The presence of which functional group generally increases a compound's solubility in water?

The presence of a hydroxyl group is known to significantly increase a compound's solubility in water due to its ability to form hydrogen bonds with water molecules. The hydroxyl group contains an oxygen atom bonded to a hydrogen atom, which creates a partial negative charge on the oxygen and a partial positive charge on the hydrogen. This polarity allows molecules with hydroxyl groups to interact favorably with the polar nature of water, facilitating greater solubility.

In contrast, groups like the alkyl group are primarily hydrophobic and do not engage in hydrogen bonding, thereby decreasing overall solubility in water. Aromatic groups, while they may interact with water due to pi stacking or other interactions, do not possess the same strong polar characteristics as hydroxyl groups and therefore generally have lower solubility. The carbonyl group can enhance solubility in some cases, but its effectiveness is often less pronounced than that of the hydroxyl group, especially when considering multiple hydroxyl groups that can cumulatively increase solubility. Thus, the hydroxyl group plays a crucial role in promoting water solubility through its hydrogen-bonding capabilities.