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

11. Radical Reactions

Radical Stability

Organic Chemistry

11. Radical Reactions

Radical Stability

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5:31 minutes

Problem 22c

Textbook Question

Use the information in Table 4-2 to explain why toluene (PhCH₃) has a very high octane rating of 111. Write an equation to show how toluene reacts with an alkyl free radical to give a relatively stable radical.

Verified step by step guidance

Step 1: Analyze the bond dissociation energy data provided in the tables. Toluene (PhCH3) contains a benzylic C-H bond, which has a bond dissociation energy of 376 kJ/mol. This is significantly lower than other types of C-H bonds, such as primary (423 kJ/mol), secondary (413 kJ/mol), and tertiary (403 kJ/mol). The lower bond dissociation energy indicates that the benzylic hydrogen is easier to remove, forming a benzylic radical.

Step 2: Explain the stability of the benzylic radical. The benzylic radical formed after homolytic cleavage of the C-H bond is stabilized by resonance. The unpaired electron on the benzylic carbon can delocalize into the aromatic ring, creating multiple resonance structures. This delocalization significantly increases the stability of the radical compared to radicals formed from other types of C-H bonds.

Step 3: Relate the stability of the benzylic radical to the high octane rating of toluene. The high stability of the benzylic radical means that toluene can resist premature combustion in an engine, which is a desirable property for high-octane fuels. The ability to form stable intermediates during combustion contributes to its high octane rating of 111.

Step 4: Write the equation for the reaction of toluene with an alkyl free radical. The alkyl free radical (R•) can abstract a hydrogen atom from the benzylic position of toluene, forming a benzylic radical (PhCH2•) and an alkane (RH). The equation is: PhCH3 + R• → PhCH2• + RH.

Step 5: Highlight the importance of resonance stabilization in organic reactions. The resonance stabilization of the benzylic radical is a key concept in organic chemistry, as it explains why certain reactions occur more readily and why certain intermediates are more stable. This principle is widely applicable in understanding reaction mechanisms and designing synthetic pathways.

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Key Concepts

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

Octane Rating

The octane rating is a measure of a fuel's ability to resist knocking during combustion. A higher octane rating indicates better performance in high-compression engines, as it signifies that the fuel can withstand greater pressure before igniting. Toluene's high octane rating of 111 suggests it has excellent anti-knock properties, making it a desirable component in gasoline.

Free Radical Reactions

Free radical reactions involve species with unpaired electrons, which are highly reactive. In the context of toluene, when it reacts with an alkyl free radical, it can form a new radical through a process called hydrogen abstraction. This reaction is significant because it leads to the formation of a relatively stable radical, which can further participate in chemical reactions without decomposing immediately.

Stability of Radicals

The stability of free radicals is influenced by their structure and the surrounding substituents. Toluene, with its aromatic ring, stabilizes the radical formed after hydrogen abstraction due to resonance. This delocalization of the unpaired electron over the aromatic system reduces the energy of the radical, making it more stable compared to aliphatic radicals, which are less stabilized.

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Related Practice

Textbook Question

a. Which of the hydrogens in the structure in the margin is the easiest for a chlorine radical to remove?

b. How many secondary hydrogens does the structure have?

Textbook Question

Rank the following radicals in decreasing order of stability. Classify each as primary, secondary, or tertiary.

a. The isopentyl radical,

b. The 3-methyl-2-butyl radical,

c. The 2-methyl-2-butyl radical,

Textbook Question

Use the information in Table 4-2 (p. 167) to rank the following radicals in decreasing order of stability.

Textbook Question

For each set of reactive intermediates, rank them in order of reactivity (1 = most reactive).

(b)

Textbook Question

A student adds NBS to a solution of 1-methylcyclohexene and irradiates the mixture with a sunlamp until all the NBS has reacted. After a careful distillation, the product mixture contains two major products of formula C₇H₁₁Br.

b. Rank these three intermediates from most stable to least stable.

Textbook Question

Explain the dramatic difference in rotational energy barriers of the following three alkenes. (Hint: Consider what the transition states must look like.)

Textbook Question

The following ethers are ranked according to their ability to form explosive peroxides. Explain this ranking based on your knowledge of the reaction mechanism.

Textbook Question

Based on the stability of the radicals produced, predict which bond in each pair would have the higher bond-dissociation energy.

(a)

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