Least Soluble in Water

Methanal, also known as formaldehyde, is a colorless gas with a pungent odor. It is the simplest aldehyde and is highly soluble in water due to its ability to form hydrogen bonds with water molecules. Methanal molecules can form hydrogen bonds with water molecules through their oxygen atom and also engage in dipole-dipole interactions with water molecules. Therefore, methanal is relatively soluble in water.

Ethanol, also known as ethyl alcohol, is a clear, colorless liquid with a characteristic odor. It is a primary alcohol and is miscible with water, meaning it can mix completely with water in all proportions. Ethanol molecules can form hydrogen bonds with water molecules through their hydroxyl (-OH) group, leading to its high solubility in water.

Propanone, also known as acetone, is a colorless liquid with a sweet, fruity odor. It is a ketone and is also miscible with water. Propanone molecules can form hydrogen bonds with water molecules through their oxygen atom, contributing to its solubility in water.

Pentanal, also known as valeraldehyde, is an aldehyde with a strong, unpleasant odor. It is a five-carbon aldehyde and has a lower solubility in water compared to the other options. The solubility of aldehydes decreases with increasing carbon chain length, as the hydrophobic nature of the hydrocarbon chain outweighs the ability to form hydrogen bonds with water molecules. Therefore, pentanal is the least soluble in water among the given options.

In summary, the least soluble compound in water among methanal, ethanol, propanone, and pentanal is pentanal.