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

3. Acids and Bases

Ranking Acidity

Organic Chemistry

3. Acids and Bases

Ranking Acidity

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6:55 minutes

Problem 53a

Textbook Question

Compare the properties of propan-2-ol (I) and the hexafluoro analog (II).

(a) Compound II has almost triple the molecular weight of I, but II has a lower boiling point. Explain.

Verified step by step guidance

Step 1: Analyze the molecular structures of propan-2-ol (I) and its hexafluoro analog (II). Compound I has two methyl groups (-CH3) attached to the central carbon, while compound II has two trifluoromethyl groups (-CF3) attached to the central carbon. The presence of fluorine atoms in II significantly increases its molecular weight compared to I.

Step 2: Compare the boiling points of the two compounds. Despite the higher molecular weight of II, its boiling point (58 °C) is lower than that of I (82 °C). This suggests that intermolecular forces in II are weaker than in I.

Step 3: Examine the dipole moments of the two compounds. Compound I has a dipole moment of 1.66 D, while compound II has a much lower dipole moment of 0.32 D. The lower dipole moment in II indicates reduced polarity, which diminishes the strength of dipole-dipole interactions between molecules.

Step 4: Consider the role of hydrogen bonding. The hydroxyl group (-OH) in both compounds can form hydrogen bonds. However, in compound II, the electron-withdrawing trifluoromethyl groups (-CF3) reduce the electron density on the oxygen atom of the hydroxyl group, weakening its ability to form hydrogen bonds. This contributes to the lower boiling point of II.

Step 5: Summarize the key factors. The lower boiling point of compound II, despite its higher molecular weight, is primarily due to its reduced polarity (lower dipole moment) and weakened hydrogen bonding capability caused by the electron-withdrawing effect of the trifluoromethyl groups.

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Key Concepts

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

Molecular Weight and Boiling Point

Molecular weight is the sum of the atomic weights of all atoms in a molecule, influencing physical properties like boiling point. Generally, higher molecular weight compounds have higher boiling points due to increased van der Waals forces. However, in this case, despite compound II having a higher molecular weight, its lower boiling point can be attributed to weaker intermolecular forces due to the presence of fluorine atoms, which disrupt hydrogen bonding.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between molecules, significantly affecting boiling points. Propan-2-ol (I) can form strong hydrogen bonds due to its hydroxyl group, leading to a higher boiling point. In contrast, the hexafluoro analog (II) has weaker dipole-dipole interactions and lower hydrogen bonding capability, resulting in a lower boiling point despite its higher molecular weight.

Dipole Moment and Polarity

The dipole moment is a measure of the separation of positive and negative charges in a molecule, indicating its polarity. A higher dipole moment suggests stronger interactions with polar solvents and greater hydrogen bonding potential. Propan-2-ol (I) has a higher dipole moment (1.66 D) compared to the hexafluoro analog (II) (0.32 D), which explains its stronger hydrogen bonding and higher boiling point despite the latter's greater molecular weight.

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