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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3:30 minutes

Problem 73

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.]

Verified step by step guidance

Consider the molecular structures of methylcyclohexane and toluene. Methylcyclohexane is a saturated hydrocarbon (alkane) with a cyclohexane ring and a methyl group attached, while toluene is an aromatic compound with a benzene ring and a methyl group attached.

Analyze the intermolecular forces present in each molecule. Methylcyclohexane primarily exhibits London dispersion forces, which are relatively weak and depend on molecular size and shape. Toluene, on the other hand, has stronger π-π interactions due to the aromatic benzene ring.

Understand how molecular shape affects packing efficiency in the solid state. Toluene's flat, planar aromatic structure allows for better stacking and closer packing in the solid phase, which increases the melting point. Methylcyclohexane's non-planar, bulky structure reduces packing efficiency.

Consider the role of molecular weight. While methylcyclohexane has a slightly higher molecular weight, melting point is more influenced by intermolecular forces and molecular packing rather than molecular weight alone.

Conclude that the higher melting point of toluene is due to its stronger intermolecular forces (π-π interactions) and more efficient molecular packing in the solid state compared to methylcyclohexane.

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Key Concepts

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

Molecular Weight vs. Melting Point

Molecular weight is the sum of the atomic weights of all atoms in a molecule, but it does not directly correlate with melting point. Melting point is influenced by intermolecular forces, such as hydrogen bonding, dipole-dipole interactions, and London dispersion forces. Thus, a compound with a higher molecular weight may not necessarily have a higher melting point if its intermolecular interactions are weaker.

Molecular Shape and Interactions

The shape of a molecule affects how closely molecules can pack together, influencing their physical properties. Toluene, being a more symmetrical and planar molecule, can stack more efficiently than the bulky methylcyclohexane. This efficient packing leads to stronger van der Waals forces in toluene, contributing to its higher melting point despite its lower molecular weight.

Intermolecular Forces

Intermolecular forces are the forces of attraction between molecules, which play a crucial role in determining physical properties like melting and boiling points. Toluene exhibits stronger dipole-dipole interactions due to its polar C-H bonds, while methylcyclohexane's interactions are primarily due to weaker London dispersion forces. The strength and type of these forces significantly influence the melting behavior of the substances.

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