Answer
🔬 Boiling Points: Structural Influences and Comparison
1. What Determines a Boiling Point?
The boiling point is the temperature at which a substance transitions from the liquid to the gas phase. Key factors include:
- Molecular weight: Higher mass often increases boiling point
- Surface area: More surface allows stronger van der Waals forces
- Branching: More branching reduces surface contact and lowers boiling point
- Hydrogen bonding: Strongest intermolecular force; significantly raises boiling point
2. Straight-Chain vs. Branched Alkanes
Straight-chain alkanes exhibit higher boiling points than branched ones due to greater surface area for intermolecular interactions.
Boiling point trend: Heptane > 3-methylhexane > 2,2-dimethylpentane
Straight-chain alkanes → Higher BP
Branched alkanes → Lower BP due to compact shape
Branched alkanes → Lower BP due to compact shape
3. Hydrogen Bonding in Alcohols
Hydrogen bonding significantly raises boiling points in alcohols.
- 1,2,3-Propanetriol (glycerol): 3 –OH groups → Highest boiling point
- 1,2-Ethanediol (ethylene glycol): 2 –OH groups → High boiling point
- 1-Pentanol: 1 –OH group and long hydrophobic chain → Lower boiling point
Boiling point trend: Glycerol > Ethylene glycol > 1-Pentanol
4. Summary Table: Boiling Point Trends
| Molecule Type | Structure | Main Factor | Boiling Point |
|---|---|---|---|
| Straight-chain alkane | Heptane | High surface area | Highest among alkanes |
| Branched alkane | 2,2-Dimethylpentane | Low surface area | Lowest among alkanes |
| Trihydroxy alcohol | Glycerol | 3 hydrogen bonds | Very high |
| Monohydroxy alcohol | 1-Pentanol | 1 hydrogen bond | Lower |
✅ Final Takeaways
- Hydrogen bonding has the greatest impact on boiling point.
- Straight-chain molecules boil at higher temperatures than branched ones.
- The number and position of –OH groups strongly affect alcohol boiling points.