Comparing the solubility of organic compounds primarily involves understanding the principle of "like dissolves like," which relates the polarity of the solute to the polarity of the solvent.
Key Factors Influencing Organic Compound Solubility
The solubility of an organic compound in a given solvent is determined by the strength of the intermolecular forces between the solute and solvent molecules compared to the forces between solute-solute and solvent-solvent molecules.
Polarity and the "Like Dissolves Like" Rule
- Polar Solvents (like water, ethanol, methanol, acetone): These solvents have significant dipole moments and can form hydrogen bonds (like water and alcohols). They tend to dissolve polar organic compounds.
- Nonpolar Solvents (like hexane, benzene, diethyl ether, carbon tetrachloride): These solvents have little to no dipole moment. They tend to dissolve nonpolar organic compounds.
Highly polar organic compounds, such as those with hydroxyl (-OH) or carboxyl (-COOH) groups, tend to be soluble in polar solvents like water. Nonpolar organic compounds, such as hydrocarbons, tend to be soluble in nonpolar solvents.
Role of Nonpolar Tail Length
Many organic molecules contain both polar and nonpolar parts. Alcohols, for example, have a polar -OH group and a nonpolar hydrocarbon chain (the "tail"). The balance between these parts significantly impacts solubility, especially in polar solvents like water.
As noted in the reference: "So because this molecule here it has such a big nonpolar tail it's going to be less soluble in water. This one the non-polar cell is smaller so it's going to have a higher solubility in water."
This means:
- For molecules with a polar functional group, increasing the size of the nonpolar hydrocarbon tail decreases solubility in water.
- Conversely, decreasing the size of the nonpolar tail increases solubility in water.
This is because a large nonpolar tail can overwhelm the effect of the polar group, making the overall molecule behave more nonpolar and thus less able to interact favorably with polar water molecules.
Other Factors to Consider
Beyond polarity and nonpolar tail size, other structural features influence solubility:
- Functional Groups: The type and number of polar functional groups (e.g., -OH, -NH₂, -COOH, -C=O) strongly influence solubility in polar solvents. More polar groups generally mean higher solubility in polar solvents.
- Molecular Size: For a given type of compound, solubility generally decreases as molecular size increases. Larger molecules have stronger van der Waals forces (solute-solute interactions) which are harder for the solvent to overcome.
- Branching: For molecules of similar size, branched isomers are often more soluble than straight-chain isomers because they tend to be less effective at packing tightly, resulting in weaker intermolecular forces.
- Temperature: Solubility of most solids and liquids increases with temperature, although there are exceptions. The solubility of gases in liquids generally decreases with increasing temperature.
Practical Comparison Example
Consider the solubility of simple alcohols in water:
| Alcohol | Formula | Nonpolar Tail Size | Water Solubility (at 25°C) |
|---|---|---|---|
| Methanol | CH₃OH | Small (1 C) | Miscible |
| Ethanol | CH₃CH₂OH | Small (2 C) | Miscible |
| Propanol | CH₃CH₂CH₂OH | Medium (3 C) | Miscible |
| Butanol | CH₃CH₂CH₂CH₂OH | Medium (4 C) | ~8 g/100 mL |
| Pentanol | CH₃(CH₂)₃CH₂OH | Large (5 C) | ~2.2 g/100 mL |
| Hexanol | CH₃(CH₂)₄CH₂OH | Larger (6 C) | ~0.6 g/100 mL |
| Heptanol | CH₃(CH₂)₅CH₂OH | Even Larger (7 C) | ~0.09 g/100 mL |
As the length of the nonpolar hydrocarbon tail increases, the solubility in water decreases significantly, demonstrating the principle mentioned in the reference.
To compare the solubility of two organic compounds, you would assess their respective polar and nonpolar characteristics and the characteristics of the solvent, predicting that the compound with polarity most similar to the solvent will be more soluble.
