Question

Which member of each pair of compounds forms intermolecular \(\mathrm{H}\) bonds? Draw the \(\mathrm{H}\) -bonded structures in each case: (a) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\) or \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\) (b) \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) or \(\mathrm{FCH}_{2} \mathrm{CH}_{2} \mathrm{~F}\)

Short answer

In pair (a), \(\left( \mathrm{CH}_{3} \right)_{2} \mathrm{NH}\) forms intermolecular \(\mathrm{H}\) bonds. In pair (b), \(\mathrm{HOCH}_2 \mathrm{CH}_2 \mathrm{OH}\) forms intermolecular \(\mathrm{H}\) bonds.

Step-by-step solution

Understand Hydrogen Bonding

Hydrogen bonds form when hydrogen is bonded to a highly electronegative atom like nitrogen (N), oxygen (O), or fluorine (F). These bonds usually occur between molecules where hydrogen from one molecule interacts with the electronegative atom of another molecule.

Analyze Compound Structures for Pair (a)

Evaluate whether \(\left( \mathrm{CH}_{3} \right)_{2} \mathrm{NH}\) or \(\left( \mathrm{CH}_{3} \right)_{3} \mathrm{N}\) can form hydrogen bonds. \(\left( \mathrm{CH}_{3} \right)_{2} \mathrm{NH}\) contains a nitrogen bonded to a hydrogen, which allows it to form hydrogen bonds. \(\left( \mathrm{CH}_{3} \right)_{3} \mathrm{~N}\), however, does not have a hydrogen bonded directly to the nitrogen, so it cannot form hydrogen bonds.

Draw the Hydrogen-Bonded Structure for (a)

Illustrate how \(\left( \mathrm{CH}_{3} \right)_{2} \mathrm{NH}\) molecules interact through hydrogen bonds. \[ (\mathrm{CH}_3)_2 \mathrm{NH} \quad \cdots \quad \mathrm{H}-N(\mathrm{CH}_3)_2 \] The dashed line represents the hydrogen bond between the hydrogen of one molecule and the nitrogen of another.

Analyze Compound Structures for Pair (b)

Evaluate whether \(\mathrm{HOCH}_2 \mathrm{CH}_2 \mathrm{OH}\) or \(\mathrm{FCH}_2 \mathrm{CH}_2 \mathrm{~F}\) can form hydrogen bonds. \(\mathrm{HOCH}_2 \mathrm{CH}_2 \mathrm{OH}\) contains hydroxyl groups (\(\mathrm{OH}\)) that can participate in hydrogen bonding. \(\mathrm{FCH}_2 \mathrm{CH}_2 \mathrm{~F}\) does not have any hydrogen directly bonded to highly electronegative atoms, so it cannot form hydrogen bonds.

Draw the Hydrogen-Bonded Structure for (b)

Illustrate how \(\mathrm{HOCH}_2 \mathrm{CH}_2 \mathrm{OH}\) molecules interact through hydrogen bonds. \[ \mathrm{HO-CH}_2 \mathrm{CH}_2 \mathrm{OH} \quad \cdots \quad \mathrm{O-H} \mathrm{C}\mathrm{H}_2 \mathrm{CH}_2 \mathrm{OH} \quad \cdots \quad \mathrm{O}-H\mathrm{ } \mathrm{H}\mathrm{OCH}_2 \mathrm{CH}_2 \mathrm{OH} \] The dashed lines represent the hydrogen bonds between the hydrogen of one molecule’s hydroxyl group and the oxygen of another molecule’s hydroxyl group.

Key concepts

Hydrogen Bonds

Hydrogen bonds are a special type of intermolecular force that occurs when a hydrogen atom is bonded to a highly electronegative atom, like nitrogen (N), oxygen (O), or fluorine (F). This bond arises because the hydrogen atom becomes slightly positive, while the electronegative atom becomes slightly negative. The positive hydrogen is then attracted to the negative part of another molecule, forming a hydrogen bond. Hydrogen bonds are crucial in many biological molecules, like DNA and proteins, stabilizing their structures. Understanding hydrogen bonds helps explain the properties of water and many other compounds.

Intermolecular Interactions

Intermolecular interactions are forces that occur between molecules. They are essential for determining the properties of substances, such as boiling points and solubility. Besides hydrogen bonds, other types of intermolecular forces include dipole-dipole interactions and London dispersion forces. Hydrogen bonds are generally stronger than these other forces.
Intermolecular interactions are vital in compound structure analysis and help us understand why certain substances behave the way they do. For example, the melting and boiling points of substances generally increase with stronger intermolecular forces due to needing more energy to break them.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold on to electrons. In the context of hydrogen bonding, it refers to how strongly atoms like N, O, or F pull electrons towards themselves when bonded with hydrogen. Higher electronegativity means stronger partial charges and more robust hydrogen bonds.
Electronegativity helps explain why some substances can form hydrogen bonds while others cannot. For instance, in the pairs of compounds mentioned in the exercise, \(\text{(CH\textsubscript{3})\textsubscript{2}NH}\) forms hydrogen bonds because nitrogen has high electronegativity and is bonded to hydrogen, whereas \(\text{(CH\textsubscript{3})\textsubscript{3}N}\) cannot, as it lacks this bonding configuration.

Compound Structure Analysis

Analyzing the structures of compounds helps us understand how they interact with each other. In terms of hydrogen bonding, look for hydrogen attached to highly electronegative atoms like N, O, or F. This feature indicates the potential for hydrogen bonds.
For example, in pair (a) of the exercise, \( \text{(CH\textsubscript{3})\textsubscript{2}NH} \) has nitrogen bonded directly to hydrogen, allowing for hydrogen bonds. In contrast, \( \text{(CH\textsubscript{3})\textsubscript{3}N} \) does not have such a bond and cannot form hydrogen bonds. Similarly, in pair (b), \(\text{HOCH\textsubscript{2}CH\textsubscript{2}OH}}\) can form hydrogen bonds due to the hydroxyl groups, while \(\text{FCH\textsubscript{2}CH\textsubscript{2}F}}\) cannot, as it lacks hydrogen bonded to an electronegative atom.