Question

Which of the molecules below would not form hydrogen bonds? (a) \(\mathrm{CH}_{3} \mathrm{OH}\) (b) \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{COOH}\) (d) \(\mathrm{NH}_{3}\) (e) None of these would form hydrogen bonds.

Short answer

The correct answer is (b) \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\), as it does not meet the conditions for hydrogen bonding (no H atoms bonded to O, N, or F).

Step-by-step solution

Analyze the structure of each molecule

First, let's examine each molecule for the presence of hydrogen atoms bonded to highly electronegative atoms (O, N, or F). (a) \(\mathrm{CH}_{3} \mathrm{OH}\): There is a hydrogen atom bonded to an oxygen atom. (b) \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\): There is no hydrogen atom bonded to a highly electronegative atom (O, N, or F). (c) \(\mathrm{CH}_{3} \mathrm{COOH}\): There is a hydrogen atom bonded to an oxygen atom. (d) \(\mathrm{NH}_{3}\): There is a hydrogen atom bonded to a nitrogen atom.

Check for unshared electron pairs on electronegative atoms

Next, we'll verify if there are any unshared electron pairs on highly electronegative atoms in the vicinity of the hydrogen atom bonded to an electronegative atom. (a) \(\mathrm{CH}_{3} \mathrm{OH}\): The oxygen atom has two unshared electron pairs. (b) \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\): There are unshared electron pairs on the oxygen atom, but the hydrogen atoms are not bonded to an electronegative atom. (c) \(\mathrm{CH}_{3} \mathrm{COOH}\): The oxygen atom in the carboxyl group has unshared electron pairs. (d) \(\mathrm{NH}_{3}\): The nitrogen atom has one unshared electron pair.

Identify which molecules can form hydrogen bonds

Now we'll determine which of the molecules satisfy the conditions for hydrogen bonding: that is, a hydrogen atom bonded to a highly electronegative atom (O, N, or F) and an unshared electron pair on another highly electronegative atom. (a) \(\mathrm{CH}_{3} \mathrm{OH}\): Meets the conditions for hydrogen bonding (H-O bond and unshared electrons on O). (b) \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\): Does not meet the conditions for hydrogen bonding (no H atoms bonded to O, N, or F). (c) \(\mathrm{CH}_{3} \mathrm{COOH}\): Meets the conditions for hydrogen bonding (H-O bond and unshared electrons on O). (d) \(\mathrm{NH}_{3}\): Meets the conditions for hydrogen bonding (H-N bond and unshared electrons on N).

Choose the correct answer

As per our analysis, the molecule \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) is not able to form hydrogen bonds based on its structure and the absence of a hydrogen atom bonded to a highly electronegative atom. The correct answer is (b) \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\).

Key concepts

Hydrogen Bonding

Hydrogen bonding is a special type of chemical bond that occurs when a hydrogen atom is attracted to an electronegative atom, typically oxygen, nitrogen, or fluorine. This attraction happens because the hydrogen atom, when bonded to these electronegative atoms, becomes slightly positive. As a result, it can attract neighboring electronegative atoms that have unshared electron pairs.

For a molecule to exhibit hydrogen bonding, it must have:

• A hydrogen atom that's directly bonded to an electronegative atom (such as O, N, or F).
• Another electronegative atom nearby, which has an unshared pair of electrons to interact with the hydrogen atom.

Hydrogen bonds are weaker than covalent bonds but stronger than van der Waals forces. They play a crucial role in determining the structure and properties of water, proteins, and DNA.

Molecular Structure

Understanding the molecular structure is key to predicting whether a molecule can form hydrogen bonds. Molecular structure refers to the arrangement of atoms in a molecule and the chemical bonds that hold these atoms together.

When analyzing a molecule, look for:

• The presence of hydrogen atoms and their attachment points on the molecule.
• Electronegative atoms that are bonded to these hydrogen atoms.
• Unshared electron pairs on these electronegative atoms.

A molecule like \( \mathrm{CH}_{3} \mathrm{OH} \) clearly showcases a hydrogen atom bonded to oxygen, an electronegative atom, which is a favorable structure for hydrogen bonding. Understanding molecular structure helps us predict physical properties like boiling and melting points, solubility, and molecular interaction.

Electronegativity

Electronegativity is the ability of an atom to attract electrons towards itself in a chemical bond. In the context of hydrogen bonding, electronegativity is vital, as only highly electronegative atoms such as oxygen, nitrogen, and fluorine can pull on hydrogen atoms strong enough to form this bond.

Here's why electronegativity matters:

• It governs the distribution of electron density in a molecule, influencing the polarity of bonds.
• Atoms with high electronegativity can create significant partial charges when they bond with hydrogen, paving the way for hydrogen bonding.

Electronegativity differences between atoms lead to polar bonds, which is why \( \mathrm{CH}_{3} \mathrm{OH} \) and \( \mathrm{NH}_{3} \) both have highly polar H-O and H-N bonds critical for hydrogen bond formation.

Unshared Electron Pairs

Unshared electron pairs, also known as lone pairs, are pairs of valence electrons that are not shared between atoms in a molecule. These lone pairs are essential for hydrogen bonding as they provide a site for hydrogen atoms to "bond" with.

For effective hydrogen bonding to occur, we need:

• An electronegative atom with unshared electron pairs as part of its electron configuration, such as oxygen or nitrogen.
• These lone pairs to be accessible to the hydrogen atoms from nearby molecules.

Take \( \mathrm{NH}_{3} \) as an example, it has a nitrogen atom with an unshared pair of electrons, providing a perfect site for hydrogen atoms from other ammonia molecules to form bonds. These unshared electron pairs are central to the formation of many molecular structures and help stabilize interactions in large biological structures.