Chapter 17: Problem 117

Hydrocarbons are insoluble in water, but some alcohols, such as methanol, \(\mathrm{CH}_{3} \mathrm{OH}\), dissolve readily in water. Why?

Short Answer

Expert verified

Hydrocarbons are insoluble in water because they are non-polar molecules, which cannot form hydrogen bonds with polar water molecules. On the other hand, methanol and other alcohols have polar hydroxyl groups (-OH), allowing them to form hydrogen bonds with water, resulting in their solubility in water.

Step by step solution

Understanding the polarity of water

Water is a polar molecule, meaning that it has a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atom. This is due to the difference in electronegativity between hydrogen and oxygen, which causes an uneven distribution of electron density in the molecule. The polar nature of water enables it to form hydrogen bonds with other polar or charged molecules, and this is an important factor in its interactions with other substances.

Hydrocarbons' non-polar nature

Hydrocarbons are organic molecules composed of carbon and hydrogen atoms. They have a non-polar nature because the electronegativity difference between carbon and hydrogen is relatively small, resulting in an even distribution of electron density in the molecule. Non-polar molecules like hydrocarbons cannot form hydrogen bonds with water, which is a key reason for their insolubility in water.

Methanol's polar nature

Methanol (\(\mathrm{CH}_3\mathrm{OH}\)) is an alcohol, an organic molecule with a hydroxyl group (-OH) attached to a carbon atom. The oxygen atom in the hydroxyl group is more electronegative than both carbon and hydrogen atoms, resulting in a polar molecule with a partial negative charge on the oxygen atom and a partial positive charge on the hydrogen atom of the hydroxyl group. This polarity allows methanol to interact with water through hydrogen bonding.

Hydrogen bonding between methanol and water

Due to the presence of the polar hydroxyl group, methanol can form hydrogen bonds with water. When methanol is added to water, the partially positive hydrogen atom of the hydroxyl group in methanol attracts the partially negative oxygen atom in water, and vice versa. This results in the formation of hydrogen bonds between methanol and water molecules, allowing them to mix and leading to the solubility of methanol in water. In conclusion, hydrocarbons are insoluble in water due to their non-polar nature, while methanol and other alcohols with polar hydroxyl groups can form hydrogen bonds with water, making them soluble.

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Hydrocarbons are insoluble in water, but some alcohols, such as methanol, \(\mathrm{CH}_{3} \mathrm{OH}\), dissolve readily in water. Why?

Short Answer

Step by step solution

Understanding the polarity of water

Hydrocarbons' non-polar nature

Methanol's polar nature

Hydrogen bonding between methanol and water

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