Problem 113 - Chemistry A Molecular Approach

Question

The boiling points of three compounds are tabulated here. $$ \begin{array}{lcc} & \text { Molar Mass } & \text { Boiling Point } \\ \hline \text { 2-hexanone } & 100.16 & 128^{\circ} \mathrm{C} \\ \hline \text { heptane } & 100.20 & 98^{\circ} \mathrm{C} \\ \hline \text { 1-hexanol } & 102.17 & 156^{\circ} \mathrm{C} \\ \hline \end{array} $$ Answer the following questions without looking up the structures for these molecules: Which compound experiences hydrogen bonding? Which compound is polar but is unable to experience hydrogen bonding? Which is neither polar nor capable of hydrogen bonding? Explain your answers.

Step-by-Step Solution

Answer

1-hexanol experiences hydrogen bonding, 2-hexanone is polar but cannot experience hydrogen bonding, and heptane is neither polar nor capable of hydrogen bonding.

Step 1: Identify hydrogen bonding capabilities

Hydrogen bonding is a strong type of dipole-dipole interaction that occurs between a hydrogen atom bonded to a highly electronegative atom (like O, N, or F) and another highly electronegative atom. To identify compounds with hydrogen bonding, look for molecules that have hydrogen atoms bonded to O, N, or F atoms.

Step 2: Determine polarity without hydrogen bonding

A molecule can be polar without hydrogen bonding if it has a permanent dipole moment. This means the molecule must have an uneven distribution of electron density, which can occur if there are electronegative atoms not bonded to hydrogen, or if the molecular structure is such that dipoles do not cancel out.

Step 3: Identify nonpolar compounds

A nonpolar molecule typically has a symmetrical structure that allows for the cancellation of dipole moments, or it is made of atoms with similar electronegativities that do not create significant dipoles. Such compounds would not experience dipole-dipole interactions or hydrogen bonding.

Step 4: Apply reasoning to given compounds

1-hexanol has a hydroxyl group (OH), which indicates it is capable of hydrogen bonding. 2-hexanone has a carbonyl group (C=O), making it polar but unable to experience hydrogen bonding because it lacks the hydrogen directly bonded to oxygen. Heptane is an alkane with a similar molar mass to the other compounds and the lowest boiling point, indicating it is nonpolar and does not experience hydrogen bonding.

Key Concepts

Molecular Polarity

Molecular polarity is a key trait that defines the characteristics of a molecule, including its interactions with other molecules and its physical properties. A molecule is considered polar when there is a difference in electronegativity between atoms, leading to an uneven distribution of electric charge. This uneven charge creates a dipole moment, with one end being partially positive and the other partially negative.

For example, water (H₂O) is a highly polar molecule because of the significant difference in electronegativity between hydrogen and oxygen atoms. The electrons spend more time around the oxygen atom, creating a partial negative charge on the oxygen end and a partial positive charge on the hydrogen ends.

In the case of the molecules presented in the exercise, 2-hexanone contains a polar C=O group. However, it lacks the specific O-H bond that would enable hydrogen bonding, which makes its interactions different from those of a molecule like 1-hexanol, which can form hydrogen bonds because of its O-H group.

Intermolecular Forces

Intermolecular forces are the forces that occur between molecules. They significantly influence a molecule’s state of matter, boiling point, and solubility. There are several types of intermolecular forces:

• Dispersion Forces: Also known as London dispersion forces, these are the weakest and occur between all molecules, resulting from temporary dipoles due to electron movement.
• Dipole-Dipole Interactions: These forces occur between polar molecules with permanent dipoles.
• Hydrogen Bonding: A strong type of dipole-dipole interaction specifically involving H atoms bonded to N, O, or F, and another electronegative atom.

1-hexanol experiences hydrogen bonding due to its hydroxyl group, making it the compound with the highest boiling point among the three. 2-hexanone, while polar, can only exhibit dipole-dipole interactions. On the other hand, heptane, being nonpolar, only experiences dispersion forces.

Boiling Point

The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure surrounding the liquid, causing it to turn into a gas. Intermolecular forces directly affect the boiling point. Substances with strong intermolecular forces require higher temperatures to overcome these forces and transition into the gas phase.

In the given examples, 1-hexanol has the highest boiling point due to hydrogen bonding, which is the strongest intermolecular force among the three compounds. 2-hexanone, with only dipole-dipole interactions, has a moderate boiling point. Heptane, limited to dispersion forces due to its nonpolarity, has the lowest boiling point, reflecting its weak intermolecular attractions.

Chemical Structure

Chemical structure plays a pivotal role in determining the properties and the reactivity of compounds. The spatial arrangement of atoms within a molecule dictates whether it is polar or nonpolar, and which type of intermolecular forces can come into play.

A molecule with a symmetrical structure is more likely to be nonpolar because any dipoles present may cancel each other out. Conversely, asymmetry in a molecule can lead to a net dipole. The presence of functional groups, like hydroxyl or carbonyl groups, also influences polarity and the possibility for hydrogen bonding.

For instance, the chemical structure of heptane is symmetrical and consists of carbons and hydrogens with similar electronegativities, rendering it nonpolar. Meanwhile, 1-hexanol features an -OH group, enabling hydrogen bonding, which is absent in the structures of 2-hexanone and heptane.