No, alcohols are generally less volatile than amines of comparable molecular weight. This means alcohols tend to have higher boiling points and evaporate more slowly than amines.
Understanding Volatility and Intermolecular Forces
Volatility refers to how easily a substance vaporizes, or turns into a gas. Substances with weaker intermolecular forces (IMFs) are more volatile because less energy is required to overcome these attractions and allow molecules to escape into the gas phase. Conversely, stronger IMFs lead to lower volatility and higher boiling points. The primary intermolecular forces that determine volatility for these organic compounds are:
- London Dispersion Forces (LDFs): Present in all molecules, increasing with molecular size and surface area.
- Dipole-Dipole Interactions: Occur between polar molecules due to the attraction of partial positive and negative charges.
- Hydrogen Bonding: A particularly strong type of dipole-dipole interaction involving hydrogen bonded to a highly electronegative atom (like oxygen, nitrogen, or fluorine) and an adjacent lone pair on another electronegative atom.
The Role of Hydrogen Bonding in Alcohols vs. Amines
Both alcohols (R-OH) and primary/secondary amines (R-NH₂, R₂NH) are capable of forming hydrogen bonds. However, the strength of these hydrogen bonds differs significantly, primarily due to the electronegativity of the oxygen versus nitrogen atoms:
- Alcohols (R-OH): Oxygen is more electronegative than nitrogen. This makes the O-H bond more polar, leading to a larger partial positive charge on the hydrogen and a larger partial negative charge on the oxygen. As a result, the hydrogen bonds formed between alcohol molecules are stronger.
- Amines (R-NH₂ / R₂NH): Nitrogen is less electronegative than oxygen, making the N-H bond less polar than an O-H bond. Consequently, the hydrogen bonds formed between amine molecules are weaker than those in alcohols. Tertiary amines (R₃N) cannot form hydrogen bonds with each other at all, as they lack a hydrogen atom directly bonded to nitrogen.
The stronger hydrogen bonding in alcohols requires more energy to break, thus increasing their boiling points and decreasing their volatility compared to amines of similar size.
Comparative Volatility and Boiling Points
This difference in intermolecular forces establishes a clear order when comparing the volatility of various organic functional groups. For compounds of similar molecular weight, volatility generally decreases (and boiling point increases) in the following order:
- Alkanes: (Only LDFs) - Most volatile
- Alkyl Halides: (LDFs, weak dipole-dipole)
- Amines: (LDFs, dipole-dipole, weaker hydrogen bonding)
- Alcohols: (LDFs, dipole-dipole, stronger hydrogen bonding) - Least volatile among these groups
- Carboxylic Acids: (Even stronger hydrogen bonding, often forming dimers) - Even lower volatility
Example:
Consider ethanol (an alcohol) and ethylamine (a primary amine), both with two carbon atoms:
- Ethanol (CH₃CH₂OH): Boiling point ≈ 78 °C
- Ethylamine (CH₃CH₂NH₂): Boiling point ≈ 17 °C
As seen, ethanol has a significantly higher boiling point, indicating lower volatility, despite having a similar molecular size to ethylamine.
Factors Influencing Volatility
Several factors collectively determine a compound's volatility:
- Intermolecular Forces: As discussed, stronger forces (like hydrogen bonding in alcohols) lead to lower volatility.
- Molecular Weight/Size: Larger molecules generally have stronger London Dispersion Forces, leading to lower volatility.
- Molecular Shape: More compact, spherical molecules tend to have weaker LDFs than long, linear molecules, making them more volatile (assuming similar molecular weight).
- Branching: Increased branching can reduce surface area for interaction, slightly decreasing LDFs and increasing volatility for a given molecular formula.
Table: Volatility Comparison
| Feature | Alcohols (R-OH) | Amines (R-NH₂, R₂NH) |
|---|---|---|
| Electronegativity of heteroatom | High (Oxygen) | Medium (Nitrogen) |
| Polarity of H-bond | Higher | Lower |
| Strength of Hydrogen Bonding | Stronger | Weaker |
| Energy to vaporize | More | Less |
| Volatility | Lower (Less volatile) | Higher (More volatile) |
| Boiling Point | Higher | Lower |
| Example | Ethanol (BP: 78°C) | Ethylamine (BP: 17°C) |
In summary, the enhanced strength of hydrogen bonding in alcohols, due to oxygen's greater electronegativity compared to nitrogen, makes alcohols less volatile than amines of comparable molecular size.
