Problem 25 - Chemistry: The Central Science in SI Units, Global Edition

Question

(a) What atoms must a molecule contain to participate in hydrogen bonding with other molecules of the same kind? (b) Which of the following molecules can form hydrogen bonds with other molecules of the same kind: \(\mathrm{CH}_{3} \mathrm{~F}, \mathrm{CH}_{3} \mathrm{NH}_{2}, \mathrm{CH}_{3} \mathrm{OH}, \mathrm{CH}_{3} \mathrm{Br} ?\)

Step-by-Step Solution

Answer

A molecule must have a hydrogen atom bonded to nitrogen (N), oxygen (O), or fluorine (F) to participate in hydrogen bonding with other molecules of the same kind. Out of the given molecules, \(\mathrm{CH}_{3}\mathrm{NH}_{2}\) and \(\mathrm{CH}_{3}\mathrm{OH}\) can form hydrogen bonds with other molecules of the same kind, while \(\mathrm{CH}_{3}\mathrm{F}\) and \(\mathrm{CH}_{3}\mathrm{Br}\) cannot.

Step 1: Identify conditions for hydrogen bonding

To participate in hydrogen bonding, a molecule must have a hydrogen atom bonded to a highly electronegative element, usually nitrogen (N), oxygen (O), or fluorine (F). This creates a highly polar bond, enabling the positive hydrogen end to attract the negative end of another molecule containing N, O, or F.

Step 2: Examine each molecule

Now, let's examine each of the given molecules and determine if they can form hydrogen bonds with other molecules of the same kind. 1. \(\mathrm{CH}_{3}\mathrm{F}\): This molecule has a hydrogen atom, but it is not bonded to the highly electronegative fluorine (F) atom. The hydrogen atoms are bonded to carbon (C), which is less electronegative. Therefore, this molecule cannot form hydrogen bonds with other molecules of the same kind. 2. \(\mathrm{CH}_{3}\mathrm{NH}_{2}\): Here, the hydrogen atoms are bonded to a highly electronegative nitrogen (N) atom. As a result, this molecule can participate in hydrogen bonding with other molecules of the same kind. 3. \(\mathrm{CH}_{3}\mathrm{OH}\): In this case, the hydrogen atom is bonded to a highly electronegative oxygen (O) atom, allowing this molecule to form hydrogen bonds with other molecules of the same kind. 4. \(\mathrm{CH}_{3}\mathrm{Br}\): This molecule has a hydrogen atom, but it is not bonded to a highly electronegative atom. The hydrogen atoms are bonded to carbon (C), and bromine (Br) is less electronegative than N, O, or F. Therefore, this molecule cannot form hydrogen bonds with other molecules of the same kind.

Step 3: Conclusion

In summary, a molecule must have a hydrogen atom bonded to nitrogen (N), oxygen (O), or fluorine (F) to participate in hydrogen bonding with other molecules of the same kind. Out of the given molecules, \(\mathrm{CH}_{3}\mathrm{NH}_{2}\) and \(\mathrm{CH}_{3}\mathrm{OH}\) can form hydrogen bonds with other molecules of the same kind, while \(\mathrm{CH}_{3}\mathrm{F}\) and \(\mathrm{CH}_{3}\mathrm{Br}\) cannot.

Key Concepts

Electronegativity

Electronegativity is a fundamental concept in chemistry that refers to the ability of an atom to attract electrons towards itself in a chemical bond. This property can vary significantly across different elements. Generally, electronegativity increases across a period from left to right in the periodic table and decreases down a group.

When it comes to hydrogen bonding, the key elements involved are nitrogen (N), oxygen (O), and fluorine (F). These atoms are highly electronegative:

• Fluorine is the most electronegative element, which makes it a strong hydrogen bond acceptor.
• Oxygen comes next, known for its strong electron-pulling capabilities, often found in water molecules.
• Nitrogen, while slightly less electronegative than oxygen, still readily participates in hydrogen bonds.

Understanding electronegativity helps us predict the behavior of molecules and their ability to form hydrogen bonds, particularly by assessing which atoms in a molecule might "pull" electrons and create polar bonds.

Hydrogen Bond Donor

A hydrogen bond donor is an atom within a molecule that provides the hydrogen atom needed for hydrogen bonding. For this to occur, the hydrogen must be bonded to an electronegative atom, creating a scenario where the hydrogen has a partial positive charge.

In the context of hydrogen bonding, typical hydrogen bond donors include:

• Nitrogen (as in amines like CH₃NH₂)
• Oxygen (such as in alcohols like CH₃OH)

These atoms not only contribute to high polarity in the bond with hydrogen but also make the hydrogen available to interact with an electronegative atom from another molecule, thereby forming a strong intermolecular attraction known as a hydrogen bond. Without such donors, molecules cannot effectively participate in hydrogen bonding, as seen in less electronegative pairings like hydrogen bonded to carbon.

Intermolecular Forces

Intermolecular forces are forces of attraction or repulsion between neighboring particles (molecules, atoms, or ions). These forces are crucial in determining the physical properties of substances such as boiling points and solubility.

Hydrogen bonds are a specific type of intermolecular force. They are particularly strong dipole-dipole attractions that occur when a hydrogen atom bonded to a highly electronegative atom (like N, O, or F) attracts an electronegative atom on a neighboring molecule. This allows molecules to cling to each other more tightly:

• Hydrogen bonds give water its unique properties, such as a high boiling point.
• Such bonds also make hydrogen bonding participants, like CH₃OH and CH₃NH₂, have higher boiling points compared to non-hydrogen bonding molecules.

Overall, understanding these interactions helps explain why some liquids, like water and alcohols, remain liquid at room temperature, while others, like methane, do not.