Question

Which compound in each of the following pairs would be more soluble in water? Explain. b. propanone or 3 -hexanone c. butane or butanal

Short answer

Propanone is more soluble than 3-hexanone; butanal is more soluble than butane.

Step-by-step solution

Understanding Solubility in Water

Solubility in water generally depends on the ability of a compound to form hydrogen bonds with water molecules. Compounds with polar groups are usually more soluble in water.

Comparing Propanone and 3-Hexanone

Propanone (acetone) has a carbonyl group (C=O) attached to a three-carbon chain. 3-Hexanone also has a carbonyl group but is attached to a longer, six-carbon chain. The shorter carbon chain in propanone makes it more polar and aids in better hydrogen bonding with water, making propanone more soluble in water than 3-hexanone.

Comparing Butane and Butanal

Butane is a non-polar hydrocarbon, whereas butanal has a polar carbonyl group (C=O). The presence of the polar carbonyl group in butanal allows it to form hydrogen bonds with water, increasing its solubility. Therefore, butanal is more soluble in water than butane.

Key concepts

Hydrogen Bonding

Hydrogen bonding plays a crucial role in the solubility of compounds in water. Water molecules can form hydrogen bonds with other molecules that have hydrogen atoms attached to highly electronegative atoms such as oxygen (O), nitrogen (N), or fluorine (F). These hydrogen bonds are strong intermolecular forces that greatly influence a compound's solubility.
When a compound can form hydrogen bonds with water, it tends to be more soluble. This is because the attraction between water molecules and the solute molecules helps to stabilize the solute in the water.
For example, in the exercise, propanone can form hydrogen bonds because of its carbonyl group. The oxygen in the carbonyl group can interact with the hydrogen atoms from water molecules, aiding in the dissolution process.

Polar Groups

Compounds with polar groups are generally more soluble in water. Polar groups have a partial positive and partial negative charge distribution which allows them to interact well with water molecules, which are also polar.
An example of a polar group is the carbonyl group (C=O). This group has a significant difference in electronegativity between the carbon and oxygen atoms. The oxygen atom is more electronegative and pulls electron density towards itself, creating a partial negative charge on the oxygen and a partial positive charge on the carbon.
In the case of the exercise, propanone, which has a shorter carbon chain compared to 3-hexanone, interacts better with water due to its polar carbonyl group. Similarly, butanal, with its carbonyl group, is more soluble in water than the non-polar butane.

Carbonyl Group

The carbonyl group, characterized by a carbon atom double-bonded to an oxygen atom (C=O), is highly influential in determining a compound's solubility in water. The carbonyl group is polar due to the difference in electronegativity between the carbon and oxygen atoms. This polarity allows the carbonyl group to engage in hydrogen bonding with water molecules.
In the given exercise, the carbonyl group in propanone helps it to be more water-soluble than 3-hexanone. The shorter chain in propanone makes it easier for the polar carbonyl group to interact with water molecules.
Likewise, butanal, which contains a carbonyl group, is more soluble in water than butane. The presence of the carbonyl group in butanal allows it to form hydrogen bonds with water, increasing its water solubility significantly.