Alkenes & Alkynes
Advanced University‑Level Study Notes
(IIT‑JEE Main + Advanced – Chemistry)
Disclaimer – These notes are meant for students who already possess a solid foundation in general chemistry, physical organic chemistry, and a first course in quantum chemistry. They are written to mimic the depth and rigor expected at a MIT‑level undergraduate or early graduate course.
1. Introduction
| Property | Alkene | Alkyne |
|---|---|---|
| General formula | CₙH₂ₙ (≥ 2) | CₙH₂ₙ₋₂ (≥ 2) |
| Unsaturation | One C=C double bond | One C≡C triple bond |
| Minimum number of substituents on sp²/sp¹ atoms | 2 | 2 |
| Hybridization of the unsaturated carbon | sp² | sp |
Alkenes and alkynes are the simplest unsaturated hydrocarbons. Their π‑bonding gives rise to a host of unique chemical properties and reactivities that form the backbone of organic synthesis and material science.
2. First‑Principles Description of π‑Bonding
2.1. Hybridization and σ‑Bonding
-
Alkenes: Each carbon of the C=C bond forms three σ bonds. By hybridizing one s and two p orbitals we get sp² hybrids oriented 120° apart in a trigonal‑planar arrangement. The remaining p orbital is orthogonal to the plane and participates in π‑bonding.
-
Alkynes: Each carbon of the C≡C bond forms two σ bonds. Hybridizing one s and one p orbital yields sp hybrids aligned linearly (180° apart). Two orthogonal p orbitals remain for π‑bonding.
2.2. Molecular Orbital (MO) Construction
2.2.1. Alkene (C₂H₄) – π System
Take two p orbitals (one on each carbon) and form symmetric (bonding, π) and antisymmetric (antibonding, π*) combinations:
π* (antibonding)
|
p (C1)---p (C2) ← π (bonding)
- Bond order: 1 (one σ, one π).
- Stability: The π bond is ~65 kcal mol⁻¹ stronger than a σ bond of similar length.
2.2.2. Alkyne (C₂H₂) – Two π Systems
Two orthogonal p orbitals on each carbon give rise to two independent π systems:
π* (antibonding) π* (antibonding)
| |
p (C1)---p (C2) p (C1)---p (C2) ← π (bonding) π (bonding)
- Bond order: 3 (two π bonds + one σ).
- Electron density: Higher at the unsaturated carbons (δ⁺) and lower at the hydrogens (δ⁻).
2.3. Frontier Molecular Orbital (FMO) Theory
- Highest Occupied Molecular Orbital (HOMO): For alkenes, the π orbital; for alkynes, the π orbital that is higher in energy due to the second π* orbital lying very high.
- Lowest Unoccupied Molecular Orbital (LUMO): For alkenes, the π* orbital; for alkynes, the π* orbital of the higher π system.
These orbitals govern the reactivity in addition reactions (HOMO–LUMO overlap) and pericyclic reactions (orbital symmetry).
3. Physical Properties
| Property | Alkenes | Alkynes |
|---|---|---|
| Boiling point | Lower than alkanes of comparable MW (γ‑bond less polar) | Slightly higher than alkenes (triple bond length ~1.20 Å) |
| Refractive index | 1.4–1.5 | 1.45–1.6 |
| UV–Vis absorption | 190–210 nm (π→π*) | 190–210 nm (π→π*) but broader |
| Polarizability | Higher than alkanes due to π electrons | Higher still due to two π systems |
4. Stereochemistry
4.1. Alkenes
- E/Z (or R/S): Determined by CIP priorities on each double‑bonded carbon.
- cis/trans: For disubstituted alkenes, cis (same side) vs trans (opposite side).
- Chiral centers: A double bond can be adjacent to a stereogenic center; elimination or addition reactions may either retain or invert configuration.
4.2. Alkynes
- Linear: sp hybridization → no stereochemistry at the triple bond itself.
- Cis/Trans: Only possible in cyclic alkynes (e.g., cyclooctyne) or in systems where substituents are on the same carbon (e.g., 1,2‑diyl–disubstituted alkynes).
- Conjugated systems: In allene (C=C=C) the central carbon is sp, the two terminal carbons are sp²; the π systems are orthogonal, giving rise to axial chirality when substituted asymmetrically.