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: \(\mathrm{CH}_{3} \mathrm{F}, \mathrm{CH}_{3} \mathrm{NH}_{2}, \mathrm{CH}_{3} \mathrm{OH}, \mathrm{CH}_{3} \mathrm{B}\) ?

Short answer

(a) For a molecule to participate in hydrogen bonding with other molecules of the same kind, it must contain hydrogen atoms bound to any of the three highly electronegative atoms: fluorine (F), oxygen (O), or nitrogen (N). This means the molecule must have an H-F, H-O, or H-N bond. (b) From the given molecules, \(\mathrm{CH}_{3}\mathrm{F}, \mathrm{CH}_{3}\mathrm{NH}_{2}, \mathrm{CH}_{3}\mathrm{OH}\) can form hydrogen bonds with other molecules of the same kind, since they contain hydrogen atoms bonded to one of the three highly electronegative atoms.

Step-by-step solution

Understanding hydrogen bonding

Hydrogen bonding is a type of intermolecular force that occurs between molecules containing a hydrogen atom bonded to a highly electronegative atom. The three highly electronegative atoms that can participate in hydrogen bonding are fluorine (F), oxygen (O), and nitrogen (N). In hydrogen bonding, the hydrogen atom in one molecule interacts with the electronegative atom in another molecule.

Identifying the atoms required for a molecule to participate in hydrogen bonding

For a molecule to participate in hydrogen bonding with other molecules of the same kind, it must contain hydrogen atoms bound to any of the three highly electronegative atoms, fluorine, oxygen, or nitrogen. This implies that the molecule must have an H-F, H-O, or H-N bond.

Evaluate each of the given molecules to determine if they can form hydrogen bonds

(a) We will now examine each of the given molecules to see if they can form hydrogen bonds with other molecules of the same kind: 1. \(\mathrm{CH}_{3}\mathrm{F}\): This molecule contains a hydrogen atom bonded to a fluorine atom (H-F bond). Therefore, it can participate in hydrogen bonding with other molecules of the same kind. 2. \(\mathrm{CH}_{3}\mathrm{NH}_{2}\): This molecule contains a hydrogen atom bonded to a nitrogen atom (H-N bond). Therefore, it can participate in hydrogen bonding with other molecules of the same kind. 3. \(\mathrm{CH}_{3}\mathrm{OH}\): This molecule contains a hydrogen atom bonded to an oxygen atom (H-O bond). Therefore, it can participate in hydrogen bonding with other molecules of the same kind. 4. \(\mathrm{CH}_{3}\mathrm{B}\): This molecule does not contain any hydrogen atoms bonded to fluorine, oxygen, or nitrogen. Therefore, it cannot participate in hydrogen bonding with other molecules of the same kind. (b) Based on our analysis, the molecules \(\mathrm{CH}_{3}\mathrm{F}, \mathrm{CH}_{3}\mathrm{NH}_{2}, \mathrm{CH}_{3}\mathrm{OH}\) can form hydrogen bonds with other molecules of the same kind.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces that hold molecules together. They are essential for determining the physical properties of substances, such as boiling and melting points.
These forces are generally weaker than the bonds within a molecule but are crucial for forming liquids and solids from gases.
There are several types of intermolecular forces. They include:

• Dipole-dipole interactions
• London dispersion forces
• Hydrogen bonding

Hydrogen bonding is a significant type of intermolecular force because it's stronger than other dipole-dipole interactions.
It occurs when a hydrogen atom, covalently bonded to a very electronegative atom, interacts with another electronegative atom in a nearby molecule.

Electronegativity

Electronegativity refers to the ability of an atom to attract shared electrons in a chemical bond.
In the context of hydrogen bonding, the presence of a highly electronegative atom is critical.
Fluorine (F), oxygen (O), and nitrogen (N) are the most electronegative elements and play a pivotal role in hydrogen bonding.
In the periodic table, electronegativity tends to increase as you move across a period from left to right, and decreases as you move down a group.
In a molecule, the difference in electronegativity between atoms can lead to polar bonds, contributing to the polarity of the molecule itself.
This results in partial charges that allow molecules to attract each other, enabling hydrogen bonding.

H-F Bond

The H-F bond is a classic example of a hydrogen bond due to the high electronegativity of fluorine.
This bond forms when hydrogen atom in the molecule bonds with fluorine, one of the most electronegative elements.
The result is a highly polar bond with significant partial positive charge on the hydrogen atom and partial negative charge on the fluorine atom.
In molecules like \(\mathrm{CH}_{3}\mathrm{F}\), these polar bonds enable the formation of hydrogen bonding with other \(\mathrm{CH}_{3}\mathrm{F}\) molecules.
This results in stronger intermolecular forces that increase the boiling and melting points compared to non-hydrogen bonded molecules.

H-N Bond

The H-N bond is another significant type of bond where hydrogen bonding can occur.
Nitrogen, being highly electronegative, attracts shared electrons, creating a strong dipole in the bond.
This enhances the molecule's ability to form hydrogen bonds with other similar molecules.
In molecules such as \(\mathrm{CH}_{3}\mathrm{NH}_{2}\), which include hydrogen atoms bonded to nitrogen, you will find hydrogen bonding.
This interaction improves the stability and structural integrity of the substance and affects its physical properties like solubility and boiling point.

H-O Bond

The H-O bond is well-known for its role in hydrogen bonding, particularly in water \(\mathrm{H}_{2}\mathrm{O}\).
Oxygen is highly electronegative, leading to a significant charge difference and creating a polar bond with hydrogen.
This polar nature makes it one of the strongest hydrogen bonds.
In substances like \(\mathrm{CH}_{3}\mathrm{OH}\), the presence of the H-O bond facilitates hydrogen bonding interactions.
These interactions contribute to properties like higher boiling points and increased viscosity.
Hydrogen bonds formed due to H-O bond interactions are crucial for maintaining the structure of many biological molecules as well.