Problem 77 - Chemistry The Central Science In Si Units

Question

The table below shows some physical properties of compounds containing O-H groups. \begin{tabular}{lccc} \hline Liquid & Molecular Weight & Experimental Dipole Moment & Boiling Point \\\ \hline \(\mathrm{CH}_{3} \mathrm{OH}\) & 32.04 & 1.7 & \(64.7^{\circ} \mathrm{C}\) \\\ \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) & 74.12 & 1.66 & \(117.7^{\circ} \mathrm{C}\) \\ \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) & 62.07 & 1.5 & \(197.3^{\circ} \mathrm{C}\) \\ \hline \end{tabular} Which of the following statements best explains these data? (a) The larger the dipole moment, the stronger the intermolecular forces, and therefore the boiling point is lowest for the molecule with the largest dipole moment. (b) The dispersion forces increase from \(\mathrm{CH}_{3} \mathrm{OH} \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) and \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\); since the boiling point also increases in this order, the dispersion forces must be the major contributing factor for the boiling point trend; \((\mathbf{c}) \mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) has two groups capable of hydrogen bonding per molecule, whereas \(\mathrm{CH}_{3} \mathrm{OH}\) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) have only one; therefore, \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) has the highest boiling point.

Step-by-Step Solution

Answer

Statement (c) explains the data best: \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) has more hydrogen bonding potential.

Step 1: Analyze Molecular Weight

Begin by examining the molecular weights of each compound. The molecular weights are as follows: \(\mathrm{CH}_{3} \mathrm{OH}\) is 32.04, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) is 74.12, and \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) is 62.07. Typically, higher molecular weight can contribute to stronger dispersion forces, which may impact boiling point.

Step 2: Examine Dipole Moments

Next, consider the experimental dipole moments of the compounds: \(\mathrm{CH}_{3} \mathrm{OH}\) has 1.7, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) has 1.66, and \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) has 1.5. A larger dipole moment typically suggests stronger dipole-dipole interactions, which can also affect the boiling point.

Step 3: Analyze Boiling Points

Observe the boiling points for each compound: \(\mathrm{CH}_{3} \mathrm{OH}\) is at 64.7°C, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) at 117.7°C, and \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) at 197.3°C. The trend shows increasing boiling points from \(\mathrm{CH}_{3} \mathrm{OH}\) to \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\).

Step 4: Consider Hydrogen Bonding Ability

Evaluate the ability of each compound to form hydrogen bonds. \(\mathrm{CH}_{3} \mathrm{OH}\) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) each have one OH group, allowing one hydrogen bond per molecule. \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) has two OH groups, allowing more hydrogen bonding, which leads to significantly stronger intermolecular forces and thus, a higher boiling point.

Step 5: Match Explanation to Observations

Compare the observations with the given statements. Statement (a) suggests that higher dipole moment should lead to higher boiling points, which is not observed as \(\mathrm{CH}_{3} \mathrm{OH}\) has the highest dipole but lowest boiling point. Statement (b) considers dispersion forces, which increase with molecular weight, supporting higher boiling points but doesn't fully explain the dominance of \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\). Statement (c) mentions the presence of two OH groups in \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) contributing more significantly to the high boiling point due to potential for extensive hydrogen bonding.

Key Concepts

Hydrogen Bonding

Hydrogen bonding is a unique type of intermolecular force that occurs between a hydrogen atom, which is covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, and another electronegative atom. This attraction is stronger than most dipole-dipole interactions, giving molecules with hydrogen bonds exceptional properties.

The ability of a molecule to form hydrogen bonds can dramatically impact its physical properties, such as boiling and melting points. For example:

• Molecules like water or alcohols (in this case, FCH}_{3}FOHF and FCH}_{3}FCH}_{2}FCH}_{2}FOHF) exhibit hydrogen bonding due to their FOHF groups.
• The compound FHOCH}_{2}FCH}_{2}FOHF can form even more hydrogen bonds, having two FOHF groups, thus leading to stronger intermolecular attraction.

While one FOHF group allows for the creation of one hydrogen bond, two groups double this potential, contributing significantly to the boiling point as seen with FHOCH}_{2}FCH}_{2}FOHF.

Dipole Moment

The dipole moment is a measure of the separation of positive and negative charges in a molecule. It indicates how strongly a molecule can interact through dipole-dipole attractions. A higher dipole moment can mean stronger interactions between molecules.

Dipole moments are determined by both the molecular geometry and the difference in electronegativity between bonded atoms.

• For instance, in molecules like FCH}_{3}FOHF, FOHF groups contribute significantly to the overall dipole moment because of the high electronegativity of oxygen.
• Despite FCH}_{3}FOHF having the highest dipole moment in the examples, it does not correspond to the highest boiling point, highlighting the predominant role of hydrogen bonding over dipole effects in this case.

Thus, while dipole moments contribute to intermolecular forces, they aren't the sole determining factor for physical properties like boiling point, especially when hydrogen bonding is present.

Boiling Point

Boiling point is the temperature at which a liquid turns into vapor. This physical property reflects the strength of intermolecular forces within a liquid. The stronger the forces holding molecules together, the higher the boiling point.

There are several factors that can affect boiling point:

• Stronger intermolecular forces, such as hydrogen bonds, result in higher boiling points.
• Larger molecular weight often leads to stronger dispersion forces, which can also increase the boiling point.

The compounds presented all have FOHF groups, but FHOCH}_{2}FCH}_{2}FOHF has the highest boiling point because its multiple FOHF groups permit extensive hydrogen bonding. While dipole moments also play a role, the key takeaway from this exercise is the significant influence of hydrogen bonding on boiling points, surpassing the impact of dipole moments alone.