Problem 12 - Chemistry

Question

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

Step-by-Step Solution

Answer

Among the given species, only \( \mathrm{CH}_{3} \mathrm{COOH} \) is capable of forming hydrogen bonds among itself.

Step 1: Evaluating \( \mathrm{C}_{2} \mathrm{H}_{6} \)

Hydrogen is not bonded to any of the electronegative atoms (N,O, or F) in \( \mathrm{C}_{2} \mathrm{H}_{6} \) so it does not form hydrogen bonds.

Step 2: Evaluating HI

Though Hydrogen exists in this molecule, it is bonded to Iodine (I) which is not one of the three atoms necessary for hydrogen bonding (N,O,F). So, HI does not form hydrogen bonds.

Step 3: Evaluating KF

Here, Hydrogen doesn't exist, so KF cannot form hydrogen bonds.

Step 4: Evaluating \( \mathrm{BeH}_{2} \)

Here the Hydrogen is not bonded to any of the required atoms (N,O, or F) for hydrogen bonding. Thus, \( \mathrm{BeH}_{2} \) does not form hydrogen bonds.

Step 5: Evaluating \( \mathrm{CH}_{3} \mathrm{COOH} \)

In \( \mathrm{CH}_{3} \mathrm{COOH} \), Hydrogen is bonded with Oxygen, one of the atoms required for hydrogen bonding. So, \( \mathrm{CH}_{3} \mathrm{COOH} \) can form hydrogen bonds among itself.

Key Concepts

Intermolecular Forces

Intermolecular forces are attractions that exist between molecules, and they play a crucial role in determining the physical properties of substances. One of the most significant types of intermolecular forces is hydrogen bonding. Hydrogen bonds are specific types of dipole-dipole interactions. They occur when hydrogen atoms are bonded to highly electronegative atoms, such as nitrogen (N), oxygen (O), or fluorine (F).
When hydrogen bonds with these elements, it creates a strong dipole due to the significant difference in electronegativity. This dipole causes a partial positive charge on the hydrogen, which then attracts the lone pair electrons of a nearby electronegative atom from another molecule. This attraction forms a hydrogen bond.

• These bonds are essential in many biological and chemical processes.
• They can significantly affect a molecule's boiling and melting points, making such substances crucial in understanding many chemical phenomena.

Without hydrogen bonds, life as we know it would not exist, as these bonds are foundational for the structure of water and biological macromolecules like DNA and proteins.

Molecular Structure

Molecular structure refers to the arrangement of atoms within a molecule. It directly influences the molecule's ability to form hydrogen bonds. For a molecule to participate in hydrogen bonding, it must contain hydrogen atoms bonded to N, O, or F.
The molecular structure impacts how the atoms are arranged and consequently how they interact. For example, in water, the bent shape and arrangement of its O-H bonds make it highly effective in forming hydrogen bonds.
When examining molecules like ethanol \((C_2H_5OH)\) or acetic acid \((CH_3COOH)\), the presence of -OH groups allows these molecules to engage in hydrogen bonding. Species like ethane \((C_2H_6)\) don’t have the necessary structural components for hydrogen bonding because the hydrogen atoms are bonded to carbon, which isn't sufficiently electronegative. The correct molecular structure aligns the hydrogen atoms optimally with electronegative atoms, enabling these important interactions.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold electrons. It is crucial when studying hydrogen bonding because only atoms with high electronegativity — like N, O, and F — can form hydrogen bonds with hydrogen.
In molecules, the electronegativity difference between bonded atoms determines the polarity of the bond. A higher difference leads to more polar bonds, which in hydrogen bonding causes the hydrogen to acquire a partial positive charge.

• This partial positive charge is what allows hydrogen to form an attractive force with electronegative atoms from other molecules, resulting in a hydrogen bond.
• Molecules with high electronegativity differences between atoms are usually more likely to engage in hydrogen bonding.

Understanding electronegativity not only helps predict hydrogen bonding but also gives us a deeper insight into molecule interaction, stability, and behavior in different environments and reactions.