Question

For each of the following pairs, indicate which substance has (a) the higher boiling point, \(\mathrm{C}_{6} \mathrm{H}_{12}\) or \(\mathrm{C}_{6} \mathrm{H}_{6}\) (b) the greater solubility in water, \(\bar{C}_{3} H_{7} O H\) or \(\mathrm{C}_{7} \mathrm{H}_{15} \mathrm{OH}\) (c) the greater acidity in aqueous solution, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}\) or \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}\).

Short answer

The higher boiling point is benzene, \(\mathrm{C}_{6} \mathrm{H}_{6}\). The greater solubility in water is propyl alcohol, \(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}\). The greater acidity in aqueous solution is benzoic acid, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}\).

Step-by-step solution

Analyze the Boiling Point

Determine the type of intermolecular forces present in the molecules. The molecule with stronger forces will have a higher boiling point. Cyclohexane \(\mathrm{C}_{6} \mathrm{H}_{12}\) is a nonpolar molecule with London dispersion forces while benzene \(\mathrm{C}_{6} \mathrm{H}_{6}\) is a polar molecule with enhanced London dispersion forces due to a delocalized electron cloud. Therefore, benzene would have a higher boiling point.

Determine Solubility in Water

According to the 'like dissolves like' concept, polar molecules dissolve in polar solvents and nonpolar molecules dissolve in nonpolar solvents. Water is a polar solvent. Propyl alcohol \(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}\) is shorter and has a higher proportion of polar (O-H) bonds compared to heptanol \(\mathrm{C}_{7} \mathrm{H}_{15} \mathrm{OH}\), thus will be more soluble in water.

Determine Acidity in Solution

An acid is a substance that can donate a proton (H+ ion) in a solution. In the case of benzaldehyde \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}\) and benzoic acid $\mathrm{C}_{6} \mathrm{H}_{5}\mathrm{COOH}$, benzoic acid can lose a proton from its carboxyl group (COOH), thus it is the stronger acid in aqueous solution.

Key concepts

Boiling Point

The boiling point of a substance is determined by the strength of the intermolecular forces present within it. These forces are the attractions between molecules that need to be overcome for a substance to change from a liquid to a gas. For example, consider cyclohexane \(\mathrm{C}_{6} \mathrm{H}_{12}\) and benzene \(\mathrm{C}_{6} \mathrm{H}_{6}\). Cyclohexane is nonpolar and exhibits London dispersion forces, which are the weakest type of intermolecular attractions.
Benzene, however, is a special case where its delocalized electron cloud enhances these dispersion forces, making them slightly stronger and giving benzene a higher boiling point compared to cyclohexane.

- **Intermolecular Forces:** Types of attraction include London dispersion, dipole-dipole, and hydrogen bonding.- **Polar vs Non-polar:** Polar substances often have higher boiling points due to stronger interactions.- **Application:** Predict boiling points by analyzing molecular polarity and size.

Solubility in Water

Solubility, particularly in water, is heavily influenced by the polarity of the substance. Water is a polar solvent, meaning it dissolves polar components more effectively than non-polar ones. Consider propyl alcohol \(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}\) and heptanol \(\mathrm{C}_{7} \mathrm{H}_{15} \mathrm{OH}\).
Propyl alcohol is smaller and has a higher percentage of its structure comprising polar hydroxyl (-OH) groups, making it more soluble in water than heptanol.

- **Like Dissolves Like:** Polar substances dissolve well in polar solvents such as water.- **Size and Structure:** Small, highly polar molecules tend to have better solubility in water.- **OH- Groups:** Molecules with hydroxyl groups, characteristic of alcohols, enhance water solubility due to hydrogen bonding.

Acidity in Aqueous Solution

Acidity in aqueous solutions refers to a substance's ability to donate a proton (\(\mathrm{H}^+\)) when dissolved in water. This proton donation capacity is influenced by the functional groups present in the molecule. Comparing benzaldehyde \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}\) and benzoic acid \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}\), benzoic acid has a carboxyl group (\(\mathrm{COOH}\)), which allows it to readily donate a proton in water, making it a stronger acid.
- **Proton Donor Ability:** Acids release \(\mathrm{H}^+\) ions in solution. Stronger acids do this more effectively.- **Functional Groups:** The presence and nature of groups like \(-\mathrm{COOH}\) directly relate to acidity levels.- **Acid Strength:** Acidic strength is often determined and compared using \(\mathrm{pK}_a\) values.