Why does water solubility of monofunctional, polar organic compounds rapidly diminish above four or five carbons?
Because the R part of the molecule (for example, ROH for alcohols) becomes large enough that the compound resembles a hydrocarbon more than it does the soluble functional group.
The water solubility of monofunctional, polar organic compounds rapidly diminishes above four or five carbons due to several factors. One of the primary reasons is the increase in hydrophobicity, or the tendency to repel or be insoluble in water, as the carbon chain lengthens.
Water is a polar solvent, meaning it has a slight positive charge on the hydrogen atoms and a slight negative charge on the oxygen atom. This polarity allows water molecules to interact with other polar molecules, forming hydrogen bonds. However, organic compounds with longer carbon chains tend to be less polar and have fewer functional groups that allow for hydrogen bonding.
As the carbon chain gets longer, the hydrophobic interactions between the nonpolar parts of the organic compounds become more significant. The nonpolar regions of the compound tend to aggregate together, minimizing their contact with water molecules. This aggregation reduces the solubility of the compound in water.
In addition to hydrophobic interactions, the increased molecular size and complexity of longer carbon chains also play a role in the diminishing water solubility. The larger size makes it more challenging for the compound to fit into the organized network of water molecules, further decreasing its solubility.
To determine the water solubility of a monofunctional, polar organic compound, it is crucial to consider its molecular structure, functional groups, and the length of its carbon chain. Experimental techniques, such as measuring the compound's partition coefficient or using solubility prediction models, can provide more accurate estimations of its solubility in water.