Question

Determine which of the following molecules would hydrogenbond with other molecules like it. For those that do not hydrogen-bond, explain why. For those that hydrogen-bond, draw a diagram of two molecules using a dotted line to indicate where the hydrogen bonding will occur. (a) \(\mathrm{Br}_{2}\) (c) \(\mathrm{CH}_{3}-\mathrm{O}-\mathrm{CH}_{3}\) (e) \(\mathrm{H}_{2} \mathrm{~S}\) (b) \(\mathrm{CH}_{3}-\mathrm{O}-\mathrm{H}\) (d) \(\mathrm{H}_{2} \mathrm{O}\)

Short answer

Methanol (\

Step-by-step solution

Understand Hydrogen Bonding

Hydrogen bonding is a type of intermolecular force that occurs between a hydrogen atom bonded to a highly electronegative atom (N, O, or F) and another electronegative atom with a lone pair of electrons.

Check Molecule \( \mathrm{Br}_{2} \)

The molecule \( \mathrm{Br}_{2} \) consists of two bromine atoms bonded together. Bromine is not highly electronegative and there is no hydrogen attached to it. Hence, \( \mathrm{Br}_{2} \) does not form hydrogen bonds.

Check Molecule \( \mathrm{CH}_{3}- \mathrm{O}- \mathrm{CH}_{3} \)

The molecule \( \mathrm{CH}_{3}- \mathrm{O}- \mathrm{CH}_{3} \) (dimethyl ether) has an oxygen atom, but hydrogen atoms are not directly bonded to the oxygen atom. Therefore, \( \mathrm{CH}_{3}- \mathrm{O}- \mathrm{CH}_{3} \) cannot form hydrogen bonds.

Check Molecule \( \mathrm{H}_{2} \mathrm{S} \)

The molecule \( \mathrm{H}_{2} \mathrm{S} \) (hydrogen sulfide) has hydrogen atoms, but sulfur is less electronegative compared to nitrogen, oxygen, and fluorine. Thus, \( \mathrm{H}_{2} \mathrm{S} \) does not form hydrogen bonds.

Check Molecule \( \mathrm{CH}_{3}- \mathrm{O}- \mathrm{H} \)

The molecule \( \mathrm{CH}_{3}- \mathrm{O}- \mathrm{H} \) (methanol) has an oxygen atom bonded to a hydrogen atom. This setup allows the molecule to form hydrogen bonds with other similar molecules. Hydrogen bonds can form between the oxygen of one molecule and the hydrogen of another.

Hydrogen Bonding in \( \mathrm{CH}_{3}- \mathrm{O}- \mathrm{H} \)

Draw two methanol molecules and show a dotted line between the hydrogen atom of one molecule and the oxygen atom of another, representing the hydrogen bond.

Check Molecule \( \mathrm{H}_{2} \mathrm{O} \)

The molecule \( \mathrm{H}_{2} \mathrm{O} \) (water) has oxygen bonded to hydrogen atoms. This setup is ideal for hydrogen bonding. Water forms hydrogen bonds between the hydrogen atom of one molecule and the oxygen atom of another.

Hydrogen Bonding in \( \mathrm{H}_{2} \mathrm{O} \)

Draw two water molecules and show a dotted line between the hydrogen atom of one molecule and the oxygen atom of another, indicating where the hydrogen bonding occurs.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between neighboring molecules. These forces are essential in determining the physical properties of substances, such as boiling and melting points. Intermolecular forces include:

• Van der Waals forces (dispersion and dipole-dipole interactions)
• Hydrogen bonds
• Ion-dipole interactions

Understanding these forces helps us predict how substances behave in different states and why certain molecules exhibit unique properties.

Hydrogen Bonds

Hydrogen bonds are a specific type of intermolecular force that occurs when a hydrogen atom is covalently bonded to a highly electronegative atom such as nitrogen (N), oxygen (O), or fluorine (F). The hydrogen bond serves as a bridge between the hydrogen atom of one molecule and the electronegative atom of another.

Key characteristics of hydrogen bonds include:

• Stronger than typical dipole-dipole interactions
• Weaker than covalent or ionic bonds
• Significantly influence boiling and melting points

This bonding is common in molecules like water (H₂O) and alcohols (CH₃OH), playing a crucial role in determining their physical properties.

Electronegativity

Electronegativity is the tendency of an atom to attract a shared pair of electrons towards itself in a chemical bond. It plays a critical role in determining the type and strength of chemical bonds. In the context of hydrogen bonding, the higher the electronegativity of the atom bonded to hydrogen, the more pronounced the hydrogen bonding.

Electronegative atoms often involved in hydrogen bonding are:

• Nitrogen
• Oxygen
• Fluorine

For instance, water molecules exhibit hydrogen bonding because oxygen is highly electronegative and attracts the shared electrons strongly.

Molecular Structure

The molecular structure refers to the three-dimensional arrangement of atoms within a molecule. This arrangement determines the molecule's shape, reactivity, and interactions. For hydrogen bonding to occur, the structure must allow certain atoms to be close enough for interaction.

For example:

• In water (H₂O), the bent shape facilitates hydrogen bonding between the oxygen of one molecule and the hydrogen of another.
• In methanol (CH₃OH), the molecular structure ensures there is an -OH group that can participate in hydrogen bonding.

Understanding molecular structure helps predict physical and chemical behaviors of substances.

Chemical Bonding

Chemical bonding refers to the process where atoms combine by sharing or transferring electrons to form molecules or compounds. Bonds can be classified as:

• Covalent bonds (sharing of electrons)
• Ionic bonds (transfer of electrons)
• Metallic bonds (pooling of electrons)

Hydrogen bonds, while not as strong as these primary bonds, significantly impact the properties of molecules due to their directional nature and formation based on specific interactions.

For example, in water (H₂O), each molecule can form multiple hydrogen bonds, leading to unique properties like high boiling point and surface tension. In contrast, molecules lacking electronegative atoms or hydrogen bonds, like bromine (Br₂), do not exhibit such intermolecular attractions.