Melting and Boiling Points, Volatility, and Solubility of Organic Compounds
How Molecular Size Affects Boiling Points
- Boiling occurs when molecules in a liquid gain enough energy to overcome the forces holding them together.
- For organic compounds, these forces are primarily intermolecular forces, such as London dispersion forces, dipole-dipole interactions, and hydrogen bonding.
- As molecular size increases, such as in a homologous series like the alkanes, boiling points also rise.
- Why? Larger molecules have more electrons, which increases the strength of Van der Waals forces.
- These forces result from temporary dipoles created by the random movement of electrons.
Consider the boiling points of the first few alkanes:
- Methane (CH₄): Boiling point = -161°C
- Ethane (C₂H₆): Boiling point = -89°C
- Propane (C₃H₈): Boiling point = -42°C
As the carbon chain length increases, the boiling point rises because the larger molecules experience stronger dispersion forces.
Effect of Branching on Physical Trends in Homologous Series
Branching significantly influences the physical properties of compounds in a homologous series:
- Boiling Point:
- Branching reduces the surface area available for intermolecular forces, weakening Van der Waals forces and lowering boiling points compared to straight-chain isomers.
- Melting Point:
- Branching disrupts regular crystal packing, often lowering melting points.
- However, highly symmetrical branching can enhance packing efficiency, leading to higher melting points.
The Effect of Functional Groups on Boiling Points
Functional groups can significantly impact boiling points by introducing polar interactions or hydrogen bonding, both of which strengthen intermolecular forces.
- Hydroxyl Groups (-OH):
- Alcohols, which contain hydroxyl groups, exhibit hydrogen bonding—a particularly strong type of dipole-dipole interaction.
- This raises their boiling points compared to alkanes of similar molecular size.
- Carbonyl Groups (C=O):
- Aldehydes and ketones, with their polar carbonyl groups, experience dipole-dipole interactions.
- These are weaker than hydrogen bonds but still stronger than dispersion forces.
Ethanol ($C_2H_OH$) has a boiling point of 78°C, while ethane ($C_2H_6$) boils at -89°C.
Hydrogen bonding occurs when a hydrogen atom is directly bonded to a highly electronegative atom (N, O, or F) and interacts with a lone pair on another electronegative atom.
- Students sometimes confuse hydrogen bonding with dipole-dipole interactions.
- Remember, hydrogen bonding is a specific, stronger type of dipole-dipole interaction.
Volatility and Solubility: Smaller Molecules and Polar Functional Groups
Volatility
Volatility
Volatility describes how easily a substance evaporates.
Substances with lower boiling points are more volatile because their molecules require less energy to escape into the gas phase.
- Smaller Molecules: Smaller organic molecules generally have weaker intermolecular forces, making them more volatile.
- Functional Groups and Polarity: Polar functional groups, such as hydroxyl (-OH) or carbonyl (C=O), can reduce volatility by increasing intermolecular attractions.
- Methanol ($CH_3OH$) is more volatile than ethanol ($C_2H_5OH$) because it has fewer dispersion forces.
- Propanone (acetone, $CH_3COCH_3$) is less volatile than propane ($C_3H_8$) because of its polar carbonyl group.
- Volatility decreases as the strength of intermolecular forces increases.
- Substances with hydrogen bonding are generally less volatile than those with only dispersion forces.
Solubility in Water
- Water, a highly polar solvent, dissolves polar and hydrogen-bonding substances well.
- This principle can be summarized as: "like dissolves like."
- Polar Functional Groups: Molecules with polar functional groups (e.g., hydroxyl, carbonyl, or amino groups) can form hydrogen bonds with water, making them soluble.
- Nonpolar Molecules: Nonpolar molecules, such as hydrocarbons, are generally insoluble in water because they cannot form significant interactions with water molecules.
- Effect of Chain Length: As the carbon chain length increases, the molecule's nonpolar character dominates, reducing its solubility in water.
- Ethanol ($C_2H_5OH$) is highly soluble in water because its -OH group forms hydrogen bonds with water molecules.
- Hexane ($C_6H_{14}$) is immiscible in water.
- Methanol ($CH_3OH$) is completely miscible in water, while hexanol ($C_6H_{13}OH$) is only sparingly soluble.
What happens to the solubility of an alcohol as its carbon chain length increases? Why?
Summary of Trends
Here’s a summary of how molecular size, functional groups, and intermolecular forces influence the properties of organic compounds:
| Property | Key Factors |
|---|---|
| Boiling Point | Increases with molecular size due to stronger London dispersion forces. Functional groups like -OH and C=O introduce hydrogen bonding or polarity, further raising boiling points. |
| Volatility | Decreases with stronger intermolecular forces, such as hydrogen bonding. |
| Solubility in Water | Favored by polar functional groups (e.g., -OH, C=O), but decreases with increasing chain length as the non-polar hydrocarbon portion dominates. |
