Question

1) Compare the solubility in water of similar size compounds like pentane, 1-propanol, and propanone. Which functional group(s) allow the compound to be soluble in water: alcohols, alkanes, and/or ketones? What do the functional groups of the more soluble compounds have in common? (What IMFs do they have with water that the insoluble function group doesn’t have)

Answer 1

Some Organic compounds dissolve in water and some are not. There are some reasons which will decide the solubility of organic compounds in water.

Requirements to dissolve in water:-

Hydrogen bonds:-

When there are hydrogen bonds ( F-H, O-H, N-H, Cl-H bonds can make hydrogen bonds ) between organic compound and water molecules, the ability to dissolve in water is high. Organic compounds such as alcohols, phenol, aldehyde, ketone, carboxylic acids, amines and more can make hydrogen bonds.
But remember that, there are organic compounds which are insoluble in water and they can make hydrogen bonds too such as phenol.

Molecular mass

If organic compounds molecular mass is small, they have a higher possibility to dissolve in water.

Size of the alkyl group

C-H bond is not enough polarized to make hydrogen bond because the difference of electronegativity between carbon and hydrogen is small. Therefore the C-H bond is non-polar. So hydrocarbon part of the molecule does not help to dissolve in water.
When hydrocarbon part of the molecule becomes simple or small, it also increases the solubility. This means when a non-polar part is so large than polar part, solubility decreases.

Hydrocarbons:-

Alkane, alkene and alkyne belong to the hydrocarbon type. In hydrocarbons, there are only carbon-hydrogen bonds in the organic compound. So polarization is weak to make strong intermolecular forces between water molecules. Therefore, alkane, alkene and alkyne are not soluble in water.

Alcohols:-

Methanol, ethanol dissolves very well in water. Propanol dissolves a little bit. Butanol is completely insoluble in water. When the alkyl group gets longer or larger, the solubility of alcohols decreases.

Aldehyde and Ketone

Aldehyde and Ketone which have less molecular mass will dissolve in water. Formaldehyde, Ethanal, butanal are soluble aldehydes. Benzaldehyde is insoluble in water. Acetone (propanone), butanone are soluble ketones.

So Order of Solubility in the water of given compounds:-

1-propanol > Propanone > Pentane.

Polarity Ranking of the Functional Groups:-

(most polar first)

Amide > Acid > Alcohol > Ketone ~ Aldehyde > Amine > Ester > Ether > Alkane.

Alcohol is more polar than ketone so 1-propanol is more soluble than propanone in water.

Alcohol and Ketone functional groups allow the compound to be soluble in water.

The more soluble functional groups have a polarized bond (F-H, O-H, N-H, Cl-H etc) which will make Hydrogen Bonding to water. So In case of Alcohol Group, there is one O-H bond which is responsible for making a Hydrogen bond with water.