Problem 41
Question
Which of the following molecules can hydrogen-bond among themselves in pure samples of bulk material? (a) methanol \(\left(\mathrm{CH}_{3} \mathrm{OH}\right) ;\) (b) ethane \(\left(\mathrm{CH}_{3} \mathrm{CH}_{3}\right) ;\) (c) dimethyl cther \(\left(\mathrm{CH}_{3} \mathrm{OCH}_{3}\right) ;\) (d) acctic acid \(\left(\mathrm{CH}_{3} \mathrm{COOH}\right)\)
Step-by-Step Solution
Verified Answer
Answer: Methanol (a) and Acetic acid (d) can form hydrogen bonds among themselves in pure samples of bulk material.
1Step 1: Examine the molecular structure of each molecule.
To determine if hydrogen bonding can occur, we should first take a close look at the molecular structure of each given molecule.
(a) Methanol: \(\mathrm{CH}_{3} \mathrm{OH}\)
(b) Ethane: \(\mathrm{CH}_{3} \mathrm{CH}_{3}\)
(c) Dimethyl ether: \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\)
(d) Acetic acid: \(\mathrm{CH}_{3} \mathrm{COOH}\)
2Step 2: Identify the electronegative atoms and hydrogen bonds.
Now that we have the molecular structures, let's check for the presence of electronegative atoms that are bonded to hydrogen atoms and can participate in hydrogen bonding.
(a) Methanol: The molecule consists of a hydroxyl (OH) group, where oxygen (O) is bonded to a hydrogen (H) atom. Oxygen is an electronegative atom, so methanol can participate in hydrogen bonding.
(b) Ethane: The molecule consists of carbon (C) and hydrogen (H) atoms. It doesn't have any electronegative atoms like nitrogen (N), oxygen (O), or fluorine (F) bonded to hydrogens, so ethane cannot participate in hydrogen bonding.
(c) Dimethyl ether: The molecule contains an oxygen (O) atom, but it is not bonded to any hydrogen atoms. Therefore, dimethyl ether cannot participate in hydrogen bonding.
(d) Acetic acid: The molecule has a carboxyl (COOH) group, where oxygen (O) is bonded to a hydrogen (H) atom. Oxygen is an electronegative atom, so acetic acid can participate in hydrogen bonding.
3Step 3: Determine which molecules can hydrogen-bond among themselves.
Based on the analysis, we now know that:
(a) Methanol can hydrogen bond among itself.
(b) Ethane cannot hydrogen bond among itself.
(c) Dimethyl ether cannot hydrogen bond among itself.
(d) Acetic acid can hydrogen bond among itself.
Therefore, methanol (a) and acetic acid (d) are the molecules that can hydrogen-bond among themselves in pure samples of bulk material.
Key Concepts
Molecular StructureElectronegative AtomsIntermolecular Forces
Molecular Structure
Understanding molecular structure is essential when discussing hydrogen bonding. Molecular structure refers to how atoms are arranged and connected within a molecule. This arrangement dictates the chemical properties and interactions of the molecule, such as whether hydrogen bonds can form.
For example, methanol has the molecular formula \( \mathrm{CH}_3 \mathrm{OH} \), indicating a hydroxyl group attached to a carbon chain. The arrangement of atoms affects how the molecule interacts with others.
Similarly, acetic acid \( \mathrm{CH}_3 \mathrm{COOH} \) has a carboxyl group, which is another key structural feature that influences its ability to engage in hydrogen bonding. Structural features can reveal which molecules have potential sites for hydrogen bonding.
For example, methanol has the molecular formula \( \mathrm{CH}_3 \mathrm{OH} \), indicating a hydroxyl group attached to a carbon chain. The arrangement of atoms affects how the molecule interacts with others.
Similarly, acetic acid \( \mathrm{CH}_3 \mathrm{COOH} \) has a carboxyl group, which is another key structural feature that influences its ability to engage in hydrogen bonding. Structural features can reveal which molecules have potential sites for hydrogen bonding.
Electronegative Atoms
Electronegative atoms are critical in the formation of hydrogen bonds. These are atoms with a strong tendency to attract electrons. In hydrogen bonding, the most common electronegative atoms involved are oxygen (O), nitrogen (N), and fluorine (F).
For instance, in methanol, the oxygen atom is highly electronegative. It forms partial negative charges when bonded with hydrogen, enabling the formation of hydrogen bonds.
In ethane \( \mathrm{CH}_3 \mathrm{CH}_3 \), there are no such electronegative atoms bonded to hydrogen, preventing hydrogen bonds. Focusing on the presence of electronegative atoms can quickly reveal potential for hydrogen bonding in a molecule.
For instance, in methanol, the oxygen atom is highly electronegative. It forms partial negative charges when bonded with hydrogen, enabling the formation of hydrogen bonds.
In ethane \( \mathrm{CH}_3 \mathrm{CH}_3 \), there are no such electronegative atoms bonded to hydrogen, preventing hydrogen bonds. Focusing on the presence of electronegative atoms can quickly reveal potential for hydrogen bonding in a molecule.
Intermolecular Forces
Intermolecular forces are the forces of attraction or repulsion between neighboring molecules. These forces significantly impact the physical properties of substances, such as boiling and melting points. Hydrogen bonding is a type of strong intermolecular force that occurs when hydrogen is bonded to a highly electronegative atom.
In acetic acid \( \mathrm{CH}_3 \mathrm{COOH} \), hydrogen bonds can form between the electronegative oxygen in one molecule and the hydrogen on another. This interaction leads to stronger cohesion between molecules, influencing the substance's properties.
Conversely, dimethyl ether \( \mathrm{CH}_3 \mathrm{OCH}_3 \) lacks hydrogen attached directly to an electronegative atom, so it only experiences weaker forces like Van der Waals interactions. Recognizing these forces allows us to predict molecular interactions and their effects on material properties.
In acetic acid \( \mathrm{CH}_3 \mathrm{COOH} \), hydrogen bonds can form between the electronegative oxygen in one molecule and the hydrogen on another. This interaction leads to stronger cohesion between molecules, influencing the substance's properties.
Conversely, dimethyl ether \( \mathrm{CH}_3 \mathrm{OCH}_3 \) lacks hydrogen attached directly to an electronegative atom, so it only experiences weaker forces like Van der Waals interactions. Recognizing these forces allows us to predict molecular interactions and their effects on material properties.
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