulfonation, Friedel-Crafts alkylation
• Does NOT undergo addition reactions easily (would lose aromaticity)

Common Mistakes in Hydrocarbon Problems

1. Confusing cycloalkanes with alkenes — same formula (CₙH₂ₙ) but different reactions
2. Applying Markovnikov’s rule to alkanes — alkanes don’t undergo addition reactions
3. Forgetting geometric isomerism rules — cis-trans requires each C of double bond to have two different groups attached
4. Not calculating DOU first — this single step prevents most errors in structure determination
5. Confusing Baeyer reagent (cold dilute KMnO₄) with hot KMnO₄ — cold gives diols, hot gives cleavage products

Practice Questions for SLMC

1. “A compound C₄H₈ does not decolorize bromine water but reacts with Cl₂ in UV light. What is it?” → Cyclobutane (not an alkene)
2. “Why is cyclopropane more reactive than cyclohexane?” → Angle strain (60° vs 109.5°)
3. “What is the product of addition of HBr to propene?” → 2-bromopropane (Markovnikov)
4. “Which alkene shows geometric isomerism: but-1-ene or but-2-ene?” → but-2-ene (both C=C carbons have two different groups)

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