Hydrocarbons – Study Notes (AIIMS Nursing Chemistry)
Hydrocarbons are the simplest organic compounds composed only of carbon (C) and hydrogen (H) atoms.
They form the structural backbone for all organic molecules and are classified mainly by the type of carbon–carbon bonds present.
1. Simple Explanation
| Category | Bond type | General Formula | Physical state at room temp. | Key features |
|---|---|---|---|---|
| Alkanes | Single σ bonds | CₙH₂ₙ₊₂ | Gases (CH₄, C₂H₆) to liquids (C₁₀H₂₂) | Saturated, only one H per C–C bond |
| Alkenes | One π bond | CₙH₂ₙ | Gases (C₂H₄) to liquids (C₆H₁₂) | Unsaturated, one double bond |
| Alkynes | One triple bond | CₙH₂ₙ₋₂ | Gases (C₂H₂) to liquids (C₆H₈) | Unsaturated, one triple bond |
| Aromatic Hydrocarbons | Delocalized π electrons | C₆ₙH₆ₙ | Gases (benzene) to liquids (toluene) | Ring with alternating double bonds (Hückel rule) |
- Saturation: An alkane is saturated (no double/triple bonds).
- Unsaturation: Alkenes & alkynes contain π bonds → more reactive.
- Aromaticity: Delocalized electrons give extra stability (e.g., benzene).
2. Key Points
| Topic | Details |
|---|---|
| Nomenclature | 1. Longest continuous carbon chain = parent name. 2. Substituents numbered from end giving the lowest set of locants. 3. Carbon chain lengths: methane, ethane, propane, butane, pentane, etc. 4. Alkene/alkyne prefixes: -ene, -yne. 5. Aromatics: benzene, toluene, xylene. |
| Physical Properties | • Melting/boiling points increase with chain length. • Alkanes are non‑polar ↓ density & solubility. • Alkenes/alkynes slightly more polarizable; show higher reactivity. |
| Reactivity | • Alkanes – only undergo free‑radical reactions (halogenation, combustion). • Alkenes – addition reactions (hydrogenation, halogenation, hydrohalogenation, hydration, oxidation). • Alkynes – similar to alkenes but with triple bond (more acidic H, more reactive). • Aromatic – substitution reactions (electrophilic aromatic substitution) preserve ring. |
| Combustion | General: CₙH₂ₙ₊₂ + (3n/2)O₂ → nCO₂ + (n+1)H₂O for alkanes. Complete combustion → CO₂ & H₂O. Incomplete → CO, soot. |
| Isomer Counting | For alkanes: n=1 → 1 isomer, n=2 → 1, n=3 → 1, n=4 → 2, n=5 → 3, ... Alkenes/alkynes have more isomers due to double/triple bond placement and stereochemistry. |
| Applications in Medicine | • Anesthetics: diethyl ether (alkane). • Solvents: benzene derivatives (toluene, xylene). • Drug precursors: many pharmaceuticals are derived from aromatic hydrocarbons. |
3. Worked Example
Question:
Identify the IUPAC name, molecular formula, and major product of the reaction of 2‑butene with bromine (Br₂) in the presence of UV light.
Step 1: Draw the structure of 2‑butene
H H
| |
CH₂=CH–CH₂–CH₃
- Longest chain = 4 carbons → butene.
- Double bond between C2–C3 → 2‑butene.
Step 2: IUPAC name
- The double bond is at carbon‑2 → 2‑butene (no stereochemistry indicated; thus cis or trans not specified).
Step 3: Reaction with Br₂ (free‑radical addition)
- Under UV light, Br₂ adds across the double bond.
- Each carbon of the double bond attaches a Br atom.
Br Br
\ /
CH₂–CH–CH₂–CH₃
| |
H H
Step 4: Product
- The product is 1,2‑dibromobutane (also called 1,2‑dibromobutane).
- Molecular formula: C₄H₈Br₂.
Answer Summary
| Item | Result |
|---|---|
| IUPAC name | 2‑butene |
| Molecular formula | C₄H₈ |
| Reaction product | 1,2‑dibromobutane (C₄H₈Br₂) |
4. Quick Revision Summary
- Alkanes:
CₙH₂ₙ₊₂, saturated, only radical reactions. - Alkenes:
CₙH₂ₙ, unsaturated, addition reactions. - Alkynes:
CₙH₂ₙ₋₂, unsaturated, more acidic H, addition reactions. - Aromatic:
C₆ₙH₆ₙ, ring with delocalized π electrons, electrophilic substitution. - Nomenclature: longest chain → parent name; substituents numbered for lowest locants; use -ene/-yne prefixes.