Question

Which of the following species are capable of hydrogenbonding among themselves: (a) \(\mathrm{C}_{2} \mathrm{H}_{6},\) (b) \(\mathrm{HI}\) (c) KF, (d) \(\mathrm{BeH}_{2}\) (e) \(\mathrm{CH}_{3} \mathrm{COOH}\) ?

Short answer

Only \( \mathrm{CH}_{3} \mathrm{COOH} \) can hydrogen bond among themselves.

Step-by-step solution

Identify the condition for hydrogen bonding

Hydrogen bonding occurs between molecules when a hydrogen atom is covalently bonded to a highly electronegative atom (such as F, O, or N), causing a strong dipole-dipole attraction with another electronegative atom nearby.

Analyze each compound

(a) \( \mathrm{C}_{2} \mathrm{H}_{6} \) lacks atoms like F, O, or N, hence cannot form hydrogen bonds. (b) \( \mathrm{HI} \) has hydrogen, but iodine is not sufficiently electronegative to support hydrogen bonding. (c) KF is ionic, not molecular, so it doesn't concern hydrogen bonding. (d) \( \mathrm{BeH}_{2} \) contains hydrogen but lacks F, O, or N, so it cannot engage in hydrogen bonding.(e) \( \mathrm{CH}_{3} \mathrm{COOH} \) contains hydrogen bonded to oxygen (a highly electronegative atom), allowing hydrogen bonds.

Conclusion

Only \( \mathrm{CH}_{3} \mathrm{COOH} \) (acetic acid) has the necessary O-H bond that allows for hydrogen bonding with other \( \mathrm{CH}_{3} \mathrm{COOH} \) molecules due to the presence of hydrogen bonded to oxygen (a highly electronegative atom).

Key concepts

Dipole-Dipole Attraction

Dipole-dipole attraction is a type of intermolecular force that acts between polar molecules. These attractions occur when the positive end of one molecule is attracted to the negative end of another molecule. This creates a directional force that holds molecules together.
One reason dipole-dipole forces are important is that they can affect the physical properties of substances, like boiling and melting points. For example, substances with strong dipole-dipole interactions generally have higher boiling points than those with weak interactions.
Hydrogen bonding is a specific type of dipole-dipole attraction that occurs in molecules where hydrogen is directly bonded to highly electronegative atoms, such as fluorine, oxygen, or nitrogen. The significant difference in electronegativity creates a strong dipole with hydrogen having a partial positive charge and the electronegative atom having a partial negative charge.

• Occurs in polar molecules with a permanent dipole.
• Strength depends on the magnitude of the molecular dipoles.
• Affects boiling and melting points, and solubility of substances.

Electronegative Atoms

Electronegative atoms have a strong tendency to attract electrons, and in covalent bonds, they pull shared electrons towards themselves. This property is crucial for the formation of polar molecules and subsequent intermolecular forces like dipole-dipole attractions.
Common highly electronegative atoms include fluorine, oxygen, and nitrogen. Due to their high electronegativity, when these atoms are bonded to hydrogen, they create significant partial charges on the atoms involved. This partial charge difference is what allows hydrogen bonding to occur.
The role of electronegative atoms is crucial in determining whether a molecule can participate in hydrogen bonding. Molecules like water (H2O) and acetic acid (CH3COOH), where hydrogen is bonded to oxygen, an electronegative atom, can engage in hydrogen bonding with each other.

• Key characters in hydrogen bonding interactions.
• Most relevant examples are F, O, N.
• Create significant dipoles in chemical bonds.

Molecular Interactions

Molecular interactions describe how molecules behave and interact with each other based on the forces acting between them. These interactions can vary in strength and type from weak van der Waals forces to stronger ionic bonds. A key type of molecular interaction is hydrogen bonding.
Hydrogen bonding is significant in determining the physical properties of substances like solubility, boiling, and melting points. It is especially important in biological systems, where it stabilizes the structures of proteins and nucleic acids.
When molecules contain polar bonds formed with highly electronegative atoms, they may engage in hydrogen bonding. These molecular interactions contribute to the unique properties of substances like water and acetic acid, giving them abilities to dissolve many compounds and support varied chemical reactions.

• Includes both weak (van der Waals) and strong (hydrogen bonds) forces.
• Critical in biological and chemical systems.
• Affects physical properties such as solubility and boiling points.

Acetic Acid

Acetic acid (CH3COOH) is a colorless liquid that has a distinct pungent smell often associated with vinegar. It is an example of a carboxylic acid, characterized by the presence of a carboxyl group (-COOH).
In acetic acid, the OH group creates the specific conditions necessary for hydrogen bonding due to oxygen's high electronegativity. This means acetic acid molecules can hydrogen bond with each other, which contributes to its relatively high boiling point compared to other substances of similar molecular weight.
This ability to form hydrogen bonds also makes acetic acid a good solvent for polar compounds, and it plays a crucial role in many organic reactions as both a solvent and a reactant. Schools or laboratories often use acetic acid diluted in water, known as vinegar, for various experiments.

• Contains a hydroxyl group (OH) crucial for hydrogen bonding.
• Acts as a solvent in organic chemistry.
• Boiling point is affected by its hydrogen bond capacity.