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

19. Reactions of Aromatics: EAS and Beyond

EAS:Synergistic and Competitive Groups

Organic Chemistry

19. Reactions of Aromatics: EAS and Beyond

EAS:Synergistic and Competitive Groups

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7:15 minutes

Problem 14

Textbook Question

Explain why fluorobenzene is more reactive than chlorobenzene toward electrophilic aromatic substitution but chloromethylbenzene is more reactive than fluoromethylbenzene.

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Fluorobenzene and chlorobenzene are both halobenzenes, where the halogen atom (fluorine or chlorine) is directly attached to the benzene ring. Halogens are electron-withdrawing due to their inductive effect (-I) but also donate electron density to the ring through resonance (+R).

In fluorobenzene, the fluorine atom is highly electronegative, which makes its inductive electron-withdrawing effect stronger than that of chlorine. However, fluorine is also a better resonance donor than chlorine because of its smaller size and better orbital overlap with the π-system of the benzene ring. This resonance donation increases the electron density in the ortho and para positions of the benzene ring, making fluorobenzene more reactive toward electrophilic aromatic substitution.

In chlorobenzene, the chlorine atom also exhibits both inductive withdrawal (-I) and resonance donation (+R). However, chlorine's resonance donation is weaker than that of fluorine due to its larger size and poorer orbital overlap with the benzene π-system. As a result, the electron density in the ring is not as effectively increased, making chlorobenzene less reactive than fluorobenzene toward electrophilic aromatic substitution.

For chloromethylbenzene and fluoromethylbenzene, the halogen atom is not directly attached to the benzene ring but is instead part of the methyl group. In this case, the inductive electron-withdrawing effect of the halogen dominates because there is no resonance interaction with the benzene ring. Chlorine's inductive effect is weaker than fluorine's, so the electron density on the benzene ring is less reduced in chloromethylbenzene compared to fluoromethylbenzene.

As a result, chloromethylbenzene is more reactive than fluoromethylbenzene toward electrophilic aromatic substitution because the benzene ring in chloromethylbenzene retains more electron density, making it more attractive to electrophiles.

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Key Concepts

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

Electrophilic Aromatic Substitution (EAS)

Electrophilic Aromatic Substitution is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. The reactivity of the aromatic compound in EAS is influenced by the nature of substituents already present on the ring, which can either donate or withdraw electron density, affecting the stability of the intermediate carbocation formed during the reaction.

Electron-Withdrawing and Electron-Donating Groups

Substituents on an aromatic ring can be classified as electron-withdrawing groups (EWGs) or electron-donating groups (EDGs). EWGs, like fluorine, decrease the electron density of the ring, making it less reactive towards electrophiles, while EDGs, such as alkyl groups, increase electron density, enhancing reactivity. The balance of these effects determines the overall reactivity of substituted aromatic compounds in EAS.

Resonance and Inductive Effects

Resonance and inductive effects are key factors influencing the stability of intermediates in EAS. Fluorine, despite being a strong EWG due to its electronegativity, can engage in resonance with the aromatic system, but its overall electron-withdrawing nature makes fluorobenzene less reactive than chlorobenzene. Conversely, chloromethylbenzene has a stronger electron-donating inductive effect from the alkyl group, making it more reactive than fluoromethylbenzene, which is hindered by fluorine's strong electronegativity.

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