Question

Among the following orders, the correct order of boiling point is/are (A) \(\mathrm{CH}_{3} \mathrm{OH}<\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}<\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}<\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{OH}\) (B) \(\mathrm{CH}_{3} \mathrm{SH}<\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{SH}<\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{SH}<\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{SH}\) (C) \(\mathrm{CH}_{3} \mathrm{SH}<\mathrm{CH}_{3} \mathrm{OH}<\mathrm{H}_{2} \mathrm{O}\) (D)

Short answer

The correct orders of boiling points are given by options (A), (B), and (C): (A) \(\mathrm{CH_3OH < C_2H_5OH < C_3H_7OH < C_4H_9OH}\) (B) \(\mathrm{CH_3SH < C_2H_5SH < C_3H_7SH < C_4H_9SH}\) (C) \(\mathrm{CH_3SH < CH_3OH < H_2O}\)

Step-by-step solution

Analyze Boiling Points in Alcohols (OH)

Increasing the carbon chain in alcohols generally leads to an increase in boiling point due to the increase in van der Waals forces related to molecular size. This means that the longer the carbon chain, the higher the boiling point. The correct order for alcohols therefore should be: \(\mathrm{CH_3OH < C_2H_5OH < C_3H_7OH < C_4H_9OH}\)

Analyze Boiling Points in Thiols (SH)

Like alcohols, increasing carbon chain in thiols (SH) generally leads to an increase in boiling point due to the increase in van der Waals forces related to molecular size. This means that the longer the carbon chain, the higher the boiling point. The hydrogen bonding is weaker in thiols than in alcohols. However, this does not affect the overall trend of increasing boiling points as the carbon chain length increases. The correct order for thiols (SH) therefore should be: \(\mathrm{CH_3SH < C_2H_5SH < C_3H_7SH < C_4H_9SH}\)

Comparing Boiling Points of CH₃SH, CH₃OH, and H₂O

To compare the boiling points of these compounds, we must consider their molecular weights and intermolecular forces. - \(\mathrm{CH_3SH :}\) Thiols have weaker hydrogen bonding compared to alcohols or water, so their boiling points should be lower than alcohol and water. - \(\mathrm{CH_3OH :}\) Alcohols have hydrogen bonding, but not as strong as water due to the presence of one less hydroxyl group. Therefore, the boiling point should be higher than thiols, but lower than water. - \(\mathrm{H_2O :}\) Water has the strongest intermolecular forces due to its extensive hydrogen bonding. This results in a high boiling point compared to both alcohols and thiols. The correct order for the boiling points of these compounds should be: \(\mathrm{CH_3SH < CH_3OH < H_2O}\) Now, let's compare our answers with the options provided in the exercise: (A) Correct (B) Correct (C) Correct (D) None. Thus, the correct orders of boiling points are given by options (A), (B), and (C).

Key concepts

Alcohols and Boiling Points

Alcohols are organic compounds characterized by the presence of a hydroxyl group (-OH). Their boiling points are influenced by intermolecular forces, particularly hydrogen bonding.
Hydrogen bonds are relatively strong, causing alcohols to have higher boiling points compared to molecules of similar size that do not form hydrogen bonds.

• Carbon Chain Length: As the carbon chain length increases, so do the van der Waals forces, which are weaker than hydrogen bonds but still contribute to an increase in boiling points.
• Order of Boiling Points: With increasing chain length, we observe the order: \(\text{CH}_3\text{OH} < \text{C}_2\text{H}_5\text{OH} < \text{C}_3\text{H}_7\text{OH} < \text{C}_4\text{H}_9\text{OH}\).

More extensive hydrogen bonding coupled with increased molecular size explains why alcohols tend to have higher boiling points as their carbon chains grow.

Thiols and Boiling Points

Thiols are similar to alcohols but have a sulfur atom replacing the oxygen in the hydroxyl group, resulting in a thiol group (-SH).
While thiols can exhibit some hydrogen bonding, it is much weaker than in alcohols.

• Weaker Hydrogen Bonding: The presence of sulfur, which is less electronegative than oxygen, weakens the hydrogen bonds, leading to lower boiling points compared to similar alcohols.
• Increasing Chain Length: As with alcohols, longer carbon chains increase the van der Waals forces, raising boiling points despite the weaker hydrogen bonding.
• Order of Boiling Points: The order follows: \(\text{CH}_3\text{SH} < \text{C}_2\text{H}_5\text{SH} < \text{C}_3\text{H}_7\text{SH} < \text{C}_4\text{H}_9\text{SH}\).

Thiols, although showing a similar trend to alcohols with increasing chain length, generally have lower boiling points due to the weaker hydrogen bonds.

Intermolecular Forces in Organic Compounds

Intermolecular forces are critical in determining boiling points for organic compounds like alcohols, thiols, and water.
These forces include hydrogen bonds, dipole-dipole interactions, and van der Waals forces.

• Hydrogen Bonding: Found strongly in water and alcohols, hydrogen bonding is the attraction between a hydrogen atom bonded to an electronegative atom (like oxygen) and another electronegative atom. Water forms extensive hydrogen bonds, giving it a higher boiling point than alcohols.
• Dipole-Dipole Interaction: More significant in polar molecules but weaker than hydrogen bonds, these interactions contribute to boiling points but are less pronounced in thiols.
• Van der Waals Forces: Present in all molecules, these forces are based on induced dipoles and are the weakest intermolecular force but increase with molecular size.
• Boiling Point Comparison: Water, due to its strong hydrogen bonds, has the highest boiling point followed by alcohols, then thiols. Thus, the order \(\text{CH}_3\text{SH} < \text{CH}_3\text{OH} < \text{H}_2\text{O}\) reflects the decreasing strength of intermolecular forces involved.

Understanding these forces aids in predicting boiling points and the behavior of organic compounds in different environments.