Question

Among the following orders, correct order(s) of boiling point is/are (A) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}<\mathrm{CH}_{3} \mathrm{OCH}_{3}<\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}<\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (B) \(\mathrm{CH}_{4}<\mathrm{CH}_{3} \mathrm{~F}<\mathrm{CH}_{3} \mathrm{Cl}<\mathrm{CH}_{3} \mathrm{Br}<\mathrm{CH}_{3} \mathrm{I}\) (C) (D)

Short answer

The correct order(s) of boiling points based on the type and strength of intermolecular forces present within the compounds are given in options (A) and (B). Thus, the short answer is: (A) \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}_{3}<\mathrm{CH}_{3}\mathrm{OCH}_{3}<\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{NH}_{2}<\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH\) (B) \(\mathrm{CH}_{4}<\mathrm{CH}_{3}\mathrm{~F}<\mathrm{CH}_{3}\mathrm{Cl}<\mathrm{CH}_{3}\mathrm{Br}<\mathrm{CH}_{3}\mathrm{I\)

Step-by-step solution

Examine Option (A)

: We need to compare the boiling points of the following compounds in option A: 1. \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}_{3}\) (propane) 2. \(\mathrm{CH}_{3}\mathrm{OCH}_{3}\) (dimethyl ether) 3. \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{NH}_{2}\) (ethanamine) 4. \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\) (ethanol) The boiling points are influenced by the presence and strength of intermolecular forces in each compound. We examine the forces as follows: 1. Ethanol has hydrogen bonding due to the presence of \(\mathrm{OH}\) group. 2. Ethanamine also has hydrogen bonding due to the presence of \(\mathrm{NH}_{2}\) group. 3. Dimethyl ether has dipole-dipole interactions due to the presence of \(\mathrm{O}\) atom. 4. Propane has only weak van der Waals forces. The strength of interactions is ranked as follows: Hydrogen bonding > Dipole-dipole > Van der Waals. Thus, the order of boiling points can be determined as: \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}_{3}<\mathrm{CH}_{3}\mathrm{OCH}_{3}<\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{NH}_{2}<\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\) Option (A) is correct.

Examine Option (B)

: We need to compare the boiling points of the following compounds in option B: 1. \(\mathrm{CH}_{4}\) (methane) 2. \(\mathrm{CH}_{3}\mathrm{F}\) (fluoromethane) 3. \(\mathrm{CH}_{3}\mathrm{Cl}\) (chloromethane) 4. \(\mathrm{CH}_{3}\mathrm{Br}\) (bromomethane) 5. \(\mathrm{CH}_{3}\mathrm{I}\) (iodomethane) All these compounds have only weak van der Waals forces. The strength of the forces increases with the size and mass of the molecule. Therefore, the order of boiling points should follow the increase in size and mass of the halogens: \(\mathrm{CH}_{4}<\mathrm{CH}_{3}\mathrm{F}<\mathrm{CH}_{3}\mathrm{Cl}<\mathrm{CH}_{3}\mathrm{Br}<\mathrm{CH}_{3}\mathrm{I}\) Option (B) is correct.

Conclusion

: Both option (A) and option (B) display the correct orders of boiling points based on the type and strength of intermolecular forces present within the compounds. Therefore, the correct answer is options (A) and (B).

Key concepts

Intermolecular Forces

Intermolecular forces are the forces that act between molecules. These forces determine the physical properties of substances, including boiling points, melting points, and solubility. Understanding these forces is crucial for predicting how substances will behave under different conditions.

There are several types of intermolecular forces, which include:

• Van der Waals forces: These are the weakest and are present in all molecules. They arise from temporary dipoles that occur due to the movement of electrons.
• Dipole-dipole interactions: These occur between polar molecules with permanent dipoles, where positive and negative charges attract each other.
• Hydrogen bonding: A special type of dipole-dipole interaction occurring when hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine.

The strength of these forces influences the energy required to change the state of a substance, such as moving from liquid to gas, which is why they are key to understanding boiling points.

Hydrogen Bonding

Hydrogen bonding is one of the strongest types of intermolecular forces. It occurs when hydrogen is covalently bonded to highly electronegative atoms, such as oxygen, nitrogen, or fluorine.

In this type of bonding, the hydrogen atom carries a slight positive charge due to the electronegativity difference, while the electronegative atom becomes slightly negative. This creates a strong dipole-dipole attraction, forming a hydrogen bond with adjacent molecules.

For instance, ethanol (\(\text{CH}_3\text{CH}_2\text{OH}\)) exhibits hydrogen bonding because of the \(\text{-OH}\) group. This significantly raises its boiling point compared to other similar-sized molecules that do not exhibit hydrogen bonding.

The presence of hydrogen bonds in a molecule dramatically increases its boiling and melting points because more energy is required to break these strong interactions during phase changes.

Van der Waals Forces

Van der Waals forces are the weakest of intermolecular forces and also the most common, as they are present in all molecules, regardless of whether they are polar or nonpolar.

These forces arise from transient dipoles that occur when electrons move around the nucleus in atoms or molecules, creating an instantaneous distribution of positive and negative charges.

Despite their weakness, Van der Waals forces can significantly affect the properties of substances. For example, in option (B) of the exercise, the boiling points of methane and halomethanes were determined by these forces. As the size and mass of the molecule increase, so does the strength of these forces, which leads to higher boiling points.

Though individually weak, the cumulative effect of Van der Waals forces in large molecules can result in noticeable effects on their physical properties.

Dipole-Dipole Interactions

Dipole-dipole interactions are forces between polar molecules with permanent dipoles. These occur when the positive end of one molecule's dipole is attracted to the negative end of another's.

These interactions are stronger than Van der Waals forces but generally weaker than hydrogen bonds. Among the molecules mentioned in the exercise, dimethyl ether (\(\text{CH}_3\text{OCH}_3\)) has significant dipole-dipole interactions due to the polar \(\text{C-O}\) bond.

These interactions lead to higher boiling points than nonpolar molecules of similar size. However, they remain less effective than hydrogen bonding, as seen when comparing the boiling points of dimethyl ether with ethanol and ethanamine.

In summary, the presence and strength of dipole-dipole interactions directly influence the energy needed for a substance to change states, affecting its boiling point.