Question

Write detailed structures and predict which compound in each pair would have (1) the lower boiling point and (2) the higher water solubility. a. \(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}, \mathrm{H}_{3} \mathrm{CCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) b. \(\mathrm{CH}_{3} \mathrm{OCH}_{3}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) ?. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH},\left(\mathrm{CH}_{3}\right)_{2} \mathrm{COH}\) d. \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}, \mathrm{HCO}_{2} \mathrm{CH}_{3}\) e. \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{6} \mathrm{CO}_{2} \mathrm{H}, \mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{7} \mathrm{CO}_{2} \mathrm{H}\)

Short answer

a: Butane, Ethylenediamine; b: Dimethyl ether, Ethanol; c: Isobutanol, Butanol; d: Methyl formate, Acetic acid; e: Octanoic acid, Octanoic acid.

Step-by-step solution

Analyzing Pair a

For the first pair, examine the structural formulas. \( \mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2} \) is ethylenediamine, which is a polar molecule with hydrogen bonding. \( \mathrm{H}_{3} \mathrm{CCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} \) is butane, which is a nonpolar alkane. Boiling point is generally higher for polar compounds due to strong intermolecular forces like hydrogen bonding; hence butane has a lower boiling point. The solubility in water is higher for ethylenediamine due to its ability to form hydrogen bonds with water.

Analyzing Pair b

For the second pair, \( \mathrm{CH}_{3} \mathrm{OCH}_{3} \) is dimethyl ether, which is an ether, and \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \) is ethanol, an alcohol. Alcohols can participate in hydrogen bonding while ethers do not, leading to a higher boiling point for ethanol. Ethanol is also more water soluble than dimethyl ether due to its ability to hydrogen bond with water.

Analyzing Pair c

The third pair includes \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH} \) which is butanol, and \( \left(\mathrm{CH}_{3}\right)_{2} \mathrm{COH} \) which is an isobutanol-like structure. Both compounds can hydrogen bond, but the branching in isobutanol disrupts molecular packing, allowing butanol to have a higher boiling point. Water solubility tends to be slightly higher for more linear structures, so butanol is expected to be more water soluble.

Analyzing Pair d

In the fourth pair, \( \mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H} \) is acetic acid, a carboxylic acid, and \( \mathrm{HCO}_{2} \mathrm{CH}_{3} \) is methyl formate, an ester. Acetic acid can form strong hydrogen bonds leading to a higher boiling point. It is also more soluble in water because it can donate and accept hydrogen bonds, unlike methyl formate.

Analyzing Pair e

For the last pair, \( \mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{6} \mathrm{CO}_{2} \mathrm{H} \) is octanoic acid, and \( \mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{7} \mathrm{CO}_{2} \mathrm{H} \) is nonanoic acid. The longer carbon chain in nonanoic acid results in greater Van der Waals forces, leading to a higher boiling point for nonanoic acid. Octanoic acid, having the shorter chain, is typically more water soluble.

Key concepts

Boiling Point

The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure, allowing it to transition from liquid to gas. It is strongly influenced by the type of intermolecular forces present within a compound. **Factors Affecting Boiling Point:**

• **Intermolecular Forces:** Compounds with stronger intermolecular forces, such as hydrogen bonds, possess higher boiling points. For example, ethanol has a higher boiling point than dimethyl ether because ethanol can form hydrogen bonds.
• **Molecular Weight:** Generally, the greater the molecular weight, the higher the boiling point, due to increased van der Waals forces.
• **Molecular Structure:** More branched molecules often have lower boiling points due to reduced surface area for intermolecular interaction compared to their linear counterparts. For instance, isobutanol has a lower boiling point than butanol.

Understanding the boiling points of substances helps predict their behavior under different temperature conditions and is fundamental in processes like distillation.

Water Solubility

Water solubility refers to the ability of a substance to dissolve in water, a polar solvent. This capability is largely dependent on the presence of polar groups and the ability of the substance to engage in hydrogen bonding. **Factors Influencing Solubility in Water:**

• **Polarity:** Polar molecules, like ethylenediamine, are more soluble in water as they can interact favorably with water molecules.
• **Hydrogen Bonding:** Molecules that can form hydrogen bonds with water, such as acetic acid, tend to be more soluble.
• **Molecular Size:** Smaller molecules are generally more soluble. However, in larger molecules like nonanoic acid, hydrophobic interactions can decrease solubility.

These factors determine how substances dissolve, affecting everything from industrial applications to biological processes.

Hydrogen Bonding

Hydrogen bonding is a type of dipole-dipole interaction that occurs between a hydrogen atom, covalently bonded to a more electronegative atom (such as oxygen or nitrogen), and another electronegative atom. **Key Characteristics of Hydrogen Bonding:**

• **Strength:** Hydrogen bonds are stronger than van der Waals forces but weaker than covalent or ionic bonds.
• **Occurrence:** Found in molecules with N-H, O-H, or F-H bonds, like water and alcohols.
• **Effects on Properties:** Compounds that engage in hydrogen bonding typically have higher boiling points and greater water solubility. Ethanol, due to its hydroxyl group, showcases these traits clearly.

Hydrogen bonding plays a critical role in the structure and function of DNA and proteins, impacting biological functionality and physical properties of compounds.

Polar Molecules

Polar molecules have a significant impact on physical and chemical properties such as solubility and boiling points. Polarity arises due to differences in electronegativity between atoms in a molecule, creating a dipole moment. **Characteristics and Importance of Polar Molecules:**

• **Dipole Moment:** Molecules with uneven charge distribution, such as water, have strong dipole moments, contributing to their interaction strength with other polar molecules.
• **Solubility:** Polar molecules tend to dissolve well in polar solvents like water, due to the ability to form favorable interactions. Ethanol is an example of a polar molecule that is highly water-soluble.
• **Chemical Reactions:** Polarity affects reactivity and interaction with other molecules. For instance, polar compounds like acetic acid are more reactive in neutral and charged environments.

Understanding molecular polarity helps in predicting solubility and reactivity, crucial for fields ranging from material science to pharmacology.