Question

Are each of the following soluble, slightly soluble, of insoluble in water? a. \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{OH}\) b. \(\mathrm{CH}_{3}-\mathrm{O}-\mathrm{CH}_{3}\) c. \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{OH}\)

Short answer

a. Soluble, b. Slightly soluble, c. Slightly soluble

Step-by-step solution

Identify the compounds

List each compound given: (a) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{OH}\) (b) \(\mathrm{CH}_{3}-\mathrm{O}-\mathrm{CH}_{3}\) (c) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{OH}\)

Determine solubility rules

Recall that compounds tend to be soluble if they have polar functional groups such as -OH (hydroxyl group) or -NH_2 (amine group) that can form hydrogen bonds with water.

Evaluate the solubility of \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{OH}\)

\(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{OH}\) is ethanol. Ethanol has a hydroxyl group (-OH), which is highly polar and can form hydrogen bonds with water, making it soluble.

Evaluate the solubility of \(\mathrm{CH}_{3}-\mathrm{O}-\mathrm{CH}_{3}\)

\(\mathrm{CH}_{3}-\mathrm{O}-\mathrm{CH}_{3}\) is dimethyl ether. While it contains an oxygen atom, the lack of a hydroxyl group (-OH) reduces its ability to form hydrogen bonds with water. Thus, it is only slightly soluble.

Evaluate the solubility of \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{OH}\)

\(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{OH}\) is hexanol. The molecule is much larger with a longer hydrophobic hydrocarbon chain compared to ethanol, making it less able to dissolve in water. Therefore, it is only slightly soluble.

Key concepts

Functional Groups

Functional groups are specific groups of atoms within molecules that have certain properties and chemical reactivity. They are key in determining the behavior of organic compounds. The most common functional groups include hydroxyl (-OH), carbonyl (C=O), carboxyl (-COOH), and amino (-NH_2). These groups can greatly influence the solubility of compounds in water.

• Hydroxyl Group (-OH): Found in alcohols (like ethanol), it can form hydrogen bonds with water molecules, enhancing solubility.
• Ether Group (-O-): Found in ethers (like dimethyl ether), it has an oxygen atom, but the lack of a hydrogen atom bonded to the oxygen limits hydrogen bonding with water, reducing solubility.

A molecule with a polar functional group is generally more soluble in water due to its ability to engage in hydrogen bonding.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction between molecules. It occurs when a hydrogen atom is directly bonded to a highly electronegative element like oxygen, nitrogen, or fluorine. This creates a strong electrostatic attraction between molecules. Hydrogen bonds make compounds like ethanol highly soluble in water. Here's why:

• The -OH group in ethanol forms hydrogen bonds with water molecules, which enhances solubility.
• Even small alcohols like methanol and ethanol can disrupt the hydrogen bonding network of water, allowing them to mix freely.

Compounds that can form hydrogen bonds are much more likely to be soluble in water compared to those that cannot, like dimethyl ether, which lacks hydrogen bonding capability.

Polarity

Polarity refers to the distribution of electrical charge around a molecule. A molecule is considered polar if it has a significant difference in electronegativity between its atoms, leading to a dipole moment. Polarity impacts solubility in water, a highly polar solvent.

• Polar Molecules: These have regions of partial positive and partial negative charge, allowing for strong interactions with water molecules and resulting in high solubility. Ethanol and hexanol, for example, have polar -OH groups making them soluble or slightly soluble in water.
• Non-polar Molecules: These have an even distribution of charge and do not interact well with water. Hence, they are typically insoluble or only slightly soluble. Dimethyl ether has an oxygen atom but lacks significant polarity and hydrogen bonding capability, making it only slightly soluble.

Understanding the polarity of a molecule can help predict its solubility behavior in water.

Solubility Rules

Solubility rules are guidelines that help predict the solubility of substances in solvents like water. A few key points include:

• Compounds with polar functional groups (like -OH and -NH_2) generally have high solubility in water.
• Large non-polar hydrocarbon chains reduce solubility, even if the compound has a polar group, as seen with hexanol.
• Ethers, despite having an oxygen atom, do not mix well with water without hydrogen bonding capability, making them only slightly soluble.

Using these rules, one can determine why ethanol is soluble (due to its -OH group and low molecular weight), dimethyl ether is slightly soluble (due to lack of hydrogen bonds), and hexanol is slightly soluble (due to its long non-polar chain despite the -OH group).