Textbook Question
The benzylic bromide shown undergoes neither SN1 nor SN2 substitution reactions. Explain.
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Ch. 24 - Benzene II: Reactions Influenced by the Aromatic Ring
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471
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Table of contents
- Ch. 2 - General Chemistry Translated: Finding the Electrons114
- Ch. 3 - Alkanes and Cycloalkanes: Properties and Conformational Analysis109
- Ch. 4 - Acids and Bases: Electron Flow144
- Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics118
- Ch. 6 - Stereoisomerism: Arrangement of Atoms in Space112
- Ch. 7 - Principles of Green (Organic) Chemistry3
- Ch. 8 - Alkenes I: Properties and Electrophilic Additions158
- Ch. 9 - Alkenes II: Oxidation and Reduction150
- Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions101
- Ch. 11 - Properties and Synthesis of Alkyl Halides: Radical Reactions108
- Ch. 12 - Substitution and Elimination: Reactions of Haloalkanes172
- Ch. 13 - Alcohols, Ethers and Related Compounds: Substitution and Elimination239
- Ch. 14 - Structural Identification I: Infrared Spectroscopy and Mass Spectrometry54
- Ch. 15 - Structural Identification II: Nuclear Magnetic Resonance Spectroscopy93
- Ch. 16 - Metals in Organic Chemistry48
- Ch. 17 - Carbonyl Addition Reactions: Aldehydes and Ketones45
- Ch. 18 - Nucleophilic Acyl Substitution I: Carboxylic Acids18
- Ch. 19 - Nucleophilic Acyl Substitution II: Carboxylic Acid Derivatives39
- Ch. 20 - Enolates: Carbonyl Addition and Substitution13
- Ch. 21 - Conjugated Systems I: Stability and Addition Reactions56
- Ch. 22 - Conjugated Systems II: Pericyclic Reactions54
- Ch. 23 - Benzene I: Aromatic Stability and Substitution Reactions64
- Ch. 24 - Benzene II: Reactions Influenced by the Aromatic Ring62
- Ch. 25 - Amines: Structure, Reactions, and Synthesis27
- Ch. 26 - Amino Acids, Proteins, and Peptide Synthesis9
- Ch. 27 - Carbohydrates, Nucleic Acids, and Lipids54
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
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Mullins 1st EditionCh. 24 - Benzene II: Reactions Influenced by the Aromatic RingProblem 38
Mullins 1st EditionCh. 24 - Benzene II: Reactions Influenced by the Aromatic RingProblem 38Chapter 23, Problem 38
In the second propagation step in the bromination of toluene, Br2 is only attacked by a radical on the substituent carbon. Why?
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Identify the radical species involved in the bromination of toluene. In this case, the radical is located on the benzylic carbon, which is the carbon directly attached to the benzene ring.
Understand the concept of radical stability. The benzylic radical is stabilized by resonance, as the unpaired electron can be delocalized over the aromatic ring, making it more stable than a radical on a non-benzylic carbon.
Recognize that in the second propagation step, the radical on the benzylic carbon attacks a bromine molecule (Br₂). This is because the benzylic radical is more reactive due to its stability and the ability to form a new C-Br bond.
Consider the mechanism of the reaction. The radical on the benzylic carbon abstracts a bromine atom from Br₂, forming a new C-Br bond and generating a bromine radical.
Note that the formation of the benzylic bromide is favored due to the stability of the benzylic radical and the relatively weak Br-Br bond, which makes it susceptible to attack by the radical.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Radical Stability
In organic chemistry, the stability of radicals is crucial for understanding reaction mechanisms. Toluene, with its methyl group, stabilizes the radical formed during bromination through hyperconjugation and inductive effects. This stabilization makes the radical on the substituent carbon more favorable for reaction compared to other positions.
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The radical stability trend.
Selectivity in Electrophilic Aromatic Substitution
Electrophilic aromatic substitution (EAS) reactions, such as bromination, exhibit regioselectivity based on the nature of substituents on the aromatic ring. The methyl group in toluene is an electron-donating group, which directs electrophiles to the ortho and para positions, making the substituent carbon the most reactive site for radical attack.
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Bromination Mechanism
The bromination of toluene involves a radical mechanism where bromine (Br₂) dissociates into bromine radicals. In the second propagation step, the generated radical preferentially attacks the carbon atom adjacent to the methyl group due to the increased stability of the resulting radical, leading to a more favorable reaction pathway.
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00:54Mechanism of Allylic Bromination.
Related Practice
Textbook Question
Suggest the reagents used to effect the transformations shown.
(c)
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Textbook Question
Suggest the reagents used to effect the transformations shown.
(a)
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Textbook Question
In Chapter 13, we learned that epoxide opening can give different products, depending on whether the reaction occurs under acidic or basic conditions. Explain why the epoxide shown opens identically under either set of conditions.
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Textbook Question
When (R)-(1-bromoethyl)benzene is treated with sodium cyanide, a single enantiomer is produced. However, upon treatment of the same molecule with water, a mixture of two enantiomers is obtained.
(a) Explain these results.
(b) Why is only partial racemization sometimes observed?
Textbook Question
Oxidation of the phenol shown gives a single quinone product. Predict this product and explain why it is the only one formed.