Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

2. Molecular Representations

Intermolecular Forces

Organic Chemistry

2. Molecular Representations

Intermolecular Forces

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3:42 minutes

Problem 8a

Textbook Question

Choose the molecule in each pair you'd expect to have the higher boiling point. Explain your reasoning.
(a) eicosane ( C₂₀H₄₂) vs pentadecane

Verified step by step guidance

Step 1: Understand the relationship between molecular size and boiling point. Larger molecules generally have higher boiling points due to increased van der Waals forces (London dispersion forces) between molecules.

Step 2: Compare the molecular structures of eicosane (C₂₀H₄₂) and pentadecane (C₁₅H₃₂). Eicosane has 20 carbon atoms, while pentadecane has 15 carbon atoms. The larger molecular size of eicosane results in stronger intermolecular forces.

Step 3: Consider the surface area of the molecules. Eicosane, being larger, has a greater surface area, which enhances the strength of London dispersion forces compared to pentadecane.

Step 4: Analyze the boiling point trend in alkanes. As the number of carbon atoms increases, the boiling point increases due to the cumulative effect of stronger intermolecular forces.

Step 5: Conclude that eicosane (C₂₀H₄₂) is expected to have a higher boiling point than pentadecane (C₁₅H₃₂) based on its larger molecular size and stronger intermolecular forces.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Boiling Point and Molecular Weight

The boiling point of a substance is influenced by its molecular weight; generally, larger molecules have higher boiling points due to increased van der Waals forces. In this case, eicosane (C₂₀H₄₂) has a higher molecular weight than pentadecane (C₁₅H₃₂), suggesting it will have a higher boiling point.

Intermolecular Forces

Intermolecular forces, particularly London dispersion forces, play a crucial role in determining boiling points. Larger hydrocarbons have more surface area, leading to stronger dispersion forces, which require more energy (heat) to overcome, resulting in higher boiling points for larger molecules.

Structural Isomerism

Structural isomerism refers to compounds with the same molecular formula but different structural arrangements. In the context of boiling points, isomers can exhibit different boiling points due to variations in their shapes and the resulting differences in intermolecular interactions, although this is less relevant when comparing straight-chain alkanes like eicosane and pentadecane.

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Related Practice

Textbook Question

List each set of compounds in order of increasing boiling point.

b. Octane, (CH₃)₃C—C(CH₃)₃ and CH₃CH₂C(CH₃)₂CH₂CH₂CH₃

Textbook Question

Choose the molecule in each pair you'd expect to have the higher boiling point. Explain your reasoning.

(b)

Textbook Question

Ionic compounds like sodium acetate have a high melting point. Despite this, they are highly soluble in water. Why?

Textbook Question

Despite methylcyclohexane (98.2 amu) having a higher molecular weight than toluene (92.1 amu), toluene melts at a higher temperature. Why? [Think about how the molecules can interact with each other based on their shape.]

Textbook Question

Draw a picture of the hydrogen bonding in methanol.

Textbook Question

Which has

g. the higher boiling point: 1-bromopentane or 1-chloropentane?

Textbook Question

Which has

h. the higher boiling point: diethyl ether or butyl alcohol?

Textbook Question

For each pair of compounds, predict which compound has the higher boiling point. Check [TABLE 6-2] to see if your prediction was right; then explain why that compound has the higher boiling point.

c. 1-bromobutane and 1-chlorobutane

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Choose the molecule in each pair you'd expect to have the higher boiling point. Explain your reasoning. (a) eicosane ( C20H42) vs pentadecane

Key Concepts

Boiling Point and Molecular Weight

Intermolecular Forces

Structural Isomerism

Watch next

Choose the molecule in each pair you'd expect to have the higher boiling point. Explain your reasoning.
(a) eicosane ( C₂₀H₄₂) vs pentadecane