Question

Give an explanation for the following observations: a. Ethanol is soluble in water, but propane is not. b. Dimethyl ether is slightly soluble in water, but pentane is not. c. 1-Propanol is soluble in water, but 1 -hexanol is not.

Short answer

Ethanol and 1-Propanol are soluble in water due to their polar -OH groups. Dimethyl ether is slightly soluble due to its polar ether group. Nonpolar substances like propane and pentane are not soluble in water.

Step-by-step solution

- Understanding Polarity

Substances dissolve in each other based on the principle 'like dissolves like'. Polar substances dissolve in polar solvents, and nonpolar substances dissolve in nonpolar solvents. Water is a polar solvent.

- Analyzing Ethanol and Propane

Ethanol has an -OH group that forms hydrogen bonds with water, making it polar and soluble in water. Propane, however, is nonpolar due to its symmetric hydrocarbon structure, making it insoluble in water.

- Analyzing Dimethyl Ether and Pentane

Dimethyl ether has an ether group (-O-) which makes it slightly polar and able to form weak hydrogen bonds with water, leading to slight solubility. Pentane is a nonpolar hydrocarbon and thus is not soluble in water.

- Analyzing 1-Propanol and 1-Hexanol

1-Propanol has a shorter hydrocarbon chain with an -OH group, making it polar and soluble in water. 1-Hexanol has a longer hydrocarbon chain which dominates over the -OH group, making it mostly nonpolar and insoluble in water.

Key concepts

Polarity

Polarity refers to the distribution of electric charge around a molecule. A molecule is polar if it has a significant difference in electronegativity between its atoms, causing a dipole moment. Water (H₂O) is a prime example of a polar molecule due to the electronegativity difference between hydrogen and oxygen.

Polar molecules tend to have one more positively charged side and one more negatively charged side. This characteristic significantly affects their solubility. Polar substances dissolve well in polar solvents (like water), whereas nonpolar substances do not. This principle is a fundamental aspect of understanding solubility and will guide us through the subsequent concepts.

Like Dissolves Like

The phrase 'like dissolves like' is central to solubility. It means that substances with similar polarity will dissolve in each other. Polar solvents dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes.

Let's take water and ethanol as an example. Both are polar, so ethanol dissolves well in water. Conversely, nonpolar substances like oils and long-chain hydrocarbons do not dissolve in water. This principle is why propane (a nonpolar molecule) doesn't dissolve in water, while ethanol (a polar molecule) does.

Understanding this concept helps predict the solubility of various substances when mixed.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction. It occurs when a hydrogen atom bonded to a highly electronegative atom, like oxygen or nitrogen, interacts with another electronegative atom.

In the context of solubility, hydrogen bonds play a pivotal role. Ethanol, for instance, has an -OH group, enabling it to form hydrogen bonds with water, hence its solubility. On the other hand, molecules like propane cannot form such bonds and thus are not soluble in water. Similarly, dimethyl ether, which can form weak hydrogen bonds, is slightly soluble in water, while pentane is not due to the absence of hydrogen bonding abilities.

Hydrocarbon Chain Length

The length of the hydrocarbon chain in a molecule dramatically influences its solubility in polar solvents like water. Shorter hydrocarbon chains, like that in 1-propanol, have less effect on the overall polarity, allowing the molecule to remain soluble in water.

However, as the hydrocarbon chain length increases, as seen in 1-hexanol, the nonpolar characteristics of the molecule overshadow the polar -OH group's influence. This makes the overall molecule less polar and hence less soluble in water. This difference explains why 1-propanol is soluble in water, while 1-hexanol is not.