Overview
This lecture explains intermolecular forces, focusing on dipole-dipole interactions and hydrogen bonding, and discusses their effects on boiling points and water solubility in various molecules.
Dipole-Dipole Interactions
- Dipole-dipole interactions occur between polar molecules with partial positive and negative charges.
- Example: In acetone, the partial negative oxygen of one molecule is attracted to the partial positive carbon of another.
- Carbon monoxide is another example, with attraction between the partial positive carbon and partial negative oxygen of adjacent molecules.
- Dipole-dipole forces are between different molecules, not within a single molecule.
Hydrogen Bonding
- Hydrogen bonding is a special, stronger type of dipole-dipole interaction.
- It occurs when hydrogen is bonded to nitrogen, oxygen, or fluorine.
- In water, the partial negative oxygen of one molecule attracts the partial positive hydrogen of another, forming hydrogen bonds.
- Hydrogen bonding raises both boiling point and water solubility of molecules.
Effects on Boiling Point and Solubility
- Ammonia and methanol, which have hydrogen bonds, exhibit higher boiling points and water solubility.
- Ethanol (has hydrogen bonds) has higher boiling point and water solubility than dimethyl ether (no hydrogen bonds), despite both being polar.
- The boiling point of ethanol is 78°C; dimethyl ether is -23°C due to lack of hydrogen bonding.
Chain Length and Branching
- Longer hydrocarbon chains increase boiling point due to more London dispersion forces.
- Increasing the nonpolar region (e.g., from ethanol to butanol to octanol) decreases water solubility but increases boiling point.
- Small-chain alcohols are highly water soluble; large nonpolar chains reduce solubility, as in octanol.
Structural Isomers and Boiling Point
- Straight-chain alkanes have higher boiling points than branched isomers due to greater surface area and more London dispersion forces.
- Example: Pentane (straight-chain) has a higher boiling point than neopentane (branched).
Nonpolar Molecules and Solubility
- Molecules made only of C-H bonds (like methane, ethane, propane) are nonpolar and do not mix with water.
Key Terms & Definitions
- Intermolecular forces — Forces or interactions between different molecules.
- Dipole-dipole interaction — Attraction between partial positive and negative charges in separate polar molecules.
- Hydrogen bonding — A strong dipole-dipole interaction when hydrogen is bonded to N, O, or F.
- London dispersion forces — Weak attractions due to temporary dipoles in molecules, stronger with larger molecules.
- Constitutional isomers — Molecules with same formula but different connectivity.
Action Items / Next Steps
- Review examples of polar and nonpolar molecules.
- Compare boiling points and solubility of various alcohols and ethers.
- Practice identifying possible hydrogen bonds in given molecular structures.