Alkanes, Alkenes and Alkynes

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Alkanes — Saturated Hydrocarbons (C_nH_{2n+2}):

Alkanes contain only C-C and C-H single bonds (σ bonds only). They are chemically inert — the C-C bond is strong (347 kJ/mol) and non-polar, so alkanes don’t react with acids, bases, or most reagents under normal conditions. Their major reactions are combustion and free radical halogenation.

Structural isomerism: butane C₄H₁₀ has two isomers (n-butane and isobutane). Pentane C₅H₁₂ has three isomers. Branching lowers boiling point: n-pentane (36°C) > isopentane (28°C) > neopentane (9.5°C) because branching reduces surface area and hence London dispersion forces.

Alkenes — Unsaturated with C=C (C_nH_{2n}):

Alkenes contain at least one carbon-carbon double bond. The double bond consists of one strong σ bond and one weaker π bond (total bond energy less than two σ bonds). The double bond is the site of reactivity — addition reactions.

E/Z (geometric) isomerism: occurs when each carbon of the C=C has two different substituents. E (entgegen, German for “opposite”) = higher priority groups on opposite sides. Z (zusammen, “together”) = higher priority groups on same side. Example: 2-butene has E and Z isomers. This is not the same as cis/trans (which is a special case for identical substituents).

⚡ ECAT Tip: Markovnikov’s rule for addition to alkenes: in HX addition, the hydrogen attaches to the carbon with more hydrogens already (the less substituted carbon), and X attaches to the more substituted carbon. For HCl + propene: H adds to the terminal CH₂ (it has 2 H vs the middle carbon’s 1 H), giving 2-chloropropane (CH₃-CHCl-CH₃) as the major product. Anti-Markovnikov addition requires peroxides (HBr only, not HF, HCl, HI).

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Hybridisation — Bonding Geometry:

• sp³ (alkanes): tetrahedral geometry, 109.5° bond angles, four σ bonds from carbon
• sp² (alkenes): trigonal planar geometry, 120° bond angles, three σ bonds + one π bond from each doubly bonded carbon
• sp (alkynes): linear geometry, 180° bond angles, two σ bonds + two π bonds from the triple-bonded carbons

This orbital picture explains why alkynes are linear (the R-C≡C-R’ angle is 180°), why alkenes are planar (all atoms connected to the double bond lie in one plane), and why alkanes adopt tetrahedral geometry around each carbon.

Addition Reactions of Alkenes:

Catalytic hydrogenation: H₂ + alkene → alkane, catalysed by Pt, Pd, or Ni. Heat is released (exothermic: ~-120 kJ/mol). Vegetable oil (liquid, unsaturated) + H₂ → margarine (solid, saturated, more stable).

Bromine addition: Br₂ (red-brown) + alkene → dibromide (colourless). This is the classic test for unsaturation — if a compound decolourises bromine water, it contains a C=C. The mechanism involves a cyclic bromonium ion intermediate (Br⁺ attacks the π electrons, forming a 3-membered ring with the C=C carbons, then Br⁻ attacks from the back — anti addition).

Hydration: H₂O + alkene (acid-catalysed, H⁺) → alcohol. Markovnikov addition: H to the less substituted carbon, OH to the more substituted carbon. Alternative: oxymercuration-demethylation (Hg(OAc)₂ + H₂O, then NaBH₄) gives Markovnikov addition without rearrangement.

⚡ ECAT Tip: Polymerisation — alkenes undergo addition polymerisation where the double bond opens and links together. Polyethene (PE): n CH₂=CH₂ → [-CH₂-CH₂-]ₙ. Polypropene (PP): n CH₂=CH-CH₃ → [-CH₂-CH(CH₃)-]ₙ. Teflon: PTFE from tetrafluoroethene CF₂=CF₂. PVC (polyvinyl chloride) from chloroethene CH₂=CHCl. These are all addition polymers — no small molecules are eliminated.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Free Radical Halogenation of Alkanes — Selectivity:

Free radical halogenation proceeds in three stages: Initiation (Cl₂ → 2Cl• under UV), Propagation (Cl• + RH → HCl + R•, then R• + Cl₂ → RCl + Cl•), Termination (any two radicals combine).

Selectivity: For propane (CH₃-CH₂-CH₃), there are 6 equivalent primary H’s and 2 equivalent secondary H’s. Relative reactivity: secondary H is ~3.5× more reactive than primary H toward chlorination. Toward bromination, secondary H is ~80× more reactive than primary H. This means bromination is far more selective — it almost exclusively substitutes the most substituted available hydrogen.

This is because the C-H bond dissociation energies differ: primary BDE ~410 kJ/mol, secondary ~397 kJ/mol, tertiary ~389 kJ/mol. Weaker bonds form radicals more easily, and the resulting radicals are more stable (tertiary > secondary > primary). Radical stability follows the same order as carbocation stability in electrophilic additions.

Alkyne Chemistry — Linear and Acidic:

Alkynes have sp-hybridised carbons with two C≡C π bonds. Terminal alkynes (R-C≡CH) are uniquely acidic for hydrocarbons: pKa ~25 (ethane pKa ~51, ethene pKa ~44). This is because the negative charge in the acetylide anion (R-C≡C:⁻) is on an sp-hybridised carbon with 50% s-character (more s = more stable anion). This acidity allows terminal alkynes to be deprotonated by strong bases (Na, NaNH₂) to form acetylide salts, which are useful nucleophiles for forming new C-C bonds.

Conjugation and Dienes:

Conjugated dienes (alternating double and single bonds: CH₂=CH-CH=CH₂) have special stability due to delocalisation of π electrons (resonance). 1,3-butadiene is the simplest conjugated diene. Electrophilic addition to conjugated dienes can give 1,2-addition (across the first double bond) or 1,4-addition (across the conjugated system, which leaves a double bond in the middle — giving a more substituted alkene). The major product depends on conditions: at low temperature, kinetic 1,2-addition dominates; at high temperature, thermodynamic 1,4-addition dominates.

⚡ ECAT Pattern: ECAT frequently tests: (1) orbital hybridisation and geometry (sp³, sp², sp) and bond angles; (2) Markovnikov’s rule with actual compound names; (3) distinguishing alkanes from alkenes using bromine water test; (4) naming geometric isomers (E/Z) for alkenes; (5) the fact that acetylene (ethyne) burns with a sooty flame (high carbon content, incomplete combustion: 2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O, but the flame temperature causes partial pyrolysis). A common ECAT question: “Write the product(s) of 2-methylpropene + HBr in the presence of peroxides.” Answer: In peroxides, the mechanism is anti-Markovnikov (bromine adds to the less substituted carbon via radical addition). Peroxides oxidise HBr to Br• which initiates radical chain. The H adds to the terminal carbon (less substituted, more H’s), Br adds to the internal carbon (more substituted): product is 1-bromo-2-methylpropane.