Hydrocarbons – Alkanes, Alkenes & Alkynes
(Advanced University‑Level Treatment – MIT‑style)
Goal – Build a deep, first‑principles understanding of the structure, bonding, and reactivity of saturated and unsaturated hydrocarbons. The notes are organized to progress from foundational concepts to sophisticated mechanistic insight, culminating in a challenging problem that tests the integration of multiple concepts.
1. Introduction
| Class | General Formula | Hybridization | Bonding | Key Distinguishing Feature |
|---|---|---|---|---|
| Alkanes | (C_nH_{2n+2}) | (sp^3) | 4 σ bonds per C | Saturated, no multiple bonds |
| Alkenes | (C_nH_{2n}) | (sp^2) | 1 σ + 1 π per C | One C=C double bond |
| Alkynes | (C_nH_{2n-2}) | (sp) | 2 σ + 2 π per C | One C≡C triple bond |
Why “hydrocarbons” matter – They represent the backbone of organic chemistry. Understanding their electronic structure underpins reactivity, stereochemistry, and synthetic strategy.
2. Alkanes
2.1 Structural & Electronic Foundations
- Hybrid Orbitals
[ \begin{aligned} \text{sp}^3 &= \frac{1}{2},\text{3s} + \frac{\sqrt{3}}{2},\text{3p}_z \ \text{σ bond} &= \text{sp}^3 , \text{on C} ;; \text{overlap with sp}^3 ; \text{on H or C} \end{aligned} ] - Bond Angles – Ideal tetrahedral: (109.5^\circ). Deviations arise from steric strain (e.g., in cycloalkanes).
- Bond Energies – C–C: ~347 kJ mol⁻¹; C–H: ~413 kJ mol⁻¹. Total heat of formation increases with chain length due to more C–C bonds.
2.2 Physical & Thermodynamic Properties
| Property | Trend with (n) | Explanation |
|---|---|---|
| Boiling point | ↑ | Larger surface area → stronger London dispersion |
| Density | Slight ↑ | Increasing mass per volume |
| Heat of combustion | ↑ | More C–H bonds → more energy released |
2.3 Reactivity: Radical Chemistry
- Initiation
[ \text{Bromine} \xrightarrow{h\nu} 2,\cdot\text{Br} ] - Propagation
[ \cdot\text{Br} + \text{RH} \rightarrow \text{R}\cdot + \text{HBr} ] [ \text{R}\cdot + \text{Br}_2 \rightarrow \text{RBr} + \cdot\text{Br} ] - Termination
[ \cdot\text{R} + \cdot\text{R} \rightarrow \text{R–R} ] [ \cdot\text{Br} + \cdot\text{Br} \rightarrow \text{Br}_2 ]
Key Insight – Radical chain reactions are driven