Question

Alcohols are more soluble in water in comparison to hydrocarbons of comparable molecular weight. Explain.

Hint:

Alcohols are more soluble in water than hydrocarbons due to hydrogen bonding between the -OH group and water molecules. The larger the non-polar hydrophobic chain, the lower the solubility, but alcohols are still more soluble than hydrocarbons of comparable molecular weight.

Answer 1

Alcohols are more soluble in water compared to hydrocarbons of similar molecular weight due to the presence of the hydroxyl group (-OH), which allows alcohols to form hydrogen bonds with water molecules. The following points explain this in detail:
1. Hydrogen Bonding:
Alcohols contain a hydroxyl group (-OH), which can form hydrogen bonds with water molecules. Water is a polar solvent, and the hydroxyl group in alcohols is also polar, making it capable of forming hydrogen bonds. These interactions increase the solubility of alcohols in water. On the other hand, hydrocarbons are non-polar and cannot form hydrogen bonds with water, making them much less soluble.
2. Polarity:
The hydroxyl group in alcohols makes them polar, while hydrocarbons are non-polar. Water is a polar solvent, and polar substances tend to dissolve more easily in polar solvents. This is the main reason why alcohols are more soluble in water than hydrocarbons of similar molecular weight.
3. Hydrophilic and Hydrophobic Parts:
In alcohols, the -OH group is hydrophilic (water-loving) and can interact with water molecules, whereas the hydrocarbon chain is hydrophobic (water-repelling). However, the hydrophilic nature of the hydroxyl group overcomes the hydrophobic nature of the hydrocarbon part, making alcohols more soluble than hydrocarbons in water.
4. Size of the Alcohol:
The solubility of alcohols in water decreases with increasing size of the hydrophobic hydrocarbon chain. Larger alcohols, such as octanol, are less soluble in water compared to smaller alcohols, like methanol or ethanol. However, alcohols are still more soluble than hydrocarbons of comparable molecular weight due to the presence of the -OH group.
5. Hydrogen Bond Formation:
In water, the hydroxyl group of alcohols can form hydrogen bonds not only with other alcohol molecules but also with water molecules. This significantly increases the solubility of alcohols. Hydrocarbons, lacking the ability to form hydrogen bonds with water, are much less soluble in water. Thus, alcohols are more soluble in water because of their ability to form hydrogen bonds with water molecules, whereas hydrocarbons lack this ability.