Question

Explain why \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) is insoluble in water, but \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) is soluble.

Short answer

\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (propane) is insoluble in water because it is nonpolar and cannot form hydrogen bonds with water. On the other hand, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (ethanol) is soluble because it contains a polar hydroxyl (-OH) group, which allows it to form hydrogen bonds with water molecules.

Step-by-step solution

Analyze the structure of \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\)

This compound is propane, which is a nonpolar molecule. Nonpolar molecules are 'hydrophobic', or 'water fearing'. This is because they lack polar bonds, and thus cannot form hydrogen bonds with water molecules.

Determine the Interaction of Propane with Water

Because propane is a nonpolar molecule, it isn't capable of forming hydrogen bonds with water. As a result, there is no interaction that is strong enough to break the existing hydrogen bonds of water molecules. Therefore, propane is insoluble in water.

Analyze the Structure of \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\)

This compound is ethanol. Unlike propane, ethanol contains a hydroxyl group (-OH) which is polar in nature. Polar molecules are 'hydrophilic', or 'water-loving'.

Determine the Interaction of Ethanol with Water

The hydroxyl group in ethanol can form hydrogen bonds with water. This interaction is strong enough to break the existing hydrogen bonds of water molecules, allowing for ethanol to dissolve. Therefore, ethanol is soluble in water.

Key concepts

Polar and Nonpolar Molecules

In chemistry, molecules can be classified as either polar or nonpolar based on their chemical structure and the distribution of electrical charges within them. A polar molecule has areas of positive and negative charge, creating an electric dipole or a directional electric field within the molecule.

• Polar molecules can dissolve in polar solvents like water because similar charges attract.
• Nonpolar molecules do not have these charge differences and tend to repel or separate from polar solvents, making them 'hydrophobic'.

Propane (\[\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\]) is a classic example of a nonpolar molecule. It is made up of only carbon and hydrogen atoms, which share electrons almost equally. Because propane does not have a zone of differing charges, it cannot form attractions, such as hydrogen bonds, with water molecules, hence it is insoluble in water.
On the other hand, ethanol (\[\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\]) has a polar hydroxyl group (-OH) at one end, creating an area of negative charge which attracts the positive parts of a water molecule. This critical difference allows ethanol to dissolve in water, showcasing how polarity significantly influences solubility.

Hydrogen Bonding

Hydrogen bonding is a unique type of interaction that plays a very important role in determining the solubility of substances. It occurs when a hydrogen atom, that is covalently bonded to an electronegative atom like oxygen or nitrogen, is attracted to another electronegative atom.
This bond is not as strong as a covalent bond but is stronger than van der Waals forces and contributes significantly to the properties of substances. It's a primary reason why water has its high boiling point and the ability to act as a 'universal solvent'.

• Sufficient hydrogen bonding can lead to the solubility of polar substances in water.
• The presence of hydrogen bonding increases the attraction between molecules, disrupting the formation of tightly packed molecular arrangements like those in solids.

For example, ethanol is soluble in water primarily due to its ability to form hydrogen bonds via its -OH group. The negative charge of the oxygen in ethanol's hydroxyl group can form a hydrogen bond with water's hydrogen, thus integrating ethanol molecules into the mass of water, allowing it to dissolve.

Organic Chemistry

Organic chemistry is the field of science that is concerned with the study of carbon-containing compounds. These include simple molecules like methane as well as complex ones like DNA and proteins. The classification into polar and nonpolar plays a significant role in understanding organic reactions and interactions.

• Most organic molecules are based on carbon, which can form four bonds allowing for diverse and complex molecules.
• Functional groups like the hydroxyl group (-OH) often determine the properties and reactivity of organic molecules.

In our case of comparing propane and ethanol, both being organic molecules, their solubility in water is contrasting due to their structural differences. Propane is a simple alkane with no functional groups that could interact with water leading to its insolubility. Meanwhile, ethanol contains a polar functional group that enhances its interaction with water molecules.
This highlights the importance of functional groups in organic chemistry, impacting boiling points, solubility, and reactivity, making the study of organic chemistry central to understanding chemical behaviors.