Question

Predict which liquid in each pair has the higher boiling point: (a) \(\mathrm{CH}_{3} \mathrm{COOH}\) or \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) or \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\)

Short answer

(a) \(\mathrm{CH}_{3} \mathrm{COOH}\); (b) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\).

Step-by-step solution

Identify Intermolecular Forces

Determine the types of intermolecular forces present in each molecule. \(\mathrm{CH}_{3} \mathrm{COOH}\) (acetic acid) has hydrogen bonding due to the \(-\mathrm{OH}\) group attached to the carbon. \(\mathrm{C}_{2} \mathrm{H}_{5}\mathrm{Cl}\) (ethyl chloride) has dipole-dipole interactions due to the polar \(\mathrm{C}-\mathrm{Cl}\) bond. Similarly, \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) (ethanol) can form hydrogen bonds due to the \(\mathrm{OH}\) group, while \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) (dimethyl ether) mostly exhibits dipole-dipole interactions.

Compare Boiling Points - Part (a)

In part (a), \(\mathrm{CH}_{3} \mathrm{COOH}\) will have higher boiling points due to the presence of hydrogen bonding, which is stronger than the dipole-dipole interactions in \(\mathrm{C}_{2} \mathrm{H}_{5}\mathrm{Cl}\). Molecules with stronger intermolecular forces require more energy to separate, leading to a higher boiling point.

Compare Boiling Points - Part (b)

In part (b), \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) will have a higher boiling point because it can form hydrogen bonds, leading to a strong attraction between molecules. \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) cannot form hydrogen bonds as effectively, resulting in a lower boiling point.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces that occur between molecules, which influence the physical properties such as boiling points, melting points, and solubility of substances. There are various types of intermolecular forces, with the three main types being van der Waals forces (also known as London dispersion forces), dipole-dipole interactions, and hydrogen bonding. The strength of these forces varies, and it significantly affects how much energy is needed to change the state of a substance from liquid to gas (boiling).

• Van der Waals Forces: These are weak forces arising from temporary fluctuations in electron density, which result in temporary dipoles. All molecules experience these forces, but they are most significant in non-polar substances.
• Dipole-Dipole Interactions: These occur in polar molecules where permanent partial charges attract each other. These forces are stronger than van der Waals but weaker than hydrogen bonds.
• Hydrogen Bonding: A special case of dipole-dipole interaction, occurring only when hydrogen is bonded to electronegative elements like nitrogen, oxygen, or fluorine. Hydrogen bonds are significantly stronger than regular dipole-dipole interactions.

Substances with stronger intermolecular forces generally have higher boiling points because more energy is needed to overcome these forces during the phase change from liquid to gas.

Hydrogen Bonding

Hydrogen bonding is a specific and powerful type of dipole-dipole interaction. It occurs when hydrogen is covalently bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine.
This leads to a strong attraction between the slightly positive hydrogen atom in one molecule and the slightly negative electronegative atom in an adjacent molecule.
Hydrogen bonds are crucial because:

• They are responsible for the unique properties of water, such as its high surface tension, boiling point, and heat capacity.
• In biological molecules, hydrogen bonds are essential for the structure of proteins and the double helix of DNA.
• Substances with hydrogen bonding generally have higher boiling points compared to those that lack such interactions. For instance, acetic acid (\(\mathrm{CH}_{3} \mathrm{COOH}\)) and ethanol (\(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\)) both have high boiling points because of hydrogen bonding.

Overall, hydrogen bonding significantly impacts the physical properties of compounds, demonstrating why it is an essential intermolecular force in both chemistry and biology.

Dipole-Dipole Interactions

Dipole-dipole interactions happen between polar molecules that have permanent dipoles, meaning that they have areas of positive and negative charge. These interactions occur because the positive end of one polar molecule is attracted to the negative end of another.
Even though these interactions are generally weaker than hydrogen bonds, they are still important in determining a molecule's physical properties.
Important aspects of dipole-dipole interactions include:

• These forces cause polar substances to have higher boiling points and melting points compared to non-polar substances with similar molecular weights since the attractions between dipoles within the molecule require energy to break.
• The strength of dipole-dipole interactions is directly related to the polarity of the molecule — the more polar the molecule, the stronger the interaction.
• For example, ethyl chloride (\(\mathrm{C}_{2} \mathrm{H}_{5}\mathrm{Cl}\)) exhibits dipole-dipole interactions due to the polar \(\mathrm{C}-\mathrm{Cl}\) bond.

Dipole-dipole interactions are crucial in understanding the boiling and melting characteristics of many common compounds, especially when comparing their strengths to other intermolecular forces.