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:Ortho vs. Para Positions

Organic Chemistry

19. Reactions of Aromatics: EAS and Beyond

EAS:Ortho vs. Para Positions

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Problem 39

Textbook Question

With a small electron-donating group on the ring, it is possible to get as much as a 2:1 ratio of ortho to para. Why might this be?

Verified step by step guidance

Identify the electron-donating group on the benzene ring. In this case, the methyl group (CH₃) is the electron-donating group.

Understand that electron-donating groups activate the benzene ring towards electrophilic aromatic substitution, making the ortho and para positions more reactive.

Recognize that the ortho position is sterically hindered compared to the para position, but the methyl group is small, reducing steric hindrance and allowing for significant ortho substitution.

Consider the resonance effect: the electron-donating group increases electron density at the ortho and para positions, stabilizing the carbocation intermediate formed during the reaction.

Conclude that the combination of electronic effects (activation and resonance stabilization) and reduced steric hindrance leads to a higher ratio of ortho to para substitution, resulting in the observed 2:1 ratio.

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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 reaction where an electrophile replaces a hydrogen atom on an aromatic ring. In the presence of a catalyst like FeCl3, chlorine acts as the electrophile, attacking the benzene ring. The position of substitution is influenced by existing substituents on the ring, which can direct the incoming group to ortho, meta, or para positions.

Ortho/Para Directing Groups

Ortho/para directing groups are substituents on an aromatic ring that increase the electron density at the ortho and para positions, making them more reactive to electrophilic attack. Electron-donating groups, such as alkyl groups, stabilize the carbocation intermediate formed during EAS, favoring substitution at these positions. This explains the higher ratio of ortho to para products in the presence of such groups.

Steric and Electronic Effects

Steric and electronic effects influence the distribution of substitution products in EAS. Small electron-donating groups, like methyl, enhance electron density at ortho and para positions, but steric hindrance is less at the para position. However, the ortho position can still be favored due to proximity effects and resonance stabilization, leading to a 2:1 ortho to para product ratio.

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