Problem 7 - Chemistry and Chemical Reactivity

Question

Which of the following compounds would be expected to form intermolecular hydrogen bonds in the liquid state? (a) \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) (dimethyl ether) (b) \(\mathrm{CH}_{4}\) (c) HF (d) \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) (acetic acid) (e) \(\mathrm{Br}_{2}\) (f) \(\mathrm{CH}_{3} \mathrm{OH}\) (methanol)

Step-by-Step Solution

Answer

HF, acetic acid (\(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\)) and methanol (\(\mathrm{CH}_{3} \mathrm{OH}\)) form hydrogen bonds.

Step 1: Identify H-Bond Donors

Hydrogen bonding occurs when a hydrogen atom is covalently bonded to a highly electronegative atom such as oxygen, nitrogen, or fluorine, imparting a significant partial positive charge on the hydrogen atom. We need to look for such bonds in the compounds listed.

Step 2: Analyze Dimethyl Ether (\(\mathrm{CH}_{3} \mathrm{OCH}_{3}\))

Dimethyl ether has an oxygen atom, which can serve as an H-bond acceptor, but it lacks hydrogen atoms directly bonded to an electronegative atom like O, N, or F, so it does not act as a donor for hydrogen bonding.

Step 3: Consider Methane (\(\mathrm{CH}_{4}\))

Methane has carbon-hydrogen bonds. Carbon is not electronegative enough to polarize the hydrogen and create hydrogen bonds. Hence, \(\mathrm{CH}_{4}\) does not form hydrogen bonds.

Step 4: Evaluate Hydrogen Fluoride (HF)

HF contains hydrogen bonded to fluorine, a highly electronegative atom. It has a strong hydrogen bond due to the large electronegativity difference between H and F, making HF capable of intermolecular hydrogen bonding.

Step 5: Examine Acetic Acid (\(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\))

In acetic acid, the hydroxyl group (\(\mathrm{OH}\)) contains hydrogen bonded to oxygen. The electronegative nature of oxygen will enable strong intermolecular hydrogen bonding with other acetic acid molecules.

Step 6: Analyze Bromine (\(\mathrm{Br}_{2}\))

\(\mathrm{Br}_{2}\) is a molecule of two halogen atoms with no hydrogen, hence it cannot form hydrogen bonds since it lacks any hydrogen atoms.

Step 7: Review Methanol (\(\mathrm{CH}_{3} \mathrm{OH}\))

Methanol has an \(\mathrm{OH}\) group just like acetic acid, meaning it can engage in hydrogen bonding. The hydrogen attached to oxygen is an H-bond donor, allowing methanol to form hydrogen bonds.

Key Concepts

Hydrogen Bond Donors

Hydrogen bond donors are essential parts of intermolecular hydrogen bonding. They typically involve hydrogen atoms bonded to highly electronegative elements like oxygen, nitrogen, or fluorine.
This creates a significant positive charge on the hydrogen, allowing it to bond with electronegative atoms in other molecules. Identifying these donors is crucial for determining if a compound can form hydrogen bonds.
In the exercise, we looked for H-bond donors in compounds such as HF and acetic acid, which both qualify because they have hydrogen atoms bonded to electronegative atoms like fluorine and oxygen respectively.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons. In the context of hydrogen bonding, this property leads to unequal sharing of electrons when hydrogen is bonded to electronegative atoms (O, N, or F).

• High electronegativity results in partial charges forming across a bond, creating dipoles.
• The stronger the difference in electronegativity between two bonded atoms, the stronger the dipole.

Understanding electronegativity helps explain why a compound like HF forms strong intermolecular hydrogen bonds, as the difference between H and F is significant.

Polar Molecules

Polar molecules occur when there is an uneven distribution of electron density, leading to partial charges across the molecule. Polarity results from differences in electronegativity between bonded atoms.

• Polar molecules like HF and methanol are prime candidates for hydrogen bonding due to their ability to form these significant dipole moments.
• The asymmetrical shape of the molecule can also contribute to its overall polarity.

In the exercise, molecules that are polar, such as acetic acid and methanol, show the capacity for hydrogen bonding because their polar nature facilitates these connections.

Chemical Compounds

Chemical compounds can be classified based on their ability to form hydrogen bonds, understanding the types and interactions in molecules is essential. For instance, bromine (\(\mathrm{Br}_{2}\)) is a non-polar molecule as it consists of two identical atoms without partial charges.

• Compounds like methane (\(\mathrm{CH}_{4}\)) do not participate in hydrogen bonding as they lack H-bond donors and significant polarity.
• Conversely, compounds with H-bond donors and polar characteristics like methanol and HF can participate in hydrogen bonds.

The presence or absence of hydrogen bonds is often what differentiates the behavior and properties of these compounds in different states.