Question

Which of the following would show hydrogen bonding? (a) \(\mathrm{CH}_{3} \mathrm{~F}\) (b) \(\mathrm{HO}-\mathrm{OH}\) (c) \(\mathrm{NH}_{3}\) (d) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{O}-\mathrm{CH}_{3}\)

Short answer

(a) \(\mathrm{CH}_{3} \mathrm{F}\) (b) \(\mathrm{HO}-\mathrm{OH}\) (c) \(\mathrm{NH}_{3}\) (d) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{O}-\mathrm{CH}_{3}\) Answer: (b) \(\mathrm{HO}-\mathrm{OH}\) and (c) \(\mathrm{NH}_{3}\)

Step-by-step solution

Analyze Molecule (a) - \(\mathrm{CH}_{3} \mathrm{F}\)

The first molecule features a covalent bond between carbon and fluorine, with three hydrogen atoms bonded to the carbon. However, none of the hydrogen atoms are attached to a highly electronegative atom such as nitrogen, oxygen, or fluorine. Therefore, hydrogen bonding cannot occur in this molecule.

Analyze Molecule (b) - \(\mathrm{HO}-\mathrm{OH}\)

The second molecule is water, with an oxygen atom sharing covalent bonds with two hydrogen atoms. Oxygen is a highly electronegative atom, and hydrogen bonding can occur between water molecules. The hydrogens covalently bonded to oxygen will also be attracted to the oxygen atoms of other nearby water molecules, creating hydrogen bonds. So, this molecule can show hydrogen bonding.

Analyze Molecule (c) - \(\mathrm{NH}_{3}\)

In this molecule, a nitrogen atom shares covalent bonds with three hydrogen atoms. Nitrogen is also a highly electronegative atom, resulting in hydrogen bonding between different ammonia molecules. The hydrogens covalently bonded to nitrogen will be attracted to the nitrogen atoms of other nearby ammonia molecules, establishing hydrogen bonds. Therefore, this molecule can also show hydrogen bonding.

Analyze Molecule (d) - \(\mathrm{H}_{3} \mathrm{C}-\mathrm{O}-\mathrm{CH}_{3}\)

This molecule has an oxygen atom sharing covalent bonds with two carbon atoms. The two \(\mathrm{H}_{3} \mathrm{C}\) groups on either side of the oxygen cannot form hydrogen bonds because they are bonded to carbon, not a highly electronegative atom. Consequently, this molecule cannot show hydrogen bonding as there are no suitable hydrogen donors or acceptors within the molecule.

Conclusion

Based on the analysis of the four molecules, the ones that can show hydrogen bonding are: (b) \(\mathrm{HO}-\mathrm{OH}\) (c) \(\mathrm{NH}_{3}\)

Key concepts

Electronegativity

Electronegativity is a crucial concept in understanding chemical bonding, particularly hydrogen bonding. It refers to the ability of an atom to attract shared electrons in a covalent bond. The electronegativity of an atom affects how electrons are distributed in a bond, leading to partial charges. Atoms such as fluorine, oxygen, and nitrogen are particularly electronegative.

When hydrogen bonds with these electronegative atoms, the bond takes on a polar character. This creates a difference in electric charge that can lead to what is called hydrogen bonding. The hydrogen atom, which has a partial positive charge, is attracted to a neighboring electronegative atom with a partial negative charge.

• Fluorine is the most electronegative element, drawing electrons towards itself more than any other atom.
• Oxygen comes just after fluorine and significantly affects the polarity of molecules like water.
• Nitrogen, also highly electronegative, is key in hydrogen bonding found in ammonia.

This ability of hydrogen atoms in certain compounds to bond with electronegative atoms is critical for the structure and properties of many substances, including water and ammonia.

Covalent Bonds

Covalent bonds are a type of chemical connection where two atoms share one or more pairs of electrons. They are fundamental to the structure of molecules, including those that can engage in hydrogen bonding. These bonds can be either polar or nonpolar, depending on the difference in electronegativity between the bonded atoms.

In polar covalent bonds, the electrons are not shared equally. One side of the bond becomes slightly more negative, while the other becomes slightly more positive. This occurs with electronegative atoms such as oxygen, nitrogen, and fluorine.

• In water ( HO- OH), the oxygen atom forms covalent bonds with hydrogen, creating a polar molecule due to the difference in electronegativity.
• In ammonia ( NH_3), nitrogen shares electrons with three hydrogens, likewise resulting in a polar molecule.

These polar bonds cause one side of the molecule to become partially charged, allowing for interactions like hydrogen bonding, where the positive hydrogen end is attracted to the negative end of another molecule.

Water Molecules

Water molecules are a prime example of hydrogen bonding, showcasing the role of electronegativity and covalent bonds. Composed of two hydrogen atoms and one oxygen atom, water has a bent molecular shape and is polar.

The strong electronegativity of the oxygen atom draws shared electrons from the hydrogen atoms, resulting in partially positive hydrogen atoms and a partially negative oxygen atom. This polarity facilitates hydrogen bonds with nearby water molecules.

• The oxygen in one water molecule is attracted to the hydrogen in another, forming these bonds.
• These interactions give water its high boiling point and surface tension.
• They also enable water's unique properties such as its solid ice being less dense than liquid water, allowing ice to float.

Water's hydrogen bonds are essential for many of its life-supporting properties, making it a unique and vital compound in both chemistry and biology.