Textbook Question
Explain why
a. H2O (100 °C) has a higher boiling point than CH3OH 165 °C2.
b. H2O (100 °C) has a higher boiling point than NH3 (-33 °C).
1278
views
2. Molecular Representations
Intermolecular Forces
6:23 minutesProblem 6d,e,f
Textbook Question
For each pair of compounds, circle the compound you expect to have the higher boiling point. Explain your reasoning.
(d) HOCH2—(CH2)4—CH2OH or (CH3)3CCH(OH)CH3
(e) (CH3CH2CH2)2NH or (CH3CH2)3N
(f) 
Verified step by step guidance1
Step 1: Understand the concept of boiling point. Boiling point is the temperature at which a substance transitions from a liquid to a gas. It is influenced by intermolecular forces such as hydrogen bonding, dipole-dipole interactions, and van der Waals forces.
Step 2: Analyze the molecular structure of each compound. For compound d, HOCH2—(CH2)4—CH2OH is a linear diol, while (CH3)3CCH(OH)CH3 is a branched alcohol. Linear molecules generally have stronger van der Waals forces due to greater surface area, potentially leading to higher boiling points.
Step 3: Consider hydrogen bonding. Both compounds in pair d can form hydrogen bonds due to the presence of hydroxyl groups (-OH). However, the linear diol can form more extensive hydrogen bonding networks compared to the branched alcohol, which may increase its boiling point.
Step 4: Examine the nitrogen-containing compounds in pair e. (CH3CH2CH2)2NH is a secondary amine, while (CH3CH2)3N is a tertiary amine. Secondary amines can form hydrogen bonds, whereas tertiary amines cannot, due to the lack of a hydrogen atom bonded to nitrogen. This suggests that the secondary amine may have a higher boiling point.
Step 5: Analyze the image for pair f. The image shows two cyclic amines: one with an NH group and the other with an NH2 group. The NH2 group can form more hydrogen bonds compared to the NH group, potentially leading to a higher boiling point for the compound with the NH2 group.
Verified video answer for a similar problem:This video solution was recommended by our tutors as helpful for the problem above
Video duration:
6mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Hydrogen Bonding
Hydrogen bonding is a strong type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. This interaction significantly increases the boiling point of a compound because more energy is required to break these bonds during the phase transition from liquid to gas. In the context of the question, compounds with more hydrogen bonding sites will generally have higher boiling points.
Recommended video:
Guided course
00:48
The definition of hydrogenation.
Molecular Structure and Surface Area
The molecular structure and surface area of a compound affect its boiling point. Linear or less branched molecules have greater surface area, leading to stronger van der Waals forces, which increase boiling points. In contrast, more compact or branched structures have less surface area, resulting in weaker intermolecular forces and lower boiling points. This concept helps in comparing the boiling points of the given compounds based on their structural differences.
Recommended video:
Guided course
08:41Review of Molecular Orbitals
Functional Groups and Polarity
Functional groups in organic molecules influence their polarity and, consequently, their boiling points. Polar functional groups, such as hydroxyl (OH) and amino (NH2) groups, enhance intermolecular attractions like hydrogen bonding, raising the boiling point. In the given question, the presence and type of functional groups in each compound determine their relative boiling points by affecting the overall polarity and intermolecular interactions.
Recommended video:
Guided course
02:36Identifying Functional Groups
Related Videos
Related Practice